What's Stroking Worth?
Same Cam Same Heads More CID
As a general rule a stock engine makes .9 ft/lbs per cid, therefore a stock 350 would gain 29.7 ft/lb by increasing cid to 383
A performance motor will make more like 1.2 ft/lbs per cid so the same 383 will gain 40 ft/lbs
Typically a 400-450hp 383 will make 450tq, but with better heads making 500hp then 500tq is possible but requires higher RPM. Poor flowing heads will not match Hp to Tq
TORQUE is increased by adding more CID more, more Compression and better flowing Heads.
Smaller intake ports increase velocity which increase torque at lower RPM but reduces horsepower at peek RPM.
The ideal head is determined by horsepower/RPM requirement. For example a racing engine will need a larger intake port to operate between 4000 to 7500rpm whereas a street performance motor operating between 2000 to 6000rpm will benifit from a high velocity smaller port head.
Ideally you want a small port with high CFM flow to provide a wide high power curve.
My experince has proven AFR heads over and over again on SBC BBC and SBF. On other engines I use other types of heads.
Peak torque will be the same if an engine has a Big Bore X Short Stroke or Long Stroke X Smaller Bore if the total CID is the same, BUT the longer stroke combination will begining making more torque at a Lower RPM and reach the Peak at a Lower RPM than the Big Bore Combination.
Example Dyno Tested Chevrolet 377 verse 383
Peak Tq 450 @ 4900rpm V 458 @ 4600rpm
Peak HP 454 @ 6100rpm V 450 @ 6000rpm
Not the differances are small and both motors make 1.19ft/lbs per cid.
If you want more torque you will need to raise the compression, use more duration to make peak torque at a higher RPM, but you will lose torque at lower RPM.
This effect can also be created by using larger intake ports or increasing the CID. eg. 590cid X 1.19 = 705ft/lbs
HORSEPOWER is related to RPM which is related to Airflow, which is increased with Larger Intake and Exhaust Runners primarily by increasing the ports minimum cross sectional area, (do not compare port volume between different types of heads, ie, small block to big block or Chev to Ford as the port lenghts are different)
and more Camshaft Duration.
Hp is NOT increased with CID. This formula show this principle
Hp = .257 X Intake CFM X Number of Cylinders.
In the above example the cylinder head intake flows a maximum of
250cfm X .257 X 8 = 514hp
But the engine is neither drawing or flowing 250cfm either because the cam duration is preventing it, the intake manifold is not flowing this much or the exhaust system is preventing it.
Using the above formula shows that the intake is only flowing 218cfm.
We know the cylinder is trying to draw more air at this RPM because if you use another head with greater flow capacity the horsepower increases.
Ported GM Bowtie 225cfm @ .500 verse AFR 180cc 262cfm @ .500
@ 6500rpm 401hp V 440hp
@ 6800rpm 386hp V 443hp
Using a cylinder head that can flow more air at a given RPM than the engine needs will not increase power, but may reduce low speed power. I test an AFR 195cc head against a 210cc head that is capable of flowing more air at 6000rpm where peek Hp occur due to the cam duration. Both heads made very close to the same Hp, but the smaller head made more Tq below 3700rpm.
A 350cid engine and a 450cid engine will make the same peak horsepower if they have the same heads, camshaft and compression ratio.
The difference will be the RPM at which the peek occurs.
To prove this statement requires two engines tested on a real word dynamometer paying very close attention to keep every thing identical except CID. However we can illustrate this principle with a PC dynamometer.
Dynamation 5 SBC AFR 195cc Heads This calculation was modeled on real world tested motor.
350 Peek Hp 494 @ 6750 Peek Tq 425 @ 5500
450 Peek Hp 491 @ 5500 Peek Tq 540 @ 4000
Below are some GM numbers. Note that their 454 makes similar Hp to their 502. With a little more duration in both engines to max out air flow I would expect both engines to produce the same maximum Hp numbers.
Never the less 450Hp 502 and 425Hp 454 are close given it is at the same RPM.
But consider the Tq difference. 454 500 @ 3250rpm 502 550Tq @ 3500rpm.
These are consider performance motors
500 :- 454 = 1.10 Tq/CID 550 :- 502 =1.09Tq/CID .
GM tested oval port iron heads avg. -240cfm @ .500 HT 502 engine.
Oval port heads 204 cam 377Hp @ 4500rpm
Oval port heads 224 cam 404Hp @ 4800rpm
Oval port heads 254 cam 441Hp @ 5300rpm
Alum. oval heads avg.-272cfm @ .500 204 cam 423hp @4700rpm
Iron Rectangle avg.- 280cfm @ .500 211 cam 450Hp @ 5200 454 same heads and cam 425Hp @ 5200
You can see from these examples more cam duration push up peak RPM where Hp occurs, but using heads with more air flow gains more Hp than using more duration.
Further while Hp remains very close 454 to 502 the bigger motor will always make more Tq and will accelerate harder, hence my motto...
BIGGER ENGINES MAKE MORE POWER!
THE RPM REDLINE
One critical issue he touches on is the limitations to stroking, specifically Feet Per Minute FPM.
As a piston movese from TDC to BDC and back up again it must accelerate from a compete stop, zero FPM to half way down the cylinder, maximun FPM, then deccelerate to a full stop again.
The stop from the journey back up the cylinder is more serve than the downward force.
The energy generated up to TDC pulls on the wrist pin and rod bolts as they are typically the weakest link, then the bottom of the piston, then the rod beam, the the rod crankshaft journal radius,
If there is to be breakage these are the places where it generally occurs.
| Generally Accepted Maximum Mean Piston Speeds |
| Factory cast-iron cranks |
3,750 ft/min |
| Aftermarket cast-steel cranks |
4,500 ft/min |
| Factory forged cranks |
4,600 ft/min |
| Budget aftermarket forged cranks |
4,800 ft/min |
| Typical race aftermarket cranks |
5,500 ft/min |
| High-dollar custom endurance race cranks |
6,000 ft/min |
| ProStock/Mountain Motors |
7,500 ft/min |
| Formula One |
7,500+ ft/min |
6000RPM in a 302CID engine is the same crankshaft speed as in a 705CID engine, but the piston in the bigger engine must travel 2 inches further in each direction, meaning it will travel much faster.
302 X 6000RPM is 3000FPM
705 X 6000RPM is 5300FPM
This is in large part where the idea that long stroke engines can not rev as high as short stroke engines.
Well they can, but they might break.
Forged 2618 pstons 4340 H-Beam rods 4340 Crankshfts increase the potention RPM limitations.
USA made parts have higher RPM limits (5500FPM) than Asiain made parts (4800FPM)
Lighter parts store less energy, but the reduce mass may take away strength.
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David Vizard Porting Heads