When does a Centrifugal blower need a blow-off valve?

[plovett]

I have a Vortech V7YS centrifugal blower and am building a carbureted Ford FE engine to use it. I am new to superchargers and am wondering about the necessity of a blow-off (by-pass) valve. The Vortech kits don't come with one, but others like Procharger do. When do you need one?

I will be running less than 10 lbs of boost on an engine of about 449 cid, at a max rpm of less than 7000. This will be a street driven car and I am looking to make 700-750 hp.

I have the Vortech carb enclosure, but am leaning toward using a carb bonnet instead. Does that make a difference regarding the blow-off valve?

Thanks,

Paul

[rebelrouser]

Not an expert, only built one procharger 410 chrysler LA engine for the street. The way I understand it is that you need a blow off valve to control boost, and too control pressure when you back off the throttle. If you are at WOT and you let off the pedal, the engine is still spinning and making boost, with closed throttle blades where does all that boost go? They tell me that this is really hard on blower drive belts, and tensioners as well.

[BLSTIC]

Blow-off valves are to avoid compressor surge and (despite what the aftermarket thinks) reduce intake noise.

You need one if

A: You think you will have compressor surge under cruise conditions AND such a condition is actually bad for your supercharger. One with a super-light spring will achieve this. You can do the math if you know the approximate air consumption of you engine at cruise and you have a compressor map. Compressor surge makes downstream pressure variations as well, so that might affect carb tuning (or it might not, I haven't seen it mentioned before, but I would rather not have random float bowl pressure variations given the choice).

or

B: PTSSSSSSCHHHHHHH sounds better than SUTUTUTUTUTU when you back off. I think it does.

or

C: You have the blower geared to develop your peak boost early and you're actually over-boosting at high rpm. A BOV without a manifold connection and with a relatively stiff spring will be a crude boost control valve.

seeing rebelrouser's post, I didn't think of the belt implications. So there's another thing to think about.

[englertracing]

If the throttle is after the centrifugal
Then every time you close the throttle.

[Joe-71]

If the throttle is after the centrifugal
Then every time you close the throttle.

X2 Joe-71

[plovett]

Thanks guys! It sounds like I should use a blow-off valve. They range in price from less than $100 to around $500. For my combo will a cheap one suffice? I don't intend to "overboost" the engine. That is, I don't intend to gear the centrifugal blower to make a lot of boost at low rpm and then vent the "overboost" at higher rpm. This will a be a drag radial street setup so I want the power to be a bit soft at low-mid rpm's. I intend to pulley the blower so it is making max boost near my redline. That will be in the 6500-7000 rpm range. So I don't think I need a blow-off valve to limit boost, but rather for bleeding off boost after closing the throttle.

When do you need a $500 blow-off valve vs a $100 one?

thanks,

paul

[BLSTIC]

When do you need a $500 blow-off valve vs a $100 one?

When you are going to drive the car a lot. In a supercharged application the blow-off valve will be opening and closing a lot more than a turbo application. Sometimes it may even 'chatter'. This wears it out, and I have actually had a worn blow-off-valve on a centrifugally supercharged car

[plovett]

When do you need a $500 blow-off valve vs a $100 one?

When you are going to drive the car a lot. In a supercharged application the blow-off valve will be opening and closing a lot more than a turbo application. Sometimes it may even 'chatter'. This wears it out, and I have actually had a worn blow-off-valve on a centrifugally supercharged car

Thanks. When it is worn out does it stay open and/or open too soon?

[BLSTIC]

No, mine functioned just fine, but the seal around the top of the piston got damaged and it made a vacuum leak. I only found out because it was on the MAP sensor line and my ecu and my boost gauge read different things

[plovett]

No, mine functioned just fine, but the seal around the top of the piston got damaged and it made a vacuum leak. I only found out because it was on the MAP sensor line and my ecu and my boost gauge read different things

Thanks. I guess it IS a moving part.

paul