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Old 11-30-2009, 10:28 PM
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What type of pistons for 11/12:1 cr?

Building a 355 cu.in.race engine that will use 110octane fuel. The heads are 040's from a 68 327 engine. I realize these heads are nothing to write home about but.....thats what we got!
Should we go with flat-tops? Or dish pistons with a postive deck height and mill the crap out of the heads? I know domes are a poor choice because of lousey flame front, although that would be the "ez way out".
Any suggestions?
6sally6

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Old 11-30-2009, 11:07 PM
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Flat top, zero deck the block or go a little farther and run the pistons .005-.010" above zero deck, and flat mill the heads. You should be able to get 11.5-1 easily this way, more than 11.5-1 will probably require you to angle mill the heads to get it.

You also have the option of doing the above, and using a solid dome piston mill the dome to a minimum height (say something like .040"-.070" tall) to acheive your target compression or more(like 13-1+) but the dome will be short enough to not have an impact on plug shrouding and flame propagation acrossed the cylinder. I have done this with no ill effects.

Use a forged piston, not that hyperpathetic junk.

Last edited by ericnova72; 11-30-2009 at 11:12 PM.
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Old 12-01-2009, 06:54 PM
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It won't take much of a dome to get you there just smooth any rough edges and run it.
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Old 12-02-2009, 08:51 AM
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[QUOTE=F-BIRD'88]A lot of bother for nothing. Only extreme high compression pistons (14:1) with the largest piston dome are going to effect the combustion flame travel if at all. Most of these extreme high cr pistons have a fire slot in the dome anyways. (race piston manufactures didn't fall of the turnip truck yesterday)

Just get a 350 dome piston that gives you 12.5:1 on a 350 with your Measured cylinder head volume and a measured 0 deck height.

If you really want 12.5:1 don't guess. Don't assume the heads are 64cc. Factory heads vary a lot. CC your cylinder heads and measure the actual piston deck clearance. and calculate the actual compression ratio.

If you guess without measuring and calculating the actual finished cr , you will get it wrong and surely give up way more power than any combustion power effect from a any domed piston.


Run it on 110 octane race gasoline and give it all the spark advance timing it requires. Many, but not all high octane (especially leaded type) race fuels require a bit more spark advance for max power than pump gas.
This has nothing to do with the piston dome.

If the cam is a big duration racey cam , lock out the mechanical advance.



Will be useing the 040 heads(327/300hp..1967) How much do they flow with just a mild port clean-up and 3 angle valve job. So I will know how big to go on the duration and lift? We're stuck with the 1.94 I/1.50E valves.
Also ..could you explain "lock out the mechanical advance"?
thanx,
6sally6
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