Forgive me, for I have searched.

I have been through pages and pages of thread but still can't find a consensus on what an MBC should be set at for 91 octane.

Some say 21 is ok, some say 19 max, some in between. Can anybody clear up the haze here?

This is assuming either stock or just with a TBE.

 

I think you're fine going 21...the ECU is just going to drop advance when the knock sensor sees it. There's a pretty good margin of error from the factory, I think.

 

I say 21 peak and taper to 19-20. We got such crappy gas here in SoCal dont we?

 

Unless my boost gauge is off, I've seen upwards of 22.5 tapering to 20 or 21. THis is with an XEDE.

 

It would be best if you had a device to monitor knock to determine if you are knocking. I've heard of other STOCK evos knocking on 91 oct. More than likely anthing over 19 psi you are knocking and ECU is pulling timing.

 

Don't know if SMART monitors knock, but it monitors AFRs and can pull timing and boost accordingly.

 

The last thing you want is to have knocks. Usually you can't hear knock activity since we got a cast iron block and if you do hear it...it might be too late. If you want something to monitor your knock counts...you can get a knock link or PLX R500 for AFR, EGT, Boost, and Knocks.

As for the max boost question, I run 1.5 bar taper to 1.4 bars on CA 91 octane. You boost level should be determined by your tuner but I would not run more than 22psi on the CA water out gas.

Thanks,
Tom

 

Thanks for the reply Tom. This will be done on a stock ECU for a while though, so would you still recommend as high as 1.5 bar?

 

I have mine set at 1.45 and leave it there. I hold boost great in every gear. I have the AVC-R. Works for me on 91 water out here in AZ.

 

Evo runs really rich stock around 10:1 so 1.5 bar shouldn't be any problem. However, I would recommend you to dial in the boost controller on the dyno.

Tom

 

I'm running 22psi on 91pisstane with EMS.

 

ha ha crappy 91 octane move to georgia 93 94 octane even pumps with 100 octane can be found ,,,boost wise 19.5 to 20 on the stocker as high as 28 have been seen on meth,,and the stock knock cant pick up on the slow cooking type of detonation the kind that you dont hear so an EGT guage should be used for assurance

 

us too. but, with a little bigger turbo than the factory
to4R 91 octane 401awhp@22psi

i honestly think this is close to the limit for the factory cams on 91octane

 

Before I added my water injection system, I found that 18 PSI worked the best for me at the dragstrip for 91 octane.