speed88

When I will be running a stock L98 bottom end, LT4 hotcam, and a TPiS Miniram. Also is AFR's hydra-rev kit worth the money??

Roostmeyer

iTrader: (3)

Stock compression is 9.3 to 1 compression. Factory our cars use a shim type gasket and whenever you use a quality aftermarket gasket you add another 4 to 5 cc's to your compression chamber. This will drop you down to high 8's compression.

So my answer is neither. I'd ask if they can mill them down to at least 58cc, but 54cc might be the best for compression. You can run 11 to 1 compression with aluminum heads and if your getting them milled anyway you might as well run as close to that as you can. 68 cc chambers would be great for blower compression, but bad for getting the last few hp out of your combo.

Hyra-rev is probably not needed. If you want extra security replace your springs with something with a little more seat pressure and call it good. I've heard AFR uses questionable springs.

speed88

But those are the only two cc's offered. So does this mean that everyone is getting them milled? I know alot of guys here are running my combo.

87350IROC

iTrader: (2)

most rebuild the engine at this time with flat top pistons to bump compression

1989GTATransAm

iTrader: (3)

Yep the above pards are correct. I have the same problem. Way back when I put on the Brodix Street heads. They came with the 68cc chambers and my compression went down to about 8.9 to one. Right now I have 256 RWHP in my GTA with the 350.
In about two weeks my car will be going into the shop to get the engine refreshed. I'am having flat-top pistons installed, a ZZ4 cam along with intake and port matching work. I'am looking for around 10.1:1 or 10.2:1 compression. May have to get the heads milled .006" or .012" to get the compression I want. Also getting the exhaust "Y" squared away and the exhaust system cleaned up.
So with a custom eprom I'am looking for around 300RWHP. If I were in your shoes I would get the heads milled to around 58cc maybe more. Other wise you will be throwing HP away. Allen

Ragtop89

Are you guys kidding?!
Less than 9:1 with 74cc AFR heads?!



I've got to pull my heads and get them machined.
I knew my compression was low but less than 9:1 . . . jebus.

OMINOUS_87

Sounds like alot more of a basic understanding of how engines work is needed here with some folk.

Its a shame to see someone build a real decent combo without attention to detail and then produce dissapointing results.

Some of yall might want to become friendly with the following article before you start slapping parts together.

Its a good look at what goes down in the comustion chamber.

http://www.popularhotrodding.com/tec...power_squeeze/

OMINOUS_87

What are you running for pistons?

You say you got AFR 74 cc right?

IROCZZ3

Yes, you'll need to have AFR mill your heads when you order them. I agree with the guys to have them milled to 58cc....I had my AFRs milled to 58cc and my compression is just a hair under 10 to 1. You need to know what you want ahead of time before you get your heads.

speed88

Thanks for all this info guys, I never knew this before. This is a great example of why is pays to ask questions before you buy. What is the stock cc of the L98 heads, 58cc??

I should also add that I am going to be running NOS as a power adder and I do not want to go to high with my compression.

SweetS10v8

Doing a little assuming here but, If your stock L98 with 64cc heads has 9.3:1 compression and you add AFRs with 74cc's your compression just went from 9.3 to 8.45:1!!!(if you used same type head gaskets)

More assuming, I would guess with your AFRs you put on quality Felpro gaskets, most likely .039 thick. Which would put you down to 8.1:1 :rockon: Go buy a supercharger, your all set! jk lol

speed88

Ok then, so your saying the stock iron L98 head is a 64cc head?? If that's the case, then buying the 68cc AFR's would not require that much milling? Does that sound right?

gixxer9

EVERY CC OF YOUR COMBUSTION CHAMBER IS EQUAL TO ABOUT .1 COMPRESSION. EVERY .002" OF GASKET THICKNESS IS EQUAL TO .1 COMPRESSION.

gixxer9

ON A 4" BORE... SORRY.

Roostmeyer

iTrader: (3)

I guess you could go down to just a 64 but it still will not get you back to stock compression with a quality head gasket.

Did some math in desktop dyno:

With 64 cc heads and a .039 thick head gasket you would have around 8.8 to 1. With 58 cc heads you would have 9.45 to 1 and with 54 cc you would have 9.9 to one. If I would were you I would mill them down to 58 cc as the heads would be more difficult to sell or reuse later if you milled them to less than 58 cc.

A shim gasket felpro .015 x 4.10 w/ 68 cc heads would put you at straight up 9 to 1, but with 58 cc heads you'd be right at 10 to 1. 10 to 1 would still have a large margin of safety from detonation, but these head gaskets might not be a good idea for nitrous. I'm not sure how long these Head gaskets last or even if they're compatible with Aluminum heads but it may be something to consider. I know everyone claims they blow out but I'm not sure how many people have actually ran these head gaskets. As long as your heads or your block aren't warped it shouldn't be a problem.

doc

When I ordered my 190cc AFR heads, I ordered 64cc chambers and had them "race prepped".

1989GTATransAm

iTrader: (3)

Let's not forget that you will be going from iron heads to aluminum heads. This even makes things worse with the big drop in compression. You need to get around 10:1 compression to even things out. Allen