Don Terrill’s Speed-Talk

all things being fairly equall, which makes the most power? has anyone done an actuall back to back optimized test? i am curious as to whether if its the same cubic inches, does it really even matter? lets say you have an 8000-8500 rpm max and 360 ci. which would be the best combo?

I think bigger bore is better if you have a good cylinder head and vice versa.

When everything else in the engine is taken to its logical conclusion, then the bigger bore - short stroke will make more power and spin faster. The most obvious benefit to the big bore is it allows a larger intake valve.
It seems to me that most Pro Stock motors are designed around the largest bore they can fit in the engine and the stroke is what ever it takes to get to 500cid. Others here would know better than I do but I think its something like a 4.7" bore and a 3.6" stroke. If you are talking a low rpm street motor then a longer stroke might be more forgiving of incorrect gearing but in a race motor its been proven over and over to go with the big bore.

Unless RPM is restricted, or there are induction restrictions, the big Bore/short Stroke engine wins every time.

My opinion is it depends on the rpm range. If rpm is restricted to 5000 to 6000 the longer stroke will prevail. If the range is 8000 to 10000+ then the short stroke has advantages.





beth

Depends on the application.

CamKing wrote:Unless RPM is restricted, or there are induction restrictions, the big Bore/short Stroke engine wins every time.

Yep, oversquare engines = high RPM !!!

Where B = bore, S = stroke, N = number of cylinders, and C = a constant for fuel quality, materials, stress levels, etc.:

HP ~ B^1.65 × S^.5 × N × C
(after F. W. Lanchester)

Removing N, most power by far, is big bore + short stroke.

What do you normally use for "C"?

For race engines, I've seen 4 used.

This is one of those on going debates. I favor the big bore short stroke, for the obvious reason, big bore allows larger valves, or take advantage of the better cyl. heads. David Reher addresses this question in his articles he writes for National Dragster. He states, torque is being applied by either a large surface area or long stroke, by engines of equivalent displacement. He argues the common belief that the longer stroke creates more torque at a given RPM. In essence the larger bore will create the same torque as a long stroke. Before the question arises, rod length is not an issue, it plays a small part in the equation. You can read it at rehermorrison.com/techTalk. RPM is basically a product of the bore and stroke combination and how much power is made in that range is determined by the induction system(ie. cyl heads, int. manifold, and carburetor) and cam selection. Pro Stock engines run very large bores and very strokes to acheive their cu. in. limit. The cyl. heads are what allows them to make the power and the stroke allows them to RPM to acheive maximum piston speed. Oversimplified RPM is Horsepower, the higher you can twist it the more power you will make. As far as torque the Pro Stocks explode off the line. This opens the other on going debate HP vs. Torque but that is another thread and can be on this sight.

Big bore = more valve curtain area / higher breathing capacity
Small bore = sonic choke occurs at lower rpms than for big bore

If you are going for max effort, choose the bore that will give you the most valve curtain area possible and where that valve curtain area goes into sonic choke just past your max desired RPM.

axegrinder wrote:When everything else in the engine is taken to its logical conclusion, then the bigger bore - short stroke will make more power and spin faster. The most obvious benefit to the big bore is it allows a larger intake valve.
It seems to me that most Pro Stock motors are designed around the largest bore they can fit in the engine and the stroke is what ever it takes to get to 500cid. Others here would know better than I do but I think its something like a 4.7" bore and a 3.6" stroke. If you are talking a low rpm street motor then a longer stroke might be more forgiving of incorrect gearing but in a race motor its been proven over and over to go with the big bore.

Question, (I know this is a really old thread) with everything else being fairly equal, which is more desirable in a N/A application:

i.e. max effort big chief headed combinations that limited to a cast single intake. A 550 ci big chief headed combination that peaks ~ 8500rpm or a 650 ci big chief headed entry that peaks ~ 7500rpm?

Both wearing the exact same cylinder heads, bore size, intake and camshaft. Both would be in vehicles of equal weight and transmissions.

Would the smaller 550 ci combination be more desirable with it's lighter weight internals, allowing it to rev quicker to it's powerband?

The answer is NO


JOE SHERMAN RACING ENGINES