Gerry - I saw twin supercharged widebody street 911 several years ago. At the time I had some info on it, don't know where it is now. From memory, the twins were centrifugal Paxton-type units. Blow off valve was used. A picture of the car was posted on this board, may have been 1999 or 2000.

Hmm...I don't get it? You want to run dual superchargers beacuse "next size SC up from the one I have may make too much boost too fast"??

Why not use bigger pulley instead of complicating matters with dual superchargers?

Twin superchargers also means twin pulleys with all inherent losses (double everything, bearings, pulley losses, plumbing etc).

I really don't see any advantage at all. If you really need to run supercharger then you pick up nearest size and then adjust pulley diameter untill you get suitable boost. You don't have freewheeling inertial masses (like in turbocharger) that you can cure by using dual units with lower masses. Supercharger (be it Whipple, Roots, Lysholm or whatever) is mechanically connected and it's speed is directly proportional to crank speed. It's like making a 6-wheel drive car believing you will accellerate faster than 4WD despite no wheelspin.

It would be cool, you would say your friends that you have 6-wheel drive car but it is still pointless...you have extra halfshafts, tires, differentials and whatnot and it's all connected to common crank.


Also, making 800hp with supercharger on aircooled 911 engine is...ehmm...almost impossible. Maybe if you invest 100000$ and blow few engines trying and then dyno it to 800hp in few pulls before it desintegrates. A SC that packs enough air to make 800hp will draw insane amounts of power from crank...100hp or such.

It all sounds good in theory but there are perfectly good reasons why almost no manufacturers use superchargers and why Natchamp pulled his SC installation and replaced it with twin turbochargers.

Please take all of this with a grain of salt because it is a) secondhand and b) info that is about 5 years old:

There was a guy on rennlist about 5 years ago that had an SoK roots blower (I think). He was very happy with it, AFTER he had rebuilt the engine to handle the boost...

I think his name was Rick and it was a Red Targa. He's from Chattanooga.

This topic was discussed in several threads in the old rennlist (when it was a mailing list instead of a BBS) and the general consensus was that the 3.2 just isn't built to handle the boost for multiple reasons: too much compression, too much heat, single plug, weak bottem end (relative to turbo). The end result was that pretty much everyone said "if you want a turbo, buy a turbo."

By the time you upgrade the brakes, etc. to keep up with 350hp, you've probably spent more than turbo money anyway.

Stephen at imagine auto had a supercharged 911 about 8 years ago or so that he had all kinds of problems with. Now, he pushes cars pretty hard, but if a 911 mechanic is having trouble keeping it running...

Of course the problem with turbos is that there is no end to the "more power is better" slippery slope, so you do end up spending a lot more money.

Porsche clearly did not design the 3.2 for boost, hence the 3.3...

-dc (down near the bottom of the slope)

I understand all you say except the "weak" bottom end of a 3.2 compared to a Turbo....don't get it.... the SC and 3.2 used a 930 case...same as Turbo....

????

Wil

Yupp..3.2 has same case and crank as turbo. Many wifes tales circulating around.

Adding a Supercharger to an engine requires the same engine considerations as adding a turbocharger. I beleive many of the supercharger stories with unhappy endings were a result of "bolt ons". You just can't do that and run any signifcant amount of boost.

A fellow PCAer back in IL had a Supercharged SC that he campained all over the midwest. He had sorted the engine and SOK whipple out completely. Drove the car everywhere, hundreds of miles. Raced the crap out of it and drove it home. It was an awsome machine because it was done correctly and conservitively. If memory serves he put down 260HP to the wheels.

I have no doubt that Gerry's machine is as well thought out. 800HP is a very ambitious goal, please keep us informed.

Rarely8, Thanks for the info on that twin SC, I will try and search the archives.

Also, I did not say I would definitely use twin SCs only that I was exploring the option vs a bigger single SC. I had good info that the type of SC I am using, which is from Autorotor, does have a bigger brother but that it makes a lot of boost which was pretty much uncontrolable with blowoff valves as it came on very fast. Also, you are limited to the speed at which the SC can spin in a safe and effecient manner so pulley size is not the only consideration. Boost is not the only problem , overboost is another problem. To get a constant 10 lbs of boost throughout the rev range you will encounter boost at times of 12lbs or greater and this can become a serious problem. Using a sophisticated engine management system such as the motec may alleviate the problem or at least ameliorate it somewhat but?. Also, my motor (top to bottom) has been completely rebuilt using the strongest components available and I have beat the crap out of it for more then 7K miles and so far the engine is absolutely OK.

Now there are many ways in which the charge can be cooled and more power made (nitrous, freon cooled intercooler, alcohol injection, 112 octane racing gas, twin plug, increased engine size etc). I am not limiting myself to a 3.2 liter engine only that it happens to be the starting point, I would love to make a 3.8 or bigger. This car runs wonderfully at this time but as I have stated multiple times it has taken two years and $50K even though not all of it was spent on the engine. As to dyno pulls , I have sheets from more then fifty pulls so please don't think I or the people helping build this car are naive ( I may be stubborn and foolish but not naive). If I were to run race gas right now and run 16 lbs of boost the engine would produce > 550 HP at the rear wheels, add an 80-100 shot of nitrous and there you have 650HP. I know this can be done as others have been there and I can make 400 RWHP with 9-10 lbs boost now. My current trans would not handle that much power (600 HP) so I need to G50 the car first which I will soon do. To finish this diatribe (which I said I would not do) I don't see where the info that no manufacturer uses SCs comes from. Please read some mags and you will find that many are using SCs and there is actually an increase in the usage. That's enough.

Maybe it's the rod bolts I was thinking about?

That is the most widely critisized shortcoming of the 3.2. My understanding is that rod bolt failures are mainly related to high rpm, not necessarily high bhp/torque at rpms below 6250 or thereabouts. The rest of the aluminum cased, 8 main bearing bottom end is as strong as Porsche has made in this configuration.

Weak rod bolts are general problem för 3.2 engine. Forces projected on rod bolts are mostly dependent on piston weight and engine speed.

With other words, it's revs that kill rod bolts, not turbocharging. Turbocharging will raise mean cylinder pressure that pushes piston downwards but that force is small compared to jolt that rod bolt goes trough when piston goes trough TDC. This is all very well explained in Corky Bell's book "Max Boost". There ae nice diagrams displaying force on pistons/rods troughout crank revolution. It's an eye-opener. I know it sounds weird as most people expect things to break and get red-hot as soon as turbochargers are involved but rod bolts are mostly killed by revs, not boost.

As turbocharged engines usually develop peak power at lower revs, it actually means that Carrera rods probably stand less risk to snap in turbo-converted 3.2 engine than in it's hard-revving N/A incarnation.

So: 3.2 engine has same bottom end, same crank as 3.3 and it's rod bolts aren't loaded with greater peak force than in it's naturally aspirated version. One more myth killed.

Juan Ruiz boosted his (otherwise stockish) 3.2 to quite respectable outputs early in it's career.

P.S. If enyone gets 800 hp from his supercharged 911 aircooled engine in more than 30 seconds I'll eat printout of this thread :D

Gerry,

Have you considered the new Ford GT blower? They have started marketing it for Lightnings and HD trucks (which I have). They incorporate a intercooler below as you most certainly know. They have very little heat build up and can maintain just about any boost level you'd ever desire. They certainly might build too much boost too quick like you mentioned above.

People believing that SC tech is old have never studied the screw blowers and their ability to reach 50psi within a couple of frames of video. I'm going TT on this project but have both 3 SC and another turbo in the garage. I also have a 9.5 liter NA car so there is more than one way to skin a cat as Gerry has shown.

I think what Goran is banking on is the air cooled engine not being able to handle the heat within the cylinders built by the compression and he may be right. But if you can make close to 700 HP with boost and add nitrous and a few other cooling tricks that I mentioned perhaps 800 HP is possible. If so, I will bring the salt and pepper :D. Thanks for the info on the Ford GT SC, I didn't think of that one. I really am not worried about making the needed boost but as Goran alludes to being able to handle the heat produced within the cylinders thus avoiding detonation and death of the engine. This is why Porsche finally went to water cooling as they increased the power in their turbo cars. However, remember boost is boost and 16 Lbs from a turbo is the same as 16 Lbs from a SC. How it gets there and stays there is different but I believe if anyone was able to get an aircooled Porsche turbo engine to 800 HP then the same is possible for an aircooled Porsche SC engine. Time will tell if I am right as I will certainly try and get there. I searched the archives and found this way cool picture courtesy of Kamikazepilot from 2001. The legendary twin SC Porsche:eek: .http://forums.pelicanparts.com/uploa...1134328907.jpg

PS There definitely are many ways to skin a cathttp://www.pelicanparts.com/support/smileys/cat.gif

Best regards,

It's all nice and dandy but there is boost and then there is boost.
Boost figures are just 20% of story. M3 makes lot's of power with zero boost. My SAAB makes 210hp with 17lbs of boost.

To put it this way: my old 930 boosted 16lbs when the wastegate diaphragm ruptured. It didn't make 800hp on this...hell, it probably never make more than 380...(what it did make is some spectacular leaks that I had to chase afterwards). It just pumped hot air around with intake pressure of 16lbs.

Imagine twin turbos boosting 16lbs on engine that is so well prepped that it will make 800hp. They will live mostly of enthalpi delta over turbine and cost maybe 20-30hp from backpressure. Yes, you will probably need big cube engine and spin it quite fast

Now imagine same engine making 800hp with superchargers. It will have to chew fuel and air worth of at least 950-1000hp in order to deliver 800hp on crank. Rest of it will dissapear into heating of bearings, belts, pulleys and air (both adiabatic and just plain losses) that is being pushed into engine.

So you would essentially have to build engine to deliver 1000hp and give away a big chunk of that to SC's.

Knock isn't the biggest problem...you can always use lower C/R pistons to bring down effective C/R but you would run into other problems. Heat is one of them (actually bigger problem for turbochargers than SC's) but head sealing is other. Separate heads don't seal well.


P.S.

Next weekend we will hopefully disasemble this engine:

http://www.itsfun.nu/itsfun2/itsfun2_194.jpg

It made 1007Nm @ 5000PRM on engine dyno. That is 723flywheel hp @ 5000RPM. Maximum power was achieved @ 7500RPM. ;) I believe it would be quite impossible to do that with superchargers.

quote:
--------------------------------------------------------------------------------
Originally posted by derickc
Maybe it's the rod bolts I was thinking about?

My answer...even here the 3.2 and the Turbo share the same 9mm rod bolts.....the SC had 10mm.

Yep...this is an RPM issue...and stock Turbo's rarely wound up too tight..

( sorry...try again...the 3.2 and 3.3 Turbo have IDENTICAL bottom ends...)

- Wil

A few things to note.

The S/C on the Lightning is a 112 Eaton (roots blower). I don't remember the GT blower displacement off hand, but it's a Lyscholm screw compressor. While similar, they are quite different.

The Lyscholm has internal compression whereas the Eaton doesn't (it just 'moves' air). The internal compression is more efficient thus it heats up air less than the Eaton. The disadvantage to the screw compressor is that it doesn't have quite the low end boost that the roots does. For an application where max power is the goal, the screw is better.

On the Lightning application, the blower draws about 45 HP, so this means the 380 HP version makes about 425 HP at the crank. Add in the fact that it still has relatively high inlet air temps (even with the intercooler), and you're providing enough fueling for what would be a 500+ HP engine normally.

Turbos are definitely the way to high power outputs but they do have some disadvantages.

Centrifugal blowers are yet another animal. They have the worst low end power, are speed limited which means a comprimise when selecting pulleys, plus the associated hardware necessary to power them. But they are the cheapest and easiest to install, hence their popularity. There's a reason why no OEM has installed one on a production vechilce...

Boost away!

I understand everything you are saying. However, your assumptions on losses from SCs are way too high. Modern twin screw SCs are extremely effecient. It is correct that there is more of a drain on engine power with SCs but you will have a hard time convincing many current manufactureres and racers that their SCs can't make power with bad cost to benefit ratio. Top fuel cars make in excess of 9000 HP and they all use SCs. I know there will be problems (heat, detonation etc.) but these are just situations for which solutions need to be found. As to your old engine making 380 flywheel HP on 16 Lbs boost, my engine makes over 400 RWHP (that's at least 460 flywheel HP)on 9 Lbs boost reliably and continuously, pretty much instantaneously also. If I pushed it up to 16 Lbs I would have to use race gas or some other detonation preventing procedure but if I did it would make 600 HP and that is a conservative estimate. All this is just talk, so until I prove my point or blow up my engine we will have to wait and see. Good luck with your engine and projects they look very interesting. BTW what boost did you use to make the 723 HP?

Best regards,

Be careful when using cars in racing venues as justification for one technology over another. Top Fuel has outlawed turbos and screw superchargers because they presented an 'unfair' advantage.

I wasn't involved in project by the time they dynoed the engine in it's latest incarnation but I believe latest pull was done with 28lbs of boost on VP-fuel.

There are different chips for different fuel qualities. Top numbers were achieved on VP. The engine made 723hp at torque peak ;) As we all know, torque peak is not where engine makes max power. Unfortunately, owner asked me not to make power figures public but it was more than 723hp ;)

This is dragrace map that is not used on street/track. Engine still has reliability problems and we'll likely go down on boost.

Modern twin-screw SC's are not extremly efficient. They are just efficient compared to Roots type. They are still less efficient than turbochargers. Then there is a little problem (that SC manufacturers often "forget" to mention): they still have to be driven of the crank. So even if you are able to manufacture SC that is as efficient as turbocharger when compressing air (which is quite possible using centrifugal supercharger as it's nothing else but half of turbocharger) you'll still have to pay for spinning the damned thing. Turbocharged engine with same compressors but exhaust-driven turbins will deliver more power as it will reuse some of heat loss (and pressure drop) over turbine.

Regards.

Turbos are definitely the way to go, but S/C do have advantages for street cars. It all comes down to priorities (and rules if racing is in the cards).

Gerry - that is the twin supercharged 911 I had mensioned previously. Glad you found it, did you find specs and info as well? I didn't know if the engine was built for show or if it really laid down the power it looks like it should. Centrifugals work like turbos which makes this twin configuration not so odd. Can't remember if it had intercoolers in the fenders.

I can add one tidbit from personal experience, my PCA buddie's supercharged SC would kick my 911 Turbo's butt on the street with 100 LESS horsepower. Instant torque is an incredible advantage.