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Over-Square vs Under-Square Bore Configurations

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BigMike:
I am wondering about bore/stroke configurations and their impact on revving.

I was under the assumption that a larger bore with a shorter stroke is advantageous for high revving applications. For instance, the 4A features an 81mm bore and a 77mm stroke.

However, the 2ZZ is the opposite. A longer stroke compared to its bore: 82mm bore and an 85mm stroke.

So which is which? Which engine is over-square and which engine is under-square? What is preferred for higher rpm power delivery? What do F1 engines use?

Thanks
Mike

BigMike:
I should add that it can be written as...

Over-Square bore/stroke   versus.    Over-square stroke/bore.

I understand that, I am just wondering which configuration is preferred for high revving applications?

Thanks
Mike

BigMike:
I've also heard that a square-engine is good for Torque. For instance, my 3RZ-FE has a square configuration of 95mm X 95mm

HOWEVER, take a look at Honda's high revving K20. It is a square engine, 86mm X 86mm


Now look at the S2000 engine, the F20C, it has the same over-square bore/stroke configuration of the 4A-GE, a larger bore with a short stroke. The F20C is 87mm bore and 84mm stroke. That is a 1.036:1 ratio compared to a 1.052:1 ratio of the 4A. Hmmmmmmm interesting

We all know the S2000 is a high revving engine, no doubt about that, so is it safe to say that a large bore with a short stroke is preferred for high revving? If so, then why is the Toyota 2ZZ the complete opposite?

Mike

BigMike:
You know, the small bore of the 2ZZ puts a restriction on the maximum valve diameter compared to an opposite stroke/bore configuration of a same-size engine.

The 2ZZ is 82 bore & 85 stroke. If it was the opposite, a 85 bore & 82 stroke, it would still be a 1.8-liter, but it'd have more room for larger valves in the head :idea:

I think this has a lot to do with high revving. If you have a large bore, then you can fit large valves -- something very much needed for high rpm. If you have a small bore, then you have to use small valves which will put a restriction of flow at higher rpm.

I also know there is valve angle theory to consider as well. The Toyota engines always seem to use a wide angle whereas the Honda engines prefer a narrow angle.

Mike

BigMike:
Valve angle comparisons

Wider-angle Toyota head (4A-GE): 50-degrees


Honda head:



Toyota did produce a narrow-angle 4A however, 37.5-degrees for the 20-valve:


I also read here that the S2000 F20C has a 51-degree angle head. WOW

Also the 2ZZ-GE has a 50-degree valve angle, just like the 4A-GE.

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