flatlander757

I was just wondering if this is at all possible and what problems would arise...

To hit on a few points first:
-Turbochargers use wastegates to bleed off excess boost along with boost controllers to make sure it doesn't keep adding boost at high rpms(ie: doesn't give more than 14psi even though it could)
-Centrifugal SCs only hit the advertised boost levels at high rpms, and increase linearly
-To my knowledge centrifugal SCs only use the wastegate when it is decellerating or something(I'd like to know the real scoop on this as well)

Proposed question:
-Would it be possible to put the biggest F-in pulley on a centrifugal supercharger say replace an 8lb pulley with a 14lb pulley, but use a wastegate in a similar fashion to how it would be used on a turbo to limit boost to 8lbs? I believe the result would be more power under the curve because instead of hitting 8lbs at 6000rpms with an 8lb pulley, it would hit 8lbs of boost at say 3200rpms with the 14lb pulley, and after it has achieved 8lbs, the boost controller would bleed off the other 6lbs of boost it would normally be exposed to during the run.

I suppose the only bad thing would be that it would put added stress on the crank snout(though I'm not positive since it won't actually be pumping more than 8lbs... just spinning faster), and likely worse gas mileage. This would probably be more of a thing to do at a drag strip.

Ideas?

MyShibbyZ28

Good idea. I don't see why it would not work, in theory. Just need to find a way to hook up a wastegate. But at the same time, I would think it would be just as easy to get a real turbo with a wastegate, instead of have a SC with one. They both have their draw backs. Depends on which creates more drag/back pressure on the engine I guess.

SC creates power faster...wastegate makes that power even faster if boost is higher...creates drag on engine.

Turbo creates power a little before max rpm...uses a wastegate also...creates backpressure on engine.

hmmmm

flatlander757

I mean, if you have a SC that limits the amount of boost you can run(I think the powerdyne is kinda limited to 450rwhp or so I have read around here), it would be cool to have that max boost for a longer period of time.

And it should still be cheaper than a turbo because there is so much extra crap you gotta do to make sure your turbo isn't gonna blow up lol.

I think the main problem would be finding a wastegate that is smooth enough to slowly open the diaphragm(or whatever the **** it uses lol, I'm not sure) to ensure a steady 7.5-8.5psi once it has maxxed out.

And as a cool bonus, it may make the SC whistle like a freakin teapot OR you could put a turkey call over the wastegate like these guys did with the BOV:
http://videos.streetfire.net/Player....6131&kw=32&p=0

Except since air is constantly escaping it would only do it at WOT which would be kinda kick ***!

MyShibbyZ28

hahahahaha

That would be hilarious.

Had a thought too, is it possible to get so much ridiculous boost with a wastegate, with an SC that the boost would start almost right away, so at like 1500 rpm. That would be awesome if someone could manage 450 hp and 500 tq at 1500 rpm on a 350.

markinkc69z

I can appreciate what you're trying to do, but you need to understand how the systems work.

Both units are centrifugal superchargers. The turbo is exhaust/piston driven and the "supercharger" is belt driven. Both take rotational - flywheel power to operate.

Centrifugal superchargers consume and move air. The rpm of the impeller determines how much air it moves. The wastegate on a turbo bleeds off exhaust gasses. Bleeding off the exhaust slows down the turbo impeller which drops the airflow. The wastegate which is referenced to manifold pressure, tries to maintain a pressure equalibrium in the system. If the turbo/wastegate sizes are matched properly then it is possible to maintain fairly tight control over the impeller rpm over a certain rpm range. As the engine rpm climbs your airflow demand increases and the wategate valve progressively adjusts to deal with the changing airflow demand of the engine and proportionate increase in exhaust volume. The pressure bled off is not on the intake side. A valve on the intake side is what I refer to as a blowoff or popoff valve such as what used to be used on indy cars. That is a poppet valve that opens at a predetermined pressure to ensure "competitiveness". This is different than a surge valve.

A surge valve opens when your throttle closes under boost to protect your plumbing and supercharger. While under boost you may only see X psi, when you slam the throttle shut the supercharger is moving air, and it doesn't care that the throttle is closed. Pressure can really spike out of control and remove tubing connections, inflate intercoolers and damage the supercharger as the pressure finds a way to escape.

The supercharger requires fuel and power to rotate and move air. Remember that it does not care, in the case of the belt drive suipercharger, whether the engine is under a load or not. It is just concerned about impeller rpm. If your supercharger is moving 1400 cfm of air but you want to bleed off 500 cfm of it you will still consume the power required to move that 1400 cfm. You will also lose a bit of the rotational assist that occurs below a certain rpm that is given back by the supercharger as it pushes on the piston during the intake stroke. (this lessens the parasitic drag of the belt driven supercharger under certain conditions and is not replicated by a turbo charger)

To do as you say requires you to look at the other end of the system. You need to reduce the volume of air pumped by the supercharger, effectively cavitating it. Thermal efficiency will take a hit, but it can (and has) be done.

flatlander757

Thanks for the great information!

So basically it's not a wastegate that would be needed, it would in fact be a surge valve? Correct?

The reason I thought the parasitic drag wouldn't increase a whole lot was because I was under the impression that more boost on superchargers = more force(pressure) that the impeller has to work against, and since it would only be spinning faster and moving more air at any given RPM, heat would be the main thing to that would be sucking power with the additional air causing just a bit more drag?

I'd like to see some dyno graphs of how this would play out, or at least how it should look relative to N/A, regularly SCed, and my idea of "overfeeding" it at low rpms

If it is in fact mostly drag that slows it down, it would probably be in your best interest to choose whatever blower that has the least parasitic loss?

We gotta go "gobble gobble" as we pass whistling Cobras

markinkc69z

Moving air is the real power absorber. Its the work that's being done. Pressure is a residual effect of moving the air as its being backed up in the intake system. The same airflow will produce differing boost levels on different engine combinations. A surge valve isn't what you're after. You need to effectively lower the pressure (below atmospheric) in the blower inlet.

flatlander757

So it would necessitate an intercooler... or two?