Vintage Race Car of the Week: Audi's Group B Quattro S1

[dieselgeek]

Re: Vintage Race Car of the Week: Audi's Group B Quattro S1

what an awesome read and video


Us turbo nerds owe a LOT to the Group B cars. Where would we be without anti-lag?!?!?

[Brian Lohnes]

Re: Vintage Race Car of the Week: Audi's Group B Quattro S1

Where would we be without anti-lag?!?!?

How 'bout 'splainin that whole anti-lag deal. Where does the air go?

Brian

[Whelk]

Re: Vintage Race Car of the Week: Audi's Group B Quattro S1

I remember the Porsche 959 homologation coming out just as Group B was being canceled. It had mind boggling specs for the day. I've often wondered how it would compare to a modern street supercar.

[dieselgeek]

Re: Vintage Race Car of the Week: Audi's Group B Quattro S1

Where would we be without anti-lag?!?!?

How 'bout 'splainin that whole anti-lag deal. Where does the air go?

Brian

As I understand it, there were a few steps in the process. These were the first guys to use BIG turbochargers in racing cars that didn't spend all their time at WOT, so "turbo lag" was a big issue. Making bigger power means "bigger turbo" but the bigger turbo has more rotational mass, which means it spins up to speed more slowly (that, combined with small displacement engines that don't create enough exhaust to spin up the turbine quickly)... what happens is, when the driver lets off the throttle, the pressurized air coming out of the turbo compressor (destined for the intake) is blocked by the throttle and that pressure backs up, slowing down the compressor (with the throttle closed, there's minimal exhaust air going into the turbine). All this means, when you're off the throttle - you have a very effective "turbo slowdown" going on.

(this is all from memory, someone here can correct me if the sequence is off)

The first big invention was the Blowoff valve. It senses vacuum (or throttle position, in some cases) and opens up a pressure relief when the driver closes the throttle so that the compressor doesn't have to fight the intake pressure without any exhaust drive pressure - effectively letting the turbo "coast" during shifts. This was a start, but still "not good enough" in the Group B engineering freakshow.

These were the days when EFI cars were still running distributors, typical EMS computers weren't fast or complex enough to handle "sequential" spark systems - which require separate spark channels for each cylinder and more complicated software... BUT, about that time the big dollar guys were playing with coilpack ignitions (the first ones were "wasted spark" where cylinders are paired together and sparked at the same time). Rusty, and the F-bomb both run Wasted Spark systems - cylinders that fire 360 degrees apart (paired together) are sparked at the same time, once per crankshaft revolution, so what you end up with is a spark at TDC on the compression stroke for one cylinder and TDC on the exhaust stroke for it's "partner" - then, on the next TDC event for that pair, the OTHER cylinder is on the compression stroke and the former is on the exhaust stroke...

What's the point of this? well - a distributor only has a narrow range of sparking capability. Say, from 45 degrees BTDC to 0 degrees TDC. This is because the rotor is only underneath each cylinder's "tower" in the distributor cap for a short duration. Try to spark too early or late, and the rotor simply sparks the next cylinder in the firing order.

But, a coilpack or coil-on-plug ignition can spark ANYTIME it wants. Enter geniuses like Lance Nist from Electromotive... they figured out that on the turbo motor, during decel, you could SKIP the spark for a cylinder or two when off-throttle, instead let the fuel/air mixture wait until the exhaust valve opens up (40-50 degrees AFTER TDC) and THEN light the mixture as it's on it's way out. You lose the compression inside the cylinder and instead direct it out the exhaust, creating pressure to keep the turbocharger spinning. Combine this with the Blowoff / Dump valve, you now have a freewheeling turbo with no pressure fighting it on the intake side, and combustion pressure driving the turbine side.

From there it gets even better, but it's over my head. I know some teams devised systems that actually let air bypass the throttle, some even had fuel injectors in their exhaust pipe with a separate throttle that opened when the normal throttle closed - to REALLY spin up the turbo during shifting... but most of that is the secret stuff that never clearly made it out to the average guys.


Us drag racers do use a version of the anti-lag feature when on a rev limiter as we stage for a drag race, though. On the Huber 4-cyl mustang, we have a rev limiter that, instead of dropping spark events when on the 2-step, we'll drop a few random 40 degree ATDC sparks out the exhaust to help spool the large-ish turbo. It's hard on exhaust parts to put so much heat into the exhaust, but it's only happening for a few seconds just before a launch. It's why you hear all the "popping and banging" out the exhaust on a lot of turbo cars as they stage.

Hope that made sense.

[Brian Lohnes]

Re: Vintage Race Car of the Week: Audi's Group B Quattro S1

Dude....that ruled.

Awesome explanation.

Brian

[Birdman]

Re: Vintage Race Car of the Week: Audi's Group B Quattro S1

For those unfamiliar with or too young to have any real knowledge of Group B rally cars, after reading this, you now understand why they were affectionately - and otherwise - known as the "Killer B's". I remember watching a few races carried live on Wide World of Sports or something; man, talk about an era when men were men!