thendawg

Im in the process of putting together parts to build my ls2 shortblock. Ive searched around but come up with alot of conflicting info. How much if any effect does static comp have on spool time? Im basically looking at the difference between 9.2:1 vs 10:1 scr. For what ots worth this will be with an m6 and running on e85.

Old Geezer

iTrader: (3)

On that fuel, I'd look real hard at the 10:1..
Gotta stay on top of the tune, tho.
At that ratio, off boost performance will be better, too.

Forcefed86

iTrader: (4)

Higher compression should spool more quickly. IMO .8 compression points isn't going to make much of a difference either way. I went from 8.6:1 to 10:1... Throttle response and overall "snappieness" didn't change much in my case.

Atomic

iTrader: (37)

Spool time is a complex thing, and a single point change in compression will not be noticeable one way or another.

Ryans99ls1

iTrader: (16)

E85 tends to help turbo's spool quicker as well. I went from 10:1 to 9.5:1 and noticed no difference in anything.

9sectruck

iTrader: (9)

10.9 comp here e85,
still on tr6 plugs and the engine is very responsive

thendawg

Sweet nice to know! Id honestly love to run 10:1 but this is a budget rebuild and Im kinda stuck on piston choices. I can go Wiseco, but the only 4.010 piston option puts me at 9.2:1 (this is even with some milling on the heads). I could go to the Autotec pistons (which would also save $75) and get 10:1, but it would be a 4032 piston, and Im a bit concerned with running a 4032 piston with boost. However, it will be on E85 and my goal is prob 700-750whp at the most, so Im still debating.

Bazman

That is interesting, was the tune optimized with the change in compression?

I think quench has a big impact on response and snappiness if the tune is adjusted accordingly. I've seen a Viper V10 with more snap and response at 9.5:1 and with twin small GT35R's than the stock 10:1 motor - due to tight quench via custom fit pistons.

stevieturbo

Go 9.2:1 then you'll be totally safe on either pump fuel or E85.

slow67

iTrader: (2)

It makes motor more powerful N/A (before boost) But it won't make it spool faster.

Heavy H.P.

I've always heard that it can effect spool time.

toytech93

iTrader: (2)

I'd choose the 2618 piston

rotary1307cc

iTrader: (5)

Static is meh. Could have an 11:1 engine and have a big *** cam and a late IVC and get away with a ton

Forcefed86

iTrader: (4)

Hard to say what optimized means exactly... If you mean running a larger cam geared towards the higher compression, then no. I don’t have a compression change only comparison, so I should probably of kept my pie hole shut. I changed too many things. (cam,heads,etc)

I’d think high static compression bleeding off cyl pressure with the cam would have a few negative side effects over a low compression turbo engine.

Higher duration shifts the power band up higher in the RPM range. This also usually requires more ign lead (counterproductive to power).

For all the OEM short blocks utilizing high compression (10:1+) and a larger cam to make things “snappy”. I’d think you’d be giving up an optimum peak torque and RPM range in boost… for a little out of boost performance. (bad trade IMO).

Ideally wouldn’t you want to…

1. Plot where you want peak torque and your redline for your specific engines top end/intake.
2. Pick a cam to match that RPM band with the least amount of overlap. (which will also allow you the least amount of ign lead)
3. Pick your fuel
4. Lastly pick your static compression based off the dynamic compression produced by the amount of boost you want to run and your chosen cam?

smokeshow

iTrader: (17)

^^ basically how it should be done.

slow67

iTrader: (2)

In an automatic car it will make more RPM on the converter (while on the transbrake). More RPM = spool faster (as side effect of more RPM, not directly from the cr change).

More compression won't change how much air the motor ingests. More compression also puts more heat in the combustion cycle instead of out the exhaust (wasted). Same cfm, slightly lower EGTs = won't spool faster (at the same RPM).