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I have an old school roots blower on my fox coupe that doesn't have an integral idle bypass like the modern blowers have. From what I understand, this valve will circulate air from the outlet of the blower back to the inlet in an effort to reduce pumping losses and heat generation. Results usually include much cooler idle and cruise IAT's and smoother idles. Mine idles and runs fine but IAT's do creep up over time so I'm interested in experimenting with this.

Has anybody else experimented with these valves and have any real experience or results to share?
 

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I did a custom bypass setup on a Kenne Bell using chrome bathroom drain pipe and Magnuson bypass valve (obsolete now - they sell the actuators, but not the butterfly valves.) Lowered cruising temps 17 degrees in A/B testing (disabled bypass, took data, enabled bypass, took data - same stretch of road.)



Here's a little write up with materials list on my home-made bypass - not same setup but might give you some ideas:

 

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Mine are somewhere around 190-200 if I just let the car sit there and idle. It's a traditional straight lobe weiand roots blower so probably the least efficient of the roots and screws. I think adding the valve could help a good bit.
 

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Mine are somewhere around 190-200 if I just let the car sit there and idle. It's a traditional straight lobe weiand roots blower so probably the least efficient of the roots and screws. I think adding the valve could help a good bit.
Do you have a ‘hot air intke’?

Is the filter exposed to the engine bay?
 

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Twin Screw
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Mine with no fuel through the screws is 150-160 idle and lower on light load. Under full boost (12-14)I inject through the TB right threw the screws and I'm right around 135-145. But I pull air from the fender under the car. Idk if you can do a bypass with fuel involved.
 

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Discussion Starter #9
Do you have a ‘hot air intke’?

Is the filter exposed to the engine bay?
No, I have mine pulling air from the passenger fender area.

Mine with no fuel through the screws is 150-160 idle and lower on light load. Under full boost (12-14)I inject through the TB right threw the screws and I'm right around 135-145. But I pull air from the fender under the car. Idk if you can do a bypass with fuel involved.
I'm wondering how much is contributed by hot coolant. may tap a different runner and see what happens. after all, many other street cars have run these blowers without much of an issue.
 

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Twin Screw
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Do you have a ‘hot air intke’?

Is the filter exposed to the engine bay?
No, I have mine pulling air from the passenger fender area.

Mine with no fuel through the screws is 150-160 idle and lower on light load. Under full boost (12-14)I inject through the TB right threw the screws and I'm right around 135-145. But I pull air from the fender under the car. Idk if you can do a bypass with fuel involved.
I'm wondering how much is contributed by hot coolant. may tap a different runner and see what happens. after all, many other street cars have run these blowers without much of an issue.
Not much. Problem is the roots is less efficient than twin screw and centrifugal units. No bypass your beating the air heating it up. If it's not detonating and making good power I would leave it be.
 

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I use a mini-cooper bypass on mine. Huge difference in IATs cruising around - like 50+ degrees, iirc.
 

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Why not? If I'm using vacuum to open it and boost to close it, and routing bypass air from under the blower to above it, why wouldn't it work?
Why not? If I'm using vacuum to open it and boost to close it, and routing bypass air from under the blower to above it, why wouldn't it work?
Where are you getting the vacuum signal from? I tired one and the vacuum signal ends up the same on both sides of that style blowoff valve, thus it never opens.
 

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I tried that exact style of valve on my M112. I don't remember the results - you'd have to find it in my massive Eaton M112 thread on here for specific results.
 

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I used this just this last summer and it worked great. Make sure the vacum line only goes on vacuum side not boosted side because it is held closed with a spring not boost like other kinds. I run 10 PSI and i actuly had to put a stronger spring on it.
 

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I used this just this last summer and it worked great. Make sure the vacum line only goes on vacuum side not boosted side because it is held closed with a spring not boost like other kinds. I run 10 PSI and i actuly had to put a stronger spring on it.
Nice, yeah that one is like the discontinued Magnuson valve and should work perfectly.
 
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