Gentlemen,
How much of a performance gain would there be going from about 9.5:1 to nearly 11:1? With todays cam designs, do any of you engine pros think pulling the stock L34 pistons out of my 402 and replacing them with the L78 pistons would provide enough of a performance boost to warrant the expense/effort? Engine does NOT need a rebuild, just want to maximize performance. Thanks

It's about a 4 percent power increase for every 1 number increase in compression!!

11 - 9.5 = 1.5 x 4 = 6 percent x say 400hp = 24hp!! For an increase in power to 424hp finally..

pdq67

PS., go from say 7.8CR. to 13.5CR. and it gets bigger!!

5.7 x 4 = 22.8 x 400 = 91.2hp!!!

IMO unless you are going to go with a cam that needs this much compression it may not really be worth it. 24 hp is not really a big gain unless you are just wanting to see slightly better #'s on the strip. If you however go with a bigger cam that would eat up this 11:1 SCR then you would see even a bigger gain. Also 11:1 with a small lift, short duration street cam may warrant you to run either race fuel or Sunoco 100 unleaded to avoid detonation. Keep an eye on your DCR. I am at about 8.3 DCR and on a hot day at the track I have problems with 93 pump gas. I usually run 2 gallons of 110 to 3 gallons of 93. Lastly if you already have a pretty large lift, long duration cam with too low of compression the 11:1 may gain you more than the theoretical 4% per point.

9.5 is not all that bad. I went from 9.5 to 10.25 on my last engine rebuild and saw no ET gain. Again it all depend on your total engine combo.

Save your money and use a 50 shot of NOS. If a ricer can live forever with a 50 shot you know a 402 can.

I don't think the L-34s are 9.5 to 1 and the L-78s are less than 11 to 1. You really aren't going to gain that much.

Originally posted by SMS:
Gentlemen,
How much of a performance gain would there be going from about 9.5:1 to nearly 11:1? With todays cam designs, do any of you engine pros think pulling the stock L34 pistons out of my 402 and replacing them with the L78 pistons would provide enough of a performance boost to warrant the expense/effort? Engine does NOT need a rebuild, just want to maximize performance. Thanks =
Also,, it isn't linear. You will see more going from say 8 or 9 to 11 than from 12 to 14.
The gains become much less as you go up in compression but you should see 3 or 4% at the lower ratios & a couple more as you get higher.
And if you stick a big dome in front of the spark plug like in a small block you could lose 50HP while raising the compression :D

Ditto Wolf,

I forgot about that!!

That's why flat-tops and small compression chambers form better flame burning fronts which makes for a more eff. combustion process then bigger chambers used with tall domes that create the same compression ratio..

pdq67