_JB_

iTrader: (33)

Why is it that turbos seem to get max boost at mid RPMs and centrifugal superchargers do not? Centrifugals and turbos are both RPM dependant, but centrifugal are basicly a turbo on a belt so why are they so different? Can't you make a centrifugal kick in a max boost in the mid RPMs like a turbo with a boost controller or something?

EPP

iTrader: (22)

Turbos build boost off of exhaust flow, the sooner you build flow, the sooner the boost builds. That is why so many people with a turbo use a two step.
Centrifical superchargers build boost off of engine rpm. Installing a smaller blower pulley will bring the boost in much quicker, although overall it will also make much more boost, which can kill a non forged bottom end. Bob

bboyferal

iTrader: (9)

The answer is in your question...

Centrifugal blower has to wait for its belt to spin its fastest for max psi.

The turbo just needs exhaust pressure adequate for its set boost (set through the wastegate spring/boost controller) and then the max boost is there... and that can happen at VERY low rpm, depending on properly sized components, of course.

300bhp/ton

I'm sure this has been asked before, but since search is a little iffy and I don't know the answer I'll ask.

Could you just run a different pulley arrangement on a centrifugal so that it makes more boost early on in the rpms. Perhaphs so it makes desired boost by 2000rpm or something (say 9psi), then have a waste gate arrangment to vent off excess boost after that, so you get a fairly stable boost level (of 9psi) from 2000rpm to the red line?

Frost

iTrader: (45)

You'd end up over-spinning most of the units I've seen if you do that.... perhaps if you picked a unit that was massively oversized for your application...

300bhp/ton

Yeah I guess, but should a D1sc be good for about 14psi on a stock CR LS1. Is so I wonder if you could have it setup for 14psi but via a wastegate limited actual boost to 9psi. Surly this would produce more low and mid end power than simply having a pulley setup for 9psi???

Maybe it just wouldn't work, although I wouldn't mind knowing a technical answer to it.

_JB_

iTrader: (33)

But the exhaust flow increases with RPMs. So shouldn't the centrifugal be the same?

_JB_

iTrader: (33)

That is the question I'm trying to get.

bboyferal

iTrader: (9)

Yeah, but the exhaust flow of an LS1 could easily peak the flow of an exhaust housing/wastegate setting/turbo combination at around 3k rpm., with the boost just staying at that peak through to redline...

Which leads to an interesting approach we have here... someone setting a blower pulley to get 10 psi, for example, at around 3k rpm (peak 20-something psi at redline, enough to blow a set-up), but venting all boost above that so that it stays at that boost... interesting. Maybe it's too difficult? Or with the existence of good roots/twin screw blowers (obviously not for us), then there is not enough incentive for the industry to design these qualities in contrast with a high difficulty?

Avengeance

iTrader: (4)

I asked this Q when I first started contimplating going with a Procharger.

Lets use these figures for example.. 14psi pulley, bleed off 6psi, target 8psi.

I vaguely remember the response.... it was something to the effect that you would have to spin the blower a lot harder, which would inevitably create a lot more heat, and that you would end up using your target psi (8psi) but your charge IAT would be that of a 14psi charge. See what Im saying?

I really wanted this to work too.... but I remember getting shot down about it.

SUPER DAVES Z

iTrader: (6)

The real problem is that the blower takes power to spin. The more psi (and rpm ) you spin the blower for, the more parasitic loss you have along with the additional heat being introduced into the system.

Example, say you have a stock procharger LS1 car with 8 psi peak you get 475 rwhp and 450rwtq, with the blower using 50 hp to spin. Now you put a smaller pulley on it to run 14 psi, which uses say 100 hp (could be more or less, but will be at least 80% higher than the psi losses) to spin plus there will be additional heat. Now you will have more torque but the hp will be much lower at the same amount of boost (that the motor sees) 8psi which could drop to 425rwhp with 550ft/lbs. (these numbers are not actual, but you get the idea)

Basically if you want peak torque at 3000rpm and full boost to redline, turbos are the only way to go.

Awake455

Consider, if you will, throttling the inlet side rather than the outlet side to limit boost. You would be drawing a vacuum in the inlet side which would eat some energy but not nearly as much as blowing off boost. You would still be moving to a different area of the compressor map (with slightly different efficiency numbers) but it could work.

TeeKay

Here's the real reason it's hard to do:
A turbo doesn't "bleed" the output to hold the boost psi. The boost in a turbo is regulated by the wastegate throttling the hot side turbine to a set impeller speed, which controls the boost. And that's independent of engine rpm once boost is in. The SC compressor runs at whatever rpm the engine rpm times the pulley multipication dictates (i.e. the impeller rpm is contantly increasing as engine rpm does). So the SC builds more and more boost as the engine rpm goes up to redline.

The equivalent to a turbo in a belt-driven centrifugal would be using some device to constantly change the equivalent pulley diameter, so that compressor rpm stayed relatively constant. To control the boost on an SC like a turbo, you would need some way to put on a constant-speed drive unit to run the compressor, not necessarily a belt. Maybe something hydraulically driven - I dunno.

Gee, I hope that was clear enough to get my idea across. HTH

thislswon

iTrader: (5)

cant go wrong with turbos period nuttin like boosted fbodies!!!!!

300bhp/ton

Yeah I see what's being said and understand why people don't try this.

In essence an auto adjusting pulley size is what's needed, so you can keep the rpms of the blower constant over a larger rpm range thus producing a more constant boost level and higher boost in the low rpms. Maybe there's som tehcnology from the Constant Velocity gearboxes that could be utilised, but hay I guess it liely to be big, heavy and complex.

As for turbo's I agree they are the ultimate, however to do properly it'll set you back a heck of a lot more than a Pro-Charger will.

Also I'm still not a fan of LAG (N.B. as in spool up time after a stall period NOT where in the rpm is max boost made) and am not convinced I'd like how a turbo setup drove cross country, point to point as it where.

Raped-Ape

iTrader: (1)

East coast supercharging makes a restrictor plate that goes in the intake inlet of the centri blower inlet. it helps get the boost on quicker , yet limiting the top end boost. i think they have the novi2000 blowers makin 9lbs @3000 and holding it . without the restrictor it would make 14lbs i believe.
hope that helps.
-jonathan

bboyferal

iTrader: (9)

HELLO!!! There we go... IF you're right about what you're saying down to the NUMBER about that set-up, then it's clear some have done this... Now, how would the foregoing problems mentioned about heat and inefficiency come into play assuming this is accurate?

TeeKay

The clue to British is bhp/ton! lol.

A real advantage of the turbo on a street car is that it drives almost like stock point-to-point.

Raped Ape, that's good to know. It's worth a check-out, thanks.

300bhp/ton

Driving gently I'm sure it is, and in a straight line at WOT I'm also sure it's great, but driving quickly on British roads requires a lot of on and off the throttle pedal and cornering on a trailing throttle.

I've driven several turbo cars and been in many more and they all suffer from throttle control (or lack of it) due to LAG. Also having watched several video's of turbo LS1's they seem no different.

Turbo = awesome power & accelaration.

But it does have drawbacks in terms of driveability hence rally cars HAVE to use anti-lag systems (ALS).

H8 LUZN

a few of you hit it right on the head. its all about efficiency! Power in=power out-losses

The supercharger/compressor system is inherently isentropically inefficient anywhere from 60% to 80% depending on the type which means that its heating the air more than neseccary for the desired boost level. It takes power to compress a certian mass flow of air to a certian pressure, and then you calculate in the extra temperature gain of the inneficient supercharger/compressor.

Also, the problem with adding a restrictor is you are heating the air more than nessecary, basically your pressure ratio (sc outlet/sc inlet) is high which means you have extra temperature yet still low boost so basically its pointless and bleeding off the excess air may be a better idea depending on the efficency range of the supercharger.

But basic laws of phyisics is that you cant create energy only hope to optimize its use for a given situation. any time you bleed air, or add a restrictor you are decreasing the efficency of the system and as a result you are wasting energy.

The best idea mentioned here was the variable pulley which basically ensures the system is only using as much energy as it needs and not consuming extra energy needed to bleed off air.