Hoyo

Ooopps...

For a moment i forgot Carl's kit for the 16v..
I was thinking about the TS for 16v's.
I do appologize.

Geir

Lagavulin

John, blather on all you want about how a turbo does this and that versus a centrifugal; except for some strange, odd reason, YOUR turbo'd car was SLOWER than Carl's, whose kit cost HALF as much.

Bottom line John, in an apples-to-apples comparison, the twin-turbo you were driving was SLOWER than Carl's centrifugal setup.

In summary, Carl's kit is FASTER, costs HALF as much as yours, and is much less complex.

Let's assume that at the same max boost level you were able to at least match Carl's numbers; your kit would still cost TWICE as much for similar performance.

Let's assume that at the same max boost level you were able to be 1/10th of a second quicker than Carl's setup; your kit would still cost TWICE as much for slightly better performance. Is that 1/10th of a second worth costing TWICE as much as the centrifugal setup, as well as the twin-tubo introducing horrid complexity?

Nonetheless, one would think that at the same max boost-level, a turbo/twin-turbo would perform better than a centrifugal. Since that's not the case at this point in time, it's reasonable to assume then, that a quality turbo kit for the 928 has not been offered yet.

tammons

Its an 86.5 with S4 Brakes 5 speed with the later long gear ratio. 17" Repro cup wheels with a Tec III and 45# injectors and an on center to4. The turbo came off a 350 chevy nova race car running in the mid 11's. Since then I have replaced the center cartridge. That would be around 450 hp which is the limit of the on center to4 turbos. You can do a custom hybred build and get around 600 out of it. I am running an early 4.5L engine. At 8psi with no intercooler and on several Gtech runs I was making 260-270 hp, which should be about right for a gtech. Suppoedly a gtech loses about 40-50 hp #'s due to rolling resistance and wind resistance. If I remember right it was in the 13's, but I did not want to break anything. As it happened I broke one of the intermediate plate fingers on that run. That would probably work out to 300-320 hp on a regular chassis dyno. Personally I think that was a bit low. My car spools up in 1st and at 20 mph will smoke the tires (285's) from a roll in first and break them loose in second.

My old setup was twin T3's up front and the spool time was about the same. I never dynoed it but at 8psi I cant tell hardly any difference from this setup. Maybe spooled slightly faster, but that car had an early short gear ratio 5 speed. I ran that car at 12 psi and it was so fast the power curve would beat me through the gears. The short 5 speed gear ratio is actually too low for a turbo street car.

One of the fastest turbo cars ever built was a Buick GNX and that was single turbo with a big intercooler. You can do just a little work to those cars and make a lot of power. I have a friend with one and it is really fast. I think he runs in the mid 11's too out of 3.8L.

One thing about turbos I will say, is in the end i feel like that you can make more civil power, if the drivetrain and engine can handle it and is the turbo is sized big enough. Gees twin t4's ot t3/t4 hybreds and you can make 800-1200 + hp depending and if granny wants to borrow the car you can just turn it down via the boost controller. That is something you cant do with a blower. Also to make in the 1000hp area with a blower robs a lot of engine power.

Well its too bad the 928 does not have a drive train upgrade. I would like to drive a 900 hp S4 !!

928fan4life

Very interesting thread so far. I have been picking up alot of info. But now I have a few more questions. What is the difference between a positive displacement blower and a centrfugal blower? I read somewhere that there is a difference but not to sure what. Also, for the superchargers, from what I have read, the max psi the car can handle is 9psi without doing any mods to the fuel line and without using an intercooler. I know for the charts that I was reading on Carls site the numbers he got was from using 6psi. What would be the difference if you used 9 psi?

Cameron

928fan,

If you are looking for really big RWHP numbers (well over 300, even over 400 if you want) in a 928, you would probably want to go S4 and then supercharge. You would be starting with almost 100 more HP stock compared to your car and there are bolt on supercharger alternatives as described in this thread that are well sorted out.

You would probably have the fastest car in town for those nights that you want to cruise Portage and Main!

Carl Fausett

Oh, Geez....here we go again....

Might I suggest you use the search utility on this fine website? That topic has been argued like the Hatfield and the McCoys.

Here is a little more....

There are three main styles of superchargers, a) Centrifugal b) Paddle (like Roots) and 3) Screw (like Whipple) that fall into two types: Internal and External compression superchargers.

The Paddle and the Screw types are grouped together as "external compression" superchargers. That is: a supercharger that moves air, but does not compress it – the compression occurs as the air is packed into the tube (or manifold). Internal compression superchargers like centrifugal superchargers compress the air within their own housing.

All superchargers are measured and compared to each other on many levels – but a main key is “adiabatic efficiency”. That is: the ability to compress the air without heating it. Because hot air expands – heating it while you are compressing it is kind of working against yourself. Our goal is a dense charge, not just pressure. We want to increase the air mass. Some blower designs are better at this than others.

For example, the Roots/GMC blower… that big impressive beast sticking out of the hood of your drag racing hot rod – will move the most air at the bottom end than any other type because it is a paddle-type "positive displacement" blower. But, it has the worst adiabatic efficiency (about 50%) because it heats the air so much. Centrifugal superchargers have the best adiabatic efficiency, usually around 75%.

Both types of superchargers, (internal compression and external compression) will do the job of compressing the air, but for the heat that they generate and the complexity of the installation, in my opinion the external compression blowers, (the Roots, the Whipple, and others) didn't fit our design goals.

In the case of the Roots and Whipple superchargers, the intercooler is often placed in the valley of the V8, below the twin-screw; between it and the motor. That limits how large the intercooler can be, and they must also fight heat gain by conduction and convection in that hot spot. That is why the drag racers often have their positive-displacement blowers sticking up out of their hood. Check the intercooler under their Roots or Twin-Screw blowers, they are often 6" or more thick!

Centrifugals do not need as big of an intercooler, and we can place it anywhere so we have room for a bigger one than we need. Also, they are mounted out away from the motor in a relatively cooler environment.

The external compression blowers also cannot use the stock fuel system because the Roots or the Whipple will mount exactly where the intake manifolds are right now. This would require the replacement of the entire Porsche and Bosch fuel delivery system by our customers, and again, this did not meet our design goals.

Another consideration in Supercharger selection: Emissions testing.

In states/counties that only do sniff (sampling) tests without a visual test, the centrifugal is more likely to pass emissions because the centrifugal comes in at about 3000 RPM and the emissions tests are done at 2500 RPM (called a cruise test) and at idle. Because the boost has not come in, and the factory Porsche induction/fuel system is still intact, this car will pass emissions – why wouldn't it? At 2500 RPM, it's still all stock.

Not so the Whipple. It will have boost at 2500 RPM, so it will have (if properly tuned) richened the mixture accordingly, and therefore is likely to have high HC emissions if tuned for HP and to avoid detonation.

In States with visual inspections in addition to Sniff testing (California for example): The centrifugal supercharger is an add-on to the front of the motor – it comes off – all the stock manifolds and fuel system is there – go get it inspected, get certified, and come back home and hang your supercharger once again. Easy. To do the same with a Whipple it means you will tear down your motor to the block – removing and reinstalling your complete intake system and all injectors and fuel lines just so you can go get inspected. No fun.

By the way – I like Whipples. They are technologically very advanced and, if I wanted a positive displacement blower to go drag racing – I'd use a Whipple because they are a huge improvement over the Roots-type blower. It's just that this application is better suited for a centrifugal-type instead.

One final consideration for us was the gearing of the Porsche 928. Where the Roots and Whipple make more boost at idle and under 2500 RPM – our design target was to bring in the power from 3,000 to 6,000 RPM for the gearing we have.

In Summary: the centrifugal superchargers are far more compact, and easier to mount and dismount from the car should emissions recertification be necessary. They allowed me to retain the Porsche fuel system in its entirety, they give me the ability to have a larger intercooler to prevent detonation (and engine damage), and they produce their boost in a RPM band very similar to the 928 gearing.

Can you supercharge one of the later Porsche 928's that have a 10:1 compression ratio? You bet. We market a 32v supercharger kit from another manufacturer as well as our own, and have 23 kits out there on higher-compression 928 engines as a result – often reporting dyno runs of 440 HP and up. But you have to compensate for the higher compression ratio with lower charged air temperatures as it enters the intake (read: "intercooler required") and you're going to have to add fuel as the boost pressure comes up so that you keep the combustion temperature nice and cool to avoid detonation. The good kits do this.

928fan4life

Carl - Thanks so much for the info! That was a good read. I always learn quite a bit after you guys post this stuff. My other question for you is, what kinda difference in performance is there between a 6 psi boost and a 9 psi boost? Do you have a dyno or something that shows the performance with a 9 psi boost? At what psi do you need to add an intercooler?

Cameron - I would like to get an s4 but the only problem is, they don't have many 928s up here. The ones they do have are way over priced...example - someone is selling a 1980 928 like mine for $18,900!! I was hoping it would be gold plated for that price! Do you know the weight difference between an s4 and a non s4?

Cameron

Fan,

I have the exact numbers, but not in front of me. I think 928GT.com might have the specs on their website. An S4 weighs about 3550 lbs. A GT is about 50 lbs less. An '80 is about another 150 lbs less in US trim IIRC. Add 10 more lbs for your all leather interior! :>).

Seriously, try 928s4vr.com or 928GT.com for the year over year specs. They are much more precise than my poor memory.

By the way, I looked up the 850. It was in a compo against a same year 928GTS. The 850 was 7.2 sec 0-60MPH vs 5.3 for the GTS. The 850 tipped the scales at 4100 lbs. The V12 in the 850 put out just a tick under 300 HP and about 330 lb-ft of torque. Both cars were autos and were about $90k at the time. The conclusion was that the 928 was well worth the dough while the 850 was all show, no go.

928fan4life

Thanks for the info cameron