Someone explain boosting to me

Supra Blues

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#1
I am new to the whole turbo world. "virgin booster".

Just got done with the 7MGTE swap in my 87 NA. Everything appears to be working normally.

Boost gauge reads about 17 vac at idle. Seems normal. I have read that if you rev the engine in neutral it will not make boost, why not? Because there is not load on the engine right? But still, why? Is it because the engine process the air through it so fast that boost has no time to develop?

Anyhoo, took the supra out for a spin last Sunday (no smog, no registration, no insurance) in first gear I really don't see too much boost but the boost gauge comes up to about zero vac. In second I see some and as i raced the engine up to approx 4,000 rpms the boost gauge maxed out at about 6-7psi (normal for stock), but then the boost started to fall. Why???? Is that normal?

Went home and inspected intercooler plumbing. Found a small tiny crack, got that fixed, but haven't driven it since.

It feels plenty strong as I got her up to about 55 in second gear pretty damned fast with only about 1/2 throttle.

Feel kinda stupid asking about boost, but I am always willing to learn something about cars.

Thanks.
 

Squid699

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#2
first off - don't drive without insurance or registration. That's just dumb.

I think you answered your own question, you had a boost leak....thats why it wasn't showing much boost and it would fall off.
 

aye mate

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#3
i can make about 4 psi in neutral. but yea boost leaks will cause you to not get as much peak boost and the boost you do make will fall off earlier. I think normal vac is about 20 isn't that right? I know mines at about 19 then when the car warms up it sits at about 20. Check for vac leaks and make sure your timing is dead on.
 

GrimJack

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#4
Ok, here's some info for you.

First - why can't I make boost in neutral? The easy answer is that you just can't. The more complex answer is that turbos are designed so they require load on the engine to make boost. Wouldn't make a whole lot of sense to design something that puts out boost when your engine doesn't need it, right? The complex answer is very complex, and explains the engineering behind the idea of a turbo. We can detail this for you if you like, but for now I'm going to leave it alone.

As for making boost on your test drive - you won't see much until you give it wide open throttle, that's where it really picks up. Maintaining speed, or slow acceleration, I never hit positive boost - actually, I rarely get to -5! Fixing the leak is a great idea, and it will work even better now. Go get some insurance and have fun!
 

lagged

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#6
its not really the AIR flowing through the fins the spools the turbo per se.... its the heat.

if your exhaust gases for some reason were ambient temperature (unrealistic example i know..) the turbo would not spool.
 

GrimJack

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#7
Blink... blink... I hate to disagree, but I don't think this is correct.

AFAIK, It's the fact that the exhaust flow is much more than the intake flow, as the exhaust also includes burned fuel. Heat is just a byproduct.
lagged said:
its not really the AIR flowing through the fins the spools the turbo per se.... its the heat.

if your exhaust gases for some reason were ambient temperature (unrealistic example i know..) the turbo would not spool.
 

Supra Blues

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#8
Oh, god lets not get into the heat vs. velocity debate again.

Yeah, I know driving without insurance is dumb, but I only drove it around the block and home. Still risky, but I will have it insured by this weekend.

I am just trying to get a feeling for if my Supra is at least in the normal range of working, the turbo, that is and it seems like it is.

I'll let you know what I find out this weekend with the little leak fixed.
 

Justin

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#9
ok, so being in neutral won't great boost, ive known this for a while... now, why is it hondas and eclipse's and four cylinders and that type can great boostin neutral?
 

shaeff

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#11
not to mention that their stock turbos are about the size of a hair dryer...

-shaeff
 

kamil

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#12
Rev high enough and you'll "boost" in neutral ;) Brake boosting is another great way to build boost from a standstill.
 

shaeff

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#13
kamil said:
Rev high enough and you'll "boost" in neutral ;) Brake boosting is another great way to build boost from a standstill.
brake boosting from a standstill only works in automatics...

-shaeff
 

bigaaron

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#15
Cars with tiny turbos can blow off at a stop light but who cares, you will smoke them as soon as it turns green!

When mine is fully warmed up at idle it is at about -18 vacuum on an autometer boost gauge and I know it is in perfect working order. I think -17 to -20 is normal. With 2.5" ic piping, maft and spearco intercooler, shouldn't the vacuum be lower then stock at idle because there is less intake restriction? With less restriction there would be less vacuum pressure right?
 
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lagged

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#17
GrimJack said:
Blink... blink... I hate to disagree, but I don't think this is correct.

AFAIK, It's the fact that the exhaust flow is much more than the intake flow, as the exhaust also includes burned fuel. Heat is just a byproduct.
eh im not an engineer so i cant say i know this for a fact. dosnt matter right? if you step on the pedal and the boost gauge gets pegged, i am happy.
 

Ckanderson

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#19
With a ct-26 I wes able to build about 4 psi if i reved it to 6k in neutral.. it is had to build boost in nutral since the engine isnt under load and the compressor doesnt spin up fast enough.

Enjoy the speed feeling for now... I remember when a stock mk3 felt fast... damn... those were the days...
 

Michael90t

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#20
I am able to get my greddy to vent easily and loudly revving in neutral.... also seems to work a bit better if you rev it a few time to get things spiining then peg it and release... but like its said who really cares only reason to do that is to show off(which I STILL do on occasion :) )
 

shaeff

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#21
gixxer750 said:
it could work in a standard, but you clutch wouldn't last long....

i wouldnt even bother, nor would i endorse it in a manual car... its not worth it, hence why i said it can only be done in automatics...


michael90T, you probably have the spring in the BOV set too loose...

-shaeff
 

Michael90t

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#23
Nope actualy set rather on the stiff side just borderline comp surge.. actualy it WILL flutter a bit on occasion.... so no loose springs for me :) got that prick set as tight as possible before comp surge because it also cuts down on bogging and backfireing vented to atmosphere

michael90T, you probably have the spring in the BOV set too loose...

-shaeff