Lovin' my MPI BOV

[shawnmcgr]

Just thought I through up a positive review post.

I installed a MPI BOV and have swapped the pullies to run 10psi but have the BOV set at 8psi (just so I don't have to run as much av gas - 200hp is good for now).

Works great!

 

[nate007]

So under full throttle you're bleeding off boost then? Seems like it'd be better to tune/swap pulleys for 8psi and know it's getting what it should? That should be making it harder to calculate A/F ratios and tuning, wouldn't it? Not to mention there is a certain loss of hp to turn your blower hard enough to make 10, then only use 8, you're probably leaving some hp on the table?
Maybe I'm wrong, but it seems a bit risky with tuning?

 

[shawnmcgr]

I'm not sure if I'm bleeding off boost at full throttle or not. My expereince with 5psi pullies was 5psi at low elevation and boost would drop off as I climbed to 2psi @ 7000ft. But I was only running about 9000rpm @ 7000ft as well.

I definitely can not hear the BOV openning at full throttle. It might be slightly open, not sure.

But I've got a steady 8psi from the parking lot (sometimes as low as 1800ft) to 7500ft. Fuel map needs very little adjustment. I have the GEMS set up for 5000-7000ft and usually do not need to touch it. It's under my cowling to keep it dry.

I've got a AF gauge so no need to calc, the AF readings are good - still rich actually.

I am leaving some hp on the table but I do not have a shim and do not want to run 50/50 race gas (race @ $5/ltr). Also, I was hoping to avoid having to upgrading drivers to Avids and rear suspension. I'm running straight av gas or 50/50 av/91 depending on the elevation of the parking lot.

Nice to know I can turn down my boost if I cannot get av gas. I'd have to pull weight out of my clutch but with the Dalton adjustable that takes about 20 min.

I might try the 10psi bov spring next year to see how much boost varies with altitude with the 10psi pullies. If there isn't much change then I may go with a head shim. What's your experience? Do you see much boost variation with altitude?

I'm very happy with 200hp @ elevation, what an animal!

 

[SRXNOMO]

we have done the same thing on centrifugal superchargers on cars. We pulley it for say 16 psi at max rpm, then use a bov or external wastegate to regulate boost to a selected level. A couple things are accomplished. First boost with centrifugal superchargers is directly proportionate to rpm. So instead of making 8 psi at max rpm, pulley it for 16psi and regulate boost to 8. Your now making 8psi from say 3500 rpm to 6000. You can also use these steps to run pump gas while crusing or commuting, then turn boost up at the track or if you want to go racing with some c12 or c16.

 

[shawnmcgr]

Thanks SRXNOMO, good to know this has been done else where with success.

I've got 4 days of mtn riding in on this set-up and I'm very happy. I made a slight adjustment to my box on the forth day just to lean it out a bit. Otherwise I have not touched it. I'm still a bit rich at idle to about 1/4 throttle. I may try adjusting the CO stock settings to help there.

 

[Ratpack]

You can run 10lbs. in the mountains on pump gas. Just don't do long pulls on the way up.

 

[shawnmcgr]

The boost calc spreadsheet I use, thanks to this site, shows I should be running 93 octane @ 6500ft at 8psi so I run 95+ octane to stay safe. I can get av gas from a small airport 20min from my house, it's not much more than premium.

What elevation are you running 10psi on pump gas? Have you got a shim? What octane pump gas are you using, something better than 91? We've got 94 at the pump here but it's got ethanol in it.

 

[Ratpack]

I run 10 psi at 8000-11000' at 91 octane. At 10000' you lose almost 4 psi from sea level, so you are really only making 6 psi. At 1500' I am running 7-8psi on 93.4 octane with 10% ethanol. Going to 10 here this year and running some torco. Ulmer and MPI have told me 10 psi on 91 octane is ok in the mountains.