Why is that?

I plan on having the block decked .030 to pop the piston .005 out, with a .039, or .041 gasket, either which way.

Heads will be milled .004-.006. With that I should come out to around 11.2:1 CR.

Is that pretty much the same then?

 

Hello,

My builder used a .051 or .052 felpro gasket and says it worked out to about 10.2. He did not want higher because I am going to add nitrous. I have added a couple of degrees of timing from stock with no knock on 93 octane. I only go to Amoco .

 

i would aim for a solid 11.5:1

i would have to find a calculator, but off the top of my head that is gonna take 52cc chambers

 

I ran 12.8:1 on the street in my 383 with no problems at all. LT1's can run higher compression than a normal motor due to 2 reasons. 1. Aluminum heads and 2. Reverse cool'd motor. You're cooling the top of the motor where the heat is generated instead of the bottom of the motor like reguluar SBC's. I've know a few people to run 13:1 -13.2:1 without detonation problems without any issues running 93 octane, and it's hot down here in Houston. More compression is free HP. I would not thermal coat a street motor, the coating reflects heat away instead of absorbing it in the pistons.

Trey I don't know why your engine builder told you to keep the compression down on your motor for nitrous? Nitrous motors Love compression. I'm running 13:1 on my 500 shot.

 

although I have run 12.5:1 on a street NA lt1, i would not reccomend it for the average joe. keeping it below 12:1 ensures it wont be a pain in the *** to tune, and it wont be touchy to variences in gas.

 

I've been using this calc here:
http://www.smokemup.com/auto_math/compression_ratio.php

You have to register though. I don't mind, there's a ton of nice calc's there.

Do you know how much milled off the heads = 1cc drop in combustion chamber?

My next concern is Piston-Valve clearance...but with a inverted dome piston I'm hoping it won't be an issue. I'm not going to run some kind of radical cam or anything...

Yeah with these pistons it should be ~11.2ish:1, so that ought to do pretty well....

 

P to V wont be a problem. just CC the heads.

also, I would mill them to get the compression ih the 11.5range.. closer to 12:1 depending on the cam

(dynamic compression)

 

Dave,
.006 is equal to 1cc

and I agree with the 11.5:1 recommendation. Alot of the max compression stuff depends on where you live (altitude, barometric pressure, etc) and as Jordon said, the dynamic compression is the biggest factor. That's where the intake valve closing point comes into play. Some good info on that can be found in Advanced Tech.

BTW, 1/2 a point or so isn't gonna make much of a difference either way. Better to stay on the safe side.

-Mindgame

 

Sweet thanks for that info. Danny (builder) said that they typically mill .004 to .006 off the heads to clean them up, so that will put me from a 54cc to a 53cc.

Decking the block should increase the CR as well.

Doing the math again...

Bore: 4.030
Stroke: 3.75
Conn Rod Length: 6
Cyl Head Vol: 53 (Was 54, now 53 due to milling the heads .006)
Deck Height: -.005 (5 out of the hole)
Head Gasket Bore: 4.125
Head Gasket Thickness: .039
Piston-Wall Clearance: .003 (SRP specs .0025-.0035)
Top Ring Land Height: .25 (What was in the calc, shouldn't make a diff)
Dish Volume: 16cc (Inverted Dome)
Compression Ratio: 11.24:1

Sounds good to me!!! Now, lets hope this theoretical math carries over!

Sound good to you guys? Sounds right on target to me!

 

Dave,

What exactly are they decking the engine to.... 9.0"? I may have missed that somewhere.

-Mindgame

 

Originally he said to leave the flat top pistons down .025 and use the .039 gasket. I didn't like that because that is too much quench for the compression ratio it would be.

With these pistons, they will sit 1.875 + 6 + 1.125 = 9". Stock deck height is 9.025. I could have him take .030 off to pop it .005 out...

Do they normally charge more for having to remove more material? Or is it just a "set the height and buzz it off" type deal...

I haven't discussed decking to that much of an extent yet with him, not until I have all my math togother. He told me $100 to deck the block though.

 

Im using the 16cc SRP dished with 0 deck and .029 gaskets and 57cc chambers....compression is 10.7:1. Im gonna spray a 150 shot so I wanted about stock compression.

 

First off, you shouldn't be the one having to do the math.
That's the engine builders job.... he should also know to keep the quench tight but within respectable limits. Along with things like manifold milling to account for decking, head milling etc..

On the other note.... the fee is just based on the setup. Whether they take .005 or .040 it's all the same cost-wise.

Good luck.

-Mindgame

 

11.8:1 here, running 91 octane. No issues at all.

 

I always like to compare # s

Heres a good calculator http://www.gafba.com/calculators/cmpratio.asp?mode=ntr