SC silly question on boost

[M3Armand]

Sorry guys....I'm know a bit more on turbos than SC's. So here's a silly question...

My understanding of SC's is that the pulley selection makes maximum boost close to redline....right? This makes the boost nice and linear all the way to redline, right? But the problem is, you only make the 8psi (or so) boost at redline - meaning that you don't really use the 8 psi up until then... So...(here's where you experts come in and show me the light), why don't tuners select the pulley to develop max boost at, for example, 3000 rpm and then add a "wastegate" to bleed off the additional boost from 3000 to redline? I understand that there could be physical limitations of the supercharger that it would be spinning way too fast past 3000 rpms. But I suppose that they could also design some form of "slipper clutch" that will allow the supercharger to "freewheel" after developing 8si or after going past 3000 rpms... :eyecrazy:

Thoughts? HELP! ....thanks....

Armand

[markesq]

That sounds like more of a turbo setup. Hence no reason to mess with a S/C configuration.

[M3Armand]

Yeah, but with the SC, there is ZERO lag. When you're on the gas, it's on. It's instantaneous and it retains the throttle response as a normally aspirated engine. You don't have to wait for anything to spool. If you twitch the gas, the SC twitches with it since it's driven by a belt. My point was, with a "wastegate" or some sort of "freewheel" action when the SC gets to a certain RPM you can have max boost from 3000 rpm all the way to redline instead of 2 psi @ 3k, 4 psi @ 4k, 6 psi @ 5k and 8 psi @ 6k (or something like that). Wouldn't you rather have 8 psi flat @ 3k all the way to 6k or 7k?

[InterSpool]

This man is a genius. I think that the pulley would have to be 30 feet long. Someone answer why this isn't possible :)

[M3Armand]

Well...no need to be sarcastic. Just asking a silly question that I'm sure someone's got the answer to. But since you made the comment that the pulley would have to be 30 feet long....what makes you say that? I thought the pulleys were round...

[InterSpool]

Whoops I meant the SC Belt as I'm sure the pulley would have to be huge to produce torque that low. Actually, I wasn't being sarcastic at all I'm just as curious as you are!

[Solid77]

I think you would want a smaller pulley for more boost..

[dingfwen]

I think this is actually a legit question. It's something I've wondered for a while too. I've also seen someone mention a BOV on a SC here on these forums.

I did a google search on the topic and came up with 70% misinformation and 30% seemingly logical information. Here's the basics:

-- Turbos have larger range of boost from idle to max power. Therefore, they must be adjusted to achieve max boost at mid-rpms and then regulated w/ a waste gate. Superchargers have a smaller range of boost from idle to max power and therefore, can be adjusted to achieve max boost at max-rpms w/o losing too much on the low end. This saves having to have a waste gate. Granted, this is also why SCs have less low-end than turbos.
-- My conjecture is this. When a turbo opens a waste gate, it is not wasting any of the engine's energy. It is simply not harvesting any more energy from the exhaust than what max-boost requires. When a SC opens a waste gate, however, it is using the engine's energy to pump air out the waste gate. Therefore, this energy is completely lost. This inefficiency at high-rpms might make it better to sacrifice low-end boost for lack of a waste gate.
-- I think cost-benefit ration for SC waste gates/boost controllers might be worse than turbos.

Okay, now that I've BSed 3 paragraphs, someone who actually knows something about turbos and superchargers please respond.

[RAiMA]

Sorry guys....I'm know a bit more on turbos than SC's. So here's a silly question...

My understanding of SC's is that the pulley selection makes maximum boost close to redline....right? This makes the boost nice and linear all the way to redline, right? But the problem is, you only make the 8psi (or so) boost at redline - meaning that you don't really use the 8 psi up until then... So...(here's where you experts come in and show me the light), why don't tuners select the pulley to develop max boost at, for example, 3000 rpm and then add a "wastegate" to bleed off the additional boost from 3000 to redline? I understand that there could be physical limitations of the supercharger that it would be spinning way too fast past 3000 rpms. But I suppose that they could also design some form of "slipper clutch" that will allow the supercharger to "freewheel" after developing 8si or after going past 3000 rpms... :eyecrazy:

Thoughts? HELP! ....thanks....

Armand

The example given above is based on a centrifugal setup. Roots and Twin screw superchargers work differently by providing max boost down low, but top end is limited compared to centrifugal superchargers.

To answer your centrifugal question above, if the car was making max boost at 3,000 rpms, by the time the car reaches 6000 rpm, the supercharger is overspun and would wear out a bearing very quickly. Overspining the supercharger also makes the blades on the impellars less efficent. The blades would end up chopping the air, instead of flowing the air efficiently. The end result would be less boost being produced.

I hope this helps answer the question.

[markesq]

I would also think that to achieve this type of post at such low rpm, the pulley ratio would be crazy. You can't go much smaller on the S/C pulley, that leaves the engine pulley being like what, 12 inches?

Also, this means that at say 500 RPM you would be producing boost and maxed out be 3,000 RPM. Besides over spinning, I am not sure the numbers would work.

The wastegate, however would probably work, if you get the other numbers to work.

[stimpee]

The example given above is based on a centrifugal setup. Roots and Twin screw superchargers work differently by providing max boost down low, but top end is limited compared to centrifugal superchargers.



On a Roots I agree, but with the twin-screw, it is AS efficient as a centrifugal at high rpm, and in some cases MORE efficient. My HP curve keeps going upward all the way TO and THRU redline. If I had a 500rpm higher redline, I could use it quite effectively with the twinscrew...

Steve

[stimpee]

BTW, this topic has been discussed and hypothesized over before, so do a search and you will find more comments. Essentiall the answer is that it CAN be done, but it will get complex, and will not be very efficient once you start dumping boost. At that point, you might as well deal with the "indirect" response of the turbo, or go with a PD
blower...

Steve

[m3boost]

In order to maintain your 8 psi at 3000 rpm, it would have be making 64 psi at 6000rpms. Why mess with the Cent. SC when you can just go with the PD supercharger.

[stimpee]

Yeah, what he said!!!

Steve

[cal_guy19]

You wouldn't necessarily need a huge pulley. S/C's have gears inside them, so they could be designed to spin the impellers at higher speeds for a given engine RPM.

Also, boost is approx proportional to the square of the RPM, so the boost at 6000 RPM
is 4 times the boost at 3000 RPM.

A waste gate in this application would be, pardon the expression, wasteful. If it were set up to bleed off any boost over say 10 psi, your S/C is still pressurizing a bunch of air up to 10 psi boost, only to have it bled off. This saps a lot of power from the engine, kind of like running a huge air conditioner compressor full out, with your windows wide open. Also, a centri S/C designed to give full boost around 3000 RPM
would have a lot of inertia, so low-end overall response would be compromised (again, like a big old air conditioner compressor, but always full on).

Positive displacement S/C's address this limitation. The blown MB's use PD superchargers and get lots of power from them.

[InterSpool]

^^ Makes sense.

[1mpowerme]

yep.

[RAiMA]

My HP curve keeps going upward all the way TO and THRU redline.

rpms would do that, regardless if a supercharger was there or not.

Getting back to the original question, why not just supercharge a car correctly? Also keep in mind, the more boost the supercharger produces, the hotter the air temps become and also the oil trying to lubricate the bearings.

Depending on how the centrifugal system is setup, 7.5PSI can be acheived @ 4000 rpms and 16PSI @ 6500rpms.

[stimpee]

rpms would do that, regardless if a supercharger was there or not.



No it would not, not if the torque curve is falling off at a rate higher than rpms are increasing, which often happens with roots blown cars, or cars with undersized turbos...

[BoostMe]

I know this would take a lot of work to make but it popped into my head when I was reading this thread

what if you could make a variable drive system for the supercharger so at low rpms you could have a drive ratio that let you have say 10 psi at 3000 rpm and then as the rpms rose the drive ratio would change accordingly so you could maintain 10 psi all the way up to the red line

how does that sound