What compression on 4.125 bore 4.00 stroke weekend driver.

 

I run 11.25:1 in my LS7. Pump gas, 42# inj.

 

around 11:1 will be perfect. too much higher and you have to pull timing to run pump gas, at that point the extra compression doesn't gain you anything. 11.25:1 would be fine too....just don't get too much higher if you wanna run 91 octane.

 

I bought a Lunati rotating assembly with -17cc reverse domes. 92 octane is the best I can get where I live. According to a calculator it seams low. Should I look for different pistons or just live with it? The whole thing was balanced and dont want to have to rebalance. Wanted to stayed NA

 

not enough info...what size engine (bore and stroke, 4.125"x4.00"?) and what is the size of your combustion chamber?

EDIT, i see it is 4.125x4.00...but still need to know CC volume....

 

....just go here http://www.aacncclub.com/crucemotor/...sionratio.html

 

Not sure what size yet. It is something like 10.6 with 64cc.

 

According to that one it is 12 with 64 I am confused

 

it is 12:1 with a .040 gasket. using a .052 head gasket i get ~11.62:1...i would have opted for a 72cc head if possible and then milled it to get the desired compression. i'm not sure how thick of a head gasket you can buy, but the thicker it is, the less quench you'll have.

you really should consult your engine builder for this sort of stuff. he/she/they can tell you what you need to do for sure.

 

12-1 espically since you will only drive on the weekend

 

http://www.c5frc.com/calculators/Com...Calculator.htm
http://www.aacncclub.com/crucemotor/...sionratio.html
Why such a difference in these two?

 

i forgot you said -17cc reverse domes....that's why the one is different. that -17cc's changes everything and the calculator i linked to doesn't take that into account unless you add it in yourself.

depending on your deck height, those pistons with your 64cc heads will yield a good, safe, power-making compression ratio that's still pump-gas friendly

 

I see. I am the engine builder at this point. Got a good deal on parts. Just want to make sure these pistons will work or if I should go with a different piston to keep more options open.

 

i used -18cc wisecos(4.125"x4.000"),.006" out the top of deck,gm headgaskets,l92(70cc) heads,10.499:1 comp runs fine on plenty of timing/low grade fuel etc.depends on tuner and cam selection and end useage.
cheers dave.

 

No point in being less than 11:1, no matter what you do with the car. I don't see a problem with going 11.5:1 as a target and wouldn't disagree with the 12:1 suggestion depending upon your ideal redline target. Static compression is just one factor. If you start with a high static compression, you can bleed some cyl pressure off with your cam choice.

 

To do that with my current pistons I would need a chamber smaller than 64cc. I would like to run L92 are LS7 heads. I may have to bite the bullet and order new pistons.

 

Try and trade the pistons in if your staying N/A. Get a 10-15cc dish. That will give you more versatility in your setup.

If you keep them.. Think about this
ported L92 64cc
0.03 cometic gaskets (comp = 11.1:1 / dcr 8.2)

However... 11.5:1 would be ideal I think...

 

That will work to a point. As rpm comes up if the cylinder head works it will fill better than even the "dynamic compression" ratio calcuator things would suggest.

In short, if you think youll have a bad combo that doesnt move as much air as it should, then run as much compression as you can. If you get the combo right, another half or point of compression will see you little to nothing in my opinion.

One of the 427s weve done, saw no meaningful gain adding near 2 full points of compression change. This wasnt with a weeny cam, this is a hyd cam car that gets shifted at 7800+ so you know your dynamic compression calculator thingys would say it would love lots fo static compression. If your making over 100% VE though it wont need the same. If you cant run full timing the engine wants (not what it is limited to reliably by octane) then run less compression. Id say 11.3 give or take a hair if you plan on getting the combo close enough that it makes pretty good cylinder pressure. You wotn leave a thing on the table you can feel

 

That is kind of what I was thinking. It will open up head options. I would not have to get them milled as much.

 

my car is at 11.7 to 1 and i run pump gas just fine, but we do have 93 here so that helps. i would try to get at least 11.5 to 1 if you can.