Theory behind using blow off valves to control boost?

[Wizbiki]

As the topic says does anybody know the theory behind this as it eliminates the need for a wastegate? Obviously the blow off valve/valves become the theoretical wastegates but how does this work?

Would the turbo be under greater stress and spinning heaps faster using the blow off valve method as all exhaust gasses go through the turbine housing. Yet with a proper wastegate some off the exhaust gas is bled off?

[Pariah082]

Blow off Valve and Wastegate are used for two entirely different purposes.

Wastegate controlls boost. BOV vents unused pressure into the atmosphers or back into the plumbing to stop the turbo trying to turn itself inside out, or spinning against itself....


and you wanna build a 9 sec car? BOV does nothing to control Boost, just its pressure.

[Wizbiki]

Originally Posted by Pariah082

Blow off Valve and Wastegate are used for two entirely different purposes.

Wastegate controlls boost. BOV vents unused pressure into the atmosphers or back into the plumbing to stop the turbo trying to turn itself inside out, or spinning against itself....


and you wanna build a 9 sec car? BOV does nothing to control Boost, just its pressure.

Yeah i do want to build a 9 sec car and im 3/4 of the way there. You can use blow off valves to control boost as platinum from bresciani uses two gtr blow off valves and no wastegate. Can you explain that with your theory?

[VLT-666]

Originally Posted by Pariah082

Blow off Valve and Wastegate are used for two entirely different purposes.

Wastegate controlls boost. BOV vents unused pressure into the atmosphers or back into the plumbing to stop the turbo trying to turn itself inside out, or spinning against itself....


and you wanna build a 9 sec car? BOV does nothing to control Boost, just its pressure.

I dont know the right answer but I'm pretty sure your wrong...
I've searched the forums and cant find the theory, but I do remember reading it on here awhile back...

[XTC16]

You find most people that use BOV's for boost control are in SuperCharged setups, not turbos

[StockyMcStock]

Would the turbo be under greater stress and spinning heaps faster using the blow off valve method as all exhaust gasses go through the turbine housing.

yes. i have absolutely no idea why some people do it with BOV's. your shaft speed would be phenomenal as ALL of the exhaust gas is going through. my only reasoning behind it is that it is done for wank factor, or ease of exhaust manifold piping.

[ATMO_BT1]

i made a thread on this a while back, i think it was called "PLATNM set up with 2 bovs?" or something like that, had good explanations from memory.

[StuiE]

It's definetely been done

Minimal lag would be one advantage, I don't see the point other than that, as said, the extra stresses/heat on the turbo wou;dn't give it massive longevity.

[PHS]

i think those bov we are talking about are really set up like overboost valves
there set to a presure the open when they go above it slightly so it stays at the right boost

the reason normal bovs don't do this is because they have a vacume line from the motor hooked up to them witch push's the gate shut under boost
till you back of the power then the presure drops off the back of the gate opening the BOV

PPPPPUUUUUUTTTTTTTTTTTTCHHHHHHHHHHHHH, tutututu

and there is a thread on BOV's or NOT
and NOT won or there's not really any diff
here it is http://www.calaisturbo.com.au/showthread.php?t=66468

[LukeVL]

Wizbiki- Don't do it. Search on here you should find a thread with a big debate on this very subject and my vies are in there. I will not go to the effort to write it again but its a stupid idea and one that should not be done.

PHS- I have nto opened the thread but hopefully I posted in there. There is a big advantage to be had with a BOV. The are only two downsides to a BOV- the noise and the popo effect. Providing they are setup right (signal line as short as possible) they help keep the turbo spinning and we have seen good results at the 1/4 mile by almost 0.5sec et. This effect can also be counteracted by not taking your foot off the throttle when you change gears but thats a bit serious for those of us with mechanical sympathy. But all this said , if you are after some yo yo fooly sick bro my cozins mate muzza has hektik doses from his VL then yes, the BOV loses.

[Wizbiki]

I wont do it. I just dont like the idead of the turbo spinning its ass off that much. Its just good to know the facts and other possible options wether good or bad. thanks guys

[6BOOST]

No offence intended, but if people don't know the answer to a question, they prob shouldn't try to answer. There are thousands of engines around the world that boost is controlled by a BOV. Nizpro were the pioneers in australia to use it, all of there 10 and 9 second RB30's of past used this method, with a pair of stock GTR BOV's used to control boost. You also need to have an understanding of the mechanical workings of an engine to say that the air would be hotter, wasted and a complete disaster.

If the air doesn't enter the engine, how can it make the turbo spin faster?? The turbo doesn't just go and spin its head off and make some stupid shaft speed, and the BOV leaks some of it off for a desired boost pressure. The turbo, and engine, REQUIRE air to enter them to prduce the output from the cylinders required to spin the turbo. No air in(as in BOV has bledd off a large amount) means less energy/air in the cylinder, less air to push the turbo, slower shaft speed, less boost.

It also not just some hash up for lazy people or cost effectiveness. Formula 1 engines of days gone by are where the idea/technology was mainly used. They made over 1200hp from 1.5 litre engines, it def works. HAving said all this, I would personally steer completely clear of the idea. Turbo matching is critical, turbo shaft speed would have to be measured and monitored(very expensive) then housings matched to suit. Its fraught with danger and for my own opinions, I can't see absolutely any reason it would be of any more use whatsoever over a wastegate. None at all. Actually, boost would have to spike between gears, not drop........

6BOOST

[Wizbiki]

Originally Posted by 6BOOST

No offence intended, but if people don't know the answer to a question, they prob shouldn't try to answer. There are thousands of engines around the world that boost is controlled by a BOV. Nizpro were the pioneers in australia to use it, all of there 10 and 9 second RB30's of past used this method, with a pair of stock GTR BOV's used to control boost. You also need to have an understanding of the mechanical workings of an engine to say that the air would be hotter, wasted and a complete disaster.

If the air doesn't enter the engine, how can it make the turbo spin faster?? The turbo doesn't just go and spin its head off and make some stupid shaft speed, and the BOV leaks some of it off for a desired boost pressure. The turbo, and engine, REQUIRE air to enter them to prduce the output from the cylinders required to spin the turbo. No air in(as in BOV has bledd off a large amount) means less energy/air in the cylinder, less air to push the turbo, slower shaft speed, less boost.

It also not just some hash up for lazy people or cost effectiveness. Formula 1 engines of days gone by are where the idea/technology was mainly used. They made over 1200hp from 1.5 litre engines, it def works. HAving said all this, I would personally steer completely clear of the idea. Turbo matching is critical, turbo shaft speed would have to be measured and monitored(very expensive) then housings matched to suit. Its fraught with danger and for my own opinions, I can't see absolutely any reason it would be of any more use whatsoever over a wastegate. None at all. Actually, boost would have to spike between gears, not drop........

6BOOST

Theres ot much you don't know is there Kyle Bloody hell ur an encyclopaedia!

[idtrp2]

so is a bov a good or bad idea even if u have a internal gate

[Warpspeed]

6Boost. What makes the turbo spin faster is the increased mass of exhaust flow as engine Rpm rises. Sure, your blowoff valve will maintain a constant boost pressure, but as engine Rpm increases, so does engine airflow. If the power curve is still rising, so is airflow.

The big problem with this is the buildup of turbine inlet pressure and uncontrolled rise in turbine Rpm. A wastegate tends to hold turbine inlet pressure reasonably constant, and turbo Rpm reasonably constant once full boost pressure has been reached.

Very high turbine back pressure at max Rpm is not going to help power.

[2filthy3]

I would also argue that this setup would also produce over the top shaft speeds, reason being ALL of the exhaust gas is going through the turbo, it has to be spinning fast because of this, and if the air the compressor pumps goes into the engine or not is kind of irrelevant (in the situation), the compressor still pumped the air that was bled of, and would have had to of spun faster to do it. but you are right 6boost (of course) that less air in is less gas out to spin the turbo, but as we know you don't need all of the gas coming out of the engine to spin the turbo fast enough.
But after saying all that it is probably totally bull**** in regards to this topic, as we have kind of related it to our own sized turbo chargers, where no wastegate would really be a bad idea, i mean all the exhaust gas going through a a turbo like mine, t3/t04 hybrid would be silly and i imagine the air flow would be past the choke point of the turbo, maybe.
However i imagine that when this has been used to success that by matching the turbo it would work fine, and be a very efficient setup. if you ever have a look at an old turbo diesel truck you might notice that many have no wastegate to control boost levels, instead they have a huge rear hosing, that is perfectly matched to turn at the right speed with all of the exhaust gas passing through it (read, very low back pressure). these systems are efficient over a small rev range, in the application i am thinking of 1200rpm to 2100rpm, the problem with matching the turbo to work with no wastegate is that you will lose bottom end (on a car) as the large rear housing required won't be coming on boost early at all.
Anyway, i'm pretty sure that the car's running bov's to regulate boost would have disproportionably large rear housing, i really don't think that these people (brescini and whoever) would have a system that has excessive shaft speed, as this would probably take most average size compressor's out of their efficiency range, and i really don't think these guy's would do that...

[LukeVL]

Scenario 1- Motor running 20psi from 4000rpm to 8000rpm. Wastegate will be open by some amount at most/all of this 4000rpm power band while boost is held constant on the inlet side. So exhaust is bypassing the turbo.

Scenario 2- Same deal with no gate- no exhaust is bypassed therefore the turbo spins faster and creates a larger restriction in the exhaust side of the motor just to be bled off. ie waste of energy and increase in power loss to drive the turbo which is the main advantage over a blower.

[idtrp2]

so is a bov good for a bb turbo or wat

[StockyMcStock]

yeah i'm with luke, tony and filthy man on this one.

but who cares about the theory, we all agree that it is a 5hithouse idea so let's leave it at that.

[6BOOST]

While I'm not going to argue with everyone about it, I don't agree with some of what has been said. And yes, you are right warpspeed, turbine inlet pressure would rise, but the key to this system working correctly is measuring turbine back pressure, turbo shaft speed, and matching turbine housing size and wheel sizes to get this in the correct range. Look at big ass trucks. O lot of them use no form of wastegate and yet don't ever make more than 15-25psi of boost. The engine simply can't push enough exhaust gas to spin them faster than that. They also aren't laggy, they're on boost most of their life.

WHile the common performance vehicle is far from a 10 litre deisel truck, the idea behind the wastegate versus BOV method is more a pure race or race/street engine with very large turbo that would be in teh same situation as the truck, not something like a T3/T4 on an RB30 or small GT BB Turbo.

And as for everyones questions about BOV's in general, yes they are a great idea to have, there is a whole argument in itself, but In my own personal opinion, every turbo engine almost should have a BOV(compressor bypass of SOME desription at least) and if you dont like the wank factor, get a plumb back one.

6BOOST

[Warpspeed]

Yes, but those big ass trucks are all diesels. Diesels require no throttle body, essentially the same airflow goes through the engine and turbo all the time. Higher engine load just increases exhaust gas temperature, which increases the boost level when you put your foot down. None of those big truck diesels need a wastegate, the whole engine works in a totally different way.

You don't see throttle butterflies or wastegates on jet engines either.

But let's stick to petrol engines.

A turbo can certainly be sized to run without a wastegate. My first turbo engine that I built back around 1970 used carby and exhaust restrictions to limit maximum boost pressure. It worked pretty well too, for what it was.

Things have changed rather a lot since then. Wastegates are still the very best way to control a turbo on a petrol engine. Do you really think that the big multinational car manufacturers, and turbo companies (like Garrett), that spend billions of dollars on R&D would still be using wastegates if there was a much better way to do it ?

I definitely agree with you about the need for a blowoff (bypass) valve to solve the surge problem, no argument there. But that is a totally different thing to controling boost pressure.

[LukeVL]

So what you are saying Kyle is it will bleed off heaps of air as it comes on boost and then keep bleeding off less and less till it almost bleeds off nothing at the top end and is maxing out the turbo efficiency and the motor can cop all of the airflow at a reasonable inlet pressure?
Doesn't sound too bad but I still wouldn't run a system like that- just seems like one of those things people do because they are bored and want some wank factor

[2filthy3]

yer, but diesel engines still have varying volumes of exhaust gas, the size of the rear housing is the main boost limiting factor. we know it's different but it was the best way we could think of explaining a point
And yes their is a better way to controll boost with vnt.
The main reaon the use of wastegates came about was to allow smaller turbo's to be used in application where they would have oversped without the wastegate. the wastegate is what lets us have some bottem end.
how about we say yeah this bov controlling pressure thing can work, but would a require a huge change to our own systems if it was to be applied.

got to be carefull with what you say, you guy's know everything, and i wasn't even alive in the 70's, barely the 80's lol.

[6BOOST]

Originally Posted by Warpspeed

Things have changed rather a lot since then. Wastegates are still the very best way to control a turbo on a petrol engine. Do you really think that the big multinational car manufacturers, and turbo companies (like Garrett), that spend billions of dollars on R&D would still be using wastegates if there was a much better way to do it ?

I think there must be some misinterpretation of what I'm saying, as what you have just said is exactly what I think and agree upon:

"Having said all this, I would personally steer completely clear of the idea. Turbo matching is critical, turbo shaft speed would have to be measured and monitored(very expensive) then housings matched to suit. Its fraught with danger and for my own opinions, I can't see absolutely any reason it would be of any more use whatsoever over a wastegate. None at all."

I make no argument about what I would use, but what you are saying is simply not correct. Forumla 1 cars of years gone by, lemans 24 hour cars, most of nizpro's engines, and actually 3 or 4 of my customers since I started making manifolds use BOV's to control boost with no problems at all. There is a red XF on gas that Jason Giller helped in teh building of that has 2 bov's for boost control, I've got 2 customers that use 2x GTR bov's, none have reported any excessive turbo failures. By CORRECTLY matching turbine wheel and housings to an engine, it is more than possible to control boost with a BOV and not overspeed a turbo.

I think some people are of the belief that spinning a turbo takes little actual exhaust flow. If the turbine and wheel were big enough to not need massive amounts of air to be bled off, such as for instance a 1000hp turbo on a 3 litre with large exhaust housing, the energy that is released from the bov would be enough to keep shaft speed well within reasonable limits while sustaining the governed boost pressure. Like I said, I'm not saying go and use it on any engine, its a race orientated measure with large turbochargers, its been proven to work, if the turbo doesn't push against a load, IE air is going out BOV, shaft speed will decrease, or in teh case of staying at the desired boost level, will maintain around teh shaft speed required to make the boost pressure needed. As it speeds up to make more, the bov bleeds more, it slows down, bov bleeds less, it all comes down to airflow versus load and a correctly matched turbo. Like I said before.

6BOOST

[trbo3ltr]

i suppose wat it would come down to is that, ur using all the exhaust gas to spool up the turbo insted of a controlled amount (throught the wastegate) so if anything it shuld spool quicker.
But as 6boost mentioned, you would still need an extremely high performance setup.

Just F uck the BOV off, boost to 14, make 90º bend on turbo intake, no filter........

dddoooooossseeeee zzzuutututututututuuttu


Carl