Supercharger extra boost blow-off | Ford Explorer Forums - Serious Explorations

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Supercharger extra boost blow-off

All -

I have a quick "why don't they do it?" question... (or maybe they do, heck, I'm just starting to research these things...)

In a turbocharger, the turbo spools up and creates boost. If extra boost is created, but beyond what is allowed to the intake, it is blown off.

So, why don't superchargers put a higher boost pully on (and I assume create boost at lower RPM) and blow off excess boost? ... say a 10lb pully but only allow 6lb?

Please correct me if I am confused! Maybe this is already done...
Thanks!
Brad
 



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If you only want 6lb, you put a 6lb pulley. I would assume thats why.
 






bkantor said:
All -

I have a quick "why don't they do it?" question... (or maybe they do, heck, I'm just starting to research these things...)

In a turbocharger, the turbo spools up and creates boost. If extra boost is created, but beyond what is allowed to the intake, it is blown off.

So, why don't superchargers put a higher boost pully on (and I assume create boost at lower RPM) and blow off excess boost? ... say a 10lb pully but only allow 6lb?

Please correct me if I am confused! Maybe this is already done...
Thanks!
Brad

What controls your boost in a turbo setup is the waste-gate not the blow off valve! The blow off valve (which is not necessary on the turbo and its sole pupose it to keep impeller speed up on a 5-spd to reduce lag in between shifts)only comes in when there is a high vacuum situation but does not regulate boost it only blows off the excess boost after the throttle blade is shut hence the "whoosh" after the throttle is let up. Turbo's regulate boost through the waste gate which has a mechanical spring to determine the amount of boost you have, the heavier the spring the longer the wastegate stays shut and the more boost is built before your wastegaste bypasses the exhaust gasses past the turbine housing slowing turbine speeds and reducing the boost. The blow off vents "cold side" air and the wastegate bypasses the exhaust or "hot side"air. On the other hand the supercharger is regulated by a pulley and some use bypass valves which work similar to a blow off valve for efficiency. :burnout:
 






where did you get that smiley? thats awsome
 






I like the smiley too.

So, if I understand correctly, if I could install a wastegate, "pressure regulator" or a way to evacuate extra boost I could in essence put a higher boost pulley on, protect my stock engine and gain more boost at a lower RPM.

I'm just thinking this could work really good with a centrifugal blower...
 






In theory this is a good idea.

In practice, it may be more trouble than it's worth. Think about all the plumbing you have to do to make it work. Also, in order to gain a lot more boost at lower RPM with a centrifugal, you have to:

-run a very very small blower pulley....it would probably have to be custom made and then since our motors have a 6-rib belt setup you run into belt slipping issues with a very small blower pulley.

OR

-Have an overdrive (larger) crank pulley made custom for you.

Having boost at low RPM is nice especially if you have very little low end torque. My take on this is that the 4.0 already has a pretty good low end (i.e., you will generally make 90% or higher of your "off-boost" peak torque by 2000RPM). That being the case, I have the opinion that the 4.0 really needs help up at the top end (either from a centrifugal blower, intake/exhaust mods, head porting, camshaft or all of these).

You could always add a small nitrous shot to make up for the lack of boost down low on a centrifugal....that's what I'm working on now :) Should be interesting to say the least....hopefully I won't lift my heads off the block with the extra cylinder pressure (since I am already pushing 7psi of boost and am about to step up to 9-10psi :D :D :D )
 






A wastegate cannot be retrofitted to a supercharger. It is only for turbo's! Check previous post of mine for bov and wastegate operations.
 






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