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Old 06-11-2012, 06:03 PM
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compression ratio and combustion Chamber size

Just a quick question here, I've got two different engines i'm building up and still in the planning phases. One is a SBC 350, the other is an SBO 403

Both engines will run an estimated static C/R of 10:1, the 350 will have either 64 or 67 CC chamber heads. The 403 will have either 64CC or 78CC (stock) heads.

My question is, would an engine that runs 10:1 make more power with a larger chamber(78CC) VS a smaller chamber(64CC) ??

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Old 06-11-2012, 10:34 PM
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Quote:
Originally Posted by camaroadam73
Just a quick question here, I've got two different engines i'm building up and still in the planning phases. One is a SBC 350, the other is an SBO 403

Both engines will run an estimated static C/R of 10:1, the 350 will have either 64 or 67 CC chamber heads. The 403 will have either 64CC or 78CC (stock) heads.

My question is, would an engine that runs 10:1 make more power with a larger chamber(78CC) VS a smaller chamber(64CC) ??
Given unlimited octane, and throwing out any differences in heat retention or dissipation, and also discounting any differences in the quench/squish effect, and optimizing the combination of parts for both, the power will be basically the same. But once all the factors are combined, there can be- and usually is- a 'winner'.

Generally speaking, look for the head that has the most quench surface available. That will give the best detonation resistance and can allow less timing be used which can even help economy. Maximize the head's available quench area by using pistons that are either flat top or a D-shaped dish to provide the most quench action. Avoid the oft-used by the OEM round dished pistons, they do not offer anywhere near as much quench surface as a D-dish can. Build the engine w/a tight (~ 0.040") quench distance. More on quench may be found by searching as well as the page here (page still under construction).

Whether a dish is needed or if FT pistons can be used will depend on the static compression ratio target vs. the combustion chamber volume. Use FT pistons anytime you can, they are about the best as far as cost, availability, durability, etc.

There are times when budget enters into the picture so much that one chamber is favored over another chamber simply because of availability, or valve sizes, or some other feature that is desired that outweighs any consideration of chamber volume. In the case of the Olds 403, many guys have found using Olds 350 heads w/the larger 2.070" x 1.625" Olds valves makes a very decent combo.

For more Olds engine build info, use the Google custom search bar above and look for olds 403 builds, and similar search terms. There have been several lengthy threads on this subject. One is- here, there are many more.

Good luck.
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