Never seen a thread on this so I thought i would post one. I have seen this mistake many, many times over the years but never realized how critical it was till I made the mistake on my own car and felt the results.

If you are plumbing your boost control to turbo you need to take that signal directly from boost out at compressor cover or dedicated line from manifold.

DO NOT SHARE LINE WITH BOV!

boost controllers control boost by BLEEDING pressure. If you are running 24 psi on a 15psi waste gate you are bleeding 9 psi of pressure. If the BOV valve is plummed on same line the BOV sees the same 9 psi pressure loss. Very important needed pressure. the blow off valve uses boost pressure to hold valve on seat. For every pound of boost run you need the same pound of extra pressure to hold valve on its seat. Whats left over is the original 9 psi spring pressure to hold valve on seat. If you bleed 9 psi pressure you now have no spring pressure to hold valve on its seat. the result is a fuel curve that is difficult to tune and very significant power loss as precious boost is bled off like huge boost leak.

the second thing that happens is BOV drivabilty is horrible. When you lift throttle the BOV receives a vacuum signal to lift valve off seat to relieve boost pressure. if the line is shared with wastegate can there is stored boost in the can that has to bleed off first before the BOV even sees vacuum. the result is a very lazy BOV that is not doing its job. Often the engine kicks and bucks on part throttle lifts as the spring pressure in the BOV is effectively larger.

I bet there are many people who have made this mistake and were never happy with how their car drove. not to mention it didnt make the power it should have.

 

Yep always try to give the BOV and boost control solenoid there own sources.

MAP sensors and FPR and such I link together.

 

+1000000 I've said that many times, and people still think they should run it the wrong way.

 

I am currently T'ing off the BOV line for my boost gauge. That shouldnt be a problem right?

And the mbc is off the turbo.

 

Why not source the boost gauge off the FPR line?

 

I dont like sourcing the FPR for pressure because if the line leaks for what ever reason then it will lean out the air fuel ratio.

 

It is the method recommended by the Mitsubishi factory for a proper boost gauge install. But, to each his own.

 

Something worth mentioning is that if you source the wastegate line from the upper IC pipe on a deticated line, you'll reduce the amount of boost taper you get. Sourcing from pre-intercooler, you will get pressure losses across the IC that will reduce the pressure in the manifold.

Sourcing it from the IC pipe means the boost controller never sees vacuum. Some ball and spring controllers can stick when a vacuum is applied, leading to over-boosting.

This does tend to cause boost spiking on smaller turbos though. It's usually only 1-2 psi and bleeds off very quickly (within about a second).

 

Good points. I have also noticed different boost levels pre and post IC. Cant remember what cooler was on the car but on manifold boost was 17psi, and moving controller to turbo boost dropped to 15psi without touching setting on controller. I concluded boost loss through intercooler was higher than what most will advertise.

 

It's interesting because there are potentially opposing factors at work. Tapping pre-IC will seemingly cause a quicker reaction, but it should limit boost prematurely only if there is a significant pressure drop across the IC. Ideally, there won't be, but that is a factor of IC volumetric capacity with respect to volumetric engine demands.

Tapping post-IC will give a more 'genuine' response with respect to actual manifold pressure, but at the same time it may create a delay in WG action (a factor of the time it takes for the pressure wave to traverse the IC), which could manifest itself as a boost spike.

But then again, with careful WG duty vs. rpm tweaking, it may end up just being a matter of 6 in one hand and a half-dozen in the other.

 

Is the after example correct in this thread?
https://www.evolutionm.net/forums/ev...-unos-mbc.html

Using the numbers in your example the manifold is seeing 24psi, and the line between the manual boost controller and the wastegate sees 9psi and the rest is bled off to atmosphere at the MBC, correct?

Forgive me, I'm new to turbos

 

that thread shows exactly how not to plumb the MBC. I shake my head that original poster didnt notice any drive-ability issues. but trust me they were there. I just corrected mine this evening and the difference is stark if you know what to look for.

 

TedB, you're dead on. If you are using something like the stock ECU to control boost, it's probably a moot point on where you mount the line. You can simply tune the boost settings for either setup.

With a manual boost controller, it can make a significant difference in how the boost acts. With the post-IC setup, you are basically adding a pure time delay to your mechanical "feedback loop." This induces overshooting that is proportional to how quickly the turbo spools as the faster it spools, the higher the boost goes during that time delay.

 

Anyone got a diagram of how to do it correctly? My MBC is coming in a few days and I really really don't want to make the same mistakes as the guy in the other thread...

 

This is from the Blitz boost controller manual with an external WG. The "IN" and "OUT" references are on the boost control solenoid as provided from Blitz.

Josh