10:1 ratio
- Thread starter vette
- Start date
The question would be, "Why would you want to?" If you put a set of 10:1 pistons in that engine, which you can do, you don't have the head flow to take advantage of the compression ratio and the cam that would need to be used with it. Just installing 10:1 pistons with no other changes and the stock heads serves no purpose at all... and the piston change with the block work needed to change the pistons will cost more than a good set of heads... so why...?
Lars
Lars
Someone told me that his 1975 had 10:1 ratio but I was skeptical because the block is the L48 style with the original heads. I could see the engine Id stamp on the block so I don't think that it was shaved to create more compression or any work done on the heads. So I was curious if it was possible to get that much more compreesion ratio from 8:5 to 10:1 without major changes. He did say the piston were changed, but is that enough to get it up to 10:1 ratio? Or is he pulling my leg?
Danny
MYBAD79
Moderator
absolutely possible...
pistons with 5cc dome and a thinner gasket or slightly shaved heads will be just around 10:1 with 76cc chambers.
5cc domed pistons and stock gasket (.040 thick) with stock heads (76cc) should be right at 9.6:1 CR. Now take .020 off the gasket, head or block and you're at 10:1
With a .030" overbore the numbers change slightly , maybe .1
If you install a hot cam with more lift and duration (that works well with 10:1 CR) you need screw in studs and better valve springs.... easier, simpler and cheaper to get better heads.....
pistons with 5cc dome and a thinner gasket or slightly shaved heads will be just around 10:1 with 76cc chambers.
5cc domed pistons and stock gasket (.040 thick) with stock heads (76cc) should be right at 9.6:1 CR. Now take .020 off the gasket, head or block and you're at 10:1
With a .030" overbore the numbers change slightly , maybe .1
If you install a hot cam with more lift and duration (that works well with 10:1 CR) you need screw in studs and better valve springs.... easier, simpler and cheaper to get better heads.....
clutchdust
Millionaire Playboy
I suppose the real question is, are you planning on doing this? Or are you just asking for shits and giggles?
Lars laid it out pretty well as to why it's not a good idea. At least if that was something that you were actually planning on. But if you're just asking if it's possible, then the short answer is yes.
Lars laid it out pretty well as to why it's not a good idea. At least if that was something that you were actually planning on. But if you're just asking if it's possible, then the short answer is yes.
big_G
Blown Member
The base engine in mid 60's to early 70's Corvettes (300 hp engines) had 10:1 scr (advertised). They used standard production heads with 1.94 inch intake and 1.55 (1.6?) inch exhaust valve diameters, 64 cc chambers and flat-top pistons. Cam selection would be critical with today's gasoline. You will need good quench (as close to .040 as possible) and de-burr the combustion chambers. All due respect to Lars, but this combo. was very common in the day.
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MYBAD79
Moderator
If you measure the as-produced 10:1 advertised engines, they were actually right at 9:1 (they were a full point lower than advertised). You had to run minimum advertised chamber volume and deck the block to spec to get the 10:1 advertised number. This was confirmed by people in GM Engineering during interviews I did for articles back in the 70's. We used to have to cut a heckuva' lot off the heads to get them down to minumum advertised chamber volume and to get the comp ratio up to the advertised ratio when we were running in the NHRA stock classes.
Any engine loses power when compression ratio is reduced from optimum. The '75 had very low compression due to emissions requirements. Bringing some compression back will certainly help, and could easily have been done if somebody changed the pistons during a rebuild. But that's a pretty costly mod for limited benefit if no other mods were made.
Lars
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