Blow through vrs draw through screw supercharger

[TTmotors]

I was wondering if it would be possible to run a blow through screw supercharger. It seems throttle response would be greatly enhanced by going to blow through ITB`s where positive pressure is waiting on the throttle plates to open. If not this a couple of large throttle plates on a sealed pre plenum opening up into a plenum for the runners to keep losses down in the runners.

The engine is designed for race, fabrication and cost are not a factor.

I was also considering an electronically controled throttle plate before the supercharger to keep pressure down so that it would be drivable at a cruise without over responding to the constant positive pressure.. Or an alternative might be a boost drain off valve. It seems mounting the supercharger like this would result in terrible part throttle efficiency. Continously venting pressure into the atmosphere. Although there must be drag associated with reducing the blower`s adiabatic efficiency as well. Im unaware of the nessasary tools to figure the difference in efficiency loss when compared to part throttle blow through vrs draw through.

Im primarily concerned with optimal throttle response, and top end power.

-Thanks-
Travis

 

[SMOKEY44211]

Although it doesn't mention it in your post I'm guessing you are considering carbureted fueling. The only advantage I can see is that you can use a smaller carb to achieve the same output. Fabrication (especially if you are considering implementing some means of intercooling) is going to produce a rather interesting looking contraption. Throttle response may be a little sharper. Adding blower intake restrictions or bleeding off some boost will actually hurt driveability once you get it tuned up for full boost. If you end up with a large volume of air between blower and carb inlet you might end up with a mini bomb should you experience a backfire after shutting down, heat soak, and trying to restart it. I've never seen this type of setup for a positive displacement blower in any automotive type applications. If you dig around you might find a piston aircraft engine that did. If you do decide to go this route I want to see the finished product.-------Phil

 

[TTmotors]

Im actaully going with an autronic or motec fuel management system with staged injection. What I thought I could do for sensors is that I can mount small "vacuum" hoses from in behind the throttle plates of every cylinder into a small alumnium box thats centralized between them (making sure the rubber tube lengths are the same in order to get a fair average pressure reading)and mounting a map sensor in the box. This would also aid in throttle response by getting the pressure signal to the computer as quickly as possible to get the nessasary transien fuel and spark tables activated. With a light wieght flywheel and decently light wieght rotating componets it would seem the throttle responce would be upto par with about any vehicle out there.

Im not sure the itb`s will work yet though. I certainly dont want to put together something that isnt going to work.... properly.

Any idea on what camshaft specs would lead towards producing the snapiest positive displacement throttle response?

 

[Majik]

You haven't mentioned the engines configuration yet? Is this like a v8 or a inline 4 or what.... sounds like your trying to boost weber IDA's almost... what are you dealin with on this beast.. I know from several who have boosted dcoe's on inline 4 engines.. they run well except the engine is very critical of throttle response... fine for a race condition but tempermental on the street... as for camshaft specs on your engine... well that has alot more to do with the size of your engine... CR, valve size... port size... type of engine... etc etc.. gonna need more info

 

[TTmotors]

Its the 3.0L Jag/mazda engine flows 270 cfm at .400 inches through a 35 mm valve. It pushes 230 all stock and 200 in torque at the brake. Im leaving the head alone and going for 8:1 compression ratio. Running an air water intercooler, all going into a 2200 pound lightened focus chassis with a higher then stock gear ratio. Main thing im looking for is alot of fun street driving and runs at a local eigth with runs in the quarter everyonce in a while as well. Its not my main vehicle so im planning on going really crazy with it.

The engine has a serious problem with short circuiting or overscavaging n/a so itll probably be worse witha blower. I can custom make the throttle bodies to any size nessasary and then im probably going to position a set of light injectors like 19lb`s up front spraying down on the valve with a set of 50`s upstream behind the plates. Current cams are stock. Probably running the whipple 3000 cc per revolution blower. Intended redline is arounnddd 7300 even though piston speed is VERY sane at that rpm because of the short stroke I could go as high as 7700 but thats the upperlimit without going dry sump. Bottem end will be built to handle what it needs to. This is more of a theoretical question then an engine specific one. But thats what im gathering information for. I can build pretty much anything that needs to be built so thats not a problem either.

 

[TTmotors]

Adding onto the last post... Some basics i left out... Bore is 3.503 stroke is 3.13. I havent cc`d any intake ports. Exhaust valve is a 32 mm flowing 230@400. Approxiamate id for intake ports, 1.85 ovalized. Valve train assembly is a roller follower setup with 1.8:1 rocker arms. Thats about all I can think of thats important.

 

[AboveRedline]

TT,

I agree theoretically you'd get better response from the system you are considering. But if the blower can pump 3000cc (one displacement) per revolution how much clock time will you be saving?
If you keep it standard, with a throttle body in front of the blower your life will be much easier. Those flow numbers are awesome, the car should really fly!

 

[AboveRedline]

TT,

I agree theoretically you'd get better response from the system you are considering. But if the blower can pump 3000cc (one displacement) per revolution how much clock time will you be saving?
If you keep it standard, with a throttle body in front of the blower your life will be much easier. If you need to do something trick, then add an intercooler. Those flow numbers are awesome, the car should really fly!

 

[patprimmer]

I agree with AboveRedline

Lag is not an issue with positive displacement blowers, in fact even with a convectional setup, the response is amazing, even alarming if you are not prepared for it.

Throttle plates downstream of the blower are more prone to shaft deflection. Even when the throttles are in front of the blower, throttle shaft deflection can cause an unstable or rolling idle.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[TTmotors]

On PD intake manifold it would seem that a large plenum size would be pretty much meaningless because of the type of blower. THe smaller the better in this case? *to a point of not limiting flow to the runners.