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.
Understand Stroke Limitations and Piston Speed
There have probably been more stroker motors built in the last 10 years than there were in the previous 90. The proliferation of inexpensive, stronger-than-stock cranks has brought an entry-level stroker kit within almost every hot rodder's financial reach. But there are limitations as to how much stroke can be used. Any time stroke length is increased we either have to accept increased piston/rod accelerations (and therefore loads) or a reduction in redline rpm. For the most part we can calculate about where to set the engine's redline rpm from a given mean piston speed. The formula is simple: Mean Piston Speed (feet per minute) x 6 divided by the stroke in inches. Fig. 1 shows the formula and an example.
|Calculating Maximum Safe RPM
|Max. Safe RPM = Mean Piston Speed (ft/min) x 6
|Divided by Stroke in Inches
|Example for a budget aftermarket forged crank in a 4-inch stroke small-block Chevy:
|4,800 x 6 = 7,200 rpm
|Maximum Mean Piston Speeds for Above Formula:
|Factory cast-iron cranks
|Aftermarket cast-steel cranks
|Factory forged cranks
|Budget aftermarket forged cranks
|Typical race aftermarket cranks
|High-dollar custom endurance race cranks
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.
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.
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.
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:
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
351/357 (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 (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
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
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
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:
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.
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.
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.
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.