Blow off valve before or after chargecooler?

[Escape]

I've always seen the blow off valve between chargecooler and plenum, and have installed there myself as well. Yesterday when going over a customers SE I noticed he had put it between turbo and chargecooler. Did a little research, and although there are arguments for each, consensus seems to be it doesn't matter. I'd like to know what the views here are.

Between turbo and chargecooler:

• boost vented close to the turbo
• no hot/warm air going through the chargecooler
• airline T-ed from the wastegate

Between chargecooler and plenum

• boost vented closer to throttle bodies, so less chance of pressure build up or pressure waves traveling back to the turbo
• cooler location for the valve
• airline from back of plenum

Filip

[andydclements]

Mine is between the plenum and the chargecooler, but I can see that it means that air which is vented still has to be cooled, and do so on a cooling system that's then getting 1) less air blowing over the rad due to slower car speed, 2) less water flow (unless it's an electric pump) due to lower engine speed.

Surely it makes very little difference, the blow-off is to enable the turbo to keep spinning or at least run-down in a controlled fashion rather than stall due to high pressure air between it and the inlet manifold.

[ramjet]

I would think by venting cooler air after the chargecooler, you may actually be venting down to a lower pressure quicker by venting cooler air (denser) versus hotter air?

Possibly lessens the pressure on the turbo quicker? Be hard to measure though.

[Steve V8]

Air pressure in the intake tracts through to the plenum isn't consistent, air is squashy, air pressure and air density are two entirely different things.

Air pressure directly after the turbo will be greater than the dense chargecooled air at the plenum, as the MAP sensor usually takes its readings at the plenum I would suggest that is the ideal place for the blow off valve. 

[Escape]

Thanks for the input guys. I realized I made a mistake in my post, of course the blow off valve isn't T-ed off from the wastegate but from the FPR connection.

@Steve V8 the MAP is in the intake, after the throttle bodies, because it has to take the pressure loss into account, especially on (partially) closed throttle. So not really relevant I think, unless I'm missing something?

I tend to agree it doesn't make much difference. Load on the chargecooler should be negligible as the turbo isn't spinning and the air only has to overcome the resistance of blow off valve (and chargecooler). 

I am wondering if the heat close to the turbo could have an effect on the valve over time. They are known to fail.

Filip

 

[Steve V8]

54 minutes ago, Escape said:

, unless I'm missing something?

Haha, the penny drops...4 cylinders and a couple of hundred bhp. 

I'll go back to making carpets....

[MrDangerUS]

IMHO, venting between  charge cooler and plenum or directly from the plenum is better for your turbo longevity.

If vented before the charge cooler, theoretically, the whole IM plumbing turns into a large Helmholtz resonator, which may  adversely affect the turbo by creating a surge ( negative gradient wave hitting compressor tries to slow down the wheel ).

However, I have seen it done both ways depending on the size of BOV and available room.

On Bosch CIS Jetronic Injected cars,  BOV should be used with Bosch K-Jet injection, as being a closed system, a loss of air for which fuel has been metered by the movement of the airflow sensor plate, can result in an over-rich condition and possible backfire. It must NOT be vented to the outside!

If  BOV is installed to an EFI system that use a MAP (manifold absolute pressure) sensor, as the MAP sensor source is inside the plenum chamber in the inlet manifold, relieving air before a closed throttle (throttle lift-off) will not affect running.

 

[Escape]

22 hours ago, Steve V8 said:

Haha, the penny drops...4 cylinders and a couple of hundred bhp. 

I'll go back to making carpets....

More to the point, you're missing 2 throttle bodies. 😛 Seriously, I do love the V8s, it's a terrible shame they were limited by the gearbox. In Flanders, the larger capacity makes the V8 a lot more expensive for road tax, so I settled for a 4-pot. And extremely happy with it I must add, especially with a V8 Range Rover (commercial) to satisfy my need for that lovely V8 burble.

On both the MAP is in the intake of course (hence the name), and the BOV should be before the throttles to be able to vent with closed throttles. I'm sure we mean the same thing, just wording it differently. 😉

@MrDangerUS that's a very clean install on the plenum chamber! I never considered that, but now I'm tempted to look into it.

Filip