Was thinking about this the other day, and, since I'm no expert on fluid dynamics, I thought I'd post this up to see if anyone could give me feedback on this.

(please note the attached diagrams for the discussion)

I've been thinking about going with better upper IC piping (UICP) and it seems that all of the aftermarket piping has the BOV attached at a right angle on a straight section of pipe (I call this design A). This is very much unlike the stock UICP piece, which has a very interesting design. Having examined the stock piece, I've come up with another possible solution (called design B), which I think may be better than the "traditional" BOV attachment.

IMO, it would seem that if the BOV were placed inline with the first straight section of piping off the throttle body (TB), then it should make it more efficient in relieving pressure in the UICP. It is my understanding that the BOV exists to relieve the pressure surge that occurs in the IC piping when the TB is closed, allowing the additional air to vent out and thus preventing compressor surge. Since the pressure buildup is caused by air being pushed back towards the turbo from the closed TB, a BOV placed inline with the piping off the TB should be better at relieving the pressure, no?

Any insights from people with FD experience would be greatly appreciated. I'm also curious to see if anyone can explain why designed the OEM piece the way they did.

l8r)

 

Diagrams of the typical aftermarked design and the alternative design, which I think might be better.

l8r)

 

The veting events are of such a short duration I don't think a change of this sort will be that beneficial, if at all.

You are also making an assumption that the air will flow all the way to the TB and then "bounce back" and travel back to the BOV and vent. In actuality, the TB will close and all the air past the BOV will stop, build up in pressure, then flow out the BOV. It's difficult to describe, but what I'm getting at is that the majority of the vented air is coming from the turbo side, not from the pressure waves caused by the closed TB.

- Steve

 

Engineering is always a compromise, same here. I think you are right and mitsu did decide to place the BOV the way they did to improve driveability. The con is that a right angle BOV attachment basically makes a pocket of "stagnant air" when the BOV is closed, which should reduce pressure loss in the flow. The OEM design (or yours) on the other hand will cause air to continously flow into the "dead end" and get pushed back out, causing more turbulence and pressure drop at high flow. So it's a trade: less pressure drop when closed, or better response of the BOV when it needs to open.

 

-The BOV can be on a right angle, its doesn't have to be advanced like a collector on a manifold

-Moving the BOV closer to the throttle body is less efficient and the wrong way to move it, ideally you want it pre-I/C much closer to the turbo

-If your not having problems holding boost or with surge then theres no need to worry about the bov

 

Thanks guys for clearing up a couple of things for me. Superhatch, your reasoning would then seem to explain the location of the BOV on the OEM pipe. Of course answers always lead to more questions:

- What if I were to place the BOV on the outside radius of the bend coming out of the intercooler? This would move it away from the TB, but still keep it close enough to the intake to allow me to recirculate the air vented by the BOV.

- Any thoughts on placing the BOV on the outside radius (BOV A in diagram) versus the straight section of piping coming off the first bend after the IC (BOV B)?

l8r)

 

although i think the BOV placement is rather arbitrary, i would keep it on the inside radius before the outside radius. reason being, even though both edges have a boundary layer, because the outside edge is longer, the air will be flowing slightly faster moving the higher velocity air slightly biased to the outside edge. therefor distrubances such as the BOV port will cause slightly bigger turbulance then on the inside.

this is all just off of assumption though, and i think the correct answer is there is no answer.

 

????

I dis-agree.

turbo surge is the turbo stopping & going again. If you were to put its relief point(BOV) next to the compressor housing, why bother. There is less time for the bov to vent before the rest of the pressure pulse goes back into the comressor. I believe one would want the bov closest to the high pressure point in an off throttel condidtion, i.e. being close to the throttle blade.

What you would want in front of the comressor housing inlet, is a throttle blade(blades in a iris form) that cuts off air supply when the main throttle is closed. This way, the turbo cant inhale any air, which prevents slowing down the turbo when the main throttle body blade closes. The turbo slows down less & @ a slower rate.

sorry to get off topic

the presure back up comes from the turbo spinning @ 100k, just doing its job, acting as an air pump.

 

Either location works, PRE I/C IS IDEAL.

the BOV relieves backpressure when the throttle body is closed so the turbo isn't pushing agaist a closed system. The most logical place for a BOV would be close to the compressor outlet. As soon as the BOV opens, pressure is relieved EVERYWHERE almost instantly. Not just near the BOV first. Mount it after the intercooler and you would make the intercooler work harder (heating the i/c up) for no reason because you are expelling the cooled air.

Think like this

When you blowoff the turbo is at a certain speed, moving a certain amount of air. You want the least amount of restriction for the turbo to push the air through so it can keep its speed up. Think about an exhaust, for performance only would you rather run a 3 foot length downpipe or a full 10ft exhaust system? Of course open DP is less restrictvie which lets the turbo spool up faster and work more efficiently.

Once you let off the gas, stop spooling the turbo, and blow off, the turbine wheel starts to slow down pretty fast. You want the turbo to keep moving as fast as possible to avoid lag, so would you rather blow it through a 2" pipe and right thru the bov or go through 6 feet of piping, and an intercooler and then a bov?


The latter is more restrictive, and the restriction will slow down the turbine wheel faster, because of more resitance to flow

-it vents hot air, not cold air after the cooler
-its the nearest to the pressure source (the compressor)
-no pressure drop across IC
-less drop in turbo speed

for there to be more surve with the BOV at the compressor the "waves" would have to bounce off the TB, travel through the charge piping, then through the IC, then all the way back to the compressor wheel which has a "hole" (bov) in front of the compressor pressurized air will always want to move from higher to lower pressure.....the easiest and fastest way is the blowoff valve. There is no way in hell that the air would pass the bov and bounce into the compressor.

Also the idea you suggest (pre turbo T/B) would induce lag by slowing the turbo down greatly and could in certain conditions create enough vacuum to suck that T/B right into the turbo......

 

Very true & proven world over
Everyone is entitled to their opinion


exactly

You said it correctly in your 2nd quote above. Now your saying it's the waves Now there are waves within the pipe, but they're not responsible for stopping the turbo.

Correct, but air pressure does build up within the pipe if the bov doesnt vent, then the surge is induced by the inability for the bov to vent in a fashion that does not cause the turbo to stall. Thats why many vendors offer different spring loads/rates to adjust / minimize surge. When that throtte blade slams shut, the presssure that is created from that event is significantly higher than any pressure the turbo will produce without anyenergy driving the turbo from the closed throttle blade, thus causing the turbo to slow down & possibly stall/stop.

I guess garrett was wrong when they developed this device with renault in the F1 turbo era, along with cosworth & honda utilizing this technique as well in CART. BTW winning multiple championships with what you think would create lag & increased vacuum I know first hand via measuring turbo speed, spending many hours reviewing data pressure data, while @ the track conducting the development work for the latter company.

In the end Superhatch's 1st sentance sums it up entirely

 

Some quick calculation can shed some light on this. Let's say we hypothetically mount or BOV immediately post-intercooler, have 8 feet of 3" diameter post-intercooler piping before the throttle, we're running at 1.5 bar or boost, IC outlet temps are around 313 Kelvins and the turbo is flowing about 250 grams/second (~33 lbs/min). The total mass of the air post intercooler would be around 31.2 grams. However, since a BOV cannot dump pressure below ambient, only 18.72 grams of air would have to reverse even if we held the BOV open indefinitely.

If we assume a 0.25 second gear-change, and bear in mind that some mass of air exists between the turbo and the cold-end of the intercooler, the mass of air which will come towards the BOV from the direction of the turbocharger during that 0.25 second period (@250 grams/sec) is greater than 62 grams which is in turn greater than the mere 18.72 grams of air forced to reverse from the direction of the throttle body.

Theoretically, in order for the amount of air coming from the direction of the throttle to be greater than the amount of air coming from the direction of the turbo, the shift-period would have to be less than 1/13th of a second. However, with a shift period that short, I doubt any significant amount of air would flow out of the BOV from either direction before the throttle opened again.

Since, for most pratical gear-change periods, the majority of the air will come from the direction of the turbocharger, an orientation which facilitates this would probably be wise. But there may still be other things at work here.

-Adrian

p.s. There is a pressure wave formed from the closing of the throttle body due to the inertia of the air moving into the engine. However, generally speaking, this pressure wave will already pass the BOV before the low-pressure wave, which travels down the BOV's control vacuum line, reaches the BOV to signal it to open. The reason for this is that both pressure waves will travel at the same speed, the speed of sound, through both pipes and the control line is often slightly longer than the piping.

 

this is wrong... pre ic is not ideal. if you expect all the air to flow backwards through the ic to get vented you're gonna push the throttle plates open or squeeze air past it. at the time of vent the entire system is pressurized, compressor surge is not the only factor for hte blow off valve and in fact it is the least of the concerns. because you can continually push air through the intercooler system right out of the bov when it's open, but you cannot however expect the already pressurized system to decompress by going through the restriction it went through initially again. that eliminates the entire purpose of depressurizing the system after the intercooler.

remember that many small turbos run without blow off valves... they just eat the surge and albeit that is not the best way to do things, it's entirely possible... but this is because they're not making enough boost to squeeze air through the throttle plates, that is the real danger... already metered air making its way into the engine during a cycle when there's less fuel and exhaust gas recirc... that's a popped motor right away.

you guys are looking to save the turbo but you're totally sacraficing hte engine... i don't get it...

also... a pre turbo throttle body is used in wrc also... for the most precise boost control. what makes you think the vacuum pressure is more than the boost pressure? if you can't shove the tb into the engine why can you suck it right out? even if your turbo were fast enough to create perfect vaccum only atmospheric pressure is pushing on the pre turbo throttle body. that's a whole what... 1 bar... you're pushing 2 bar onto the throttle body after the turbo...

you should stop talking.

in an ideal case for no surge you'd have two blowoff valves... one on each side of the intercooler, but why would you even bother? so you can have less turbo lag? with that much venting you'd have to completely repressurize the system, PARTICULARLY THE INTERCOOLER and you'd lag prolly just as much while spending twice the money. there is a lul in the air travel when you vent and some of the intercooler stays pressurized, dwell time. this means you pressurize it less when the valve is resealed (assuming quick shift, at slow shifts everything is just equilibrium anyway) so imagine every time you shifted was slow... and that's how much you'd have to wait to repressurize the system if you purged it so cleanly.

turbochargers aren't whining little *****es, they're whining jet like robust pieces of equiptment. they won't cry over a little surge, so you shouldn't either.

 

Errrm. MAF cars with BOV's typically run rich between shifts because lots of metered air escapes through the valve causing the ECU to inject more fuel than is necessary.

MAF cars sometimes run lean after the shift because, when the throttle is snapped back open, an increase in flow travels away from the throttle plate at the same speed in BOTH directions. However, because the intake ports are often closer, the increase in flow reaches the intake ports before reaching the MAF sensor, which causes a temporary lean condition.

Various ECU/throttle programming changes can alter, or correct, that, but it's what generally happens on MAF cars with no specific correction.

 

That's why we recirculate the air back into the intake on the Evo, no? That way the air that has already been metered stays in the system, since it is 'vented' back into the intake, but after the MAF.

l8r)

 

well... maf cars with bovs don't really tend to have restrictive purge either... if you let enough air through it would be bad lean yeah?