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View Full Version : If you rebuild an engine and the boost goes down without touching MBC-good or bad?



turbovanmanČ
01-23-2006, 11:02 PM
I have the flu and can't think, not that I could before but, just put new rings, rebuilt head on a stock 89 Shadow, 2.5L 5 spd. Before I took it apart, his boost was set at 11-12 psi. After I did the work, and I took alot of carbon off the piston tops, the boost is now at 7-8 ish. I haven't touched the MBC and the timing is the same. Soooooooo, is this good or bad, meaning does the boost change mean its more or less efficient?

Tony Hanna
01-24-2006, 01:05 AM
The only way I could see the boost dropping off that much is if you increased the ability to flow enough to exceed the volume of air that the turbo can provide.

Out of curiosity, what kind of MBC is he running? If it's a bleed, a big change in temperature can cause a big change in boost. I saw the Daytona go from 8-9 psi when it was warm outside to cutout when the temperature dropped.
Something like this seems to me to be a more likely culprit.
HTH,

Bardo
01-24-2006, 01:11 AM
The only way I could see the boost dropping off that much is if you increased the ability to flow enough to exceed the volume of air that the turbo can provide.HTH,
in going to go with tony on this one. the way i see it. boost is "backed up air" air thats not getting in and out of the engine fast enought. the less efficient a head,extman and intake man are the easyer its gooing to be for that air to build up.
But thats just my .02 cents

turbovanmanČ
01-24-2006, 01:36 AM
Well, the rings are fresh, the valve are cleaned of carbon, back cut and the seats with a 5 angle, head milled. Sounds like a plan. BTW, outside temp is about the same as before.

Frank
01-24-2006, 07:54 AM
Hmmm. I dont know man. You arent flowing enough air to loose 5psi of boost by cleaning up the head. In theory yes it could happen, but its not probable.

It must be something else.


Frank

cordes
01-24-2006, 12:26 PM
I am not sure when/how you removed the MBC, but that could have something to do with it. If it is a G-valve style, and it was josled around enough to move, then it could make that big of a difference. Someone might have picked it up and played with it etc.

Also, f you changed the WG at all that would certianly do it.

Subliminal
01-24-2006, 12:30 PM
Vacuum Leak somewheres? Or rather. maybe there was a vacuum leak, and now it's fixed..?

bn880
01-24-2006, 12:34 PM
It's probably a good thing. :) I'm sure if you retard your timing you will get back to those PSI's. The faster your engine goes through it's RPM the less boost your turbo will be able to generate. (it has to keep up sort of speak) You probably have a fairly small turbocharger? Anyway, you will see similar things happen if you port intakes, or change cams for more agressive ones. It takes more air to make the psi.

Otherwise, yeah, some change to your valve. Or your timing was retarded before now you advanced it.

turbovanmanČ
01-24-2006, 01:28 PM
Oh well, we'll chalk it up better vacuum, maybe I bumped the MBC etc. Maybe its the TWILIGHT ZONE, muhahahahaa, :lol: :nod:

The S is Silent
01-24-2006, 01:32 PM
Oh well, we'll chalk it up better vacuum, maybe I bumped the MBC etc. Maybe its the TWILIGHT ZONE, muhahahahaa, :lol: :nod:

The squirrels in my neighborhood have been known to carry around wrenches. They like to untorque bolts that I KNOW I have torqued down. Like the bolt holding my alternator tensioner on...or the bolts holding my intermediate axle shaft bracket onto the block.

Damn squirrels.

cordes
01-24-2006, 01:56 PM
The squirrels in my neighborhood have been known to carry around wrenches. They like to untorque bolts that I KNOW I have torqued down. Like the bolt holding my alternator tensioner on...or the bolts holding my intermediate axle shaft bracket onto the block.

Damn squirrels.


Actually my roomate proved that you can run without the bolt that holds the alt. tensioner bracket on there for at lest a year.;)

Tony Hanna
01-25-2006, 04:09 AM
The squirrels in my neighborhood have been known to carry around wrenches. They like to untorque bolts that I KNOW I have torqued down. Like the bolt holding my alternator tensioner on...or the bolts holding my intermediate axle shaft bracket onto the block.

Damn squirrels.

Maybe that's what's been happening with John's bellhousing bolts. :confused: :D

tryingbe
01-30-2006, 10:12 AM
7-8 psi sounds fine...

That's the way the engine is telling you "I need to be broken-in first.";)

Bardo
01-30-2006, 10:14 AM
but it is broken cause it was higher