Its a 400 with flat top 30 over pistons, .020 deck, and 64cc chambers. My machine shop calculated it out to be at 11 using the thickest felpro head gasket they make. He is telling me that the chambers need to be open and the valves need to be unshrouded, to get around 10.5 for the street.I would like to use pump gas.
What do you think of this, is he right??
Thanks!

 

Ummm what are the pistons rated at?

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1987 GTA L98 MD8
355, TFS Heads, LT4 Hot Cam
My GTA

The Minnesota F-body Club

 

With .039 thick gaskets I think that would put you just over 11:1.

 

Matt, what do you mean by the rate of the piston? Thanks

 

If you use the Chevy gasket .051 compressed and had valve reliefs of 4cc's you would have about 10.8

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Chronologically challenged. But still kicking!
'92 CAMARO RS
305 TBI 5/spd (now TPI)
B4C Alt.
Cam change:TRW 198/210 ,450/.460 114 LSA
Stock TBI manifolds
'85 IROC Ex.
14.94-92.00 mph
SPDC base and "350" Vortec coming soon!
'69 camaro-BB/Lenco,1050 dominators, /Pump Gas/no drugs 8.97-150+/3200lbs.
SoCal Thirdgen F-Bodies

 

First thing to do is to put the head (and gasket) on the motor with no crank or rods. Turn the engine upside down and scribe a line on the head where the cylinderbore mates with the bottom of the head. Now take head off and roll the chamber back to the scribed line. That should add a couple of ccs. Find out what your true size is for valve reliefs.
Are there 4 valve releifs or 2?

If you have the right cam timing you can run 10.8 on pump gas. The LT1s run 12 to 1 (and reverse flow cooling does not help that much).

A thick gasket can cause more problems that it helps. It will disrupt quench (same with deck heights more than about .015). Try to stick with the .039 gasket.

[This message has been edited by jcb999 (edited November 08, 2001).]

 

My pistons have 2 reliefs, Thanks

 

You can get an idea of the volume by measuring the depth. If they are about .25 inch or deeper, they are probably about 6ccs. If they are shallower than that, i would think they are about 4ccs.

Is this page to calc CR.
http://home.tampabay.rr.com/yz250/calc.htm

 

I meant that we needed a little more info about your pistons, as others have discussed. But I think they got it locked down for ya now .

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1987 GTA L98 MD8
355, TFS Heads, LT4 Hot Cam
My GTA

The Minnesota F-body Club

 

I would open them up like he is telling you. I have a 406 with flatops,2 valve reliefs,Aluminum 64cc heads. My compression is right around 11.1, im using a 224/234 cam. And the motor is still detonating at anything over 20* of wot timing, 14* in the chip.

Dont run your motor on the limit of compression for 93 pump gas. The small increase in power will be lost and then some having to run retarded timing. learn from my mistake or you just might be in my shoes having to pull the heads.

My motor made 306 rwhp and 419 rwt with a bad ignition coil and whith this low amount of timing it will make even more when i pull the heads and have the chambers opened up so i can run full timing.



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90 IROC
SuperRamed 406
Trickflow 23* heads

 

Based on what you say for specs your machinist is right and MikeH has the right idea....so there ya go

 

So now that I must open them up, how much do you think that will cost? Am I better off getting new AFR heads and this time go with their competition 195cc? Do you think they will flow much better(or worth it?) I had bought those other heads used. Thanks again guys! Mike

 

I'd get some pistons that will give you the cr you need and sell off those ones.
If you "0 deck the block" find a piston with
a 22 cc dish. this will give you a cr of 9.75:1.. just right for street gas.
Check out Keith Black's pistons KB168+.030"
works with a 5.7" rod.

 

Rolling the chamber back can be done by u with a die grinder. Zero cost. But, you should make sure that the end result will be worth it. Cam timing has a lot to do with compression and use of pump gas. With 230 degrees duration at 050" on a 110lsa, an aluminum head motor can go to about 10.5 to 1 compression and still run on pump gas.

What heads are we talking about?
I personally would not go below 10 to 1 on a aluminum head motor (its not required)




[This message has been edited by jcb999 (edited November 09, 2001).]

 

The heads are AFR 190s. My cam is a 230/236 @.50 but its 112 lobe. Thanks