[Herr-Kuhn]

There seems to be a lot of bad information floating about regarding my 928S4 twin turbo system. Consider this post an effort to eliminate this bad information by using facts to debunk these myths:

Myth 1: The twin turbo system will cost 2X what a supercharger system will cost.
Fact: No it won't!

Myth 2: Turbochargers don't make the midrange power of a supercharger.
Fact: This depends on the supercharger used and your definition of "mid-range". Additionally, statements like this have to be based on equal paramaters, such as; same engine, same compression ratio, same fuel, etc. Pound per pound of boost a twin screw supercharger will nearly always outpower a turbo to about 2,500 or so rpm, where the turbo will then come on full song and make more power on the same boost. This is because the twin screw pump makes nearly immediate full boost pressure whereas the turbocharger requires more RPM to get on full song. Make no mistake, a centrifugal supercharger with its centrifugal compressor usually will make less HP across the entire RPM band than the turbocharger and likely less HP than the twin screw pump below about 4,500 RPM. A centrifugal supercharger will never make the off idle power numbers of a twin screw pump because it is down on boost there. Furthermore, one must consider that the centrifugal supercharger is the slowest of the three (by a long shot!) to reach full manifold pressure, because it only does so at maximum engine speed. The turbo can make all its boost by mid range RPM when it is loaded up and so can every twin screw pump. Engine torque follows cylinder pressures and cylinder pressures are a function of boost pressure (assuming all other variables equal). Engine torque applied across engine speed produced HP. All one has to do it look at the test data available. The twin turbo system was designed to provide power in the range where a performance car like the 928 is driven, namely 3,000 to 6,000 RPM and do this without taxation on the longevity or drivability of the vehicle for daily use. Looking at the data available run at the same paramaters on the same engines shows the significant HP and torque advantage the turbocharged car has in the real world. One other point to add, with a twin screw pump and a turbocharger the boost is modulated based on RPM. If you ever over-rev a turbo or screw pump car you are not likely to "overboost" the engine. Over rev a centrifugal car and you will build higher boost than the the setting at which system was designed to work.

Myth 3: The twin turbo system won't make any more power than a supercharger system
Fact: First off comparisons based on this statement must have several variables which are fixed, those being the same engine, same fuel, same boost pressures, same compression ratio, etc. When one says "it makes the same power as the turbo car", ask them at what boost pressure that power was made and furthermore take a look at the powerband width which is made at that same boost pressure setting. In simple terms "area under the curve per pound of boost", then pick the pump that works for the area of the curve you want to be driving around in. I can see where one might prefer a twin screw pump over a turbocharger, but I can't see on what performance basis one would pick a centrifugal over either. Consider the twin turbo system has produced in excess of 530 ft-lbs on the road and roughly 550RWHP on less than 12 psig. Too much torque for you, okay we'll crank the pressure back to 9 psig and still make the same torque than you'll see off of what the centrifugally boosted car is making on 11 or 12 psig, maybe even more. Better yet, install a cockpit adjustable wastegate control and go to town. One should consider what pressures are required with other forms of boosting systems to make these numbers, again on the same engine. Remember, you never drive your car hammered up onto the rev limiter all day long. You have to shift at that point. Even a full blown track car benefits from a wide powerband in the upstairs region. It's real simple, if off-idle instant boost is your game, pick a twin screw pump. If a peak high end HP number is your game, pick a centrifugal. If you want excellent road manners down low with massive mid range and top end power, pick the turbo. For further details, see debunk to "Myth 2"

Myth 4: The twin turbo system places far more heat into the engine bay than a supercharger system
Fact: Is this statement based on running at the same HP level or the same boost pressure level? Since the turbo engine doesn't have to burn as much fuel as supercharged engine to make the same power, there is less overall heat produced per pony. About 1/3 of the heat energy created by burning the fuel goes out the tailpipe...it is this heat energy we harness to power the turbocharger. The rest is letoff as heat (out the pipes or into the cooling system). Since superchargers require more fuel to be burned per HP this means there is more heat going out the tailpies and into the cooling system as compared to the turbocharged car (at the same power level). The twin turbo system has been tested in high ambient heat conditions (100 F) and has exhibited no issues with overheating or melt-down as some would like to believe or fabricate stories about. Additionally, please consider the twin turbo system and most twin screw systems available retain the 928's wondeful dual high-speed radiator fan system and shrouding as designed at the factory. A centrifugal supercharger system typically requres that you remove the large fan packs to fit the supercharger and furthermore some have intake boxes that block off a large area behind the radiator. More fuel burned per HP with a cooling system which has lost its high-power fan system and has a partially blocked radiator is not conducive to a cooler engine bay. Who's running hot again?

Myth 5: A turbocharged 928 will burn up exhaust valves while one with a supercharger system will not.
Fact: Is one supposed to assume a supercharger places no additional heat on the exhaust valves as compared to its normally aspirated sibling? I'm still perplexed on this statement. Granted there may be slightly more heat on a turbocharged system exhaust valve compared to a supercharged system exhaust valve, but that difference is nowhere near the difference either system places on the exhaust valves as compared to what the normally aspirated system originally had. Also, please remember that you'll have to burn more fuel with the supercharged car to make the same power as the turbocharged car did and about 1/3 of that heat is going out as raw exhaust energy. Both types of systems place additional heat on the exhaust valves...how can they not, you are burning more fuel.

Myth 6: The twin turbo system forces you to cut "lots of holes" in the engine bay.
Fact: Two 2.5" holes are required to get the intercooler discharge pipes back into the engine bay. I'm sorry, I was not aware people cherished that 10 square inches of material so much.

Myth 7: The twin turbo system will require more maintenance than a supercharged system will.
Fact: This one really confuses me . The turbocharger is such a simple device as the rotating assembly is one part housed in dual ball-bearings. A supercharger has more moving parts, especially a centrifugal supercharger with its overdriven gearing system. Additionally, any belt driven pump will require retensioning of the belt and replacement of the belt. I can guarantee there will never be a broken crankshaft bolt, broken crankshaft pulley, slipping belt or broken belt with the twin turbo system. There is no additional maint. required with the turbocharger beyond routine oil changes. It's really that simple.

Myth 8: The twin turbo system suffers from excessive "turbo lag"!
Fact: This has to be the most over-used statement by a supercharger salesman . True, a twin screw pump makes nearly immediate off-idle boost which is why they are really nice for stoplight to stoplight racing. On the other hand, a centirfugal supercharger simply can't make any appriciable boost until it is a full 2/3 up the RPM scale. When you hear statements like this ask them how much boost their system makes and ask to see the boost curve the system produces on your engine. Then remember that torque (and consequently HP) at that point in the speed range follows the manifold pressure (minus any power robbed to drive the pump).

Myth 9: It's simply not worth it!
Fact: Beauty is always in the eye of the beholder. In my opinion the twin screw system makes a nice functional and aesthetic addition to the engine bay of any 928. Of course, I also think the twin turbo system also makes a very nice addition to the engine bay of the 928.

Myth 10: A centrifugally supercharged car is faster in the real world than an equally boosted turbo car!
Statement: Prove it.

Myth 11: The centrifugal boys are in reactionary mode
Fact: Well, I guess this in one myth that actually IS true!

Additionally I'd like to add that if people are so sure about all of their fabricated negatives of my system, why don't they come here and state these negatives to me? I think I'm pretty reasonable and use facts and not opinion to back up my statements. I would also recommend that before you make a significant purchase like this to go on and drive an equally prepared car with each of the three systems before you purchase. Come to KY any time if you would like a ride in my car as part of your comparison. Then buy what you want to buy, we live in a free market economy!

We should all be happy right now, it's nearly Christmas!