Dubstep Shep

So I'm attempting to manually calculate the compression ratio for the LS7 and the results I'm getting are slightly off from the 11.0:1 spec for the engine.

Long story short, I use this calculator to let me easily determine how a change will affect other specs or measurements.

I think my issue is with the specs I'm using to calculate the compression, not the formulas. I'd like to see if anyone like Katech or an engine builder can confirm these specs.

Anyways, here's what I have for the specs on the LS7:
Number of cylinder: 8
Bore: 4.125"
Stroke: 4.000"
Deck Height: 9.240"
Head Gasket Bore diameter: 4.150"
Head Gasket compressed thickness: 0.051"
Cylinder Head chamber Volume: 70cc
Piston Dish Volume: 5.3cc
Piston Dome Volume: 0cc
Piston compression height: 1.170"
Connecting Rod Length: 6.067"
Actual piston diameter: 4.124"
Depth of compression ring groove: 0.3" (I pulled this one out of my *** )

Anyways, all that gets me a total combustion chamber volume of 5.326977897 cubic inches (significant digits can bite me ) with swept volume of 53.4561625 cubic inches. That puts the compression ratio at 11.035...:1, which is close but still seems slightly off. The total combustion chamber volume should be 5.34561625 cubic inches, which is 0.018638352 cubic inches off from my calculation.

I realize this is being pretty finicky and that TECHNICALLY 11.035 rounds down to 11.0, but I can't help feeling like I'm slightly off somewhere.

Thoughts?

c5racr1

Well since the factory only went .0 you are correct

Dan_the_C5_Man

I didn't check all your parameters for accuracy, but, who said a stock LS7 has EXACTLY 11.0000:1 static compression ratio? Could it be as simple as GM rounded down as well?

You fared a bit better than this guy did. http://ls1tech.com/forums/generation...-11-8-1-a.html

juanvaldez

How can you have a 4.124" piston in a 4.000 bore?

Total volume piston down/total volume piston up. Piston down bore volume + head gasket + piston dish + chamber/piston dish + head gasket + chamber.

c5racr1

4.00 stroke, 4.125 bore

atljar

You also drive a 427.7 inch motor

MTPZ06

So I have to change my hood emblems to "428"? Dammit...

Dubstep Shep

Yea, it's perfectly possible. I just don't feel like that's the case. If it weren't 11.00 I feel like they would have made it 11.04:1

Whoops, mixed those up. I'll correct them shortly on here. They're correct in my calcs though.

But yea, that's sort of how I did it. Basically the only thing that you didn't use that I did is the volume on the side of the piston between it and the cylinder walls. It's a super small volume though.

427.6493, TECHNICALLY.

Apparently so...

Dan_the_C5_Man

In the history of marketing, in both cars and bikes, I don't ever recall seeing a SCR with a greater resolution than a tenth. Never saw anything other than 10.x:1 or 11.x:1, etc.

phipp85

Top ring groove is .150" deep by .050" wide.

Dubstep Shep

Sure, I get that on the marketing side.

I meant from the technical spec side. Like what they put in the tech manual. I'd think if it were actually 11.035, they'd call that out in the specifications on the service or tech manual.

Is that .150" from the top of the piston or you're saying the groove itself is .150" deep?

I need to know how far down the groove starts.

phipp85

That dimension is .180"

Dubstep Shep

That seems really small to me.

It's this dimension here:

phipp85

Yes it is.

Dubstep Shep

Man, no wonder these motors don't like boost... That top ring is reaaaaally close there.

vertC6

So to clarify, the CR when the heads are milled 0.030 and everything else is the same is what? I have heard so many different numbers from qualified people over the years from 11.4 to 11.8. What is it? Some same 1 CC equals .006 but others say that is not correct.

Dubstep Shep

I'd be interested to know as well.

Texas Speed quoted me that .007 is 1cc.

The fact is that it isn't linear. The combustion area on the head is a dome, not a cylinder. The volume reduction per linear amount removed gets lower the further you cut into it.

phipp85

Chunks of piston breaking off like it's made of peanut brittle if your tune isn't spot on.

Dubstep Shep

Well yea, detonation is always a problem on boosted motors.

What I'm saying is that even if you don't have any detonation, the depth of that ring land means a lot of heat gets to your top rings. Soon as that ring gap reaches zero, you have biiiiig issues.

Boosted motors make more heat, so generally that ring gap gets smaller faster on that motor than a stock or NA motor.

Unless that's what you meant lol.

phipp85

Yeah that's pretty much what I meant. I measured an LS9 pistion once but I forgot what the measurement was. End gaps(top ring) were crazy tight for boost too, right at .010" if you put it in the very top of the liner that had no wear. It was being taken apart because..........you guessed it, rings closing up lol.

Peanut brittle for everyone!!!