I need some help...


I have installed a mbc set at 24 psi in 4th gear. (everything is stock) I boost perfectly up to 24 psi and then drop slowly as my rpm increase... BUT, when I reach 6000 rpm (close to 18 psi) then the boost drop very quicly to reach 9-10 psi at 7000 rpm...


My mbc is installed the right way since I have tried inverting the hoses connections...


What the hell is wrong!!!


Thanks,

 

Likely you are seeing the Stock Wastgate Actuator in action here which has a weak spring. You can try preloading it 1-3 turns but your minimum boost will go up so you will have to back off your boost controller so you don't run boost too high at the low to mid rpm's. A good starting point is 2 turns and see how it goes. There was some variation from the factory on how much preload there is on the WGA, so some can only get 2.5 turns while others can get 3.5 turns due to having less factory preload which is likely your case. You can search the archives for how to get to it and adjust it.

Still even with an upgraded Forge WGA the stock turbo will only allow you so much boost at higher RPM's and taper boost as it runs out of flow. I think from 3000-5500 you can get 25+ psi and then from 5500-7500 it will taper from 25 psi to 19 psi no matter what WGA you use or how high you turn your boost controller.

The one test I haven't done is to wire the wastgate shut, throw some race gas in for safety, and only go WOT at 5500+ RPM's with a logger or camera on the boost gauge. That would once and for all confirm that the turbo can't flow any more above that RPM and that the WGA isn't to blame at all. Just don't go WOT between 3000 and 5500 because this turbo can still go over 30 PSI at lower RPM's and that could potentially be a really bad idea.

*** Not responsible for anyone blowing an engine attempting this test!

 

Chris, Just suck it up and do it; very interested in the results (not blown engine but whether the OEM T can exceed 25psi at top end)

Later, Ken

 

MBC = ****ty way to control boost on an Evo X. Get a real tune.

 

Seriously... the ECU can control the boost better on an X than an MBC can. Its better by leaps and bounds than the previous generations from what tuners have been saying with experimentation.

 

I would agree except that if you want to change the boost it's not so easy to re-flash the ECU with a laptop. Granted I'm only changing the low to mid range boost on a stock turbo, but once I have my GT3076R to play with it will be very handy to have 2 preset boost levels to select from inside the cabin.

 

Since My boost drop as much as 10-12 psi I don't think playing with the wastgate actuator will make that much of a difference... would it? Also, I am unable to locate the rod that I am supose to adjust. Can you please clarify how to since I was not able to get pictures or clear explanation... The heatshield is now off put don't know what to do next.


Thanks,

 

I think you mean without.... so going on that...

True and you may be referring to situations of spontaneous performance testing. Any sort of track use that you would be knowing ahead of time for, could easily be prepped with a laptop and tactrix cable, or a carputer installed (perfect way to go). Having a map of pure performance waiting to be flashed while you drive to your destination on a map tuned for economy is better than just changing your boost levels (since it can also include timing changes and AFR's). So yeah, cheap way to be able to have a high amount of boost on tap when you want it... MBC. Ideal way... carputer with ECUFlash or something similar installed and swapping maps on the fly.

 

There might even be guides and pictures of tightening the WG rod out there, kinda hard to explain besides reading those posts. The stock wastgate is designed to open at 11 PSI so that just means you are at like zero preload or close to it.

 

Actually I meant with a laptop... haha. Not everyone wants to pull over and reflash their car with a freakin computer to change the boost a couple PSI. :P
Granted if I am tuning the car it's no big deal but to have a low and high boost setting available at the flick of a switch is great for when you need it right away, not after pulling over for 5 minutes. I guess everyone is entitled to their opinions.

 

Thanks a lot for your help. I was finally able to locate the rod late last night. I have ajusted only one turn to see how the car react to be sure and log some data monitoring. I saw 1/2 psi increase or so accross the rpm range so I'll try giving it 2 more today and see the result.


One thing that is bugging me is that I have a litle dangeling or ratle at low speed (under 15km/h) ... I guess it might be because the heatshield is off and I didin't reinstall everything yet... I'll see once I am done.

 

EH my point was that we were talking 2 different parts of the same thing. You can change boost with yours... i can change all the settings with mine. Depending on needs... either one could work for you. Good points!

 

Somehow you don't have a clue what you're talking about. An MBC works just fine as many great tuners have tuned with them with great results.

 

Alright... I have turned the rod 3.5 turn (witch is the max I can do) and gain only 1.4 psi maximum troughout the entire RPM range... Still at 11.5psi at 7500rpm...

Some people are telling me it could be my mbc witch I think is fine since I am able to reach 24 psi easy witouch any prob.


One question about my mbc... it's a Turbo XS (standard and basic one)... on their website they say "Bleed type boost controller allows you to increase boost approximately 12psi over your wastegate boost setting."

What do they mean by that? Could it be my mbc that is making my boost drop past 6000rpm. ???

 

If the WGA is set from the factory to open at 10-12 psi with a hose going straight to it, having a properly working boost controller set for 22-24 psi in the lower RPM's should hold the boost much higher than what you are seeing. Granted the stock turbo will taper at higher RPM's due to flow but I would check to see if there is a bleed hole in your boost controller design. If not try putting a small pin hole in a hose barb and see if that will relieve the pressure. If boost is being trapped between the MBC and WGA it will make spoolup slow and keep the wastgate open until it bleeds off.

Another possiblilty is you have a lose hose somewhere and the boost is leaking out of your intercooler pipes somewhere. Usually that will cause other issues though since any large leaks will make the car upset about the missing air.