Hey Guys,

Quick question.....(learning as I go)

Is a Blow off Valve and Wastegate use similar?
If not, what is the difference? What is the blow off valve use for? To make noise?

VIC

 

No they are not the same. A wastegate is to control the boost by throwing out some exhaust gases before they reach the turbo, if it was not there the spooling of the turbo would accelerate causing the boost to creep until the engine blows up. A blow off is mounted on the intake pipes activated by a vacum hose whenever you step of the gas petal, and it releases unused intake gases before they get back to the turbo to cause damage to it (surge).

These are the basics. For further details do some search and learn how a turbo car works.

 

Thanks....

 

So when you let off the gas the back pressure is relieved by the blow off valve. Does anyone think it might be possible to set up a wastegate on the intake to control boost pressure?

 

No, two different set-ups.

 

I actually thought about that one today. I''m sure there is some sort of pressure activated 1 way valve that would work.

 

A pop-off valve will only allow so much pressure to build up in the intake piping before it opens. Its more of a safety item so you dont overboost if the wastegate sticks shut.

 

what was a really funny question

 

Yes, I know of the pop-off valve, and I'll mount one when I get my Greddy elbow, but I was thinking more along the lines of a valve that stays on, so you don't have to **** with it so much if indeed you do overboost.

 

Um, that's what a pop off valve does. It's not like you need to replace it if you overboost. The valve opens and then it closes.

The difference between a blow off valve and a pop off valve is that a blow off valve is designed to open when the throttle slams shut during a shift to prevent excess pressure from building up and surging back to the compressor thus damaging it, while a pop off valve is designed to open at a particular boost level and vent some of the air in the intake tract and prevent it from making it to the open throttle and into the motor.

You do not want to use a pop off valve as a means of boost control. The wastegate is the primary means of boost control because it actually slows the turbo down and only lets it spool a certain amount. It has a vacuum line plumbed to it from the intake manifold so that it can "sense" boost. When the boost reaches a certain level, the wastegate (which is either mounted on the exhaust manifold or to the hot side of the turbo) vents exhaust gasses past the turbo and generally into a dump pipe which then meets up with the down pipe after the turbo. The exhaust that is vented past the turbo rejoins the rest of the exhaust gas at this point.

A pop off valve (or any other kind of hypothetical release valve in the intake tract) will do nothing to slow the turbo down which means that your turbo will keep spinning faster and faster and doing a bunch of work to pump more air and then all the extra work it does gets vented out of the intake.

 

err.. just checking... you know a pop-off valve is not the same as a Blow-off valve right?
A blow-off valve/bypass valve/diverter valve is actuated by manifold vacuum/pressure and has a vacuum line routed to it.
A pop off valve works like a wastegate and is spring operated. All you have to do is set the spring tension and forget about it. If and when you overboost when you have creep or your wastegate malfunctions, the pop-off valve will pop-open until boost returns to normal levels and it will shut of again.

 

Oh, so it doesn't "pop open"? I was always under the impression that it had to be reclosed.

You're wrong about the turbo continuing to do more and more work though. The only reason the turbo can do more and more work, is because there are more and more combustion gasses from the extra air provided by the turbos. Yes, it will be doing more work than it normally would, but that is hardly a large consideration since we are talking about 1-2psi difference at the most. Remember, if you're making 10psi with the stock IC in the way, the turbos are producing more like 13-14psi right?

 

No, he's not. The turbos WILL continue to overspool with a pop-off valve. That is why they should only be used as a safety feature and NOT for boost control.

 

They will only overspool to a point. Looking back that looks a bit harsh, but I still hold firm that the turbos will be doing less work than they would without the popoff valve. If your pop off valve keeps things at 10 psi, they won't be working nearly as hard as they would be were you to be boosting to 14 psi without the popoff valve due to the smaller amounts of exhaust gasses.

 

https://www.rx7club.com/forum/showth...hreadid=115448

 

Technically you could regulate boost by venting post-charge, pre-manifold to the atmosphere. However it would be a seriously bad idea as you would be throwing away charged air (waste of energy) in addition to bringing the turbo into surge since the turbine side no longer has direct regulation of the exhaust gas/turbine load via a wastegate.

There's absolutely no positive to venting charged uncombusted oxygen as compared to venting post-combustion exhaust gas (which will be sent out the exhaust-side anyways).

 

I couldn't agree more. Thats why wastegates are where they are. Why make the turbos work harder than they have to? With a pop-off valve or similar device controlling boost, the turbos will be spinning faster and faster as engine revs increase. Those bearings only have a limited number of spins in them, so why make them wear out faster? If you can control boost by controlling turbo speed, they'll last a hell of a lot longer, which is exactly what the purpose of a wastegate is.

 

If I remember right, On the old turbocharged F1 cars, there was a homologated device that all cars had called a pop off valve, which had an indicator that would stay popped out - (almost like a turkey temp timer thing) to show that boost was higher than allowed and a penalty was in order. The indicator could not be reseated by the pit crews (although officials had a tool to reseat them). Perhaps this is where you remember the term/got the impression from..

 

Pop off valves are good on the FD as a safety measure for preventing overboost if the turbos start creeping on you. This happens when the wastegate can no longer vent enough exhaust gas past the turbine to slow it from spooling more. But, a wastegate is still a far more effective means of boost control than only using a pop off valve.

So yes, a pop off valve is a good thing to have. It is not however, the preferred method of limiting boost.

 

Does the wastegate valve that releases the pressure from the turbo have to go back into the exhaust system or can it vent into the air????

 

there is external and internal

external can be vented and make a very loud noise

 

I have an external 38 mm tial? Would it make a diffrence if it stayed out of the exhaust? I kind of got it with the turbo kit I ordered. I just didn't know what it was.

 

Normally wastegates route exhaust gas into a dump pipe that rejoins the down pipe after the turbo. In effect this limits the amount of exhaust gas that enters the hot side of the turbo and spools it up. Both internal and external wastegates can be set up this way. An external wastegate can also be set up to dump exhaust gas out into the atmosphere instead of into the down pipe. This tends to make more power with the obvious downside of you dumping raw exhaust gas into the air. If the car is running without a catalytic converter that's not really a big deal but if you're trying to keep it emissions legal then you'll want to use a dump pipe.