engine compression

[BIGCHRIS]

I am wanting to freshen my engine up and checked the compression on it before i tare it down.It had 125 to 128 psi across the board ,what can i gather from that figure right know.Is there a way to figure total compression from that this engine is in a boat i bought so i know little about it

[SmokinLowriderSS]

If my memory serves me right, typical tight chevys at 8.5:1 or so run a bit over 150 PSI as an average. That pressure though will depend a lot on how good your induction system is. I know somebody here will have a better call than my balpark. Biggest concern IMO is the similarity between cylinders. Should be within 10% of each other. Low ones need investigated. Looks decently tight and, unless planning an upgrade program (like I started last year), I would hesitate to dismantle a good solid motor. 'Course, I don't run one as hard as a lot of guys here so I get a lot of years between a "refresh".

[Carnivalride]

I am wanting to freshen my engine up and checked the compression on it before i tare it down.It had 125 to 128 psi across the board ,what can i gather from that figure right know.Is there a way to figure total compression from that this engine is in a boat i bought so i know little about it
If there 125-128 psi it sounds like your pretty good on consistancy. Did you spin it warm with the throttle butterflies open and all the plugs out? If so how bigs the cam, long duration cams bleed compression bad with starter rpm. Weak batteries also make this number low. The best way is to know for sure disassemble cc everthing. I'd check everything out really well if you plan to flog it.
just my .02

[BIGCHRIS]

Alright,i took the heads off and it has a small dome piston in it GM#3999295 says it is a LS6 piston from what i gather. It also has steel crank and 990 heads .It has a hyd. cam in it and i am defiianatly going to go to a solid. My question is will these pistons hold up or do i need to replace them.this engine is pretty fresh by looking at a couple of the bearings.I want to make a little more power with this engine therefore the cam change and some minor head work.Thanks :idea:

[Carnivalride]

Alright,i took the heads off and it has a small dome piston in it GM#3999295 says it is a LS6 piston from what i gather. It also has steel crank and 990 heads .It has a hyd. cam in it and i am defiianatly going to go to a solid. My question is will these pistons hold up or do i need to replace them.this engine is pretty fresh by looking at a couple of the bearings.I want to make a little more power with this engine therefore the cam change and some minor head work.Thanks :idea:
Is it a 2 bolt or a 4 bolt block? What kinda rods do you have? If the pistons are forged they should be fine if they'll give you the compression you want. Are the small domes .095" tall or taller, if they're the .095" ones you'll have to mill the heads to get 9.5:1 even with a "zero" deck. What are you wanting to do with your boat? :confused:
Loren

[BIGCHRIS]

Is it a 2 bolt or a 4 bolt block? What kinda rods do you have? If the pistons are forged they should be fine if they'll give you the compression you want. Are the small domes .095" tall or taller, if they're the .095" ones you'll have to mill the heads to get 9.5:1 even with a "zero" deck. What are you wanting to do with your boat? :confused:
Loren
C ride,it has the dimple rods,and it is a 4 bolt main.I will check the domes tonight and see what they are.This boat is used for play mostly,we like to ride alot but we are always lake racing and thrashing when we get the chance.I like to roll as much as possible :) any cam suggestions

[steelcomp]

Is it a 2 bolt or a 4 bolt block? What kinda rods do you have? If the pistons are forged they should be fine if they'll give you the compression you want. Are the small domes .095" tall or taller, if they're the .095" ones you'll have to mill the heads to get 9.5:1 even with a "zero" deck. What are you wanting to do with your boat? :confused:
LorenI'm thinking that if it's truely an LS6, it's going to have more than 9.5:1 compression. What did they come with stock? (Cast iron head version?)

[steelcomp]

I'm thinking that if it's truely an LS6, it's going to have more than 9.5:1 compression. What did they come with stock? (Cast iron head version?)OK...here it is...new P/N 6262976..."This 454 standard, pressed pin, high performance piston has 9:1 compression ratio and a closed dome. For 1971 LS6."
I'm not sure what they mean by a closed dome, but there you have it. 9:1 is what you get.

[GofastRacer]

Right out of the Chevy HP book!.
LS-6 1970 460/450BHP= Holley 800cfm 4-bbl carb, mechanical cam, cast-iron large port cylinder heads, 11:1CR, 4 bolt mains, forged Tuffrided crankshaft, forged pistons with pressed-in pins!.
LS-6 1971 Corvette= 425BHP Holley 800 CFM 4-Bbl carb, mechanical cam, aluminum large port open chamber cylinder heads,9.0:1 CR. Al other specs as other specs as 1970 LS-6

[BIGCHRIS]

Right out of the Chevy HP book!.
LS-6 460/450BHP Holley 800cfm 4-bbl carb, mechanical cam, cast-iron 1970 large port cylinder heads, 11:1CR, 4 bolt mains, forged Tuffrided crankshaft, forged pistons with pressed-in pins!.
LS-6 425BHP Holley 800 CFM 4-Bbl carb, mechanical cam, aluminum 1971 large port open chamber cylinder heads,9.0:1 CR. Al other specs
Corvette as other specs as 1970 LS-6
Thanks gofast,iwas hoping it was aroun 9-1 if i can get the right camshaft picked out i will be alright.The engine ran strong but i am not crazy about a hyd cam that and a couple of oil leeks to fix and i think i will be ready to go.Always wanting to go faster u no how it goes :rollside: :rollside:

[Carnivalride]

OK...here it is...new P/N 6262976..."This 454 standard, pressed pin, high performance piston has 9:1 compression ratio and a closed dome. For 1971 LS6."
I'm not sure what they mean by a closed dome, but there you have it. 9:1 is what you get.
The TRW # is 2399 they have a .095" tall dome (13cc) for a closed chamber head, coupled with open chamber heads (119cc) you'll get about 8.8-9.1:1 with a .041" gasket depending on piston to deck. I used to have the TRWs in my engine their forged tough and heavy 639gms.
Loren

[lilrick]

The TRW # is 2399 they have a .095" tall dome (13cc) for a closed chamber head, coupled with open chamber heads (119cc) you'll get about 8.8-9.1:1 with a .041" gasket depending on piston to deck. I used to have the TRWs in my engine their forged tough and heavy 639gms.
Loren
I have this piston in my lil ol jet and they take alot of abuse!!!!!!!!!!

[Squirtin Thunder]

I have this piston in my lil ol jet and they take alot of abuse!!!!!!!!!!
Rick is that Lil old jet boat ready ???