91Z28

I'm pretty sure it is 6cc, but I've seen some people claim 4.5, 5, etc. I'm in the process of doing a heads and cam upgrade and am shooting for an 11:1 CR and need to know the piston volume so I can pick the right head gasket thickness and combustion chamber size. I'm sure somebody here knows for sure. Thanks!

tbsimmons

LT1 I have read, not measured, are -1cc each relief, so taht would put it at -4cc per piston.

91Z28

I use this link for calculating CR:

http://www.rbracing-rsr.com/compstaticcalc.html

The only unknown variable is the piston volume, and I know right now my engine is 10:1. The only number I put in there that makes 10:1 is 6cc. It is just really confusing, you have so many people that will swear that it is 4cc, 5cc, 6cc, etc. I think on the beginners faq on this board it claims 4.5cc.

91Z28

According to the sources I've found there are 3 different part #s for LT1/LT4 pistons as well. Since aluminum head LT1s and LT4s used a 54.4cc chamber and the Caprice iron head used something different (either 58 or 64, I don't remember) I'm sure they used different pistons since they also used different gaskets to get the desired compression ratios. I'll bet that is why different people have come up with different #s. I'll go with 6cc since it has to be so otherwise I wouldn't have 10:1 right now, I was just hoping someone could make me 100% sure.

tbsimmons

Iron headed LT-1's are 64cc chambered heads.
The LT1 aluminums are 58 and LT4 are 54 give or take.
With -4cc pistons I am getting the 10.0 with the Iron heads and correct head gaskets.
The compression ratio's of all the LT1/4 is different.
If you just change the combustion chambers to the correct values, 54-58-64, then you get about the right numbers for compression ratio.
Having the pistons be higher than -4, the compression ratio is not even close.

96capricemgr

iTrader: (1)

The LT1 shortblocks are the same LT1 irons are 64cc, aluminums are 54cc despite popular thinking otherwise and LT4s 58ss but they still had the most compression due to different pistons and actually a THICK gasket.
All numbers obviously have production variances.

The iron head motors got a .029 gasket the aluminums .049 the LT4 .051.

You are going to have to do a lot of searching and paying VERY close attention to who posts what ijn order to decide for yourself what is right. Frankly there is more bad info out there on this than good but a select few have posted good info and IMO I have done a good job of identifying who is worth trusting.

GIZMO

iTrader: (8)

12553160 LT1 Head Gasket (iron head) 0.028"

10168457 LT1 Head Gasket (aluminum head) 0.050"

12551488 LT4 Head Gasket 0.043"

96capricemgr

iTrader: (1)

Any chance of multiple LT4 gaskets being available????
You are certainly someone I pay attention too.
Can I get your take on the CC of the various heads because you would have more hands on knowledge than probably anyone else on this board?

GIZMO

iTrader: (8)

As far as I know that is the only factory available gasket for the LT4.

I have never actually checked the valve pockets on a set of LT1 pistons. But I can tell you that the spec. that GM sent to the NHRA is -4cc.

10159437 LT1 Piston

12528828 LT4 Piston (I have only seen one LT4 piston, but I can tell you that compared to a LT1 piston the valve pockets are much shallower).

The NHRA lists the aluminum head LT1 and the LT4 combustion chambers at 51.9cc. The iron head is listed at 62.0cc. As the blueprint specs. allow for "cleaning-up" the head surface you could probably add about 2cc's and get a good number for OEM.

tbsimmons

Well I stand corrected. The information I have gotten out of a mechanics program and research must be wrong.
After seeing both a Aluminum headed LT1 and LT4 engine apart, I do not recall smaller reliefs in the pistons.
Seeing LT4 heads, what I helped install on a friends engine, were smaller than LT1 aluminum counterpart.

Dwyane referring to someone that another is not giving good information, I would watch, espicially me. The information that I give is not from hear say, actually either from research or from actually seeing a product, which I have seen all three and installed. I usually dont open my mouth unless I am sure on what is being disussed.
So try not to take punches at me on every forum.
I should have stated I am changing to the correct head gasket volume for the compression ratio.
The following is what I have read about the gaskets, I have only seen the LT1 aluminum and Iron, not the LT4.
LT1 Iron - .028-.029 Compressed. - 12553160
LT1 Aluminum - .049-.050 Compressed - 10168457
LT4 - .043 Compressed - 12551488
Getting 10.0 or Iron, 10.7 for the LT4, 10.2 for the LT1 Aluminum.
This is with the following in common.
4.00 bore
3.48 stroke
.025 in the hole - This can very with the blocks that I have seen.
-4 CC valve reliefs.

The book I am looking at right now says the following.
Pistons
LT1 - 10.4:1 with 58 CC heads - 10159436
LT4 - 10.8:1 with 54.4 CC heads - 1258828

Piston in hole stated to be .033-.034, mine was .035 with the stock pistons when taken apart.

So take it for what it is worth.

GIZMO

iTrader: (8)

The LT4 piston that I saw was in the IHRA tech trailer. We were comparing it to an LT1 piston. Both new in the box. Did all LT4's get the right pistons? Who knows?
As far as this whole compression ratio question goes, you really need to measure everything. There are variations from one casting to the next.

Good Luck.

Daren

96capricemgr

iTrader: (1)

I was not taking any shot at you.
A rough measure of my shortblock with a straight edge and feeler gauge was .022 down, I know that is less than normal though.

I have not CC'd any yet but trust my sources and can find posts here even where others measured what I am claiming. Seems most who have measured multiple heads do find signifigant variance in factory castings which is likely the source of conflict here.

After fairly signifigant chamber porting and then a .032 milling my chambers are 53.1cc, best number I can find as far as how much milling needs to be done for 1cc is .006 using that as a rough number suggests my heads were 58cc after the chamber work before milling.

tbsimmons

Well put Daren I agree with you.
Even what Dwayne said about his piston being .022 down in the hole and mine at .035 give or take shows the tolerances GM used, which might explain the differences in what is stated above..

lethal93ta

I know that I had did 3 sets of LT1 heads now and CCed them before we did the chamber work, mine were all around 56cc after the work in the chambers polishing everything and we trued the heads Im at 58 now, we took a lot out of mine blending everything in for my bigger valves and didnt take much off sence Im trying to lower my compression for boost, my pistons were about .030 in the hole from factory and we decked mine to about .025 in the hole now, every block is diffrent CC everything if you really want to know what gasket you want to run.