Craig, Scott, Boost and blow off valves?

[Aussie Mike]

Hi Guys,

I have a couple of questions on blow off valves. I understand they have a couple of functions with supercharged cars. 1. they control the maximum amount of boost the engine gets 2. they give the boost somewhere to go on closed throttle.

I've been looking at the Procharger setup on Garrett's Superformance and it doesn't seem to run one (I could be wrong and I'll have a closer look this week). It's only running about 5-7 PSI of boost though so it probably doesn't need one I guess? So what it the threshold for needing a blow off valve? I'm thinking if we spin the blower a little harder and make 9-10 PSI we'll need to fit one. Which valve would you reccomend? Is there any restriction on how far away from the throttle body they should be?

Cheers

[750hp]

Mike - sorry for the delay in replying. I've been minding my sister's house (zoo), and they don't have a computer.....

How can I cope under such adversity?!?

I can't remember exactly how the Procharger was set up on Garrett's car, but I think he had the serpentine belt.... In that case, while it would be nice to have even the smallest Bosch blow-off valve, it isn't absolutely necessary. Any pressurised air that hits the closed throttle and rebounds back towards the blower needs to be vented to prevent spiking the blower and potentially damaging the impeller and bearings. With moderate boost and a serpentine belt, the belt will simply slip on deceleration. This means that the impeller doesn't cop much abuse.

Now - the function of a blow-off valve is purely to dump the pressurised air under closed throttle. This is all they do. They simply get a signal from the manifold, and as soon as there's vacuum in the manifold, it draws the valve / piston inside the blow-off open. Boost control is done via a wastegate in a turbo application, or you can do some tricky boost control via Motec, Autronic and other good ECUs with a blower.

Garretts car - is it running MAP or MAF? If MAF, you'll have to see if it's mounted before or after the blower. Generally, they're mounted before. In this case, the ECU has used the Mass Air value to determine fuel injector pulse. If you vent that pressurised air on deceleration, the ECU will still inject LOTS of fuel because the MAF "read" a lot of air going past. You would have to run a hose from the BOV to somewhere between the MAF and blower so that the air is recirculated. A happy by-product of this is that you don't get the racer "WHOOOOSH" when you back off. My car isn't all that noticeable when you back off, but Scott should have his car in Fast and Furious 3 !!!!! He came up along side my friend's SS Impala and boosted it then backed off - the blow-off valve nearly scared the sh!t out of me.....

With more boost, I would definitely fit a BOV. Almost without exception, they will all work OK. It's a personal preference thing as to whether he would want the billet look, anodised look, or cast alloy

With blowers, you want the BOV pretty close to the blower. If he has an intercooler, definitely between the blower and intercooler. Turbos are the other way around....

Jeez - it would have been quicker for me to just pick up the phone and call you rather than type all this out!

[Aussie Mike]

Thanks for that Craig, I didn't realise they detected engine vacume. I thought they just went on intake pressure.

On Garrett's setup the blower blows through the MAF sensor whigh is almost directly before the throttle body. I've been thinking that removing the MAF and geting a MAP tune done would unleash a bit more power. I could also put the BOV on the tube section that will replace the MAF but unfortunately this is after the intercooler. What's the reason for having the BOV closer to the blower on a blown car and the other way on a turbo?

Cheers

[750hp]

Does the LS1 Edit program do MAP tuning?

Mike, at anything under 15psi I wouldn't be too concerned where the BOV goes. It wouldn't be any problem at all to position it where the MAF was.

Blown / Turbo differences? No idea - probably just the Fast and Furios turbo guys needing to be differentiate themselves from the knuckle dragging supercharger guys...

[750hp]

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[Aussie Mike]

Hi Craig

Garrett came over on Sunday and we had a good look under the hood. It's got a blow off in there but I'm not sure on the manufacturer. There is a 1" diameter tube running from the high pressure side of the blower to the low pressure side. There is a butterfly in the middle of the tube and it's actuated by an arm off a diafragm attached a bracket on the side of the tube.

LS1 Edit does do MAP tuning no trouble. Holden's GTS 300 gets some of it's extra Go courtesy of not running a MAF. The Edit guys seem to be generally getting 20 KW more tuning these motors MAFless. The stock ECU seems to cope quite well with big blower set ups too. I believe the Commodore with the big whipple blower that Boxhead showed us is making about 400KW at the rear wheels using the stock ECU and a fairly stock bottom end.


Cheers