Raced it!

[cygnusx1]

I never noticed boost dropping off but I do have a much higher flowing turbo. I also don't go to 6K rpms. The motor just feels nice and fat between 3K - 5K...I dont feel the need or urge to rev more than that with my current setup. The boost drop off in yours may be the stock T3 running out of breath at 6K and 14psi. Tough to say. I never ran my T3 at 14psi. It was old and blew the bearing shortly after adjusting it to 10psi.

 

EDIT: I reread your post. If you can maintain 14psi all the way to 6K when tapping MBC off the manifold, then your turbo is NOT running out of breath, nor is your intercooler causing more pressure drop than the turbo can handle. Think about it.

[Guest Cronic]

My boost falls off at 4500rpm. I can never maintain anything above 15psi.

Sometimes, durring the day, the turbo will constantly hold 20 throught he gears, but very rarely. When it does though, damn... I've had many races with well modified LS1's and come out well in front of them. Some as much as 5 cars.

[Guest bastaad525]

the pressure drop theory wasn't mine, but made perfect sense to me. I wouldn't suspect it's the turbo running out of breath by that point either.

 

The pressure drop theory goes (get ready for another long Bast post here):

 

The MBC works by adjusting the spring tension to open at a given boost pressure. I currently have mine set to open at ~14psi. Whether I set it to run from the intake manifold or the compressor housing, initial boost is right around there at onset, as it should be... the thing is going to open at that boost level no matter what. When it's getting it's signal from the compressor, it SHOULD BE maintaining the turbo speed to give 14psi at the compressor, however, at the manifold (where my boost gauge reads), boost is slowly falling off as revs increase. So as revs and flow are going up, pressure is getting lost somewhere. I've read many times that pressure drop goes up in relation to flow, so this makes sense, that it may be outflowing my I/C. However, given the size of the I/C, the inlet and outlet's, the size piping I'm running... it's odd to be losing ~2-3psi, so I'm kinda stumped. Unless there's a leak but I've tried to find one with no luck. And knowing what my a/f ratio looks like, and the fact that the car doesn't buck or stumble like I'd expect, a leak like that doesn't seem likely. I also had the MBC getting it's signal from the compressor housing before I installed the I/C, set to 10psi, and boost did NOT fall off as revs went up, it only started once the I/C was in.

 

 

When hooked up to the manifold, the MBC is maintaining the turbo speed as such to maintain 14psi in the manifold, and it makes sense that it would compensate for any pressure drop happening between the compressor and the intake by the allowing the turbo to spin faster and faster. It's very possible that by 6k (and btw I dont usually rev this high on the street either, but do for dyno testing) the turbo is actually putting out more like 16-17psi at the compressor. This thought scared me a bit, which is why (for now) I've settled on running the MBC from the compressor housing. I have chatted with a couple other guys on here that said they saw similiar patterns on their setups. I thought it was an inherent 'flaw' of the MBC. And am very surprised that you don't get this at all on yours.

 

if I had the time and money, I'd set up a second boost gauge on or just after the compressor housing to confirm if this is really the case. As it stands I don't really know what I can do about it. I think one possibility is that the BOV might be leaking. Since it's recirculated, it wouldn't cause any symptoms of a regular boost leak. I dunno if a leaky BOV could cause boost to drop like that. But anyway I highly doubt it. Those things are rated to hold 20psi stock, and I crushed the cap on mine a bit to increase the spring rate as well. And man that valve is TIGHT... I've tried pushing it open by hand... NOT easy. Maybe there is some dirt or something holding it open? I'll try removing it and cleaning it asap.

 

otherwise I'm stumped.

 

EDIT: now that I think about it the BOV leaking seems even more unlikely. Because I've run the BOV with it's boost reference disconnected before, which caused it to leak bad under boost, but the boost gauge still read the same boost at WOT.

[Guest Cronic]

Im using an EBC - Greddy profec B, and still have the boost fall issue.

[cygnusx1]

I see your point bast. I wouldn't worry about it too much. One way you stress the turbo more, one way you run more conservative. Like you said, two gauges will give you a read on your entire compressed side losses due to flow....if you want to know what they are. It's all in the name of "science" but dont lose sleep over it.

 

I just ordered speedbleeders. When I get my brakes bled and the fluid changed, I'll take it for a spin again and watch the boost...I dont think it drops though. I use 2.5" I/C tubing and a pretty big Spearco.

[Guest bastaad525]

it only bothers me that my torque is falling off so fast and costing me top end (hp), but I don't want to put so much strain on the turbo just to save that last who-knows-how-much hp (I'm not Jersey ). Like you said... the flipside is a more conservative top end... less risk of blowing anything I guess/hope. Another side effect is the fuel curve closely follows this boost curve... super rich at onset, leaner as revs increase. Though... I'm interested to know how the FMU is reacting to the changing boost... I need an in car fuel pressure gauge and a wideband SO bad... Judging from your test a while back though (thanks for indulging my curiousity btw) fuel pressure shouldn't be changing much.

 

I think my brakes need to be bled as well... I haven't abused them or anything, but lately they're getting soft, but come back if I pump them. Time to bust out the ol' haynes manual... for all the engine work I've done I have no idea how to go about bleeding the brakes

 

I'm running that Trust/Greddy I/C, 2.5" inlet and outlets, medium sized (like 2' x 9" x 3"), and 2.25" piping for all pipes except the 2" pipe immediately out of the turbo.

 

Speedbleeders huh? Not really sure what those are... something I can pick up in a regular parts store? I'm all for anything that makes any car job easier....

[Guest Cronic]

At idle, press your brake pedal several times. If your idle goes up and down a couple hundred RPM, it's probably your Brake booster. Check your fluid as well, as this could be low.

[Roostmonkey]

Everyone should have speed bleeders in their car.They make brake bleeding a fast one man job and they're under $30 for the set of 4.

[cygnusx1]

http://www.speedbleeder.com

 

The idle normally moves a little bit when you pump the brake quickly...That's because the booster is "stealing" vacuum from the motor when you do that. Bleeding brakes is pretty easy and should be done at least every two years for normal use. Follow the bleeding procedure in the manual. Old fluid can get spongy due to moisture absorption and foaming. Don't use silicone brake fluids unless it's just for a show car. Use a good DOT 4 fluid. I am switching to ATE Super Blue brake fluid. It is not the best, but it is affordable and probably good enough for me. The fluid I boiled at the track was DOT 4 but it was just a common store brand and I have no air ducts to the brakes. If I run more track events, I plan on either adding ducts, the better fluid, and/or looking for a dedicated track car....maybe a low budget 240Z only for the track

[JMortensen]

You can get a 2' length of vacuum hose and an old Gatorade bottle for $.50 and get the same effect.

 

Just put a little brake fluid in the bottom of the bottle and stick the hose under the surface of the fluid. Then hook the other end up to your bleeder, and use it just like you had speed bleeders on there.

 

If you're really snazzy you can make a hook out of a wire coat hanger so that you can hang the bleeder bottle from the suspension.

[cygnusx1]

Yeah, I have been using a weighted hose (to keep the end submerged) and bottle for many years and it works fine but I guess I am getting spoiled. I just like the fact that old fluid or air has no chance of getting sucked back into the speedbleeder no matter how lazy or incompetent I get...as long as the threads are providing a good seal.

[Guest bastaad525]

Man I feel like a newb makes you wonder how I can pay so much attention to going so fast, and never once pay attention to the area of STOPPING

 

Well (as usual) I like Jmortensen's idea a lot, but you still kinda need two people for that yes? Just went thru the haynes right now to kinda get an understanding for the procedure. You would still need two people so that one could open and close the bleed screw as the other person holds the pedal? Or with the gatorade bottle trick you can just leave the bleeder open?

 

if I were to go with the speed bleeders, is it worth 2x the price to get the stainless ones?

 

P.s.: why is that animation on the speedbleeder homepage making me want to laugh

[JMortensen]

Thanks Bastaad. If the end of the vacuum hose stays under the brake fluid, then you don't need to close the bleeder between pumps.

[cygnusx1]

...as long as the threads on the stock bleeder screws dont leak and as long as the hose stays on the nipple tightly it's a one person job. I always have thread leakage when I loosen the bleeder screws. I suppose some "thread compound" could help that situation. However, for $7 a corner, I'll give them a try. Getting new toys also gives me more incentive to get under the car and play. The toy factor.

 

Bast.. brakes are important for the street, but on the track, to be competitive, you can have ALL the horsepower in the world, if you dont have enough brakes, it's worthless. I was pulling away from all kinds of cars on the straights, in turns, and out of turns, However, I was losing all my ground at the ends of straights entering corners. Very frustrating.

[Hugh]

Car looks great on the track. Take care of it!

[Guest bastaad525]

that's definately the situation I'm in now....

 

meh car has plenty of go, but not nearly enough stop or stick!

[cygnusx1]

Hehehe, check out this creative brake cooling Idea from donmei over at imprezawrxsti.com forums!

 

donmei wrote: 280Z - You may want to try my patented (joke) brake water spray system that I made for my Dad's '58 Corvette. He raced it at Limerock on Labor day with the VSCCA. I rigged 2 junkyard windshield washer systems into the brake light circuit so it sprayed the drums whenever he applied the brakes.

http://www.imprezawrxsti.com/postnuke/index.php?name=PNphpBB2&file=viewtopic&p=156524#156524