Gerry

John, very nice work on your project. I can tell that you really love that car because it is something very special.

Now, please explain why the older motors like yours and "quick carl's" and "onebad928" blew the ring lands out of the pistons with moderate boost and the S4 guys with superchargers are still holding togeather with 10:1 compression?

Are the S4 pistons really that much better than the early motor pistons?

You know, your gonna have to bring your lap top on your honey moon to keep in touch with reallity, right John?

Fastest928

I wonder what the parasitic losses are for the SC vs. the turbo? Higher? Lower?

Hmm....I remember a conversation about that ....comments?

Marc
DEVEK

bcdavis

Turbos are generally way more efficient at making horsepower.

It's just that these are older turbos, and an older car, with a fairly restrictive intake, exhaust, etc...
Stick some big turbos on an s4 built for boost, with big intercoolers, and I would almost guarantee it would put down much larger horsepower numbers than the supercharged crowd...

But it would probably be a lot more complex, and a lot more expensive.

Nathan Valles

I gotta find my Corky Bell books, but if I remember correctly superchargers of the range we are talking about (4.5-5L 8-12psi) consume between 40 to 60 hp to drive the air. Turbo's will consume about 3 to 5 hp to do the same work.

The trade off is all in the plumbing. (Warning: Gross Oversimplification ahead)
MECHANICALLY: superchargers are much closer to plug and play systems with some pvc and silicone tubing required, while turbo's need both the intake and exhaust systems to be redone, requiring lots of welding.

Z

Some thoughts and observations on the supercharger vs turbo issue again:

It appears that John's setup makes about an additional 15-16rwhp per psi of boost. The centrifugal supercharged 32 valve cars have generally been getting around 20rwhp per psi of boost. That figure isn't just the Wisconsin centrifugals, but includes others that have been dynoed as well.

As Marc said, a stock '81 makes maybe 185rwhp. John's now making 366rwhp, so that makes it a gain of 181rwhp with 11psi-12psi of turbo boost, an engine rebuild and a MAF conversion. I don't know about cats or exhaust modifications so I can't comment on those, but will give him the benefit of the doubt and assume they're stock and there's no power increase over stock from any modifications there. Lag's car made 265rwhp stock. It then made 492rwhp, so it's a gain of 227rwhp with 11psi of centrifugal boost, the original maybe 50K or so mile engine, and no other modifications.

Some of the Vortech centrifugal superchargers have adiabatic efficiencies listed of up to 79%.

Various figures and calculations are floating around showing how much power is required to power a supercharger. That same information for turbos doesn't seem to be as available. Instead a lot of people seem to think it's "free" horsepower. Power reduction from running the turbos comes from increased exhaust back pressure. There are published measurements of a stock 911 turbo's exhaust back pressure before the turbo. Those measurements show over 17psi of exhaust back pressure that the engine sees at 6K RPM when making 11.6psi of boost. Anybody want to put their car on the dyno do a run, restrict the exhaust until they see 17psi of back pressure at the engine, and then do another run with the exhaust restricted like that? Might be pretty interesting to see how much the measured power is reduced doing that, and how "free" things really are with that exhaust back pressure.

In the Pro Mod 5.0 class of drag racing, turbo cars were and may still be competing. The last I saw, the fastest cars were centrifugal supercharged, and the number of turbos seemed to be in decline. The top centrifugal cars in that class are running the 1/4 mile in the 6.5 second or less range, at 210+mph, and making easily over 2,000hp.

There was apparently a time when centrifugal superchargers couldn't make the high boost levels that turbos were capable of, or at least not easily. There are current off the shelf centrifugals making 40+psi or more now. That's the pressure above atmospheric, not absolute pressure.

Mercedes, Ford, Jaguar, Toyota, General Motors, and maybe more other big auto manufacturers have new supercharged cars out, and coming out. I haven't noticed any apparent increase in new turbo models being produced and developed, at least not to the same extent if they are.

Superchargers are becoming more and more popoular on 911s and Boxsters. Latest European Car magazine

I saw a program last night on automotive technology and developments that are in the works for the future. Reduced emissions, non-reliance on fossil fuels, and increased efficiency were a fair part of it. Besides having things like fuel cells, one of the major auto manufacturers had a car featured that's currently being tested with promising results at this point. It's an internal combustion engine/electric hybrid. It's different from the current gas/electric hybrids that are already available now though. The electric part is basically the same, but the internal combustion engine in this one is hydrogen fueled and supercharged.

Nathan Valles

Comparing the 16V to the 32V is like comparing apples and oranges. Until we see numbers from both systems on the same engine comparisons are useless. The 16V and 32V heads are so different that we are comparing two completely different engines with completely different capacities for HP.

John..

As best I can tell, the reason I was able to get to that much power without having one bar lies in the exhaust system. For those who have seen the pictures of the assembled engine, the downpipes curve downwards and then tie into the exhaust system....

The exhaust consists of two 2.25" pipes that feed back to a small stainless steel Borla muffler....it is all mandrel bent and would appear to be quite high flowing. There are no cats on the car as of today. On the NA side, that is probably worth 10-15 HP, so this may explain it.

I have heard it is possible to make more than twice the HP with less than one atmosphere of boost...I suspect is also has something to do with the crossover point, where the boost is not only in the manifold, but is also helping to increase the pumping efficiency of the engine....the turbos are pumps afterall and will force even atmospheric air into the engine under part throttle loads. I guess I am saying they might have an effect on increasing the cylinder fill values even on the NA side.

It would be easy to calculate the HP required for a mechanically driven blower to make the same volume of boosted air, but I have heard around 30- 50 HP for a medium sized V8.

I do not know the exact boost numbers the car ran on the rollers, but it was right around 12 psig, certainly not over that value at all. I was just watching I didn't step on the passenger's side roller and get launched! I still find it strange the car will pull 1 to 2 more lbs on the road than on the rollers...no real explaination except for maybe more load on the road and better charge cooling.

Anyway, RE turbos on an S4. I now have a sketch of how it all would work on an S4, but the fabrication would be massive! It is hanging on my file cabinet here at work...it is a crude sketch. Tim and Lag's S4 numbers are wicked and very impressive!, but I have zero doubts that mid range torque would be up 80 or 100 ft-lbs more with a proper turbo setup. A proper turbo setup on an S4, you could be on the wastegates at 3500 +/- RPM and hold boost up to the redline. The torque and HP curves would be way different than what you see with the centrifugal blowers as of today. Remember, you have to shift anyway, so it isn't just about HP at redline, because you spend only a tiny portion of time there anyway. I would say you want a great curve from 3000 to redline to make the most of it.

My estimate with one bar on a 5.0 liter S4 engine with twin turbos would be in the neighborhood of 600-640 crankshaft HP. I firmly beleive with proper exhaust, turbos and intercooling that this is easily within reach for the S4...maybe it would take 15 lbs, 16 lbs?, but it is easily possible.

Here is what I would do:

Pull it and drop compression to about 8.5:1 or 9:1
Forged Pistons
Custom Header manifolds cut through the inner fenders
Hand two Garretts or KKKs with external wastegates under fenders
Route feed pipes under fenders to front of car
Two 3.5" X 12" X 12" air to air charge coolers under each fender
Cooled pipes cut back through inner fenders and join behind radiator
One pipe to one throttle body to a reverse intake manifold
Intake air would be metered with MAF (or two) before the turbos..goal would be to have stock airbox to make it look trick....fab a new bottom side that has two outlets to each turbo....a fiberglass nightmare!

As for the pistons, well I don't think there is really much difference in their make-up until you get to the GTS which had forged units. The cast pistons will handle moderate boost for short bursts....the way most of us use it anyway. The issue lies in the cast piston's weakness under sustained high temperates as well is it's inability to handle any form of detonation. Lots of guys out there running cast pistons on boost, but when it goes south it gets ugly. The cast pistons don't expand much, which is good, but their strength gets significantly lower with the loads of turbocharging or supercharging. In addition, detonation will kill them in short order. Am I saying forged pistons will handle sustained detonation....no way! But, ask yourself this...if a cast 928 piston can snap off a ring land before the head gasket lets go, is it not a weak link? The 951 has a long history of handling boost and the Mahle forged units are hard to beat (I attempted to get Mahles for the Bastard, but they were not available, so JE was the ticket for me). I do think I would have run into issues with the cast units up over 10 lbs of boost, because during the tuning I did get some pinging and maybe some light detonation. The ugly side of the forged units is the wall clearances you have to run to make it work....0.0024" in my case, stock is 0.0016"

I know Marc at Devek and 928 International tore down a few Callaway cars between them, and both had ring land issues. I know the one at Devek was not charge cooled (had water injection), not sure about the Intl. experience. My car and another out of NY state were the two survivors and I never had any issues with internal engine components excapt for a scratch on cyl. 1! It always ran exceptionally strong, especially out in the cold weather. My car had a decent air to air unit, but it was restrictive by todays standards, so mid range was suffering a bit....now boost comes on much faster and harder.

There is a European auto repair shop here in Cincinnati and the owner swears there was a turbocharged 1980 928 US spec built in about 1993 that made well over 500 HP at the crank. It is supposed to reside in California. Anybody heard about this?

Tony

Ya know, off all the thing you listed, i bet that is would trun out to be the hardest!

Theres room in the fenders but you also have other systems there..FUEL for one! Cruise Control...washer fluids etc. Would be quite the feat to do it all and still keep the all the cars components 100% operational. The other trick i bet is finding the room in the engine compartement to actaully cut the hole to the fenders!

My bet is on Tom Clouiter to get a twin turbo at those numbers.
http://www.thepowerbroker.net/scavenger.htm

BC

John - Have you actually laid under an S4 without the belly pan? Have you worked on one? Honestly, I just took one apart. Its a tub now, and will be rebuilt with cleaned or better parts. Turbos on an S4 is your dream. Thats Fine. I am into supporting dreams. But go look at an S4 engine IN the car, and sit under it for a while. Then get back to that sketch.

The SC route for the S4 is just too easy, and there is too much power available. And after 550, I think the tires, no matter what size, are traction limited.

Z

The weight of the dyno rollers is less than that of the car, so you might be right that there's more load on the road. Going slower through the RPM range on the road giving more time for boost to build?

Stay tuned.

I'd think it should definitely be possible. Lag and Tim are both over 600 crank horsepower with the 11psi and 13psi on their cars.

That is kind of an interesting thing. There have been broken ring lands on boosted early cars, and apparently on one or more boosted GTS cars. I haven't heard of any on a pre-GTS 32 valve car yet though. There have been failures of head gaskets in those pre-GTS 32 valve cars, including at least one that was pretty much a new stock gasket, and there was no damage that occured to the stock pistons in those cases.

There's an '83 twin turbo in the Milwaukee area that's supposed to be a Callaway. I haven't seen the car, but several other area 928 owners have.