Bored & Stroked

Increasing the stroke lenght increases Cubic Inch Displacement (CID), which increase Torque (TQ measure in pounds per foot {ft-lbs}), but also increases friction, which consumes Horsepower (HP).

Brake Horsepower (BHP)is measured at the flywheel on an engine dyno. RWHP is measured at the rear wheels on a chassis dyno.
I recommend using low drag rings and pistons to help offset this fictional power loss and lighter weight pistons and rods.
Additionally the increased stroke lowers the peek TQ Revolutions Per Minute (RPM), which is a good thing for street machines.
More torque at lower RPM means more rapid acceleration.
The follow examples are extremes to illustrate the point. 
 
Bore 5.00" X Stroke 2.44" = 383   421TQ/4000rpm 380HP/5200rpm 
Bore 4.03" X Stroke 3.75" = 383   417TQ/3700rpm 368HP/5000rpm 
Bore 3.00" X Stroke 6.78" = 383   387TQ/3500rpm 313HP/4700rpm
       

Increasing the engine's CID by stroking increases the torque, but as you can see CAN also reduce horsepower as you can see.
This is true whether the increase comes from a longer stroker or a larger bore diameter.
However increasing the bore diameter produces more torque less horsepower lost to friction than stroking. The horsepower is lost to friction. It is more desirable to increase bore diameter to make power, but once the bore is bigger, more stroke will make more power again. However bigger engine do make more power, see bottom of page.
 5.00" X  6.78" = 1065   1150TQ/4000rpm   378HP/2000rpm   2HP/4200 
 
 
When stroking a mild performance or stock engine, the top end HP can be a negative number, but the lower RPM numbers will be a gain.
Increasing valve lift can help to offset this BHP loss, and helps supply more air/fuel to fill the greater cylinder displacement.
For example a stock 350 to 383 conversion. Two sets of figures show the low speed power and the peek numbers. Bore X Stroke = CID
 
 
4.03" X 3.48" = 355         355TQ/2000rpm 175HP/2500rpm 395TQ/4000rpm 365HP/5500rpm

4.03" X 3.75" = 383         389TQ/2000rpm 191HP/2500rpm 417TQ/3600rpm 368HP/5000rpm
 
                                          +34TQ/2000rpm +16HP/2500rpm     Peek Power +22TQ +3HP

Notice the torque and horsepower went up in the lower RPM range and the gain in TQ remained at the peek output RPM. In this application we did not loose BHP, but the gain was minimal. Increasing the stoke to theoretical 4.1" (too long to fit in block) will loose BHP as you can see below.
 
4.03" X 4.10" = 418         432TQ/2000rpm 210HP/2500rpm 447TQ/3300rpm 364HP/4900rpm
 
                                           +77TQ/2000rpm +35HP/2500rpm     Peek Power +52TQ -1HHP
 
The longer stroke also reduced the RPM where the peek power numbers occured.
This is ideal for street performance. Increasing camshaft lift will move the power band back up.
Here is the same 418 with 1.6:1 rockers to increase the lift.
 
4.03" X 4.1" = 418         432TQ/2000rpm 210HP/2500rpm 448TQ/3500rpm 369HP/5000rpm
 
                                          +77TQ/2000rpm +35HP/2500rpm     Peek Power +53TQ +4HP
 
Here we see the bootom end power remain the same, but the extra lift help pick up the top end.
 
 
 
To further illustrate the results of boring and stroking, let's look at some numbers generated by a computer dynamometer.
Entry level computer dynos like the one I use can not duplicate the "real world", but they can come close and are very useful for evaluating various combinations. More expensive P/C dynos can better duplicate the "real world",  but require exhaustive data entry.
 
Let's start with a Small Block Chevy 350, first boring it over size, then make it a 383.
Then use a 400 block and stroke it some more, then use an aftermarket block.
Bore X Stroke = CID

 
 
4.00" X 3.48" = 350   390TQ/4000rpm 364HP/5500rpm +  0TQ + 0HP
 
4.03" X 3.48" = 355   395TQ/4500rpm 365HP/6000rpm +  5TQ + 1HP
4.06" X 3.48" = 360   400TQ/4500rpm 366HP/5500rpm + 10TQ +2HP
 
4.03" X 3.75" = 383   417TQ/3800rpm 368HP/5000rpm +27TQ + 4HP
4.03" X 4.00" = 408   439TQ/3500rpm 367HP/4800rpm +47TQ + 3HP
 
4.15" X 3.48" = 377   414TQ/4000rpm 370HP/5000rpm +24TQ  +6HP
4.15" X 3.75" = 407   438TQ/3600rpm 371HP/4800rpm +48TQ  +7HP
4.15" X 4.00" = 434   460TQ/3400rpm 366HP/4500rpm +80TQ  +2HP
 
4.25" X 3.48" = 426   429TQ/3800rpm 373HP/5000rpm +39TQ  +9HP
4.25" X 3.75" = 426   455TQ/3600rpm 370HP/5000rpm +65TQ  +6HP
4.25" X 4.12" = 468   495TQ/2500rpm 367HP/4500rpm +105TQ +3HP
4.25" X 4.25" = 482   510TQ/3500rpm 364HP/4500rpm +120TQ +0HP

 

As you can see stroking makes TQ, but the biggest HP gain came from increasing the bore and leaving the stroke at 3.48" To make the BIG BHP power gains, we add more cam duration, compression and better flowing heads. Typically a 383 will make between 400 to 520BHP in a street performance application. Keep in mind that horsepower is related to RPM, so the high you rev the engine the more BHP you can make. Torque is increased with more cubic inches and better volumetric (air in exhaust out) and thermal (burning of the fuel) efficiency.
 
 
Why Increase The Stroke?
It makes the engine bigger. BIGGER ENGINES MAKE MORE POWER
 
The increased engine size will accelerated harder with the extra torque and stroking is the most affordable way to increase engine size. As you can see both increasing the bore diameter and increasing the stroke increases the torque. However increasing the bore diameter is more desirable to make both more torque and more horsepower. As you can see from the 426 verses 482 combos the smaller motor looks like the better choice for making horsepower, but keep in mind that you can exchange torque for horsepower. Or in other words bigger valves, ports, carb, cam and headers will give the following results.
4.25" X 3.48" = 426   429TQ/3800rpm 373HP/5000rpm +39TQ  +9HP
4.25" X 4.25" = 482   510TQ/3500rpm 364HP/4500rpm +120TQ +0HP
 
Which motor would you choose?
The end result being more torque with no lost in horsepower.
 
Why Increase The Bore?
Increasing the bore sizes results in more net power, but .060" isn't enough to make an impressive difference, and is the limited over bore for most blocks. The only options are stepping up to a big block if using a small block, or in the case of SB Chevys using a 400 block, or buying an aftermarket block such as a Dart Iron Eagle.
These blocks close close to $2000
 
As you can see from the exaggerated extremes at the top of this article, the big bore short stroke engine made the biggest gains, while the small bore long stroke gave up all the horsepower.
The realistic .030 over with a 3.75" stroke made almost as much torque as the large bore motor, but wasn't impressive in producing HP.

To compensate increasing airflow at the high RPM and trading torque for horsepower is effective. For example changing the intake manifold from a duel plenum to a single plenum will produce these results.
Duel Plenum    4.03" X 3.75" = 383   406TQ/4000rpm 374HP/5500rpm
Single Plenum 4.03" X 3.75" = 383   416TQ/4000rpm 396HP/5500rpm
+10TQ +22HP

While these numbers look better, we need to look at the lower RPM number to decide if a single plenum is what we really want.
Single Plenum   4.03" X 3.75" = 383   348TQ/2500rpm 166HP/2500rpm
Duel Plenum   4.03" X 3.75" = 383   372TQ/2500rpm 177HP/2500rpm +24TQ +11HP 

At this speed the duel plane/plenum manifold will be accalerating faster. What this reveals is power can be produced with more cubic inches or from more RPM, YOU CHOOSE!
 
In the real would some combination work better than others.
The Stroker Engine Company will help you find the best combination for you application and budget.
 

My motto is bigger engines make more power. While the computer may predict only a small increase in BHP, in the real world more BHP is made at lower RPM.

 Here is a 355cid verse a 427cid as tested by Popular Hot Rodding Magazine using the same heads and camshaft.

RPM    355 TQ - BHP    427 TQ - BHP                                    
2600        346 - 171           507 - 251 increase 161TQ 80BHP
3600        422 - 290           549 - 376 increase 127TQ 87BHP
4600        456 - 399           556 - 487 increase 100TQ 88BHP
5600        444 - 474           475 - 507 increase   31TQ 34BHP
6000        425 - 486           418 - 478       loss      7TQ  8BHP

Peek TQ 5000 458            4000 570

Peek BHP 6000 486          5400  512

 Let's Talk about Rod Lengths Click Here

Camshaft Talk

The Theory of Horsepower

By Charley Rockwell

 

Ten years ago, the only reliable way to produce more power from your engine was to purchase high performance parts, install them in your engine, and then test them on a dynamometer or race track. It as an entirely trial and error process. Some professional engine builders managed to spend enough on parts and testing to blunder into a special combination of parts and machining that produced winning horsepower.

 

Today, you do not need to perform so many trial and error tests to produce horsepower. With the aid of this series of articles and some modern computer programs, you can predict the power improvement of performance modifications with an accuracy of nearly 2%. These articles will cover the general theory of engine power improvement and inform the reader how to quickly evaluate engine modifications with a calculator. Computer programs are only required to get the accuracy down to a few percent.

 

 

 

PART I - Valve Area Determines Horsepower

 

The piston creates a vacuum as it moves down the cylinder during the intake stroke. The atmosphere pushes air into this vacuum through the intake valve. The faster the piston moves the faster the air has to flow through the intake valve. Simply stated, "The faster you rev your engine, the faster the air has to flow through the intake."

 

Engineers have observed that air cannot really flow through the intake at speeds exceeding 650 feet per second. This appears to be a critical speed at which it takes more power to shove air through the intake than you get by burning the air in the cylinder. For engine that burn very efficiently, the speed could be as high as 710 feet per second. an inefficient burning engine may have a critical intake speed of only 600 feet per second. An efficient burning engine would be a 4 valve per cylinder Cosworth Formula 1 engine. An example of an inefficient burning engine is a Ford Model T sidevalve engine. A maximum intake speed of 650 feet per second works very well for engine developing peak power between 4000 and 8500 rpm.

Now sit back and think about what you have just read because I will use it to explain the two most important concepts in engine design:

 

  1. The faster you rev your engine, the more power you will make UNTIL the intake air speed reaches 650 feet per second.

 

2) The larger the intake valve, the faster you can rev your engine before the intake air speed reaches 650 feet per second.

 

The above two concepts lead to the most important conclusion:

YOUR ENGINE'S MAXIMUM POWER IS DETERMINED BY YOUR INTAKE VALVE AREA.

 

 

 

Yes, it is the engine with the biggest valves that wins the races, not the engine with the most cubic inches of displacement. A 180 cubic inch Formula 1 engine with 32 inches of intake valve area makes 720 horsepower and a 427 cubic inch racing Ford with only 28 inches of valve area can only make 600 horsepower. It is interesting to note that the Formula 1 engine has to spin at 13,500 rpm for maximum power and the Ford engine at only 6800 rpm for maximum power. This illustrates the following important concept:

Valve area determines total potential horsepower and displacement determines how fast your engine has to rev to produce maximum power.

 

Let me explain the above concept. Suppose we had a single cylinder engine with a valve area of one square inch, and at 5000 rpm the piston was moving air throught the intake valve at 650 feet per second. If I rev the engine faster, I will not make any more power because it consumes too much power to shove the air through the valve faster that 650 feet per second. If I double the area of the piston, than air will be going through the intake valve at 650 feet per second at only 2500 rpm. If the intake valve remains equal, the bigger the piston the sooner the intake speed reaches 650 feet per second. The horsepower will remain the same with the small piston at 5000 rpm or with the big piston at 2500 rpm.

 

Let me repeat the concept one more time:

Valve area determines total potential horsepower and displacement determines how fast your engine has to rev to produce maximum power.

 

 

 Revs: they are a measure of how quickly an engine can complete a cycle and repeat it over again. Revs are primarily limited by the life of the reciprocating parts. Giving for granted that the valve train will survive higher revs than the piston-rod assembly (and it's not always SO granted) the upper rev limit is usually obtained as a first approximation by the "mean piston speed". If you want to build a hot rod that will be rebuilt every 1000 turns of the crank, you can go up to a mean piston speed of 28 m/s (92 feet per second). If you want your engine to last a race, you can risk 24 m/s (79 fps), if you want your engine to last a season without worry, you'd better focus on 20 m/s (66 fps). On a road going bike that is supposed to last several years, 18 m/s is the highest safe figure. These figures are for 1980's technology motors. You can increase them by 25% if you are using 21st century technology.

It's always a good idea to look at mean piston speed and consider the following:

1. Internal inertia stress on the bottom end components increases with the SQUARE of mean piston speed.

2. Sonic velocity places a limit on aspiration efficiency at high mean piston speeds - the airflow just can't keep up with piston motion, no matter how short and efficient the inlet tract is.

The first issue has been dealt with over the years with better design and materials technology.

The second limit is basic physics and there is no solution other than to eek out incremental improvements with better detail design.

The current "limit" is about 5200 FPM, which is the regime of F1 and NASCAR engines.

The new LS7 is running 4733 FPM at the 7100 rev fuel cutoff, which is VERY Impressive for a production engine, even DOHC, but if you have enough budget for titanium rods, inlet valves, dry sump, etc. it can be done with Gen IV architecture components.

If you want to approach this level using vintage SB architecture components you better have at least a $20,000 budget, but it still won't be reasonably streetable.

About 3800-4000 FPM is a reasonable goal for an enthusiast using vintage architecture SB components with a few thousand dollars to spend on a good street or dual purpose high performance engine.

Connecting Rod Length Comparison

 

By Rick Draganowski

dragan@harborside.com

 

 http://www.rustpuppy.org/rodstudy.htm http://www.rustpuppy.org/

Piston movement was computed by simulating the crankshaft/connecting rod/piston assembly in several precise engineering drawings (DesignCad) and then determining the exact amount of piston movement for each of 256 divisions of one rotation.

 

The piston movement data was then used as an input vector in a MathCad program to calculate velocity, acceleration, and dynamic forces.

 

The simulation of an infinitely long connecting rod, which imparts true harmonic motion to the piston, is the starting point.

 

The motion generated by a finite length connecting rod is quite distorted by comparison. It has much more velocity and acceleration at the top of the stroke compared to the bottom. A graph of the movement is peaked at the top of each cycle and rounded and flattened at the bottom. This is caused by the rod angle increasing and pulling the piston down and adding to the motion caused by the crankshaft rotating down from top dead center. At the bottom as the rod journal slows the angle decreases. This retards the movement of the piston by subtracting the rod angle component that was added at the top of the stroke from the crankshaft movement component at the bottom of the stroke.

 

Compression and combustion pressures are in opposition to the inertial forces so the top of exhaust and intake strokes generate the largest forces on the rod.

 

 

1) Maximum Piston Acceleration

 

This table is for a 3.75" stroke used in a 400 0r 383 small block Chevy engine.

------infinite rod--------6.0" rod---5.7" rod---5.565" rod

5000rpm 1332G 1749G 1776G 1790G

6000rpm 1933G 2525G 2558G 2578G

7000rpm 2631G 3437G 3482G 3509G

 

Percent difference due to rod length in above table.

Difference between 6" rod and 5.565" rod 2.34%

Difference between 6" rod and 5.7" rod 1.54%

Difference between 5.7" rod and 5.565" rod 0.79%

 

This table is for a 3.48" stroke used in a 350 or 305 small block Chevy engine.

------infinite rod---------6.0" rod---5.7" rod

5000rpm 1240G 1600G 1623G

6000rpm 1786G 2305G 2338G

7000rpm 2432G 3138G 3182G

 

 

2) Maximum Connecting Rod Dynamic Load (Tension)

 

This table is for a 3.75" stroke used in a 400 or 383 small block Chevy engine. The forces are based on the weight of the piston and pin assembly and do not include the percentage of force generated by the acceleration of the end of the connecting rod. The reference piston is the stock replacement Silv-O-Lite piston for a 400 engine.

 

------infinite rod-----------6.0" rod-----5.7" rod----5.565" rod

5000rpm 2249LBS 2938LBS 2976LBS 3000LBS

6000rpm 3239LBS 4232LBS 4287LBS 4320LBS

7000rpm 4409LBS 5769LBS 5834LBS 5849LBS

 

Percent difference due to rod length in above table.

 

Difference between 6" rod and 5.565" rod 2.34%

Difference between 6" rod and 5.7" rod 1.54%

Difference between 5.7" rod and 5.565" rod 0.79%

 

 

3) Maximum Rod Angularity

 

This is the angle the connecting rod makes with the axis of the cylinder bore at 90 degrees after top dead center (maximum excursion from bore axis. This measurement is for the 3.75" stroke of the 400 and 383 only.

 

6.0" rod-----18.21 degrees

5.7" rod-----19.20 degrees

5.565" rod-19.69 degrees

 

 

4) Cylinder Wall Load

 

Percentage of compression and combustion force against the top of piston transmitted to the major thrust face of the piston and then to the cylinder wall.

 

This table is for the 3.75" stroke.

6.0" rod----32.89%

5.7" rod----34.83%

5.565" rod-35.64%

 

This table is for the 3.48" stroke.

6.0" rod---30.31%

5.7" rod---32.05%

 

 

5) Piston Speed

 

Maximum piston speed for the 3.75" stroke at 5000 rpm.

 

Infinite rod---81.68 feet per second, 55.69 MPH

6.0" rod------85.64 feet per second, 58.4 MPH

5.7" rod------86.01 feet per second, 58.6 MPH

5.565" rod---86.20 feet per second, 58.8 MPH

 

 

6) Effective Stroke

 

Because of the mechanical advantage provide by the toggling effect of the rod the shorter rods act as if they were in a longer stroke engine at the top of the stroke. This effect would make the short rod engine rev faster from 2000 to 4000 rpm and the circle track people claim that acceleration out of the turns is significantly improved with the shorter rod. In all other factors the longer rod comes out superior...

 

Effective stroke as compared to the infinite rod model for the 3.75" stroke.

 

infinite rod-=- 3.75"

6.0" rod------- 4.20"

5.7" rod------- 4.23"

5.565" rod---- 4.25"

 

Note that the differences are subtle...

 

 

7) Dwell Time

 

This measurement is of the number of crankshaft degrees the piston is within 0.250 inches of top dead center. It is the subject of much conjecture and controversy in the automotive literature.

 

This table is for a 3.75" stroke used in a 400 0r 383 small block Chevy engine.

 

Infinite rod---59.853 degrees

6.0" rod------52.397 degrees

5.7" rod------52.071 degrees

5.565" rod---51.915 degrees

 

Percentage difference in dwell time between the 6.0" rod and the 5.7" rod is 0.626%.

 

Percentage difference in dwell time between the 5.7" rod and the 5.565" rod is 0.3%.

 

Percentage difference in dwell time between the 6.0" rod and the 5.565" rod is 0.928%. (Still less than 1 percent)

 

 

This table is for a 3.48" stroke used in a 350 or 305 small block Chevy engine.

 

Infinite rod---62.188 degrees

6.0" rod------54.929 degrees

5.7" rod------54.605 degrees

 

Percentage difference in dwell time between the 6.0" rod and the 5.7" rod is 0.593% at the 3.48" stroke.

 

 

8) Author’s comments:

 

The data in this report seems to indicate that the differences between the rod lengths are exaggerated in the literature. In many (most) cases claims are anecdotal and represent the vested interests of the suppliers. I have seen no objective dyno testing of rod lengths but keep hoping for one.

 

There are real gains to be had by going to longer rods but they are small, usually a lot less than 2 percent. However, the hard-core racers are grasping at every tiny bit of performance and can justify the expense. For the more average rodder I would suggest staying with the rod length specified by the factory. Money would be far better spent on improving the heads, cam, and induction and exhaust systems. (and perhaps a supercharger..)

 

Piston Speed Calculator

Here is the rule of thumb. Figure out the piston speed in feet per minute. (stroke X max rpm you want to run,devided by 6) never go over 3500 feet per minute for stock engines, 4000 feet per minute for modified engines and 5000 feet per minute for all out race engines. example, a 4.25 stroke length stroker motor should not go over 5600 rpm for a modified engine.

 

This calculator requires the use of Javascript enabled and capable browsers. This calculator is designed to give the speed of a piston on the upward and downward travel within the engine. Enter the given engine stroke in either millimeters (MM) or inches, and the engine RPM. Then click on Calculate. The speed is returned in multiple designation units. The formula the stroke x 2 x RPM.

 
Unit
Number
Required Data Entry
Engine Stroke MM  Inches 
Engine RPM x1
   
Calculated Results
Piston Speed FPM Feet Per Minute
Piston Speed FPS Feet Per Second
Piston Speed Rounded FPM Feet Per Minute
Piston Speed Rounded FPS Feet Per Second
Piston Speed MPM Meters Per Minute
Piston Speed MPS Meters Per Second
Piston Speed Rounded MPM Meters Per Minute
Piston Speed Rounded MPS Meters Per Second
Version 1.3.2

http://www.valvespecialist.com/calculation.htm
http://www.epi-eng.com/ET-PistnVelAccel.htm

 

 

http://www.moparmusclemagazine.com/techarticles/mopp_0309_buyers_guide_to_choosing_the_best_piston_parts/index.html

http://www.fordmuscle.com/archives/2003/09/stroker/

 

A stroked engine is often looked upon as a great way to increase power N/A.

This thread should help in understanding what different combinations are out there and what they can do for them.

Stroker Explanation Links:

Stroker Motors Explained

331 Stroker Build-Up

No matter what the configuration, keep in mind that all crankshaft strokes, rod lengths, and piston heights need to correspond within thousandths of an 8.200" deck height, which is the modern day 302's deck height.

Piston Diagram 1
Piston Diagram 2

Stock 302/306 CI:

A factory 5.0L of 302 cubic inches consist of 4.00" bore, 3.00" stroke, 5.090" rod, 1.600" compression height piston.

A 306, which has the cylinder walls overbored .030", to 4.030", to create a fresh cylinder wall, maintains the factory 3.00" stroke, 5.090" rod, and 1.600" compression height piston. A 306 is a form of a budget build and is not intended to make extra power, but to create fresh cylinder walls for longer engine life and revive lost compression through high mileage engines.

A long rod 306, uses the same as above but with a 5.4" rod. The longer rod pulls away from top dead center slower, which is bad for bigger/larger volume intake ports. In short, it will reduce air speed. The increased dwell at top dead center makes the engine more prone to detonation. Low rpm torque suffers as well, due to the longer rod preferring higher rpms. This also requires a custom cam to get the dwell times to match up, in which off the shelf cams are designed for stock 5.090" rods. Unless you are trying to be different, increasing the stroke is a much better hp/dollar ratio.

Stroker Combinations:

327/331 CI Stroker:

A 331 needs a new crankshaft, new rods, and new pistons to account for the different geometry over stock. It utilizes a 4.030" bore, 3.25" stroke, 5.315" or 5.400" rod, and either a 1.250" or 1.175" compression height piston. A 327 has all the same attributes but keeps the factory 4.00" bore.

A couple companies, like DSS Racing, offer a 5.315" rod with a 1.250" compression height piston for their 331 kits. The more popular kits are typically the 5.400" rod with a 1.175" compression height piston.

342/347 CI Stroker:

A 347 needs a new crankshaft, new rods, and new pistons to account for the different geometry over stock. It utilizes a 4.030" bore, 3.40" stroke, 5.315" or 5.400" rod, and either a 1.175" or 1.090" compression height piston. A 342 has all the same attributes but keeps the factory 4.00" bore.

A few companies, like Probe Industries, offer a 5.315" rod with a 1.175" compression height piston for their 347 kits. The other option is a 5.400" rod matched with a 1.090" compression height piston.

The vast majority of 347s and an occassional 331 need extra cylinder skirt block clearance at the bottom of the cylinder walls to clear the rod bolts as a crank rotation is being made. The throw of the crank is too large to have safe tolerances to rotate.

Commonly referred to as a "big bore" 347, you can use a 331 crankshaft of a 3.25" crankshaft and a 4.125" bore.

364/369 CI:

Requires honing/boring beyond stock block bore capabilities. A Dart block, which has a safe bore range of 4.185" is a very capable block to do this.

A 363.5 (actual cubic inch) uses a 4.125" bore, 3.40" stroke, a 5.315" or 5.400" rod matched with a 1.175" or 1.090" compression height piston.

A 369 uses the same as above, but with a 4.155" bore piston.

If going with a big bore setup, like above remember a few things:

1. Dart recommends a 4.185" maximum bore, but they have been sonic-checked to well over 4.200"
2. Piston scuffing that causes wear and damage, can only be fixed with a sleeve, and the less material, the more problems.
3. With thin walls between each cylinder bore, blown headgaskets can become a problem, particular with boost/nitrous.

Engine Facts:

Piston speed in feet per minute:

A 302 with the 3.00” stroke moves 3,000 feet per minute.
A 331 with the 3.25” stroke moves 3,250 feet per minute.
A 347 with the 3.40” stroke moves 3,400 feet per minute.

The more stroke, the more piston speed is created. This creates harsher starting and stopping of the piston at top dead center and bottom dead center. The piston acts as a rock in a sling. A lighter piston becomes more and more ideal as the stroke increases. A lighter piston is linear to a shorter compression height piston.

Piston Design for 347 Strokers:

A commonly discussed issue and often exaggerated problem is the case of the wrist pin area intersecting the oil ring land.

A combination that can be tricky to accomplish for longevity, is the 347 stroker with the 3.40" stroke, 5.40" rod, and the 1.090" compression height piston. The decreased 1.090" compression height pushes the wrist pin area into the oil ringland which can cause oil consumption issues if not addressed with true engine builder prowess. The 347 with the 3.40" stroke, 5.315" rod, and the 1.175" compression height piston avoids this problem. The piston is tall enough to keep the wrist pin out of the oil ring land, and also has a longer piston skirt for better stability and oil control from bottom dead center to top dead center. Stroker pistons typically have a shorter piston skirt and in this case, the 1.090" piston has a shorter skirt than the 1.175", which originally was designed to provide crankshaft counterweight clearance at bottom dead center. Shorter skirts increase piston skirt and cylinder bore load. With the use of good pistons that have detailed tolerances and an aftermarket block, it is not much of an issue.

If long engine life and reliability are your goal (daily driver), keep the piston pin out of the ring area, by utilizing the 5.315” rod and 1.175” compression height piston. Having the piston pin close to the hot piston crown is just asking for premature engine blow-by or even failure. The oil struggles to stay on the wrist pin/boss because heat chases it away. The taller compression height also directly strengthens the piston crown.

The picture below shows the difference and how it is possible to get the 347 stroker and keep piece of mind for most:

1.090" CH Piston vs. 1.175" CH Piston

To still get the 347 stroker combination many will choose for a daily driver, piece of mind, or just overall oil control, the 5.315" rod with 1.175" compression height piston is often the most sound choice.

However, the 1.090" piston is not the issue it used to be. A proper engine builder can set proper piston to wall clearance, even tighter with better piston properties.

The use of a good dimpled oil support rail (needed for 1.090" pistons) that can't rotate in the groove (due to the dimple facing down into the pin bore and effectively locking it in place), you can help combat "extra" problems with oil consumption. Bad ones, use what amounts to a 3rd oil ring wiper with no dimple that clamps against the back of the oil ring groove and they can rotate and/or roll in place. Ideally you want a oil support rail to grip, which comes with it being the correct size, against the back of the oil ring groove and also have a dimple. A good builder and parts used are key. Here is a picture of an oil ring support in an aftermarket ls1 stroker piston, which helps put text to visualization:

Oil Ring Land Support Ring for 1.090" Pistons on a 347 Stroker

The oil ring support goes underneath the oil control expander on the very bottom, if looking at the piston from it's 'in engine' orientation. The oil support rings go directly above and below the oil control expander, with the oil ring support on the furthest wrist pin side.

Also, a less popular idea to keep "extra" oil consumption problems from occuring, the use of a wrist pin button is used. It effectively acts as the name implies. It buttons into the wrist pin, creating a near solid piston for the 1.090" compression height piston. A picture of one below:

Wrist Pin Button

Again, good machining and a good engine builder can pretty much alleviate any extra oil consumption problems that could occur.

If in doubt of your engine builder, and you want the cubic inches, go with the 347 with the 1.175 compression height piston, which does not require "extra" piston parts. On the contrary, if you have an excellent engine builder that knows what he is doing, consider the 5.40" stroke combination.

It can be summarized like so in my opinion:

331 (5.315"/5.400" rod) or 347 (5.315" rod) - Daily Driver
347 (5.400" rod) - Street/Strip or Track Car

Mark O'Neal at CHP/Probe has recommended the 5.315" rod for street engines.

Keep in mind, there is a reason for 99% of OEM pistons to not have the wrist pin intersect the oil ring land.

Here is some more stroker information from a Hot Rod Engine Information article:

If the stroke is increased by 10 percent, the reciprocating loads will, at any given rpm, go up by 10 percent. Although reciprocating loads are proportional to the mass involved, they go up with the square of the rpm. What this means is that if the engine is turned at 10 percent higher rpm, the reciprocating forces go up by 21 percent (1.1x 1.1 = 1.21). To offset the inevitable combination of the greater stroke and the desire for more rpm, we need to look for a lighter-than-stock piston. Checking through various manufactures' catalogs looking for pistons that are toward the lighter side is time well spent. Here, ROSS, Mahle, JE and KB are worthwhile starting points. If the piston is offered with a lightweight pin upgrade, then, budget allowing, this is well worth considering.

Weight Comparison of Piston and Rods for 347 by Probe:

A 5.40” (1.090” piston) combo is sometimes considered lighter than a shorter rod – 5.315” rod with 1.175” CH.

Probe’s lightweight 4340’s are 510 grams for the 5.090” rod, 520 grams for the 5.315” rod, and 530 grams for the 5.4” rod.

The ultra light 4340’s are 531 grams for the 5.40” rod, 530 grams for the 5.315” rod, and 518 grams for the 5.090” rod.

The 5.40” rod and 4.030” with the 3.4” stroke’s piston weighs 474 grams.
The 5.315” rod and 4.030” with the 3.4” stroke’s piston weighs 474 grams.

Using this example, the piston/rod combo, whether using a 5.315” rod or a 5.4” rod, the rotating weight is virtually the same. If not the lighter side, going to the 5.315” rod in this example.

The weights are approximate and could very well go the other way.

Ring Location Changed by Stroke:

A post by FastDriver:

Here is some info posted by FastDriver about picking a 3.25 inch stroke over a 3.4" stroke:

"I forgot to mention the biggest reason CP didn't like the 3.4" stroke. The ringlands on high boost application pistons has to be lower, which runs you into a bind if the pin is already intersecting the oil-ringland.

There are three reasons as I understand them:

1. The easy explanation is that the higher you place the rings, the more heat that they are exposed to making more prone to fail - there is a thermal barrier between rings that are lower on the piston and the combustion chamber that is created by less efficient burning of the gasses between the crown of the piston and the cylinder wall 2. The thinnest part of matrial at the crown of the piston is the "meat" between the top of the piston and the 1st ringland making this the most likely part of a piston to fail in many applications, and 3. the higher the ring the more prone it is to fail due to mild detonation. As you can see from the article I quoted below, this is not optimal for a naturally aspirated engine:

The "dead space volume" above the piston up to the top of the cylinder wall usually traps unburnt fuel and burns less completely...producing more emissions. Reducing this volume, by moving the top ring up , decreases emissions. The top ring is now exposed to hotter temperatures and must be stronger.

However, moving the top ring up is not just for emissions purposes either:

Here you see a higher top ring and piston pin location placed at the level of the oil ring groove, both of which allows for a longer rod and better rod ratio in these forged race-only strutted pistons.

Moving the top ring down improves durability but at the same time, creates a situation where more entrapment of unburned gases will occur locally in that area, leading to a less efficient burn.

If you want more technical information concerning the subject talk to a tech named Mike at CP. He once explained the subject to me and at the time, I felt I had a very good understanding and I was in agreement with his assessment that I should go with the 3.25" crank instead of the 3.34, 3.4, and 3.5" billet cranks I could get at the time."

Rod to Stroke Ratio:

Rod to Stroke and How It Can Affect Performance

A lower number rod to stroke ratio does affect efficiency in a slight manner by applying more thrust to the thrust side of the block, but is very often blown out of proportion. The ratios are all quite close. Piston speed is actually works against wear issues more-so than rod to stroke ratio. Setting up a proper hone, bore, and ring gaps are crucial as well. A taller deck height helps the original low rod to stroke ratios, as seen in the 351 stroker engines, which have more stroke than the 302 strokers, and yet it has the same, if not similar rod to stroke ratios. The larger engines, like the 351 have more rotating mass as well.

Rod is Divided by Stroke:

302 Blocks

289/293 (5.155"/2.87") - 1.79
302/306 (5.090"/3.00") - 1.70/1.80 (5.400")
327/331 (5.315"/3.25") - 1.64
327/331 (5.400"/3.25") - 1.66
342/347 (5.315"/3.40") - 1.56
342/347 (5.400"/3.40") - 1.59
352/355 (5.205"/3.50") - 1.49

351 Windsor

351/357 (5.956"/3.50") - 1.70
387/393 (5.956"/3.85") - 1.55
387/393 (6.200"/3.85") - 1.61
402/408 (6.200"/4.00") - 1.55
412/418 (6.200"/4.10") - 1.51

351 Cleveland

351 (5.778"/3.50") - 1.65
383 (5.850"/3.75") - 1.56
396 (6.000"/3.85") - 1.56
408 (6.000"/4.00") - 1.50
426 (6.000"/4.17") - 1.44

Big Bore

427 (6.200"/4.00" - 1.55

429 and 460 Strokers

429 (6.605"/3.550") - 1.86
460 (6.605"/3.850") - 1.72
501 (6.800"/4.150") - 1.64
532 (6.800"/4.300") - 1.58
557 (6.800"/4.440") - 1.53

Modular:

4.6L 2V (5.933"/3.543") - 1.674
4.6L 3V (5.933"/3.543") - 1.674
4.6L 4V (5.933"/3.543") - 1.674
5.4L 2V (6.657"/4.165") - 1.598
5.4L 3V (6.657"/4.165") - 1.598
5.4L 4V (6.657"/4.165") - 1.598

GM:

LT1-1.638
LT4-1.638
LT5-1.568
383-1.520
383-1.600
LS1-1.684
LS2-1.684
LS3-1.684
LS6-1.684
LS7-1.517
LS9-1.657
L92-1.684

Mopar:

5.7L HEMI-1.744

Others:

NASCAR (9.00" deck height) - 1.93
Formula 1 - 2.0 +
Pro Stock - 1.71

Some thoughts by some very bright individuals on R:S ratios:

Larry Meaux says:

From all the various Rod Ratio engines ive had on my Dyno so far, once you go under 1.50:1, blow-by CFM steadily increases, but you can resolve this with Vac-Pump or use some stages of DrySump Pump to scavenge/vacuum..and this will make more HP/TQ.

But if the particular Block you have is moving around and the Rod Ratio is small with a lot of stroke, + a lot of windage ..those small rod ratios have increasing BlowBy issues that have to taken care of.


Darin Morgan stated this below as well:

Most people tend to overgeneralize this issue. It would be more accurate to compare different rod-to-stroke ratios, and from a mathematical stand-point, a couple thousandths of an inch of rod length doesn't really change things a lot in an engine. We've conducted tests for GM on NASCAR engines where we varied rod ratio from 1.48- to 1.85:1. In the test, mean piston speeds were in the 4,500-4,800 feet-per-second range, and we took painstaking measures to minimize variables. The result was zero difference in average power and a zero difference in the shape of the horse-power curves. However, I'm not going to say there's absolutely nothing to rod ratio, and there are some pitfalls of going above and below a certain point. At anything below a 1.55:1 ratio, rod angularity is such that it will increase the side loading of the piston, increase piston rock, and increase skirt load. So while you can cave in skirts on a high-end engine and shorten its life, it won't change the actual power it makes. Above 1.80- or 1.85:1, you can run into an induction lag situation where there's so little piston movement at TDC that you have to advance the cam or decrease the cross-sectional area of your induction package to increase velocity. Where people get into trouble is when they get a magical rod ratio in their head and screw up the entire engine design trying to achieve it. The rod ratio is pretty simple. Take whatever stroke you have, then put the wrist pin as high as you can on the piston without getting into the oil ring. What-ever connects the two is your rod length.


Pros and Cons of Long vs. Short Rods:

Long Rods:

Pros:

Provides longer piston dwell time at & near TDC, which maintains a longer state of compression by keeping the chamber volume small. This has obvious benefits: better combustion, higher cylinder pressure after the first few degrees of rotation past TDC, and higher temperatures within the combustion chamber. This type of rod will produce very good mid to upper RPM torque.

The longer rod will reduce friction within the engine, due to the reduced angle which will place less stress at the thrust surface of the piston during combustion. These rods work well with numerically high gear ratios and lighter vehicles.

For the same total deck height, a longer rod will use a shorter (and therefore lighter) piston, and generally have a safer maximum RPM.

Cons:

They do not promote good cylinder filling (volumetric efficiency) at low to moderate engine speeds due to reduced air flow velocity. After the first few degrees beyond TDC piston speed will increase in proportion to crank rotation, but will be biased by the connecting rod length. The piston will descend at a reduced rate and gain its maximum speed at a later point in the crankshaft’s rotation.

Longer rods have greater interference with the cylinder bottom & water jacket area, pan rails, pan, and camshaft - some combinations of stroke length & rod choice are not practical.

To take advantage of the energy that occurs within the movement of a column of air, it is important to select manifold and port dimensions that will promote high velocity within both the intake and exhaust passages. Long runners and reduced inside diameter air passages work well with long rods.

Camshaft selection must be carefully considered. Long duration cams will reduce the cylinder pressure dramatically during the closing period of the intake cycle.

Short Rods:

Pros:

Provides very good intake and exhaust velocities at low to moderate engine speeds causing the engine to produce good low end torque, mostly due to the higher vacuum at the beginning of the intake cycle. The faster piston movement away from TDC of the intake stroke provides more displacement under the valve at every point of crank rotation, increasing vacuum. High intake velocities also create a more homogenous (uniform) air/fuel mixture within the combustion chamber. This will produce greater power output due to this effect.

The increase in piston speed away from TDC on the power stroke causes the chamber volume to increase more rapidly than in a long-rod motor - this delays the point of maximum cylinder pressure for best effect with supercharger or turbo boost and/or nitrous oxide.

Cam timing (especially intake valve closing) can be more radical than in a long-rod motor.

Cons:

Causes an increase in piston speed away from TDC which, at very high RPM, will out-run the flame front, causing a decrease in total cylinder pressure (Brake Mean Effective Pressure) at the end of the combustion cycle.

Due to the reduced dwell time of the piston at TDC the piston will descend at a faster rate with a reduction in cylinder pressure and temperature as compared to a long-rod motor. This will reduce total combustion.

Basic 351 Stroker Information:

Stock 351 CI:

A 351 utilizes a 4.00" bore, 3.50" stroke, 5.956" rod, and a 1.772" compression height piston.

351 CI Strokers:

A 393 utilizes a 4.030" bore, 3.85" stroke. 5.956" rod, and 1.608" compression height piston. It uses a 2.300" crank journal. The 5.956" rod is a stock 351 Windsor rod. The 1.608 piston is the same as the 302. The 393 is typically known to save money as compared to the 408. A 393 can also be created with a 6.200" rod, with a different compression height piston.

A 408 utilizes a 4.030" bore, 4.00" stroke, 6.250" rod and a 1.250" compression height piston. It uses a 2.100" crank journal, which has less friction.

The 393 moves 3,850 fps at 6,000 RPM.

The 408 moves 4,000 fps at 6,000 RPM.