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Quick easy BBSC question......

Brian,
If by chance you broke a belt and they did not have the right length of belt in the middle of of nowhere, you could run use (purchase) a shorter belt on the car. Basically, bypass the SC pulley completely to get where you needed to go. The car will run fine.
 
would this be a useful manuever for emissions, or (arizona) will it be a non-issue... i.e. i'll still pass, or they will take one look and say "uh oh, you messed with the engine, you automatically flunk.."
 
Yes you can!

Actually, you can turn off a supercharger. Check out the MB AMG superchargers. They use a magnetic clutch, just like our AC compressor to turn the screw or not according to the ECU.

But in the long run, it is not really needed. Not much parasitic losses with the NOVI 1000, and you only make boost under load.
 
I want to see someone hook up a continuously-variable drive to a supercharger, so it spin the supercharger at a higher ratio at lower engine speeds.

-Mike
 
kpond said:
For the SC, boost is tied exclusively to rpm. Turbos produce boost under load.

Nit, nit, nit.

Kendall,

Just trying to learn here, but I thought that even with a belt driven blower, that there is very little boost at constant throttle? I have watched a fellow tracker's boost gauge as we blasted down the front straight of Roebling Road. Six plus pounds of boost during the first part of the straight (while we were accelerating), but would drop to zero as soon as he started lifting the throttle, even though the RPM's were still very high.

'splain, 'splain, please:confused:
 
ncdogdoc said:
Kendall,

Just trying to learn here, but I thought that even with a belt driven blower, that there is very little boost at constant throttle? I have watched a fellow tracker's boost gauge as we blasted down the front straight of Roebling Road. Six plus pounds of boost during the first part of the straight (while we were accelerating), but would drop to zero as soon as he started lifting the throttle, even though the RPM's were still very high.

'splain, 'splain, please:confused:

When you lift of course the throttle plate starts to close so I would guess that a valve is opened to prevent the boost from "dead-heading" against the throttle plate. Solution: Don't lift. :)
 
Hi Gary,

See, I've turned in to a nit picker!

The SC is driven by a belt and pulley and there is absolutely no magic that at a given RPM the SC produces the same amount of boost *almost* regardless of load. For example, in a supercharged car at 4,000 rpm with 3/4 throttle, you're going to have the same amount of boost as in the same scenario at full throttle. I am conveniently assuming more throttle = more load.

In a turbo application, the above scenario is much different. The speed of the turbo varies with engine load. So, a turbocharged car - let's take an NSX, for example, can produce maximum boost levels at or possibly even below 4,000 rpm. If you cruise along in a turbo'd car and gently accelerate from 3,000 to 4,000 rpm, virtually no boost is produced. Do the same 3K-4K run at full throttle, and you can have full boost.

OK, back to the SC and to the example you cited. Virtually all SC applications employ a very simple boost bypass device (typically a Bosch part) that sits in the intake line between the SC outlet and the engine's throttle body. There is a traditional engine vacuum line that connects to this bypass valve. Vacuum from the engine will hold the valve open and as soon as vacuum goes to zero, the valve is closed (no bypass).

As a BBSC car, for example, sits at idle, the SC is still spinning pretty good creating a volume of air with nowhere to go. No worries, vacuum is high and so the bypass valve is open and the air just escapes out the bypass valve before the nearly-closed throttle body. Now, go for a drive in that BBSC car, mash the gas pedal and the vacuum goes to zero almost instantly... until boost starts to build at 3500 rpm or so. Keep that throttle mashed, and you'll see 6 psi at 8K RPM. Ooops, suddenly you see the black and white in the approacing intersection and you let off the gas pedal - still turning somewhere north of 7K RPM. The throttle plate slams shut, engine vacuum instantly returns and sucks the bypass valve open. The SC is still spinning like an SOB (with the same amount of work by the engine), but the compressed air is vented out the bypass valve.

OK, so there has to be a moral to the story. That moral is that all other things being equal - a turbo is a much better answer than a SC to the NSX HP deficiency woes. A turbo will give boost at lower RPMs (where it is needed) while simultaneously complementing the high-end performance.

If you size the SC pulley to give you 'x" amount of boost at 4000 rpm, you will have approximately '2X' (or more) at 8k rpm. This is a generalization (for all of the fellow nitpickers), but I use it to illustrate the concept. In current SC applications, if you want 4 psi at 4K rpm, you will likely have 10psi or more at redline which is usually undesireable.

However a properly designed turbo car can give you full boost way down low and still not blow out your top-end boost. The turbocharger, unlike the SC achieves this largely due to its characteristic responsivness to load.... which the SC doesn't have.

I hope that helps.
 
kpond said:
Hi Gary,

I hope that helps.


Thanks so much, never to old to learn!:D
 
If you hold a SC at a given rpm the bypass valve will eventually shut it off. So because of this the SC will only work under load as well, boost is still entirely rpm dependent.

So if you’re cruising down the highway at 5000 rpm in third gear you won't be under boost until you mash the pedal more.

With a roots and twin-screw blower the air cannot pass thru it unless it is turning. This is why if the belt brakes you’re not supposed to drive them. I guess if you wanted to limp back you could hold the bypass valve open, but it isn't something I would want to do. With a centrifugal, the blower is like a big fan, air can still pass thru it even if it isn't spinning.
 
What the "easy bypass" comment refers to, is that if you had, say- an idler pulley go bad on a road trip. The factory alternator belt (which can be kept in the trunk) can go right back on and driven that way. The engine breathes okay thru an "idling SC- only costs about 12hp on our dyno "what if" testing.

MB
 
Now, go for a drive in that BBSC car, mash the gas pedal and the vacuum goes to zero almost instantly... until boost starts to build at 3500 rpm or so.

Mine begins at 2500 RPM.

If you size the SC pulley to give you 'x" amount of boost at 4000 rpm, you will have approximately '2X' (or more) at 8k rpm.

Technically, incorrect. Centrifugal machines operate under a mechanical engineering term called "The Affinity Laws". This is a simple family of equations that let you calculate various parameters at different machine speeds. Pressure is related to the square of the RPM ratio. Therefore, in the above example there will be 4x pressure.
 
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With a roots and twin-screw blower the air cannot pass thru it unless it is turning.

I have seen an NSX with a CT blower on it (sans the belt) at engine idle that still runs, because it is strong enough to pull a vacuum under the blower, to cause the screws to spin, albeit rather slowly. (Say, has a grammar teacher told Saint that he uses commas too much?) Point is that you could drive the engine to safe haven w/o the belt on a positive displacement blower.
 
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I work for Mr. Basch, We in fact run the BBsc's w/ the sc. belt off for the first 500 miles to break in the new low compression motors that we're building. Its ok to take off the belt, but you'll need a stock belt to run the alt. Also a stock belt has 4 ribs and the sc' belt has 5, besure to line them up correctly!
 
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