Just have a few questions. I am currently rebuilding one of my shortblocks and have been turbo'd at 8.8:1 at 15psi. Should I throw in some 10.5:1 pistons and run 10psi? My only means of tuning, as far as fuel goes, is the sfmu at the moment. Am I going to have any knock problems at that compression, or should I just stay at 8.8:1?

Depends on many things. Intake temps, fuel octane rating, injector size, fuel pressures, and air fuel ratios.

You need to ensure during tuning that you are monitoring knock and AFR. If you are not prepared to tune then do not venture.

And I am moving this to the Turbo Section.

Originally posted by CODE4
Depends on many things. Intake temps, fuel octane rating, injector size, fuel pressures, and air fuel ratios. You need to ensure during tuning that you are monitoring knock and AFR. If you are not prepared to tune then do not venture. And I am moving this to the Turbo Section.

Sorry about that, thought I was already in the Turbo forum. Anyway, I would be using 93 octane of course, 310cc injectors, I have a wideband and can data log knock. I have an HRC intercooler as well. Do you think I would be fine at 10psi with the SFMU?

Should be. As long as you aware that the SFMU is not really a tuning tool. The only thing you really can do with it is make sure you have a target AFR at WOT by adjusting your fuel pressure.

I would not personally go that route. If you have money to spend upgrade to a full EMS.

IMO those injectors are way to small... I would get 450cc or higher and I second the EMS you will be a lot better of tunning your fuel and timing curve. And this will be a lot safer for your setup....

Mechanically tuning EFI with 10.5:1 compression...

I see a big boom in your future.

Like others have said, you really should up your injector size and use a full EMS. Getting your AFRs to stay in the right place will be crazy difficult at that compression ratio. It can be done and make awesome power (bullett has proven this) but just trying to hit the correct AFRs with just the SFMU will be nearly impossible.

Originally posted by RoninEclipse2G
Mechanically tuning EFI with 10.5:1 compression... I see a big boom in your future. Like others have said, you really should up your injector size and use a full EMS. Getting your AFRs to stay in the right place will be crazy difficult at that compression ratio. It can be done and make awesome power (bullett has proven this) but just trying to hit the correct AFRs with just the SFMU will be nearly impossible.

and thats a surprise?

Originally posted by Star Turbo Talon

Originally posted by RoninEclipse2G Mechanically tuning EFI with 10.5:1 compression... I see a big boom in your future. Like others have said, you really should up your injector size and use a full EMS. Getting your AFRs to stay in the right place will be crazy difficult at that compression ratio. It can be done and make awesome power (bullett has proven this) but just trying to hit the correct AFRs with just the SFMU will be nearly impossible.

and thats a surprise?

To everyone but 97allmtr? No.

Originally posted by RoninEclipse2G

Originally posted by Star Turbo Talon

Originally posted by RoninEclipse2G Mechanically tuning EFI with 10.5:1 compression... I see a big boom in your future. Like others have said, you really should up your injector size and use a full EMS. Getting your AFRs to stay in the right place will be crazy difficult at that compression ratio. It can be done and make awesome power (bullett has proven this) but just trying to hit the correct AFRs with just the SFMU will be nearly impossible.

and thats a surprise?

To everyone but 97allmtr? No.

Actually I was considering it as it isn't impossible, but wanted to see others input first and see if anyone had done it. It didn't sound very good to me either, as at 8.8:1 the sfmu still isn't good either. I wasn't really even going to attempt it except for the fact that others were telling me I'd be okay. I was thinking more along the lines of 5psi with it if that. I am upgrading to a stand alone here in the next few months. But, okay! You got me!

I ran the same setup with a star kit @ 10PSI. SFMU and 30lb injectors. It ran fine pulled 215WHP on the dyno. Man it was nice too it boosted fast now I'm locomp and it takes forever to get it spoolin. Boost still hits full at 4k but with hicomp it just gets there faster.

Anything past 10 psi I'd make sure you've got some good tuning aids.