Only holds 5 psi with vacuum brakes!

[Tom87GN]

Did a bit of searching and couldn't find anything that answers my question directly.

The question is: Why can't my car hold any more than 5 psi of boost at the line after I put the larger and softer rear pads and S-10 wheel cylinders in? My GN has vacuum brakes instead of the slugmaster system.

My current best 60' time is shown in my signature below, but that's only leaving at 5 psi. I figured I would at least be able to hold 10 psi without problem.

What gives?

 

[Sinful6]

Maybe your header is leaking causing your TCC to stall early? At anyrate, a bigger convertor would be a plus

 

[Crazy D]

Uhh, dude, I wish I had your problem. 5 psi is decent. We couldn't hit that before we started hopping in my GN. Also have S10 brakes and vacuum brakes on slicks.

 

[Bryan 94 Z28]

I've concluded that the reason for the lack of ability to build higher boost with vacuum brakes is because of the fact that the brake system needs vacuum in order to operate correctly.

Essentially, what happens is that when you reach boost while stalling it up at the line, you lose all the power in the braking system. Even though your check valve prevents boost from entering the brake booster, you lose all vacuum that is going to it, which is what it uses to apply the extra pressure needed to properly operate the brakes. Even though the small amount of vacuum that the booster uses doesn't seem like much, when you multiply it and spread it across the booster's diaphragm it provides a large boost of power to the master cylinder.

The other conclusion that I've drawn is that 5 psi produces just about the right amount of power to push through the weakened brakes. It seems to me, from reading through these posts for awhile now, that 5 psi is about all that people with vacuum brakes are able to stall up to.

I've also been trying to think up ways to solve this problem. The only somewhat feasible solution I've come up with is the installation of a vacuum canister/electric vacuum pump that would be attached to the line going to the booster. This would allow for a constant vacuum source that is not dependant upon the intake pressure in the engine. Anyone else have any other ideas? (besides going back to the slugmaster, of course )

 

[azgn]

Guess I would have to disagree....I have vac brakes with long shoes only....I can easily hold 10 lbs, and on a good track, 15

it all has to do with the condition of your brake system (both front and rear), not heating the brakes during burn out (line lock), and technique (after the burn out, get into neutral and replenish the vac before staging....) not to mention a properly matched converter and turbo

 

[Tom87GN]

Well, that all makes sense, but I have a line lock and I do attempt to replenish the vacuum booster after the burnout. The condition of the brake system is good IMO I guess. Maybe there is something there to look at????? My 51 turbo (63 housing) should be matched well with my 3200 converter, right?

 

[azgn]

I would say something is wrong if all you can build is 5 lbs.....are you sure your converter is stalling where it should? What rpm are you seeing 5lbs? It should flash well by stall speed under wot...

I can use a 2 stage rev limiter set at 3,000 and see 10 lbs on the boost gauge....it'll just sit there (my converter is 3600 stall)
off the trans brake, I launch at 4,000 rpm

the other problem would be too much fuel at spool up....the turbo may just be coming up very slow because it is too rich........you aren't using Red's chips are you? Take a look at your 02's at spool up......mine are in the 300's

looking at your combo, with some boost (25) you should be in the mid 11's easily......you may have other issues?

 

[Sinful6]

You could adjust the porportioning valve...

 

[Tom87GN]

Please explain how to adjust the proportioning valve. When I installed my line lock, I didn't see how you could adjust anything there. I learn new things every day!

AZGN, my converter seems to be working fine. I leave the line at 3300 rpms or so and it flashes from there. Don't know what it flashes to since I'm usually to worried about where I'm going than looking at the stock tach. My 02's at spool up are not out of line either. When I leave, the number is right at 800mv and then drops from there.

 

[azgn]

well, that is what I am saying......if your 02's are at 800 when you launch, you are waaay rich.......look on DS or Tlink and see what is happening when you are building boost....it needs to be real lean to spool properly

(the best chip for doing this is the MaxEffort, BTW....)

 

[Sinful6]

LOL, to adjust a proportioning valve, one must first install a proportioning valve!

 

[Tom87GN]

O.K., I'm reading ya AZGN. I'll get my computer out and look at my runs from Saturday and see if I was correct by saying my 02's are up there while building boost. I just happen to have my laptop here with me at work. Go figure! Something else to take me away from what I get paid for.

 

[Tom87GN]

I checked my turbolink files from Saturday, and they all show that I'm around 790mv holding 5 psi of boost at the line. Is that something that I need to get changed in the chip?

 

[azgn]

oh yeah.........that is definitely hurting spool up.....think I mentioned that my 02's 0-4000 rpm at the start line are in the 300's

Joe's chips are usually purty good, but check it out

 

[Phatman]

Well....

That gives me something to think about, same problem, new brakes, stem to stearn and all points in between and Joe's chip...

The chip pulls hard on the big end but seems to be off on the start, runs fat down low but leans out on the other end. He said he thought it would help with street racing and I guess he was right, it's not as hard to control tire spin and it's always fun to play catch-up....

 

[gndriver]

Hey you think you got it bad my car cant even hold 1 that's right 1 lb of boost

 

[Tom87GN]

Hey, thanks for the info AZGN! I think I may contact PTE and see what they say about that. They're only an hour from my home, so I may drive up there this week and have them go through my chip and take some fuel out down low to get that turbo humming!