Full boost half throttle

Splinter · Post by **Splinter** » Wed Feb 08, 2006 9:22 pm

Ive got my MBC connected up to where the stock solenoid was connected (with new hoses), and Im boosting up to 13PSI in 4th (4EAT) when Im cruising with the throttle half down.

It also has a tendancy to boost up to 12 or 13 and drop down to 10 when Im at full throttle.

That is, it holds boost BETTER at half throttle than full throttle.

Vac leak maybe??

dscoobydoo · Post by **dscoobydoo** » Wed Feb 08, 2006 10:37 pm

Maybe, but my leagcy pulls better about 3/4 throttle. If I push it full throttle, it's like it does not push as hard. You can tell the difference.

renaud · Post by **renaud** » Thu Feb 09, 2006 5:03 am

me the 1/3 trottle pedal is where i got the more power boost... lol
Happy to see that I'm not the only one to have that shit on my car

kidatari · Post by **kidatari** » Thu Feb 09, 2006 6:52 am

I had that problem really bad with one of my previous Legacy SS' (4EAT). It was causing high EGTs, which is no good at all. But this was all under factory boost control, which makes me wonder if the boost solenoid took a crap, or the wastegate was acting 'iffy.

Usually it's referred to as PTFB- Part Throttle Full Boost. Basically your turbo is trying to push 12-14psi through a partially closed throttle butterfly.

Of course, I never figured out how to fix the problem before the car got drifted into a K-wall at Thunderhill, so I don't really have a fix.

I plan on expiramenting with some aftermarket boost control on my 91 5MT SS when I get back to the 'states.

NICO · Post by **NICO** » Thu Feb 09, 2006 10:32 am

wow i had that problem a long time ago.

you need to open up the exhuste get headers and a bigger turbo, and get a electronic boost controller.

best boost controller in the world i say turbosmart e-boost.

rallysam · Post by **rallysam** » Thu Feb 09, 2006 3:55 pm

This sounds weird. Could you show us a picture of how you have it hooked up?

Splinter · Post by **Splinter** » Thu Feb 09, 2006 6:26 pm

Digicam is shot...

Ive got a vacuum hose from the compressor nipple to the inlet on the MBC, and the outlet of the MBC hosed to the waste gate actuator.

rallysam · Post by **rallysam** » Thu Feb 09, 2006 7:13 pm

Well, that sounds like you've got it right. Most people get PTFB because they hook up their MBC to the intake manifold instead of the compressor outlet, but luckily you knew better.

As you know, it's a pretty simple control system, not that much can go wrong with it.

All I can think of is:
* One of those two hoses is leaking. Inspect them closely, and clamp all connections with some quality clamps.
* Your MBC isn't working right, is clogged, or you hooked it up backwards.

One more important question, how do you have your boost gauge set up? Where is it plumbed to?

Splinter · Post by **Splinter** » Thu Feb 09, 2006 7:36 pm

boost gauge is hooked up to the intake manifold, between the manifold and the canister purge hose

I know its accurate because I was hitting fuel cut right where I was supposed to when I was tuning the MBC

The MBC isnt hooked up backwards, when its backwards the boost is totally uncontrolled.

Theres an excellent chance it isnt working properly though, I'll try and find a different spring to use.

rallysam · Post by **rallysam** » Thu Feb 09, 2006 7:51 pm

Splinter wrote:MBC it hooked up to the intake manifold, between the manifold and the canister purge hose

You mean the boost gauge is hooked up to the intake manifold, right? The MBC isn't - you said it's hooked up to the compressor outlet.

This behavior makes no sense to me.

One thing that might shed light, is you could temporarily connect the boost gauge to the compressor outlet. To do that, you could tee off one of the lines for the boost controller. That would tell you more about what the turbo and MBC are doing. Right now, you're trying to see what the turbo is doing, but your view is blocked and confused by all this stuff between the gauge and the turbo that introduce their own pressure variations (the throttle plate, the intake restrictions, and intercooler).

Splinter · Post by **Splinter** » Thu Feb 09, 2006 7:56 pm

rallysam wrote:You mean the boost gauge is hooked up to the intake manifold, right? The MBC isn't - you said it's hooked up to the compressor outlet.

Yea sorry

rallysam wrote:One thing that might shed light, is you could temporarily connect the boost gauge to the compressor outlet. To do that, you could tee off one of the lines for the boost controller.

I'll try that.

Splinter · Post by **Splinter** » Mon Mar 13, 2006 6:35 am

Fixed the problem by punching a tiny hole in the hose from the MBC to the wastegate

legacy92ej22t · Post by **legacy92ej22t** » Mon Mar 13, 2006 1:51 pm

That doesn't sound good.

You fixed it by creating a leak?

sammydafish · Post by **sammydafish** » Mon Mar 13, 2006 4:33 pm

legacy92ej22t wrote:That doesn't sound good. You fixed it by creating a leak?

not a leak, a vent. all MBCs need this. It only needs to be a very small hole in order to relieve pressure on the actualtor side of the valve. If not then the pressure in the line will hold the valve closed requiring more pressure to open the valve than the spring is set to. Usualy this causes crazy boost spikes, not the behavior seen in this post. Seeing more boost at part throttle than you do at WOT means one of two things to me, bad tunning or to small a compressor (can't flow the air required at WOT).

Splinter · Post by **Splinter** » Mon Mar 13, 2006 4:34 pm

Yea, I think what was happening was that high pressure was trapping the wastegate open

legacy92ej22t · Post by **legacy92ej22t** » Mon Mar 13, 2006 4:50 pm

I know that MBC's need a bleeder hole but the MBC should already have the hole on it. Why would you need to put a hole in the vacuum line then? Wouldn't that make two holes?

Still doesn't sound right to me.

Splinter · Post by **Splinter** » Mon Mar 13, 2006 4:58 pm

Homebuilt MBC

BAC5.2 · Post by **BAC5.2** » Mon Mar 13, 2006 5:00 pm

Unless the MBC used was a pile and didn't have a bleed hole in it.

rallysam · Post by **rallysam** » Mon Mar 13, 2006 5:26 pm

Splinter wrote:Homebuilt MBC

Ok, that explains it

vrg3 · Post by **vrg3** » Mon Mar 13, 2006 5:54 pm

I wouldn't rely on a pinhole in a rubber hose. Just drill a little hole in the MBC. If you don't have the means, ask a friendly jeweler.

legacycentral bbs

A forum for 89-94 BC-BF(BJ) Legacy Owners and Fans

Full boost half throttle

Full boost half throttle