mbc no good for low psi?
 

both times, year or 2 ago on old setup, and just yesterday i was trying to set a ball and spring type mbc for 9-10 psi and it just doesnt happen, are they no good for low psi ? kept seeing 11-12 psi and going up before i let off, screw was backed almost all the way out when i was done

Re: mbc no good for low psi?
 

Depends on the MBC design, and depends on your wastegate setting. Details!

Re: mbc no good for low psi?
 

i had this same problem, bought a turbo smart manual boost controller and the damn this would stay at 11-12psi. even turned down all the way, i asked the shop i got it from, and on one side there is a little ball a spring, tha manual boost controllers are set from 12psi to whatever you want,

so if you need to take out the small ball and spring and you will be able to go to wastegate pressure.

Re: mbc no good for low psi?
 

If it is a ball and spring setup it will have a bleed hole to let the pressure off to allow the gate to close. So even when it is loosened up all they way it will be hard to get boost all the way down to the pressure of the gate spring since you are still sort-of running a bleed valve with the loosened ball and spring setup.

Or at least I think that is how it is.

Re: mbc no good for low psi?
 

you need to cut the spring a little, i do that, now i can go from 6 to any psi

note: i use an internal wastegate set at 6psi

Re: mbc no good for low psi?
 

im using an external gate with a 6psi spring, the mbc is just a cheap ebay style one

wouldnt cutting the spring make it so it wouldnt have a "flat" top anymore for the screw to push on and probly start getting caught in the threads?

Re: mbc no good for low psi?
 

Be a man and just run 12 psi.

Re: mbc no good for low psi?
 

lol, I dont understand why you would want to lower you're boost pressure, unless you're running a gt42 and cant stop spinning tires.

Re: mbc no good for low psi?
 

GT42 isn't going to make significantly more power than a T04E at 12 psi. In fact, the last GT42 powered 2.0 I tuned made about 15-20 less than the last T04E powered 2.0 I tuned - that's also a K vs B series comparo, for what it's worth, with the 84mm B-series making more power off of the smaller less efficient turbo and "shittier" unported B-series head. Both K-series and GT-series are hype, don't buy into it. :1

If you are spinning tires at 12 psi you probably have an alignment problem or need to upgrade from cheap beater tires. All the turbo cars I've tuned that are fast launch in the 6500-8000 rpm area at whatever psi puts them at or around 400 whp, on 24.5" to 26" slicks, so there is a LARGE window of traction for drag radials given more conservative launching and lower power levels in an ungutted streetcar.

Re: mbc no good for low psi?
 

Yup, I went for a ride in ELspool's car the other night on 225 BFGs, it hooks well with 2 people in the car at 15-16 psi of boost.

Re: mbc no good for low psi?
 

i got an 11.somthin psi spring too, might just throw that in there, what will 11psi on a ford 60/63 on a Ls motor do?

Re: mbc no good for low psi?
 

Be a lot of fun to drive?

Re: mbc no good for low psi?
 

Why not shim the 6 pound wastegate spring to what ever you need and not even use the MBC?

Re: mbc no good for low psi?
 

i made a post about that actually, i was thinking about that but its gona be a bitch taking it apart and shimming it until its right, probly just gona run 11psi :1

Re: mbc no good for low psi?
 

Run the 11psi spring and it will end up being about 9.67psi anyway due to IC pressure loss DEPENDING on your vaccum source (manifold = higher, turbo = lower).


JP

Re: mbc no good for low psi?
 

+1

Easiest thing to do...

Re: mbc no good for low psi?
 

One question... If you were to use an electronic boost controller would that help reduce spool time by keeping the wg closed until the last possible second?

Ok I looked into it and the answer is yes.. But does anyone here know if its really that big of a difference? It doesnt sound like a big deal unless I'm competing in drag events trying to shed one tenth of a second...

Re: mbc no good for low psi?
 

You will probably fry tire regardless of how quick your turbo spools, so until you fix your traction issues, don't worry about it.

Re: mbc no good for low psi?
 

i had a cheap egay mbc that did the same thing. couldn't go any lower than 11psi. ended up just not using it for the time being because even cutting the spring didn't change anything. maybe i didn't cut enough off.

-Luke

Re: mbc no good for low psi?
 

If I'm not mistaken, that's how external gates work already, but I think I'm wrong...I forget. Internal bleed and fluctuate and suck.


JP

Re: mbc no good for low psi?
 

Both internals and externals will see pressure in them before full boost (where they open almost full tilt), but internals are cheap mass-produced garbage with almost no spring preload. Put a Tial on with the firering, and you'll get an idea of what preload is.

A MBC has a spring in it thats preloaded to not move ONE BIT until target psi is met. Technically it will, but when target psi is met, that tiny 'pre-leak' has just pre-presurized the tubing/wg so you don't spike.

The cheap MBC I was given has set at ~14psi 2/3 of the way out, and would not go below 10psi w/o cutting the spring...in half.

Re: mbc no good for low psi?
 

Well wouldn't that be the problem? You have an 11psi spring in there and want to run 7psi. Dont manual boost controllers allow you to only turn up the boost? The lowest boost you should be able to get is limited by spring that is in the wastegate. What you could do is get a 3psi or whatever size spring and then use the manual boost controller to limit the amount of boost that the wastegate sees.

Re: mbc no good for low psi?
 

you suck at reading.

i put the 11psi spring in there and am going to switch vac source from the manifold to the comp housing, we'll see. while i was at it i took my 38mm knock off all the way apart, i mean all the way, after i noticed the diaphram assembly was wobbly , the 4 set screws had came loose

Re: mbc no good for low psi?
 

welp. i guess the big red spring was 6psi too. took the small red out and put the big red in, connected the wg line to the comp housing figureing my enging would get about 9-10psi. still get 5-6. stupid ssauto springs

Re: mbc no good for low psi?
 

Put in both.

Re: mbc no good for low psi?
 

ok.


also what if i seal the bleed hole on the mbc and cut the spring? will that let me do lower boost?