I used to spend some time with a hybrid - base Impreza with a WRX drivetrain bolted in and really loved the car except for the turbo lag. Below 3500 RPM it was pretty lacking. Now it's well documented that you can get a turbo WRX well above the stock 227 hp, but has anyone tried putting a roots or scroll type supercharger on one in place of the Turbo?

I know that you're going to loose on the top end potential - no question. But I'd think that you could get similar HP to a stock turbo with a broader powerband. For street and auto-X I'd rather have a motor that comes on at 1500 RPM and makes 225 HP, than a motor that makes 275 and comes on at 3500.

Set aside cost, complexity and smog check issues for the moment - what do you think of the concept, and has anyone done it?

 

"Crusifictions?"

"Yes."

"Good, line on the left, two doors down, one cross each."

Couldn't resist.

 

Okay, for starters I must say that I am currently driving a 2000 RS. I sold my WRX a few months ago. There is a lot of talk on the RS forums about the whold supercharger thing. One advantage we have is that our engines are 25% bigger than yours. Our compression is quite a bit higher than a WRX, so when you add a little boost--like the five or so pounds that a small subercharger gives--you can get some pretty nice results (compared to what you had before). One of the conclusions that the other RS guys have some to is that superchargers for our relatively small engines do not make a lot of sense. You would need a really good supercharger to even come close to the power you make on your stock WRX engine. A much better solution would be to get a different turbo for your WRX. A lot of people have said that a P&P on their stock turbo (usualy done by deadbodt @ www.deadboltspeed.com) when coupled with a truly good exhaust system and engine management can yield astounding results. You should spool a lot faster. Try searching the forums for more info on this. Im not sure if he is a member here, but Jerry from deadbolt is a member of NASIOC... If you find out anything on other forums, keep us posted.

 

See the last two issues of Sport Compact Car Magazine for in-depth feature articles on street supercharging for imports. Yes, they can work great but no one has developed a bolt-on kit for the Subaru flat-four that I'm aware of. Packaging the blower is the major hurdle on any installation.

--
0==WW==0
"…axles of evil…" - george w. bush

 

That's useful information. Thanks! I'd prefer a turbo if I was confident that I could get significant boost by 2k, but waiting to 3.5k doesn't appeal to me.

Specifically I'm thinking of another kit car build.

Here's my current car: http://www.smm.com/gallery/Berkeley-Choate-24?page=1

The Stalker put's out 200 HP and 200 ft. lbs. and weighs about 1400 lbs. I'd like to get similar power out of my next build while dropping a couple hundred pounds. Since I don't have to deal with smog, A/C, power steering et cetera, I've got plenty of room to do mods.

So if I go Subaru (which I'm definitely leaning towards) the least expensive way would be to put a N/A 2.5 in there, and I could always add a SC in the future. OTOH if I could get a WRX modded to put out power over a broader band, I'd have a lot more giddiup from the start...

Any other feedback?

 

I wouldn't say with any confidence at all that you could "always add a SC in the future." There are a ton of serious issues involved in supercharging a flat-four motor, which explains why, as WW noted, there is no one making a supercharger kit for any boxer engine.

Just a small sample of the problems:

1) a supercharger is a belt-driven acessory, but none of the EJ20/EJ25 pulleys turn fast enough to actually drive it. There is a LONG and detailed account of a NASOIC member trying to mount a Mercedes "Kompressor" supercharger on a 2.5l N/A block, and that was just one of a huge host of problems he faced; he ended up using outrageously oversized pulleys and gears to get up to the speed the SC expected. Most superchargers turn at 10,000 RPM or more just to generate 3-4 PSI.

2) Where do you mount it? In straight and V-mounted engines, the supercharger likes to sit right on top of the intake manifold, which is why they poke up right through the hood of blown big-blocks; where to mount it is much less clear with a horizontally-opposed block.

3) Boost creep at high RPMs is a very serious problem; a friend of mine has the new Stillen supercharger on his 350z and he has to watch his RPMs and fuel pressure constantly; the engine leans out like crazy and detonates constantly, and I'm certain it won't last six months. He's trying to rig an external wastegate, but it is requiring lots of custom fabrication.

Look up that thread about the Mercedes supercharger on NASOIC, you'll see what I mean.