The clutch is not the problem. If you have parallel systems you have to isolate them. That requires a means to sense the pressures in each system and in the intake, and solenoids/throttlebodies to restrict or cut off flow the the system providing less flow and/or pressure.

If you just hook up the two systems in parallel, the super charger will backfeed the turbo. Once the super charger clutch opens, the turbo will start to spool but won't build pressure because it will all backfeed through the supercharger.

Notice that I said rear turbo. The air charge from the rear turbo would be cooled significantly before it reached the supercharger. The slightly warmer, much denser air wouldn't have to be compressed as much by the supercharger to get a desired pressure so less heat would be added by the supercharger. You could run a clutched supercharger to elliminate the double heating at higher rpms if you wanted, which would be better in a non-intercooled application. A clutched SC would also be needed if the systems were in series and there was no cooling between the turbo and superchargers.

Tony, I know you were kidding (but you're thinking about doing it). For those of us without enough money to build our ideas, this mental masturbation is all we've got!

 

I was going to do a "twin charge" setup on my audi. I was planning on using the mercedes amg ihi superchargers. Those are beasts. Nice, twinscrew (not roots, like the other mercedes eatons), clutched, and support over 500hp on near stock motors. The lobes are designed to allow enough air to pass through, so the engine can run normaly aspirated when the clutch is disengaged. Nice unit, and perfect for what you guys are talking about.
Hans

 

Good info blau928 !! The previous Lancia Rallye car was the Lancia 037, which was supercharged, and often fueled debates on SC vs turbo when compared to Audi Quattro or other choice of Rally Car. Problem was, that the Lancia 037 was 2WD mid-engine SC car, vs the Audi, which was...well....quattro with front turbo engine, and the debate continued, as the comparison was between two completely different concepts, running in the same race! Was really fun following Gruppe B back then!

 

Rear mount turbos have been executed successfully for many years now. IF they are well-executed, they can be indistinguishable, drivability-wise, from a "manifold end" style install. That was a pretty clean setup from the pics, but I would tend to agree, a couple more days of design/build might make it better.

Sure, there's a little more length to deal with - BUT - most, if not all of that length is (relatively) straight. Many factory engineered turbo installs have more twists and turns than your average line dancer, by virtue of being required to fit completely in the engine compartment. As has been observed, they add a bunch of heat to the compartment as well, which is NOT good.

A well executed mid-to-rear-mount turbo install can make up a lot for it's distance from the engine by having fewer velocity killing, friction and turbulence-inducing bends in the system. Especially those 90 degree+ bends which are just brutal on the fluid dynamics. Yes, there is still more friction in 10 ft of straight pipe than 5 feet, but that's not the only 'x' in the equation.

Also, don't forget the latest generation of ball-bearing turbos are pretty durn quick to spool.

If you're just bound and determined to get every last bit of spool speed out of an exhaust-driven turbo, plumb an injector and stick it in the exhaust side - one can then quit worrying about all that lost heat energy...
(BTW- also a really old strategy - but those pesky safety types disapprove)

Greg

 

THATS WHAT THEY DID ON THE 924 TURBO. lag is the last thing i'm worrying about on my rear mount setup. i got the 924 method to keep the turbo spool, i have carls's FES kit to supply the added fuel. i got the lower compression ratio of all the years, and i have unlimited race gas 100 oct. my engine is fresh so 1bar is nothing for my car. only weak point is non-forged pistons

 

I believe you are correct, but it could mean either. I always understood it to mean they spooled up in sequence... at different times instead of the same time. Mazda and Lexus have used sequential turbos on production cars, and they both use parallel systems.

 

Matt,
Thanks for restoring thefacts I had in mind. Would be nice if someone (may be Jim Bailey) could clarify what sequential meant for the 959. Jim Bailey are you still reading?

 

I just looked through PET5. The turbos are parallel. We're sane afterall.

 

The 959 twin turbos were arranged to operate sequentially for the benefit of responsiveness - below 4,000 rpm, since there was not enought exhaust gas to drive both turbines efficiently, all the exhaust gas was fed to a single turbo. Between 4,000 and 4,200 rpm, the second turbo was in "pre-spin" mode, that is it was spun up to be prepared to make boost. Above 4,200 rpm both turbos operated simultaneously - in parallel - to provide full boost.

 

Victor is correct: Sequential setups are all the same: Mazda RX7, Toyota Supra (last generation), and yes even the 959. One smaller turbo gets the engine on boost, then the other comes online and supplies top-end (at which point they are parallel as you cannot turn off/disengage a turbo) . No one is using this setup anymore as the plumbing is terrifically complicated to manage.

Anyone modifying a RX7 or Supra ditches the twins in favor of a larger single... far easier to manage with a far better powerband. It was proven that the powerbands didn't turn out to be as smooth as they 'theoretically' should have been.

As for the newer ball-bearing turbo's spooling faster: while they do (to a small amount), their biggest advantage is 'boost transition' (from negative manifold pressure to positive) and staying there throughout a gear change. I have a Garrett GT35R (pretty big *** turbo, actually a good match for a 928) on a 2.0l motor and can achieve up to 5 pounds of boost very quickly on it's way to 21 pounds. During a shift, I'll lose 7 pounds (maybe), but have it back in the blink of an eye (literally).

A properly sized turbo (to engine displacement), with wheels sized proportionately to each other (efficient), and at least a 'full-race' bearing or ball-bearing, mounted as close to exhaust ports as possible is the best possible solution.

As for complying with inspections: the 928 engine bay is tight enough that 99% of those monkey's would never even see it/them. I'll pull stuff apart to make it better or fix it, but not because someone might see it.