Cylinder head flow vs stroke

[Backis]

This is something i been wondering about lets say you have an Engine 4"bore 3.48 stroke vs 3.9" bore 3.75 stroke if you have identical
cyl head flow same lift on cam same duration what Engine would produce the most horse Power at 7000 rpm

[Ozonkiller]

One thing to consider is the larger bore MAY increase air flow due to a decrease in shrouding...

[BOOT]

How many valves? What degree head?

[digger]

Bigger bore not due to VE differences but friction will be lower

[cardo0]

I believe the heads, cam and manifolding determine how well/much the motor can breathe. Keeping those items the same but changing the bore/stroke combination I would expect move the power curves to reflect the change in lever arm of the stroke which also changes the piston speed. What I'm saying is the VE may remain close to the same but one may have a lower power curve which can be wider with more useful rpm. I think sbc owners are always looking for their perfect combination while the options for the sbc are nearly unlimited. I don't have the data to prove this but will say I'm convinced you can build a motor greatly oversquare or greatly undersquare enough to become a worthless flop but your welcome to prove me wrong.

Something I would like to know but can't seem to resolve is what stroke to bore is optimum? How much stroke is enough before a motor becomes a flop?

[cab0154]

for 2 engines, of a given displacement, and a given port cross section; the port doesnt know what the bore and stroke are.

[cardo0]

So I looked in my library for bore vs stroke info and I found 2 full pages on this by Smokey Yunick (RIP). In summary he says while a larger bore with longer rods will make more hp in the same displacement a longer stroke motor car can be faster as it should make more mid range torque to help accelerate. But on the longer straight way tracks the larger bore/longer rod motor should be faster.

Find it in Smokey's Power Secrets. Hope this helps.

[Krooser]

Back in the mid-60's it always seemed to me that a 327 Chevy or a 340 Mopar just seemed to have it going on.... a near perfect combination.

They just felt "right" on the butt Dyno.

Sure there were faster cars with bigger cubes later but my tri-power 389 got beat enough by those two combos to scratch my noggin more than once.

[chevyfreak]

Back in the mid-60's it always seemed to me that a 327 Chevy or a 340 Mopar just seemed to have it going on.... a near perfect combination.

They just felt "right" on the butt Dyno.

Sure there were faster cars with bigger cubes later but my tri-power 389 got beat enough by those two combos to scratch my noggin more than once.

I actually swopped my 350 for a 327. Just love them.
Like you said, they just feel right.
I favour the big bore shorter stroke combo's more.

My buddy took a 153 and over bored it to 4". And installed it in an opel manta. With the std 13"wheels and 3.46 ratio. 1st and 2nd you just put your foot down and the ass is all over the place, some great fun, actually made him use 15" wheels just so it could have better street manners.
Its amazing the difference the way bigger bore makes.

Chevyfreak.

[David Redszus]

So we have two engines, a large bore (4.0"), short stroke (3.48"), compared to a small bore (3.90"), long stroke (3.75").

The displacements are not identical (350 vs 358). When adjusted to identical displacements the large bore engine bore becomes 4.05". The larger bore allows the possibility to install slightly larger valves.

If mean piston speeds are compared, the short stroke engine could run to higher rpms (8800 vs 8200), thus increasing power.

If it were my decision, I would run the big bore, short stroke, larger valve engine.

But the question still remains; what is the optimum bore to stroke ratio?

[plovett]

So we have two engines, a large bore (4.0"), short stroke (3.48"), compared to a small bore (3.90"), long stroke (3.75").

The displacements are not identical (350 vs 358). When adjusted to identical displacements the large bore engine bore becomes 4.05". The larger bore allows the possibility to install slightly larger valves.

If mean piston speeds are compared, the short stroke engine could run to higher rpms (8800 vs 8200), thus increasing power.

If it were my decision, I would run the big bore, short stroke, larger valve engine.

But the question still remains; what is the optimum bore to stroke ratio?

I don't see why there would be an optimum bore to stroke ratio, except for a very specific set of circumstances?

[peejay]

So we have two engines, a large bore (4.0"), short stroke (3.48"), compared to a small bore (3.90"), long stroke (3.75").

The displacements are not identical (350 vs 358). When adjusted to identical displacements the large bore engine bore becomes 4.05". The larger bore allows the possibility to install slightly larger valves.

If mean piston speeds are compared, the short stroke engine could run to higher rpms (8800 vs 8200), thus increasing power.

If it were my decision, I would run the big bore, short stroke, larger valve engine.

But the question still remains; what is the optimum bore to stroke ratio?

I don't see why there would be an optimum bore to stroke ratio, except for a very specific set of circumstances?

For a given compression ratio, the bore to stroke ratio defines what the chamber and piston can be shaped like.

This is why automakers have mostly settled on undersquare setups in the 500cc range. It makes a good combustion chamber shape for the kind of compression ratios that are used. Highly oversquare engines end up having to sacrifice compression or combustion, going the other way forces you to sacrifice valve area potential.

[digger]

This is something i been wondering about lets say you have an Engine 4"bore 3.48 stroke vs 3.9" bore 3.75 stroke if you have identical
cyl head flow same lift on cam same duration what Engine would produce the most horse Power at 7000 rpm

when ever you have a bigger engine vs smaller engine there is a rpm cross over point where the outputs will match, below which the bigger engine will make more and above which the smaller engine will make more. the cross over point depends on how good the induction and exhaust system is. there is no one answer

[Bob Hollinshead]

I don't see why there would be an optimum bore to stroke ratio, except for a very specific set of circumstances?

You cannot continue to increase the bore, decrease the stroke and turn more RPM to make more power. There will be a limit which can be dictated by a few parameters, such as the flame speed of the fuel being used. So the optimum bore/stroke ratio would be much different for a 358 ci v8 using gasoline and one using 90% nitromethane/10% methanol.

So if a person could optimize combustion area shape and size for the fuel being used the bore ratio would be fixed The stroke would adjust the engine size and rpm range?

[David Redszus]

It is possible to increase the bore, reduce the stroke and increase rpm and thereby increase power to giddy levels.

A bore to stroke ratio of 2.46 to 1, is not unusual; it has been done many times when the rules permit.

Typically, the design approach begins with displacement and number of cylinders allowed.
Next the stroke would be determined based on mean piston speed, followed by the bore diameter.
This would allow the selection of number and size of valves, followed by camshaft and valve train.

That is for NA engines. Forced induction engines would follow another path.

To compare engines of various levels of performance and design, simply compare their respective BMEP values.
Even better is to compare peak torque BMEP and peak power BMEP, for selected engine designs.