I ran into some threads on another site that states that a BOV is useless and a waste of money. Is it really necessary to run one and if one is not used what are the consequences if any? If anyone has any real world experience and knowledge on this matter it would be greatly appreciated. Please don't reply with hear say. Because if I don't need it on my engine I'm yanking my 50mm Tial and selling it. If you want to look it's long and drawn out http://ausrotary.dntinternet.com/for...ic.php?t=17113

 

not using a b.o.v. will pop your i.c. pipes, after you spool up and let the throttel close air will remain traped and the freespining turbine will keep boosting, it will even make the compressor wheel push back and rub into the compressor housing.

 

I think this is the best post from that thread:


So yes, it is there for a reason..


I have seen some speculate that they have had the front nut come off their compressor section from backspin as well.. Note i said speculation.



-Zach

 

Besides all of the OEM cars that didn't have blowoff valves (Buick GN's, Ford 2.3T's, others that are beyond my tired grasp), dedicated race cars (rally cars, other turbo thingies) don't use blowoff valves....

 

This was posted by NZ off of the Autospeed website...




So how often have you see a BOV actually tested for its ability to reduce the pressure spike that occurs when you shut the throttle on a hard-blowing turbo? Not very often, right? And in fact, what does that pressure spike actually look like?

We decided to do some high-speed logging of the pressures that occur in the intake system between the turbo and the throttle blade when you snap the throttle shut. We used a Fluke 123 digital Scopemeter and a Fluke pressure transducer.



This screen grab shows the pressures with the electronically controlled Goyen BOV working. Each vertical division is the equivalent of 20 kPa (about 3 psi) manifold pressure and each horizontal division is 200 milliseconds. So from far left, the car is holding 70 kPa, grading down slightly to 60 kPa (about 9 psi) before the throttle is suddenly closed. The pressure abruptly spikes by 10 kPa (about 1.5 psi) but the spike is very short-lived (about 50 milliseconds) before it rapidly and smoothly falls away. In fact, the pressure drops from the spike of 70 kPa down to 20 kPa in less than 100 milliseconds (ie one tenth of a second). The fall to less than 10 kPa takes about 650 milliseconds (ie 0.65 seconds) in total.

And what a different story there is without the BOV working!



Again the throttle was abruptly closed at 60 kPa boost. The immediate pressure spike was about 15 kPa (only a little higher than without the BOV) but what follows from there is completely different. Rather than dying away quickly, the pressure is both much slower to fall off and is also accompanied by very rapid pressure waves, with these waves starting off at about 20 kPa peak-to-peak and then gradually dropping to about 10 kPa. The frequency of these varies from 15-20Hz. In other words, there is a pressure wave of up to 20 kPa racing up and down the intake system between the throttle and the turbo. It is very likely that this wave battering against the turbo compressor is potentially much more damaging that the initial pressure spike itself. Also note that the trapped pressure takes a lot longer to decrease – to drop to 20 kPa takes about 500 milliseconds (half a second) compared with one-fifth of that when a BOV is fitted."

Gooble Gooble Boost

 

The energy of these air particles stuck between the throttle and the compressor in reality have no effect on durability of the compressor or the bearing system. On the other hand, "wasting" these compressed air particles via a device such as BOV is negative for performance, which is why they were not ever used in the pinical of turbo motorsport (F1 circa 1979 to 1988).

As a personal testimonial to the thread starter I have not been running BOV's for almost 10 years now and have never had a turbo failure because of not running such a device..... total millage probably 60 000 miles plus across all different vehicles and at very high power levels. The evidence is not there to warrant me using one personaly nor recomending them to people I deal with.

 

This is the first time I've ever heard someone sucessfully running without a BOV. I've always heard that without a BOV would increase the reverse flow of air in the turbo's leading to engine damage.

 

Doesnt a wastegate do the same thing as a blow off valve? Sorry, I am an idiot when it comes to this stuff. Cant learn without asking.

 

RACE CARS do not use blowoff valves. (Even Rally cars with antilag - no BOV!)

MILLIONS of PRODUCTION CARS do not use blowoff valves.

If this is the first you've heard of it, that's kinda scary...

 

I said sucessfully, meaning you'll have the same life-span an engine/turbo setup using a BOV. I'm sure it could be done but I think your repair and matainence bills would be quite costly. I don't pretend to be a guru of car knowledge so I wouldn't know whats under the hood of every turbo car but which production cars running turbos don't have BOV's? No flame, just curious.

 

peejay, Idon't know what race cars you were watching but any rally car that is turbo'd uses a bov, I spent the day at pikes peak hill climb and you can hear the bov open three or four time during the corner as they get in and out of the gas. very impresive how acurately they have them tuned. the cart series running in denver last was a road track and every one of those cars ran a bov making alot more boost and power than you i'm sure. I'll give you that production car that are cheap don't use bov with thier turbos but they are make 6-7psi not fifteen to twenty + and throttle response is not really the prime concern, increased power from a small displacement while maintaining the gas milelage is what those companies are looking for.

Ians, to answer your question no a bov and waste gate do not do the same things, the bov is between the compressor and t/b and open to relieve excess pressure when the throttle is shut allow the turbo to keep spin at the hi rpm ready to make boost as soon as you need it. A waste gate opens once the boost level you are set to run at is reached it dumps the exhuast around the turbo into the exhaust so the turbo is not spun any faster thus not allowing the boost to continue rising and stabilising it at the level you want.

 

peejay =

 

That chirping is the wastegate and the air trying to sneak past the throttle blade. Not a blowoff valve.

Ford 2.3T - many of these were 15psi nominal and then the wastegate solenoid would basically disable the wastegate over (IIRC) 4400rpm, and you'd see 18+psi. (Automatic cars were limited to something lower and there was no wastegate-disable, in order to save the crappy autobox they used)

No BOV.

You only really started to see BOV's when Japanese companies started using teeny tiiiny turbos that have to spin to ungodly RPM instead of what the more reasonable larger T3's would need to do for a given pressure ratio.

Throttle response? BOV hurts throttle response...

 

Open up an issue of hyper rev magazine on RX7's. It's interesting to note that alot of japanese tuners do not have BOV's underneath the hood of their cars, even after converting to a single turbo setup.

Can it be done? It has.

Does it have any detrimental effects to the turbo? I don't know, but maybe someone on this forum should try it out and let us all know =)

 

I'm not a rally expert, but I am a CART expert

CART does not run a bov, they only run a pop-off valve that is used by the sanctioning body to limit their max boost. The stuttering you hear on the upshifts is the ecu killing the ignition for an instant for the clutchless, hold the throttle to the floor upshifts.

 

I love how the people who claim it is 100% absolutely neccesary and then flame anyone who dissagrees have <100 posts, makes me laugh. I was also under the impression that a BOV was neccesary for a higher boost set-up but now I know, it really isn't. That said if I ever go turbo I'll probably include a simple one just for the rice.
Grant

 

This is apparently beyond my expertise

 

I was in a hurry earlier today.... so I completely forgot this little bit of info:

What opens a blowoff valve? Intake manifold vacuum.

Kinda useless if you don't *have* intake manifold vacuum. Let off the throttle on a car equipped with most types of antilag, and intake manifold pressure will stay at or above atrmospheric! At "idle" they hold the throttle open and kill power by retarding ignition timing 30 or 40 degrees... so they're still running under boost even though they are just making enough power to keep the engine spinning.

Of course, with that much ignition retard, it kind of becomes an external-combustion engine, which just makes the turbo spool up that much more nastily And that poor turbo is trying to actively pump *hard* into a mostly-closed throttle blade. (And no BOV!)

Modern systems use throttle by wire to do this much more smoothly than the old semi-mechanical throttle kicker/ignition retard flamethrowing bang-bang systems, which is why WRC cars are so much less exciting to listen to than they were five years ago. When the computer controls *everything* there are certain synergies to be had, since the accelerator pedal is not longer something to control airflow into the engine, but simply a power-demand lever, and the computer decides how much airflow and what kind of ignition timing is best to make commanded power in a given situation.

 

One reason for using them is to keep the compressor spinning as quickly as possible between shifts. It is obvious that a bov can accomplish this; although I doubt it makes much difference.

That being said, slowing the compressor down more quickly by having a backsurge of air is not going to wear down the bearings much more at all. Its the same as speeding the turbo up.......

I think that while a bov can help, it is not really worth it. That having been said, I still use one on my car.

 

Does having drive-by-wire vs. traditional throttle cable make a difference whether or not a BOV is necessary? and why is it that throttle response is reduced with a BOV?