barcode

iTrader: (1)

I find it hard to believe that would work since a turbines rotation has much to do with the heat of the exhaust gas as well. This would be why people go to the effort of finding materials that do not absorb the heat, but rather maintain it. Also, the amount of air that a bypass valve would release is relativey small and would do little to air in turbo spool. On that note, Psi (pressure) has little place in this argument, we're talking about sheer volume of air, pressure doesn't have a lot to do with causing compressor surge or "back spin".

scathcart

By pumping air into exhaust, the axhsut air/fuel mixture continues to burn off the excess fuel, giving a larger expansion of gases. By allowing for this more complete combustion before the turbo, you would essentially develop a slight anti-lag effect.
The air blown into the exhaust would not cause a cooling effect, if that is what you were thinking.

Slacker7

iTrader: (2)

Well the whole point of this discussion is about whether or not having a BOV will relieve stresses and actually help extend the life of a turbocharger. By reducing the amount of vibration of the rotating assembly you reduce the dynamic loadings on the shaft, bearings etc. If you have vibrations some of it will definitely translate to bending, sideloads etc etc. So it cannot be ignored. I still don't see the point in just focusing on static side loads.



Just picture this scenario. The pressure in the TB piping tries to slow down the compressor too much which causes the pressure before the turbine to build up causing the assembly to speed up. This process then repeats itself(everything here is relative)

In your scenario where you let off the BOV pressure in the manifold is relieved so there is nothing trying to spin the rotating assembly backwards. I never said it would be enough to build boost I just said it is significant to effect the magnitude of surge.



I don't know about that. I'm sure they have learnt alot more about surge and bov's might have been one of the advancements

barcode

iTrader: (1)

To my understanding compressor surge is a bad thing right, it causes damage to the turbo, or am I just completely wrong?

SmogSUX

iTrader: (12)

This whole forum is a huge arguement....I still don't know if I need a bov..I do know my turbo has 150+k on it so ill need a new one soon..but this arguement left me more confused than I was about bov

White_FC

I havn't been focusing on static sideloads at all... I've been focusing on dynamic ones just not vibrations. Why? because it would be hell hard to quantify vibrations and how they translate into sideloads..
But if they're only say 1% of the other sideloads created, why bother?
1% is important, but 99% is more important.
The vibrations caused by 38Psi hitting the compressor wheel or 44psi hitting the turbine wheel will be probably only less than 1% of the sidelaod created by the actrual force.

Why would pressure infront of the turbine build up? ...I can't see any good reason..

Ok.. for the risk of repeating myself if its large enough to effect surge, why wouldn't it effect the during of the gush of air comming out of a BOV?

Well they _definatly_ were introduced as a pollution measure.
And I still think that a big company with millions going into the R&D of their newest model sports car complete with a turbocharged engine would not do any study on compressor surge..

Obviously it wasn't a big concern for them before emmision laws became much tougher.

wargoblin

I've seen a couple cars witout Recirculation Valves, or Blow off Valves. If its noise you want, theres nothing like the intake pressure bouncing off your intake turbine fins. TOOT TOOOTas;kldjf;laskdjf

Claudio RX-7

iTrader: (1)

No, you are not wrong, that is the whole principal of the thing, what the BOV is there to do. But these guys are going on about turbo resistance, and stuff that doesnt even relate. What i wrote about how the BOV works and its importance is the main deal of this topic.

Later,

White_FC

Er, no... It directly relates to it.

The principal of the thing is emmissions, not saving the turbo from instant death like so many people seem to think.

But hey if you'd like to tell me how it saves the turbo's bearings and/or compressor wheel from death go right a head because i'm sure im not the only one that would want to hear it...

SmogSUX

iTrader: (12)

Jeez I wish 2 people could agree on somthing....I don't know either of them so I don't know whos smarter. If it did help I would buy one. If it doesn't then I wasted $300 on a "cool" noise.

BlaCkPlaGUE

We should get someone from like fc3s.org or HKS to answer this question.. heh.

SmogSUX

iTrader: (12)

Somehow I think that would end up in another arguement =/ I asked my friend (drives a 93 Mr-2 Turbo) and he said that bov make it easier on your turbo, but make it a little harder on your engine. He said it a bit differently, but I can't remeber exactly all that he told me.

patman

iTrader: (3)

Ok, I skipped a couple pages since the same arguments were presented over and over again, so i may have missed someone already saying this. Boost pressure on throttle letoff can hit high 40s low 50s psi. 40-50 psi is a very substantial amount of force, especially in a dynamic situation. Ever seen a tire blow and blow chunks out of a metal rim? That can happen at 45 psi. The fact that the exhaust gasses are stronger than the boost spike is exactly the problem as this means that the compressor wheel will keep spinning regardless of the extreme forces acting on it. If the area of the compressor wheel is 4 sqin, that means that the total force on the wheel is 45*4=180 lbs! tell me a sudden force application of 180 lbs erratically over a few months will not fatigue the metal of a compressor wheel, and wear the bearings it spins in.

point #2. When talking about street dirven cars, you CANNOT use race car habits/ technology as a basis for arguments. Any engineer will tell you that designs for cars driven daily by mechanically incompetent people must be much tougher than a car driven, no matter how hard, by an experienced race car driver, and inspected after each event. The coices that people make when building race cars are per their own personal preference, and not necessarily generally technically sound. Also, it is not good practice to base all your arguments on something which you have no personal experience in, and therefore must theorize as to the motives and successes of others.

Point #3 A BOV has no emissions purpose whatsoever. think about it. WHat could it do to improve emissions? the stock computer does not even recognize boost levels that high, so dont say that it eliminates a rich condition caused by correction for the boost level. Noise? what noise? just compressor whine? every turbo car makes compressor whine, which is caused by THE FORCES ON THE COMPRESSOR WHEEL. so, if a BOV reduces compressor whine, that is a good thing, as it means that it is reducing stress on the turbo.

point #4. Obviously you have not yet learned about metal fatigue and the effects of repeated stress, as in, even if the increased force of the high pressure surge is not enought o rip the turbo apart, which it almost never is, it will still put unnecessary stress on the parts, causing them to break down on a molecular level, and after some number of wear cycles, form small cracks, and then break.

pat

White_FC

4sqin is a huge area. The whole compressor wheel wouldn't even have an area anywhere near that big, let alone the small section that the force would be acting on (at any one time). The surge spike would be acting on maybe something more like 40sqmm (~0.5sqin) and thats being generous.

But if you wanna talk about forces, while under boost, call it 20psi, the exhaust pressure acting on the exhaust wheel would be at least 40 psi. So.. uhh.. the bearing comment goes right out the window. If a boost spike(~40psi) acting on the compressor wheel would kill beairngs faster, so would everytime you hit normal boost (because of said exhaust wheel load..)

If your talking about the two racing examples I used earlier, one was an all out race car that ran 40psi around a track racing for 24hours and won, I can't remember how many gear changes they do around the bathurst track, but.. it's alot.
If a wheel was going to fatigue from constant cycles, thats where it would happen.
But since you obviously have much personal experience in this field i'm all ears.
how many compressor wheel failures have you seen that you can directly attribute to compressor surge?

The other example I used was a production race car which is much more closely related to a street car. As I said before a turbo repair place that sponsored the car said running a BOV would not improve turbo life. Why would they lie? They certainly wern't making money when the turbo blew and a new one was needed...

Wrong, it is an emmisions device. The FSM even says it's a noise emmisions device. Noise being compressor surge, not whine. Most people would be a bit freaked out by a 'choof choof chooff' noise happening under the bonnet of a car..
And I never ever said it preveted a rich mixture from having a high pressure in the intercooler piping?
On other cars, that use MAF's, its also there to prevent a rich mixture because compressor surge would push air past the MAF, but in the wrong direction, so it would measure the airflow twice that the engine is not actually ingesting. Not that this has anything to do with our AFM's.

I agree with you on fatigue, however I don't feel a 40psi pressure spike would cause much concern to a compressor wheel. How many turbos have you seen fatigue like this though that can be directly attributed to compressor surge and not the wheel collecting the housing when the beairng **** themselves?

patman

iTrader: (3)

if the compressor wheel had a diameter of 1.128 inches (that would be a rediculously small turbo), the face area of the wheel would be 4 sqin. cmon you are obviously relatively intelligent, couldnt you have figured that out?

the exhaust pressure does reach that high, but:
the exhaust side of the turbo is designed to withstand it, and more importantly, the compressor side is at 20 psi, so the energy goes into compressing the air, rather than destroying the turbo. however, when you get 40 psi on BOTH sides, the force on the bearings and shaft are much greater.

most MAFs are designed in one direction, so that they would only measure a small fraction of the air that flowed past backward.

As to racing examples:
1) a racecar uses a much better turbo than a stock one, which is better able to withstand said pressures.
2) 24 hours of continuous driving is a lot of shifts, but a good percentage of them are not made at 40 psi, because they are made in turns.
3) if i own a car for 10 years, and drive it to work for 30 mins each way 5 days a week, thats 2600 hours, not counting side trips and weekends. also, a good portion of this time the car is cold, so the parts are poorly lubricated and brittle.

did i miss anything?

pat

White_FC

Point being that the force isn't acting on the whole face of the wheel at once. It will only act on a very small point, the face of one of the fins at any one time. Now I dunno if you've done this, but it takes ALOT of pressure to even just bend a fin. Took near on my whole body weight acting as a point load (90kg!) to bend one of the large fins (I have a busted turbo here.. Wheel was already shot from contact of the housing).

The bearings on the exhaust side are the same as on the compressor side. Fair enough the wheel itself is made of a more suitable material for the heat. But the bearings are certainly the same. And you will NEVER _ever_ have 40Psi on both sides.
When you back of the throttle there is no longer any much force at all pushing against the turbine. So you go from a 40Psi force creating a 20psi force on the other end, to a 40psi force on one end trying to reverse the direction of the exhaust wheel which has no load on it (effectivly).
So uhh.. as far as the bearings are concerned they're stuffed either way except that since theres no load on the exhaust wheel when the compressor surges there wont be as much load transfered into the bearing because most of it can be used to change the momentum of the exhaust wheel.

No, most AFM are designed uni-directional, most MAF's aren't.. At least the ones i've seen anyhow.

A much better turbo? How? theres only so much you can do to a turbo. They might have slightly better designed fins(might not be stronger though..) and what not, but i'd bet they use the same bearings.

A good percentage of shifting on the road is made in turns or going into turns aswell..

And you also wouldn't be driving it like a race car or with as much load on the engine at full throttle.

Nope

patman

iTrader: (3)

actually, the force is acting on the whole wheel. of course, the force is divided up among the fins, but if the fins are .4 in by 1 in, then there is a force of 20 lbs or so (just from my head, i just did calc 3 for 2 hours no more math tonite) on each fin. another thing to take into consideration is that these are dynamic pressures, rather than static, so the forces are much greater.

in fact, there IS a time when there is 40 psi on both sides, that is why you need a BOV. it takes longer for the exhaust pressure to clear than it does for the intake pressure to build. this is because when you let off the gas, the engine is still running at WOT for a few mS before it has time to react. in this period, the exhaust pressure is constant, but the intake pressure builds very rapidly.

the shifting going into turns we can neglect, btw, because this is deceleration, with no boost. however, in a race, you dont stop. therefore, you do not have intervals of very high throttle straight line driving like you often do on the street. it is true that street driving is at a lower average throttle %, tho.

as for better turbos in race cars, it is important to remember that race cars generally have bigger turbos, which can easier deal with boost spikes. also, many stock turbos are junk, with poor quality bearings and cheaply made parts, whereas people building a race car tend to buy nice stuff for their toy.

pat

patman

iTrader: (3)

ps i dont know the equations off the top of my head for dynamic pressure, so i cant throw any big numbers at ya, but believe me it makes a big difference.

White_FC

Everything we're talking about here are dynamic forces.. so thats a moot point
(it's 1/2 pv^2 by the way).

be a bit hard to 'work out' because...

p = ~1.3 kg/m^3 before the turbo.. would obviously change after the turbo heats it up, and again after the engine burns it..

no idea what the velocity would be for the intake side of things when its surging but i'd bet my leftballsack it'd be smaller than the exhaust pulses at WOT in the high revs.

But anyway, neither here nor there really, you will NEVER get 40psi in the exhaust side when you let off the throttle! pressure builds on the intake side because it's a closed system after the TB shuts.
The exhaust however, is not.. you have a vrey free flowing exhaust compared to the shut TB.


And uhh yeah you do certainly have periods of straight line acceleration when racing.... Like.. on a straight?.. I know at my local strip I strart the straight at about ~ 40km/hr after the last turn and hit an indicated 220km/hr at the end (probably more like just under 200km/hr..).
I don't too many people that accelerate flat out like that for that long on the street..

Guzman_FC

Compressor blowoff and bypass valves play a vital role in any turbo system

because they not only improve the performance of your vehicle but they also

increase the longevity of the turbocharger as well. The primary function of any

blow-off or by-pass valve is to prevent compressor surge by relieving the

compressed airflow that reverses direction as the throttle is closed. Blowoff and

bypass valves reroute the pressurized air, that is normally forced backwards into

the turbo compressor, into the intake stream or by venting it to the atmosphere.

Subsequently, engine performance is then increased from quicker turbo spool up,

as the compressor speed is not reduced by the backward airflow. The longevity

of the turbo is also increased, as its bearings will not suffer from the impact that

is also produced by compressor surge. Though many vehicles incorporate factory

by-pass valves, they are often undersized and quickly are beyond their capable

operating range as airflow levels are increased.

vice

heres the way i see this argument. if you have a turbo, why not use a BOV? it cant hurt. if your worrying about lag, then get a ******* supercharger. or, if your racing, equip your car with a "misfiring" system. if you drive well, you dont have that much lag.

White_FC

Vice, originally the thread was about the acutal purpose of a BOV..

Your completly correct, it wont hurt, you might loose some power between shifts if you run one, but then again you might not.

But if it makes you sleep easier at night you should run one.

Slacker7

iTrader: (2)

Do you just ignore the effect of vibrations because it's too complicated?


Pressure inside intake pipe slowing down the rotating assemly at a higher rate than it would if it were to free spin causing the turbine to slow down at the same time causing a pressure build up behind turbine.

Same principle as above

White_FC

Er.. No.. It is complicated, no question about that. Although I ignored it because it'd be worth about 1poofteenth of a percent compared to the other dynamic loads on the bearings. And when most things in this day and age are over engineered by 300%.. well.. 1poofteenth of 300 doesn't equal very much.

But if you wanna gimme some numbers that shows it isn't, i'm all ears.. I certainly can't be bothered working it out.
No idea what your trying to get at with that, because even with no load on the compressor wheel the turbine wheel slows down _rapidly_. If there were pressure behind the turbine wheel, that would be causing it to spin faster, hence a longer BOV release (which doesn't happen....). Just because the wheel slows down does not mean less exhaust gas can get through. The turbine is _not_ a compressor like you seem to be relating it to?
...
I know I am pretty stupid but if you could tell me how your theory works I'd love to know . More rapid decel of the ext. wheel = more exhaust manifold pressure? Was that how I was supposed to read it?

are_one

The same question of if one is actually needed was asked on turbobuick.com. There was a massive cyber civil war that went on for weeks, the opinions are still split 50/50. I guess this is a touchy subject for some people. What we concluded over there was you don't need it, but it's good insurance.