924Board.org

[B]

Hi everyone
I have been kicking around the notion of switching forced induction paths from the bae set up to a supercharger. Is one typeof supercharger better than another? It seems as though several kinds have been used successfully. Roots type, twin screw, I had even seen one on here with a built in wastegate or bov(I can;t remember which) that would limit boost to about 7 or 8psi. There seems to be quite a bit of the gm setups on ebay for a reasonable price.
I guess the second question then too would be about the intake for the charger - how difficult was this to bring to life? It is hard to tell in some of the photos that I have seen.

Any help and information is much appreciated

Sean
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1979 924 n/a
BAE turbo
5-speed snailshell

[ideola]

Most of the GM ones are going to be way too big, and many of them have a completely custom arrangement where the SC attaches directly to the intake, which is not the kind of arrangement you want. Until I purchased the IHI twin-screw unit from the Millenia, I was very very close on several occasions to purchasing the Eaton unit that comes on the Chevy Cobalt SS & the Saturn Ion Reline. IMO, it is probably the best Eaton available, but I can't tell you how it fits in the engine bay. As it is with the IHI unit I have, I am probably going to need to customize it significantly in order to get it to fit. It is very long, and I haven't yet sorted out how I'm going to get the intake plumbing up past the brake booster & master cylinder.

In terms of small packaging, the best units are probably the SC12 & SC14 units that are common on JDM cars but pretty hard to come by here in the states. You need to figure out what your target BHP is going to be because that will determine what size SC you want. If you're only looking for ~6psi of boost the SC12 is probably fine, but will not do well for much more than that.

I would recommend picking up Corky Bell's book on supercharging before you invest any money. Short of that, in general, the best SC you can get is a twin-screw, but they are typically orders-of-magnitude more expensive, the IHI Millenia unit being about the only one I've found that is available for low $$ used. The Eaton roots style put a lot of heat back into the charge, so you'll almost certainly want an intercooler if you go that route. You could also look at a centrifugal setup, but IMO, that would be pointless since you loose the primary benefit of the positive displacement superchargers, which is near-full boost from idle. You already have a turbo, so switching over to a centrifugal SC would probably be underwhelming.
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erstwhile owner of just about every 924 variant ever made

[simsport]

I have to agree that its a good idea to get the book, it tells you a lot about blowers and selection. Ideola is spot on with his comments and he has used another type of blower to me. For what its worth my thoughts are......

The Eaton twin screw M62 is available as a plain unit for custom builders. It has basic mounting faces for the inlet and outlet and I beleive it to be an excellent unit having thrashed mine for years to give between 10 and 12 PSI flat out.

When I started along this road I wanted reliability and ease of fitting and the Roots blowers are great on both counts as (in the Eaton anyway)they come with a sealed oil supply and optional nose drives and pulley sizes. They also have a bypass assembly that is built in to the latest models.

The downside is indeed heat as when pressed the Eaton blower knocks out a lot of heat, although at lower boost levels its acceptable. I need an intercooler to run at 12psi (13000 rpm).

One of the key problems is finding a unit thats not too big as most are made for much bigger engines than ours. The Eaton M62 is correct for a 2ltr although the M45 may also work. Whipple told me they cannot offer a unit for my engine but Opcon Autorotor in Sweden said they could provide a Lysholm type for about 3000 dollars.....eek!

I've heard the centrifugal type getting good reprots is the Rotrex unit which is very small and easy to mount.

Cheers
simon
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Blown is always best!

[B]

Thanks
I probably will start with the book just to get a sense of what I'm in for, if I go that route. right now it is just a thought that is beginning to become an idea - I thought I would look to those that had experience before I go any further.
when you talk about heat out of these - are you talking in the area of a non -ic turbo? mine now only runs about 6 (I think) - no ic currently and it can get toasty under the hood - so I imagine the charge is getting pretty warm as well

thanks again
Sean
_________________
1979 924 n/a
BAE turbo
5-speed snailshell

[ideola]

You're talking about two different heat gain issues. The turbine on the turbo scavenges heat from the exhaust to spin up the turbo. There will be heat gain under the hood from the exhaust setup, but that has nothing to do with heat introduced into the charge path.

It is a physiological fact that compressing a gas causes its temperature to rise. The more efficient the compressor, the less heat gain will be experienced, but you will have heat gain nonetheless. Conversely, a less efficient compressor will introduce much more heat into the charge air, which is the stuff we're really concerned about because it's what's being ingested into the combustion chamber. Hotter air makes combustion more detonation prone, etc. etc.

You want the most efficient compressor you can get for a targeted horsepower range. You have to match the compressor to the target HP range. A bigger compressor will compress air more efficiently, but will take longer to spool up and may not be suitable for smaller engines. So its a matter of balance. This is where the Corky Bell books will really help you, because they will provide the basis for selecting the compressor size you need for intended use.

As a general rule, relatively modern turbocharging compressors tend to be more efficient than roots style supercharging compressors. As an example, our K26 is about 70% efficient in its optimum pressure ratio range, which produces somewhere between 6-9 psi of boost. By contrast, the Eatons tend to only be about 60% efficient in the 6 psi range. And by the way, the heat gain goes up exponentially (not linearly) with efficiency loss. There's a lot of complicated math involved, too much to repeat here. In any event the twin-screw Lysholm style S/Cs are usually in the range of 80% efficiency, so MUCH better than the roots style, and as good as the most modern turbos.

The centrifugal S/Cs are comparable to turbos, usually in the 70% range + or - 5%. However, I really dislike the centrifugals because they rob HP from in order to produce HP (unlike a turbo which scavenges exhaust to produce HP), and they don't give you any of the other advantages of supercharging, the biggest of which is linear boost, no lag, and boost right from idle the positive displacement style (i.e. roots & twin screw) offer. Since you already have a turbo, going to a centrifugal SC would be a step back, in my opinion. If you were starting from scratch, a centrifugal SC is probably the cheapest easiest path to forced induction on an NA 924, but IMO far from the best or ideal approach.
_________________
erstwhile owner of just about every 924 variant ever made