Rebuilding my lt1. I told my machinist to give me pistons that would yield a stock compression ratio, 10.5:1. I wanted to go with a small cam but hit it with some nitrous. So he recoomended I go with a set of je pistons, they didnt make anything with a big enough dish so I had to have them custom made. He cc'd my heads at 53ccs. I never asked him what cc the dish on the piston was. I took it upon myself to cc my pistons so I had an input on my compression ratio calculator. I came up with 17 ccs. keep in mind I did not use a piece of glass or anything to put ontop of the dish, me and my 2 buddies eyed it. I used a medical syeringe, which i figured had to be pretty accurate. If my calculator is correct I got alot more dish than I needed. Let me run down what I input here, im 99.9% sure this is all right.
4.030 bore
3.48 stroke
53 cc chambers
.040 gasket - cometic mls
4.100 gasket bore - comtic mls
.020 piston to deck not sure exactly, but stock is supposed to be 9.025 and it was decked, it doesnt make a huge impact as i changed it to see
17cc dish piston


with all this i got 9.7:1 compression.... not really what I wanted. Is my math correct? I checked it many times, and to be honest I'm kinda mad. This guy has been building engines since the 60's and has done quite a few for many friends that have turned out nicely. He told me I shouldnt run 10.5 on the street...... but I told him it was stock and he said ok. I'm wondering if he took it upon himself to make it 9.7 and figure i wouldnt notice a difference, maybee he didnt want to fight with me. I dont know. I'm not very happy right now. thanks for any help -Dave

 

It's 10.25 if it's zero deck and 9.8 with a 0.020" deck.

Rich

 

looks like I did it right, thanks for the help I apreciate it. -Dave

 

You're welcome. FYI:

GV (Head Gasket Volume) = Bore(in) X Bore(in) X 12.87
X (Head Gasket Thickness in Inches)

DV (Below Deck Volume) = Bore(in) X Bore(in) X 12.87
X (Inches Below the Deck)

HV (Head Volume) = CC's

VV (Dish, Valve Pocket, Dome Volume) = CC's
(Minus for dish or valve pockets, Plus for dome)

PV (Volume displaced by Piston) = Bore X Bore X Stroke X 12.87

Compression Ratio = (GV+DV+HV-VV+PV) / (GV+DV+HV-VV)

Cubic Inches = Bore x Bore X Stroke X Number of Cylinders X .7854

Convert Cubic Inches to CCs = Cubic Inches X 16.386
Convert Cubic Inches to Liters = Cubic Inches X .016386

Engine in Liters = (Bore(mm) X Bore(mm) X Stroke(mm)
X Number of Cylinders X 12.87) / (16386 X 1000)

Engine in CCs = (Bore(mm) X Bore(mm) X Stroke(mm)
X Number of Cylinders X 12.87) / 16386

Convert CCs to Cubic Inches = CCs / 16.386
Convert Liters to Cubic Inches = Liters / .016386

 

hey, thanks

 

Before you get too mad at your machinist you ought to realize how little difference that <1 point change in compression is costing you. Maybe 10hp, and that's being really generous. If it saves your butt from some detonation you'll be grateful, if you ever realize it anyway.

That said, i'm all about getting what I paid for.

Jeremy

 

Play with the cam to help with the compression loss...and spray more.

 

no way, i paid for a 10.5:1 motor. these were $700 pistons. ill throw them at him before i put em in my motor.

 

What you did was close but not exact. I would find a clear piece of plastic and use that for a cover to find out the exact cc's. Sounds like you know what to do.

I have had some of the same problems. I was told my heads were 61.5cc after milling. They turned out to be 64.2cc. Not good. I bought pistons that were supposed to be 3.4cc but turned out to be 2.8cc. That was a good thing. So knowing what I had I could make the proper decisions to correct things. Bottom line is you have to measure like you did. Can't take anything for granted on a build up for max power.

 

You could also use a thinner head gasket....

 

Why $700 customs when JE, Mahle, and Wiseco make off the shelf -16cc dish pistons???

 

thinner head gasket only brings it up to 10:1

i didnt need -16cc dish pistons for it. i dont know what he did...

 

stock is 10.5:1, there should be no issue with it at all. im not even sure what happened here.

 

Since he didn't think you should run the higher compression,it sounds to me like he's a "knows best" guy. Apparently he's not dialed in to the LT1 scene. All these suggestions for crutches are BS. You're spending good money to have it done right, & not asking for a grenade (regardless of his opinion or screw-up), so it should be done like you ask.

 

yeah, hes one of "those" guys. im gonna go down there, worst case scenario hopefully i can sell these pistons and at least make most of my money back so i can pick up something else. i'd reallly like to get a nice set of mahles.