Compression Ratio

[akart]

If I have a 350 w/ a compression ratio of 8.5:1 @76cc chamber volume. What would be the compression if I put on heads w/72cc chamber volume?

[Guilty-by-Design]

Originally Posted by akart

If I have a 350 w/ a compression ratio of 8.5:1 @76cc chamber volume. What would be the compression if I put on heads w/72cc chamber volume?

http://www.csgnetwork.com/compcalc.html
this should help you out

[akart]

I looked at that chart, and I do not understand all the data input required. Da! Is the question not answerable without more data?

[Guilty-by-Design]

after playing with the numbers i got it to just about match @ 8.51:1 then i reduced the chamber volume to 72cc and got 8.84:1 that should give you some idea where it will be but you wont know for sure unless you add the exact measurements from your engine

[MikeB]

If a 350's comp ratio is 8.50:1 with 76cc chambers, it will be 8.82:1 with 72cc chambers. Milling .030" off the decks of the 76 cc heads will get you 9.04:1. (I was wrong here. The chamber is not a round cylinder shape, so volume can't be calculated, only measured.)

Is this by any chance a base Goodwrench 350? if so, it is closer to 7.8:1 as delivered.

[akart]

Originally Posted by MikeB

If a 350's comp ratio is 8.50:1 with 76cc chambers, it will be 8.82:1 with 72cc chambers. Milling .030" off the decks of the 76 cc heads will get you 9.04:1. Got it right here in my spreadsheet.

Is this by any chance a base Goodwrench 350? if so, it is closer to 7.8:1 as delivered.

How do you figure the compression is 7.8:1 as the advertiser ratio is 8.5:1?

[MikeB]

Originally Posted by akart

How do you figure the compression is 7.8:1 as the advertiser ratio is 8.5:1?

To get 8.5:1 with a 76 cc combustion chamber, a .038" gasket, and the piston .025" down the hole (9.025" deck height), you need a piston with 7 cc valve reliefs. That's a flat top piston.

The Goodwrench engine uses a dished piston with 4 valves reliefs, which is around 16 cc total (or maybe even more). My spreadsheet shows that CR as being 7.8:1, which is what Hot Rod magazine came up with.

I have no idea why GM rates the engine at 8.5, but I've read on the Corvette Forum where they did the same for 60s and 70s engines.

BTW, I was wrong in my post above about milling the heads .030" to get 9.04:1. it's hard to calculate how much the volume is reduced because the chamber is not a round cylinder shape.