High Compression + Low Boost: Your Thoughts

Spd-Kilz · 09-27-2009, 11:37 AM

I had never planned to play with boost, but it's becoming increasingly attractive to me. I'm getting used to the setup I have now and would like more power.

My main problem is my compression ratio is 11.5:1.

Is the only main issue with high compression + boost pre-ignition? If so, could I counteract this with 94 octane pump gas and methanol injection?

Can this be done relatively safely?

I realize there is another thread, but I'm really looking to get more insight into the issue.

I'm not looking for people to say "buy new, thicker gaskets" or "get bigger chamber heads" etc. I'd like to keep the setup I have now.

383lt1impala · 09-27-2009, 12:59 PM

Well u can run low amount of psi with pump gas without meth. Me I plan to run 7-10 psi on c16 but im running 12.56 compression.

DevonsZ · 09-27-2009, 02:54 PM

I ran 11.3cr with 10-11lbs of boost on 91oct and meth.

fknsmshn · 09-27-2009, 03:12 PM

My favorite thing about high comp and low boost is the spool times. Lag is pretty much non existant.

From what I know, tuning is a bitch so dont slouch on that. Just watch for detonation and if you do run meth, keep an eye on the bottle.

BoostnTBSS · 09-27-2009, 03:29 PM

I run 10.6-1 with 15psi with E85 and no I/C and 25*'s timing. and it spools great 348 ci with Gt42 w/ 1.15 ex i see 7 psi as soon as i touch the throttle. like 1/8 throttle... it is instant..

Got Me SOM · 09-27-2009, 04:35 PM

You may have to run higher than 94 to be safe, more like 100 octane. It'll want to detonate, but the cool thing is it'll be responsive. With 94 you'll be able to run about 4 psi depending on what blower/turbo you use. kinda defeats the purpose if you ask me.

when i had 10:1 compression with 9-10 lbs on my vortech my car was fun.

Spd-Kilz · 09-27-2009, 06:12 PM

Interesting. I'd really like to use pump gas and stay away from using race fuel since the car is a street car, and that can get rather expensive. I really don't have much of an issue running about 4psi...that would put me around 570rwhp which imho is not bad at all. As a general rule of thumb, how much are the stock internals good for? I really don't want to exceed that anyways. I don't want to play with fire. What supercharger would be good for low boost levels? A P1SC?

Got Me SOM · 09-27-2009, 06:32 PM

I would use the SEARCH feature as this has been asked many times before. It's almost impossible to run only 4 psi, you have to run a super large pulley unless you go custom. Most kits start off at 6-7 psi.

Stock bottom ends are limited by the cast pistons and the ringlands. they go when they go, they don't fail at a certain amount of hp.

Your compression is really pushing it and asking for detonation. Make sure you have a credit card when not if the bottom end fails. Also invest in a good fuel system.

There is no such thing as cutting corners or cheaping out when you go forced induction so either do it right or don't do it at all.

black_z · 09-27-2009, 06:50 PM

Is E85 an option? If not, I would definitely mix race gas. Tuning and good fuel will hold it together.

Zombie · 09-27-2009, 09:09 PM

without higher octane it's not the greatest of ideas. On 100 oct or e85 it's possible (especially e85) but it's far from ideal and limits you.

Spd-Kilz · 09-27-2009, 10:29 PM

What all is involved with getting the car to run E85?

DVS99TRANS · 09-28-2009, 02:21 AM

I am in the same boat as you Spd-Kilz, i have been researching which is the best route. I just had my 408 built and am running a static 11-1cr. Should have had it built 9.5-1

I'm thinking about getting a new set of heads, and dropping the cr as low as I can without having to swap pistons. Even if that is the case I would only be able to run around 7-8psi max on pump gas if I'm lucky.
Stock internals: you shouldn't really go above 8psi is the norm. Hell you'll still net over 500whp

Spd-Kilz · 09-28-2009, 07:51 AM

At least i'm not the only one. I really don't want to go through the hassle of taking off the heads. 8 pounds would be ridiculous on a 408. I figure i'll be at ~600rwhp with 5 pounds. Hell, i should be over 500 with 2psi.

Quote:

Originally Posted by DVS99TRANS

I am in the same boat as you Spd-Kilz, i have been researching which is the best route. I just had my 408 built and am running a static 11-1cr. Should have had it built 9.5-1

I'm thinking about getting a new set of heads, and dropping the cr as low as I can without having to swap pistons. Even if that is the case I would only be able to run around 7-8psi max on pump gas if I'm lucky.
Stock internals: you shouldn't really go above 8psi is the norm. Hell you'll still net over 500whp

SSdreamz · 09-28-2009, 08:47 AM

same boat as you...been kicking the idea around about the higher compression and boost... im at 11.7:1 but ive also have a forged bottem end.... ill be watching this thread to see where it goes

cfon159 · 09-28-2009, 12:34 PM

im kinda new still to the ls crowd, but im pretty sure u can get away with up to 10 psi at 11-11.5:1 ratio. i say this because back in the civic crowd (where i once resided till opening eyes to v8) many guys would run that on their all aluminum, thin-walled motors with as big as a gt35 turbo. so long as it is tuned properly to ~11:1 afr, u should be set. i only wish i had the resources so i can do my turbo build to help prove this... gl tho and keep us informed.

Zombie · 09-28-2009, 01:51 PM

Quote:

Originally Posted by cfon159

im kinda new still to the ls crowd, but im pretty sure u can get away with up to 10 psi at 11-11.5:1 ratio. i say this because back in the civic crowd (where i once resided till opening eyes to v8) many guys would run that on their all aluminum, thin-walled motors with as big as a gt35 turbo. so long as it is tuned properly to ~11:1 afr, u should be set. i only wish i had the resources so i can do my turbo build to help prove this... gl tho and keep us informed.

The smaller bore of those motors are less prone to detonation. Larger bore = more likely to detonate.

DVS99TRANS · 09-29-2009, 06:21 AM

^^^exactly, my overall goal with the car is to run it on pump gas period. Just like Spd-Kilz is going for. The other good thing to do I think would be to swap up to a custom FI cam that might help out a higher compression motor on FI which will be a must IMHO. Meth is nice but am going to steer clear of it just in case. I am also running PRC 2.5 5.3 heads undecked also, which aren't the best and are going to have to be replaced. In that area, I am looking at getting the biggest combustion chambers possible getting me back into the 10-1cr range again which is easily possible. That and running a T76 @ 8-10psi-ish would become a possibility without detonating. You can always run race gas if you want to turn it up at the track or limited amounts of time. That is the direction I am thinking...
A buddy of mine that is on here is thinking the same thing. He has a high compression 408, and we have been talking about it quite a bit. Hopefully he sees this and adds to the thread too.