blow through roots charger...why not? 3

[deraffe]

I have a 1991 BMW 325i with a 2.7lt "stroker" engine (885 casting cyl head, 292 degree cam, long tube headers, 10.2:1 comp,82mm stroke, lightened stock rods, cleaned up intake and exhaust ports/chambers, balanced rotating assembly).
The engine is currently running Dbilas 45mm ITB's (individual throttle bodies) and i am building a mega squirt EMS for it.

Once the engine has had enough break in time, I would like to add a M-90 gen 5 Eaton supercharger.
the superchrger is from a 2002 (?) Buick Regal and has a built in by pass valve. I am planning on running a max of 12 lbs boost. I am also going to be running an air to water intercooler.

In re reading Corky Bell's supercharger book, I noticed that he states that roots superchargers should be used in a draw through configuration. I how ever would like to use it in a blow through application though. I can not find a reason stated anywhere to explain why he makes this statement. Can anyone give me a reason why this will not work or is less than ideal?

thank you for your help.

 

[MikeHalloran]

What Corky is probably thinking of is that putting the fuel in before the charger reduces the polytropic exponent in
the charger, so the charge comes out a little cooler.








Mike Halloran
Pembroke Pines, FL, USA

 

[deraffe]

OK, the fuel is still going to go in to the engine in the same spot(s) I am just going to have the charge (if that is what you would call it since there is no boost pressure inside the roots) come out of the supercharger, into the intercooler, then into the ITB's via an airlog.

The Dbilas have sealed throttle shafts and can handle 2 bar according to the manufacture.

I also have a vacuume activated blow off valve, will that be needed (or a good idea to add) to prevent throttle plate damage? Will it just be redundant as I already have a built in by pass valve on the supercharger. I realize the by pass valve is meant for "crusing" or low vacuume situations, but does it not also preform a similar function?

 

[MikeHalloran]

It's possible to gear the Roots so there's no pressure gain, but there's no point in doing so. It's an air pump. It acts a compressor by stuffing air in faster than the engine can take it out. The compression actually takes place in the gaps between lobes as they circulate around the outside of the housing, which will get hot when the manifold pressure goes up.

You need a _big_ blowoff valve, usually a plate secured with springs, between the Roots and the engine, to prevent damaging the Roots when the engine backfires.

Some systems use a bypass valve around the Roots, that allows the engine to draw air even if the Roots stops. It may not make a difference in normal operation. It is _not_ the same thing as a blowoff valve.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

If you close the downstream throttles and the bypass sticks closed, you bend the throttle blades and get a runnaway engine.

Do a google search of the site. This has been done to death already, as twincharging I think.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[deraffe]

OK, thanks for the clairification. I have a Vortex blow off valve that is designed for systems makeing in excess of 2 bar. At a max of 12 psi i should be safe running this. as if i try to turn much more boost out of the supercharger i come close to exceding the rating for the bearings.

i am familiar with turbo charging, but this my first venture into supercharging, so i just wanted to clairify some points.

thank you again.

 

[BrianPetersen]

Depending on how you control the bypass around the supercharger, a blow-through configuration could give very high pumping losses at part load (if the bypass valve doesn't open until a set pressure even at part load), which will kill fuel consumption and heat up the intake charge. If the bypass is mechanically linked to the throttle, it shouldn't have this problem, but there might be some interesting peaks and valleys to the throttle response behaviour.

 

[deraffe]

The by pass is vacuume activated as it comes from the factory. That is why I am using a 5 gen found on most of the GM models as opposed to the earlier ones found on Fords like the thunderbird that have nothing.

Did I make things more complicated for myself with this choice?

 

[BrianPetersen]

If you make it so that the bypass is controlled in the same way it was on the OEM application of that supercharger and the throttle is positioned the same way in the flow stream, it should work equally well. Was the OEM main throttle in front of the supercharger, or behind it?

A vacuum operated bypass might not act in a fail-safe manner if you have a leak in the hose leading to the bypass actuator.

 

[deraffe]

Orginally the throttle body was located infront of the supercharger (it actually bolts to the rear of the housing/inlet just before the bypas that is located in the housing).

I am no so concerned with the by-pass being a fail safe as much as it helping with fuel economy at low vacuume or cruise. I am treating the blow off valve as more of the "fail safe" to keep from damaging the ITB's (individual throttle bodies) that I can only get from Germany at great expense and to keep back flow to a minium when the throttles are closed. I assume that it will act in a similar fashion to the way it works with a turbo charged system to reduce lag and back flow also.