matt1289

I plan on purchasing summits Pro LS Piston and Rod kit. I was researching and mathing out my compression ratios. It appears to me that Summits advertised compression rations are off.

https://www.summitracing.com/parts/sum-pr36l9037 - Pro LS 4340 Piston and Rod Kit, 347 Cid., 3.903 bore, 3.622 stroke, 6.125 rod, 7cc dish,

Summit site - 9.7:1 @ 65cc - I get 10.35:1
Summit site - 9.4:1 @ 68cc - I get 10.00:1

I am missing something?

G Atsma

You didn't figure the dish. I just used Summit's compression calculator, and while my numbers were not exact to these, figuring w/wo the dish came close

RonSSNova

iTrader: (3)

The better CR calculators ask the bore dia of the head gasket. The head gasket bore isn’t necessarily the same as the cyl bore.

The other thing not always specified by the piston manufacturer is deck height. Stock pistons protrude approx .007”.
I’m using Wiseco turbo pistons and verified they are 0 deck.

I took the time to CC the heads and valve reliefs, measure the deck height and run the numbers on my L33 5.3.
Ended up at 9.3:1. Wiseco said 9.75:1.

matt1289

I did account for that. I am using the excel compression calculator here - https://ls1tech.com/forums/generatio...alculator.html

RonSSNova

iTrader: (3)

Good! The diff isn’t huge, but it all adds up

KCS

iTrader: (20)

What are all your inputs? I used the Wallace Racing calculator and it matches what Summit’s came up with.

Kfxguy

iTrader: (1)

when I figured mine up, it was within .15 of what summit published. unless you are trying to split hair and get it exact, you may be overthinking it. .2-.3 point different aint gonna matter either way.

R6cowboy

iTrader: (3)

I concur, using stock head gasket thickness & diameter and piston -.007" above deck, CR matches Summit's.

matt1289

Below are the parameters I am using in the calculator. It comes out correct for everything else.

65cc


68cc

matt1289

Looks like to me using the compress spread sheet they are using 6.098 rod lengths vs the rod length that comes with the Summit Pro LS rod and piston package of 6.125. So they are in fact advertised incorrect.

I don't really know how much 0.6 difference matters but I do plan to run a Procharger. With my heads using that piston and rod kit I will be at 10.23:1. I think that will be fine for 7-8 lbs of boost on E85.

Summitracing

All Pro LS pistons are advertised with .045 Piston to head clearance factored in. The pistons are targed to be .005 in the hole at 9.240. The reason we went .005 down is we've found some iron blocks as low as 9.230 from the factory -which could get some people in trouble in the ran a taller piston. We also have customers that deck blocks or run .040 gaskets etc., so we are cautious on height.

matt1289

Is there a value in the compress calculator I can adjust to account for that? Looking at the screen shots I attached.

KCS

iTrader: (20)

You have the wrong piston compression height. The link you provided is for pistons with a 1.299' compression height, but you have the compression height at 1.338". If you clink on the "Kit/Combo Contents" tab in your link, you will see what I mean.

Then set the head gasket thickness at .040", which should get you the same .045" quench that Summit uses. Your number will be a little higher, mostly because the spreadsheet is hot garbage and doesn't consider the head gasket bore in it's calculations. The calculations use the cylinder bore diameter, but the head gasket bore will be a litttle larger than the bore itself so there is extra volume not being accounted for and therefore contributes to a higher number.

IMO, delete the spreadsheet from your computer. Use an online calculator, like the one from Wallace Racing, and forget that spreadsheet even exists.

Summitracing

One thing about calculating with the gasket o.d. is a 4.100 gasket at .045 thick on a 4.000 bore amounts to .028 cc. It is something, but there are other factors as simple as the thermostat temperature and plug heat range that can get us in the weeds real quick.

matt1289

"The link you provided is for pistons with a 1.299' compression height, but you have the compression height at 1.338" That did it thanks!

spent21

iTrader: (6)

Thread digging here....
i'm building a 6.2 stroker & had the same issue of the compression calculators not coming up with the Summit advertised Compression Ratio.

What I found is that nearly no one accounts for the first compression ring land height. I was coming up with anything from 13.5 - 13:1 CR until I found the calculator on Diamond Piston's website. When I entered the same exact info in addition to the top ring height, the resulting CR was the same as Summit's advertised.

Che70velle

iTrader: (6)

Ahh, the crevice volume. I have a formula for figuring that. I got it in early 90’s. We called Smokey Yunick whenever we needed things like that. There was no internet….gasp.

G Atsma

Smokey was DA MAN!!!