blow off valve...

[DavidT]

what does it do?

 

[Dr. Tweak]

When your turbo is spooled and boosting and you suddenly let off the throttle, the massive boost spike in the intake would cause damage to the turbo and some other things possibly.

The BOV opens when pressure gets too high in the intake tract to prevent this from happening.

 

[Off Road SHO]

It relieves excess pressure in the intake tract. They (the valves) are usually vacuum operated. When on boost (positive pressure in the intake tract caused by the super or turbo charger) and with the throttle body open, there is no vacuum only pressure. Now when you let up on the gas pedal, you close the throttle body. Since the turbine is still spinning it is still trying to force air past the now closed throttle body. It can't and the resulting pressure build up will stall the compressor wheel....Unless...

Unless you have a place for the pressure build up to go. As soon as you closed the throttle body the downstream side of the throttle body immediately went to a high vacuum situation because the pistons are still trying to suck air in and the only path is the idle air bypass now. This high vacuum situation causes the vacuum operated blow-off valve to open and vent the built up pressure to the atmosphere.

Would you need a picture now that I have rambled on?

Tom

 

[Off Road SHO]

BTW, don't confuse the blow-off valve with a waste gate. A waste gate will start to bleed off exhaust before it gets to the turbo to keep from boosting too high.

On my Toyota buggy the waste gate is integral to the turbo and is operated by a tube running from the pressurized side of the intake tract, only in this case the valve opens when the intake tract reaches 5 psi of positive pressure.

Tom

 

[DavidT]

Why is it adjustable?

 

[Dr. Tweak]

That's how you control how much boost you are running.

 

[Toolman]

BOV does not regulate boost, or at least it should not be used to do so.

 

[Dr. Tweak]

BOV does not regulate boost, or at least it should not be used to do so.

I thought he was asking about the wastegate?

 

[Camarok]

**HIJACK....

I know about BOVs I know about Wastegates.... but what does it actually do?

I've seen that off road sho noted that it bleeds exhaust off before it initially spools correct.

Now... can you adjust your waste gate to spool earlier? IE... is that ONE of the reasons you have turbo lag in exception to a big turbo...

So what a waste gate is... is that it is a temporary bypass to the turbo.... around it so to say? At the point the waste gate is open..... exhaust is going passing around it through the manifold.... then closes and exhaust gas is now passing through the turbo splines?
Is that right? I'm just kind of trying to figure out the wastegate and how/when it is working/ or how it works. Thanks guys

 

[Dr. Tweak]

The wastegate is what controls how much boost you are making. The turbo spools up, starts boosting, and when it hits the point that you've set where you want boost to stop, then the wastegate opens and starts bleeding the exhaust and keeping the turbo at that point. This is why turbos are so great, you can increase/decrease boost with just adjusting the wastegate, which on most cars in controlled right from the cabin! (ie, Toolman's car)

On a supercharger you would acually have to change the pulley for the same effect.

 

[AutoSHO]

A wastegate is how a turbocharged engine controls boost pressure. The wastegate has a port pre-turbocharger that allows it to vent exhaust gasses around the turbine once the desired boost level is achieved. Until full spool is reached the wastegate is closed. When you put a boost controller on a turbocharged car, they manipulate the signal to the wastegate to modify the amount of boost achieved before the wastegate opens, allowing some of the exhaust gas to divert around the turbine.

A Blow-Off valve is only a means to relieve excess pressure in the intake tract when there is a pressure differential between the pre-TB tubing and the intake manifold. It is not used to control boost.

 

[Camarok]

YA i know... boost controler...... but that's the function of the waste gate then.... so theoretically you can turn the boost so low that you would not have any??? a N/A car then

but like is there a little passage that the exhaust goes "by" as it is bleeding around the turbo? assuming... I mean... it just doesn't leak right that as an open headder...

 

[Chris Benvie]

David,

I'm glad to see that you are asking questions



BUT..


I would seriously read up a little bit more on the car that you now own. Either that or start one big post. There are alot of easy to find questions already answered for you out there on the internet. I think that you might now know what you havn't gotten yourself into with this car. You will grenade that tranny. There is also something very wrong with that motor missing. Please Please Please, for the love of S/* SHO's learn a lil bit more and stop driving it until it is fixed. If you think the SHO is temperamental a S/* SHO is even worse.


Good luck with the car, and have fun learning about it. I am still reading and still learning and having fun while doing it.

 

[Camarok]

.....

 

[Dr. Tweak]

Yes, the wastegate is a bypass around the turbo. It sometimes is vented right to the open air, resulting in that sssssssssssssssssshhhhhhhh.... sound that high power turbo cars make at full boost.

And yes you can turn the boost all the way down. This is where you begin when tuning a turbo engine.

 

[Camarok]

Yes, the wastegate is a bypass around the turbo. It sometimes is vented right to the open air, resulting in that sssssssssssssssssshhhhhhhh.... sound that high power turbo cars make at full boost.

And yes you can turn the boost all the way down. This is where you begin when tuning a turbo engine.

COOL thanks man... I'll stop asking questions now.

 

[Dr. Tweak]

Dude, check out howstuffworks.com they explain turbo systems pretty well.

 

[Camarok]

never thought of that... grrr... I've been there for TONS of things before.... i'm done now...

 

[Off Road SHO]

Yes, the wastegate is a bypass around the turbo. It sometimes is vented right to the open air, resulting in that sssssssssssssssssshhhhhhhh.... sound that high power turbo cars make at full boost.

And yes you can turn the boost all the way down. This is where you begin when tuning a turbo engine.

Actually a waste gate vents to the exhaust pipe, just down stream of the turbo. On buggies they just use a little bitty exhaust pipe of its own. And if you're hearing a ssssssssshhhhhhhhhh sound, it's the pop off valve relieving the air pressure in the intake tract, not the wastegate. When the exhaust wastegate opens, you should hear....exhaust.

Tom

 

[Mike Kopstain]

Dave,

You definetly want to track down that miss. If an injector is dropping or clogged and you are at high rpm you could easily destroy that new motor, which is something no one here wants to see.

As far as the transmission is concerned, I drove that car and if you indeed are driving it as you said, I.E. no burnouts, no hole shot's, etc, then I think you will be fine with the transmission. It's when you're pushing all that power through one wheel that you are gambling a $2500 Quaifed rebuild (on a good day).

I would also, at the minimum, spring for some gauges. If the car were mine, I would want boost, EGT, and fuel pressure all within eyesite.