Z06d

I'm trying to calculate my engine's static compression ratio, however, I don't know what head gaskets were used. The engine builder is searching for the info, but it's been about 5 years, so who knows if he'll find it...

-How much can the head gasket thickness & bore vary from gasket to gasket?
-How much can the CR vary as a result?
-What are the most common gasket specs used for a naturally aspirated build?
-Am I missing any other pertinent info for this calculation?

Displacement: 427 ci / 7.0 L
Bore: 4.125
Stroke: 4.000
Rod length: 6.100
Comp height: 1.140
Dome vol.: -12.3cc
Cyl. Head Chamber Volume: 72cc
Gasket thickness: unknown
Gasket Bore: unknown

FYI, the CR would be 10.97:1 with 65cc chamber and 4.160x.048 gasket, according the the shortblock builder (shortblock purchased elsewhere).

Thanks!

PREDATOR-Z

~10.36 SCR with .052 gasket
Your pistons are 12.3 cc reliefs correct?

intenseblue

iTrader: (1)

What did you use for deck height Predator?

Assuming 0 deck I got 10.22 with the .048 gasket he mentioned

With the same values except for a 65cc chamber I got 10.96

intenseblue

iTrader: (1)

Consider using Cometic .040 gaskets. Would be good for a .2 bump and help with quench.

Z06d

So, I slid a feeler gauge between the head and deck where there was a slight indentation in the gasket, and came up with .056 gasket thickness. Now, I'm not sure how accurately that represents the gasket immediately around the bore, but I think it's probably pretty close...

To be honest, I'm not 100% sure. I was leaning towards the -12cc being the reliefs as well. The spec sheet just says "Dome Volume: -12cc".

SummitRacing's CR calculator says "Effective Dome Volume:
Use (-) for Dome and (+) for Dish", which seems to be flipped...

But, if I use -12cc I get 13.13:1, which I know is impossible. With +12cc, I get 10.11:1 which seems right on...

Are "deck clearance" and "deck height" the same thing? Is that the distance from the top of the piston to the block's deck?

Deck height isn't on my spec sheet. It's a sleeved LS2 block if that helps.

I'll definitely be considering my options, including: changing gaskets, milling heads, and swapping heads entirely. If 10.11:1 is correct, isn't that a relatively low CR for an NA LS motor?

intenseblue

iTrader: (1)

Deck height and deck clearance are indeed on and the same. It tells you how far in or out of the bore that the piston sits at TDC. To get a general idea without meausuring, which truly isn't the proper way, use:

block deck height - piston compression height - rod lenth - stroke/2

In this case 9.24-1.14-6.1-2 which equals 0. If the block has been decked/milled .xxx thousands of an inch assume the pistons stick out that far.

And IMO that compression ratio is too low. A stone stock LS2 comes in at 10.9:1 as does an LS7. I wouldn't shoot for anything lower unless I planned on boosting it. You're going to give up hp/tq, throttle response, and fuel mileage. Compression really has no downside until you go too high for pump gas

bww3588

iTrader: (49)

Deck height and clearance are not the same at all, but go hand in hand.

Deck height is the measurement from crank centerline to the top of the deck. Deck clearance is difference between top of the piston at TDC and top of the deck

intenseblue

iTrader: (1)

Good call on that one. I stand corrected. Always called it the same.

Z06d

what do you guys think of how I measure the gasket thickness? Any problems doing it that way?

Thanks. I really appreciate the formula!

I agree regarding the compression being too low, but who knows? Maybe the heads were milled before I bought the car...? The only way to know, assuming the builder can't find his notes, is to remove the heads, right?

Thanks for the clarification! I had a strange feeling something was off

batboy

iTrader: (6)

My guess is that GM MLS head gaskets were used which generally has a .051" or .054" compressed thickness (I'll plug in .054" since that's close to what you measured). Most gaskets for a 4.125" bore are actually 4.150". You will need to "guess" what the deck height is, but most LS engines tend to be around .006" out of the hole. Using my favorite compression ratio calculator, I get 10.25:1 CR. The 12cc valve reliefs and 72cc combustion chambers are going to give you low CR for N/A. But, looks good for forced induction.

Z06d

I just found out the deck clearance is .005". The gasket is a metal type from what I can tell. The tabs that stick out are a bronzish color. Is the GM MLS a metal type as well?

I'll be looking around for a good deal on maybe some ported L92's, or maybe afr/tfs, or whoever makes a nice head. I want something comparable in flow to LS7 heads.

When I asked the shortblock builder if LS7 heads would work, they said "Not with the pistons as-is. Max intake valve diameter is 2.100”.

So, do I still have good choices with a max 2.100" intake valve? I'll do some more research, but any recommendations are appreciated...

BTW, I have a FAST manifold that says "Wilson" on it. Is that a 92mm? If so, does it present any compatibility issues with certain heads?

intenseblue

iTrader: (1)

Did he give a reason as to why an LS7 head wouldn't work? At the moment that makes zero sense to me.

Z06d

That's all the builder stated in the email, but doesn't the LS7 have 2.20" intake valves? Sounds to me like the valve reliefs in the pistons aren't large enough...