Cylinder head flow vs stroke

[LoganD]

Larger bore engines are less thermally efficient generally, so there's going to be a crossover point between the extra flow enlarging the bore gives you and the reduction in thermal efficiency.

If you can run 4V heads and are turning less than 8000 RPM, making the engine square is about perfect. I'd probably make it undersquare if it's less than 7000 RPM.

[plovett]

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.

Yeah, I guess I am saying there are a lot of variables, so there is not one magic number.

In general, I like bigger bores and shorter strokes. Just for the breathing. That is probably only in relation to 90% of factory engines built in the last hundred years. But even that is not always better.

Now a particular combination might have a sweet spot.

JMO,

paulie

[swampbuggy]

In response to what Krooser and Chevyfreak said on page one about the old 327, 4" bore X 3.25" stroke . I always thought that engine was ideal overall, this is why i made a decision to build a 327 on steriods, 4.185" bore= 4" plus 4.6% 3.375" stroke= 3.25" plus 3.9% 4.185 x 3.375 x 8 holes=371 C.I. Mark H.

[Stan Weiss]

RPM has to be part of the optimization process. Fuel type and flame speed come into play. A bore that is perfect for 11000 RPM maybe to big for the flame to totally travel at 15000 RPM.

Stan

[hoffman900]

RPM has to be part of the optimization process. Fuel type and flame speed come into play. A bore that is perfect for 11000 RPM maybe to big for the flame to totally travel at 15000 RPM.

Stan

Ducati went as large as 4.56” (116mm) on the bores of their last Superbike twin, which has since been replaced by a V4. They have publicly stated any future twins will be limited to 3.93” (100mm) for combustion control. Granted, we’re talking engines that spin 11,000rpm and north.

https://www.cycleworld.com/new-ducati-p ... p-forward/

What’s pretty impressive is Ducati making 14:1 compression work on pump gas, with large bores, and high rpm.

[Stan Weiss]

Bob,
Before the RPM limit of 10.5K some NHRA 500 ci Pro Stock engine were turning 11.5K with 4.7x" bores.

Stan

[Erland Cox]

To determine the bore size I woukd calculate the port size needed to get the power I want.
Port size gives the valve size you need and the valve size and the valve placement gives
you the bore diameter needed to not shroud the valves.
Then I would go for the smallest non shrouding bore size.
You can look at sportbike engines, 600 versus 1000cc.
It is possible to run 15:1 in a 1000 but only 13:1 in a 600.

Erland