detroit_903

iTrader: (8)

What would be my compression ratio with JE -31cc PISTONS with bone stock 94 lt1 aluminum heads

aboatguy

There are some variables that have to be factored in for compression ratio. For example:
Has the block been decked? Head gasket thickness? Whats the stroke? Bore? etc.

Do a search for engine calculators and you'll be able to plug them in yourself, with your paremeters.

the_merv

iTrader: (88)

Here is a Calculator..
http://www.rbracing-rsr.com/compstaticcalc.html

Steve in Seattle

Bore: 4.030 (assume a typical rebuild)
Deck Height: 0 (typical 0-decked block)
Gasket: 0.039" (just a guess... typical FelPro 1074's)
Chamber: 58cc's (stock LT1 heads)
Stroke: 3.48? 3.75? 3.8? 3.875? (355/383/388/396 what size engine?)

SCR Ratios:
355 - 8.46:1
375 - 9.04:1
388 - 9.15:1
396 - 9.31:1

more importantly, you need to pick a cam (or at least a Intake Valve Closing angle) and calculate the Dynamic Compression ratio. With a GM 847 cam (79 degress ABDC) and 6" rods I get:

DCR Ratios:
355 - 5.97:1
375 - 6.41:1
388 - 6.49:1
396 - 6.61:1

Unless you plan on running boost I'd pass on those pistons. Your DCR should be between 8.5 and 9 if you want to get the most out of premium gas. I wouldn't go any lower than 8:1.

((8.9x14.7)/DCR)-14.7= Target Boost
(130/DCR)-14.7 = target boost

You need about 6psi of boost to run those on pump and get any gains from it.
DCR tool: (http://www.kb-silvolite.com/calc.php?action=comp)

detroit_903

iTrader: (8)

Thanks everyone for the help - Mike

the_merv

iTrader: (88)

His Pistons are -34cc, they are dome pistons. Stock LT1 Heads are 54cc chambers.

Gun5

iTrader: (37)

so what would mine be 355, stock stroke, LE2 heads felpro gaskets SRP pistons

the_merv

iTrader: (88)

Plug in your numbers in that formula I posted..

Boodyrider

iTrader: (2)

What are the needed specs for a STOCK LT1? I can't find deck height, relief CCs, or stock gasket bore/thickness anywhere.

Yeah, I KNOW the Felpro .039 is a common rebuild gasket, but I've never been able to find ALL the stock specs to get the calculators to work right.

Doe anyone have these numbers, or just smart answers? ALL I want is STOCK numbers that work... so i can play with modded numbers i DO know.

Gun5

iTrader: (37)

got it all cept combustion chamber.. it changes with heads I got LE2s

I just put 58

Steve in Seattle

You're kidding right? 34cc domes under stock heads? good luck with those valve releifs :p

Stock heads I've measured float closer to 56cc's... I use 58 from minor porting.

If that's the case:

Bore: 4.030
Deck Height: 0
Gasket: 0.039"
Chamber: 54cc's
Stroke: 3.48? 3.75? 3.8? 3.875? (355/383/388/396 what size engine?)
Rod: 6"
ICP: 79 ABDC (GM 847 cam )

SCR / DCR
355 - 24.07 / 16.38
383 - 25.86 / 17.72
388 - 26.19 / 17.97
396 - 26.69 / 18.35

Again, this is with a zero-decked block, proper quench heights, 6" rods, and a the GM 847 cam. A milder cam will make this even more insane (sooner ICP).

Unless this is a diesel engine I'm gonna say those pistons are -31cc's, or you need to find new heads.


Good news though... if you get a custom 1.9" stroke crank you can down to 13.6 SCR / 8.98 DCR.

Granted it'd only be 3.2L V8... but it wouldn't blow up on start up and you'd get to use stock heads with a nice big dome.

Steve in Seattle

1. clam down.

2. This info is everywhere. I suggest you buy and read "How to Rebuild Small-Block Chevy LT1/LT4 Engines" by Mike Mavrigian (ISBN 1-55788-393-9), it's a good start and has what you need. Also, the GM Helm's manuals are an invaluable tool for detailed torque specs and tolerances / clearances needed.

3. Here's a head start:
GM 10168457 LT1 head gasket: 0.050" compressed
GM 12551488 LT4 head gasket: 0.043" compressed
GM 12553160 iron head gasket: 0.029" compressed
Gasket bores: 4.166"

Bore: 4.0007-4.0017"
Deck height: -0.025" (9.025" crank center to LT1/4 deck)
Rod length: 5.7"
Stroke: 3.48"

LT1 chambers: 58cc's (though measure around 55 to 56)
LT4 chambers: 54.4cc's (less variation reported here)

LT1 pistons: -4.5cc to -8cc... depends who to talk to or what you read. I have a feeling -6cc is about right, but I'm pulling my stock '97 TA apart in a few months and can check then.

good luck.

the_merv

iTrader: (88)

Some places label their domed pistons with negative cc... but I see what you mean. In that formula it says enter the domed piston in as a negative value, but that is cause that is less space in the chamber.

the_merv

iTrader: (88)

I get 54cc for the Chambers, this is comparing the LT1 Heads to the LT4 Heads...
"Combustion chambers are almost identical with a negligible .4cc increase in volume for the LT4 Combustion chambers at 54.4cc's."

Pulled it from here, in the section talking about the Head designs.
http://www.grandsportregistry.com/lt1vslt4.htm

Boodyrider

iTrader: (2)

Steve, thanks. I've searched for the stock numbers online only.... but had no real luck with finding them.

As for head CCs... really got to have it measured, but FWIW, mine came out roughly 55 with a valve job and Manley FloMaster stock size valves, just a little cleanup. Every head varies a bit, especially with used heads that you don't know exactly what was done, so if you HAVE to know... get them measured (at least twice for repeatability).