somebody asked what size the lt4 chambers are... i could be wrong but i'm thinkin that maybe they use a thinner head gasket to get the higher compression. is that right or am i imagining that i read that somewhere?

 

i think they have smaller cc
you can change gaskets when doing headers or whatever to get that, that's probably what you're thinking about
i know the impys had a different size gasket in their lt1s than the camaro/corvette lt1s
but no, i think lt4 used standard size (although you might be right, i'm not totally sure on this)

 

Yet another source for confusing info:

http://www.grandsportregistry.com/lt1vslt4.htm
Everything I have ever read about the LT4 indicates the change in CR for the LT4 to result from the pistons, not the heads or the gasket:
Another reference that conflicts:

http://www.automotiverebuilder.com/ar/ar99928.htm

And finally, the LT1/LT4 rebuild book:

http://www.amazon.com/exec/obidos/AS...145653-6316010

That source is obviously as confused as we are, because in one paragraph they attribute the difference in CR between the LT4 and LT1 to 3cc smaller piston valve reliefs in the LT4, and in a photo caption, they attribute the difference to the LT4 having "smaller 54cc chambers to achieve 10.8:1 compression ratio, as opposed to the LT-1's (sic) 10.5:1 ratio." They state in various places in the book that the LT1 CR was 10.4:1, and in several other places that is was 10.5:1.... another mystery that seems to pop up frequently.

Answer is.... cc the heads BEFORE you select the pistons.

 

i really don't see a problem with ordering pistons and you are off by 2-4 cc due to the comb.chamber you can always change the gasket quench size.....now if you were off by say the difference of a 76cc head vs. a 58cc head then you may have more compression than you wanted or the opposite..

 

Well im hoping my heads are the minimum of 56cc considering they are ported but i won't know ti'll they get here =/.

 

For some it wouldn't matter but 4 cc mean the difference between a static compression ration of 11.01 and 11.55 on my setup with a .039 head gasket.
That is reflected in my Dynamic Compression ratio of 8.20 and 8.59 respectively.
Had I been really close to the 9.0 limit for pump gas I would be in for a shock when I had to much DCR for pump gas.
The "small stuff" adds up in this case.

 

Can someone give me a hand, it seems every compression calculator use gives me a different compression ratio.

4.030 bore
stroke 3.750
-3.4cc flat top pistons (Speed pro part# TRW-L2491F30)
.039 gasket (Speed Pro Part #: 9966PT Im not sure what size the speed pro gasket is , even called speed pro and they don't have the specs on their computers )
deck height .025
not sure on the gasket bore.
not sure on the chamber sizes, I tired 54, 56 and 58 and every calculator I use gives me a different static compression result.

Can someone give me a hand here please?

Im getting 13.6:1 with 54cc and a .039 gasket =/ and some are telling me 11.6:1

 

Help

 

I am reminded of an old quote about representing oneself in a court of law...

Hire a lawyer....err, professional engine builder to solve your problems. Of course that would mean believing and taking the advice you pay for....

 

THIS informational page states towards the bottom that the LT4 heads had "smaller 54cc combustion chambers to increase the compression ratio to 10.8:1."

on the lower left column on THIS page summit advertises the LT4 heads @ 54.4cc and a compression ratio of 11.1:1 on a stock LT1.

are there different methods of measuring them????

from what ive seen on this thread and also reading other info, im leaning toward 56cc for LT1 and 54cc for LT4, roughly.

 

Just got my heads back from the machine shop, they were 58cc.

 

It could well be that there are BOTH (two LT1 heads). Gm/Chevy has done this sort of thing in the past.
Any one know if all the casting are from the same plant?

 

The compression should only go up .4 points compression. From 10.4 to 10.8, at least thats what my book says. How to rebuild the lt1/lt4 engines by HPbooks. Good reading.This same book lists both head chambers as 54.4 cc. I can't remeber if the pistons were different and/or the heads were. I too have heard different info. My GM service manual says 10.5:1. I can't seem to find the chamber size in the manual.

 

As noted in my post above, that book has conflicting statements, inlcuding one paragraph and the picture right next to it that say two different things:

 

I wish I had read thru the posts completely. I missed the 2 page, so my fault. I know what you are saying once I began to look this stuff up. Maybe they assigned different people to write the chapters