On my Evo with the following mods: 272 cams, 3" TBE with straight pipe, Helix Intake, Hallman MBC, Walbro FP, Forge BOV and Reflash, I am getting the following readings on my Defi D series boost guage: It holds about 22 psi and tapers to 20psi. I sometimes see spikes of up to 23 and tapering down to 19psi, but typically it tapers to right about 20 psi at 7k.

My friend with similar mods says he sees 21-22psi all the way through 4th according to his Defi D series boost guage.

Am I experiencing unusual taper or is my friend's boost guage just off?

 

Tough to really tell on a gauge. A map sensor is really the only way to see what is really going on. My Defi Gauge looked rock solid but the Map sensor revealed big boost taper.

 

Which spring is in your MBC? What about his? How many turns clockwise are your MBCs turned?

 

I have the red, green and yellow one still left...so not sure which one is inside. The vendor I bought it from said it was good to go as purchased for an Evo.

No idea how many turns our MBCs are turned.

 

Sorry...I was wondering which spring is in your Hallman...heavy or light. The light spring does not hold boost as well. If you do put in the heavy spring, you will definitely want to turn the MBC down to start with and ease it up cautiously. 20psi will probably come somewhere just shy of 4 clockwise turns with the heavy spring...

 

Oops...I misread. We both have the hallman Pro. AFAIK, they didn't come with multiple springs, or did they? I bought mine used and it didn't come with additional springs.

 

Mine didnt either, only 1 spring, is that heavy or light that normally comes with it.

 

Oh yeah...I believe you only get the extra spring with the Pro RX (and Evolution RX). The extra spring is the heavy one, so yours should be light. I had to move to the heavy spring to hold solid boost...

 

That amount of taper is what i experience as well but I don't have a flash yet, hoping a custom Dynoflash will remedy next month.