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:
- 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.
