Originally posted by "Email from friend"



BMW M3 Intercooled Turbocharger System

Fellow BMW enthusiasts,

Our M3 Turbo systems (OBDI & OBDII) are in production. This three year adventure has not been easy or cheap. After the fourth iteration of forced induction BMW's, with almost every conceivable blower type, we simply cannot justify, either technically or functionally, any other mechanism than the turbo. We have unhesitatingly set our sights on building the best "spare no expense" system that twenty-five years of experience permits. It is hugely evident after these last three years that the "best" had to have the turbo.

With the considerable advantage of viewing all the competitor's efforts prior to our own, we have the unique position of knowing all their merits and shortcomings. At the outset, we had no specific agenda regarding blower types. The perception after one trip around the block in an M3 with a centrifugal supercharger (CS), clearly showed that it was not an acceptable machine. It developed little power, with even less potential for more. The response was the worst of any known form of forced induction. Any blown automobile that cannot achieve boost until 3300 RPMs is not going be satisfactory with us, and certainly not what the enthusiast tells us he wants. This system was built by the current "top dog," and is representative of the best offered today.

Our next effort was with the fixed displacement blower and was met with modest success. After months of evaluation we concluded this approach offered the best performance available under 2000 rpm, but real power was going to be very illusive. It did not take long to realize that 2000 rpm was not the performance range that fast motoring was all about. When the extreme complexity and cost required of the fixed displacement blower installation was summed up, the low rpm advantage was not considered worth the trouble and expense.

Then came the turbo. That experience was like stepping out into the light. The whole spectrum of performance of the turbo just came alive and easily proved vastly superior to the dark experience of the CS. Except for boost below the 2000 mark, the turbo performance so badly eclipsed the fixed displacement blower that our path to the intended perfection was clearly spelled out.

We had many objectives in mind when designing this system. Simplicity and serviceability were of urgent priority, but the two greatest objectives were; first, to weed out all the "fussy" little problems that have thus far kept forced induction systems for the BMW from being either acceptable or successful. Second, to build a system technically and functionally correct. Performance would be a slam-dunk as we knew the turbo would pull any supercharger, even spotting 'em a gear. Regardless of fancy ads, chrome plating and polished castings, not one of the current competing products has delivered a logical answer to the question of how to produce a powerful, responsive, trouble free system. That had to come to a stop.

The simplicity, elegance and component quality of M3 turbo are going to solve all these problems and create many happy owners. The performance of the turbo will create even more incredulous owners. The long term durability and fuss free operation of the turbo will produce a whole new cadre of firm believers. The amateur state of affairs in aftermarket forced induction for the M3 is over. RIP.

The Enthusiast's Objectives
Your, and our, objectives for performance and reliability are virtually the same. Only the subtle difference exists that we provide and you use. With you as the user, take a minute and spell out the objectives you personally have for the M3 as equipped with forced induction. This paper will try to make the case that the turbocharger will meet those objectives very well and that the currently popular centrifugal supercharger (CS) will not do so. I at least hope to pique your interest to investigate these things further than just listening to the pitch of a supercharger salesman. Or myself, for that matter.

I would like to suggest that we are all looking for the best combination of the following facets of performance:

Power
Engine durability
System durability
Simplicity
Low speed response
Mid-range torque
Ease of maintenance
Lack of fuss
Rpm at which max boost occurs
Overall quality of driving
Economy
Price

My contention is that the turbo wins every category, hands down.

Why the Turbo?
Don't suggest to me the turbo has been eclipsed by the supercharger. Rather, suggest that the aftermarket supercharger salesman has so distorted technical accuracy as to be an engineering travesty, and his volume of diatribe has been somewhat convincing to many people. Yet, almost completely wrong. For technical excellence and engineering judgement, we should all stick with the industrial leaders such as Rolls Royce, Porsche, Mercedes, Audi, Lotus, Mitsubishi, Toyota, Nissan, Saab, Volvo, and Ferrari. These guys are turbo adherents and lead the world in automotive engineering. May the SC salesman worship Pontiac until higher authority sets him straight. And that one is still not a centrifugal blower.

If racing stirs your blood, and so it should for any real BMW enthusiast, check your history and see when a supercharged car last won a race of any significance. It was 1954, Juan Fangio, in the French Grand Prix driving an Alfa Romeo. Great though these events were at the time, if anyone comes to the grid today with a supercharger, for a race that also permits turbocharging, they will not even make the starting lineup. Over the last 45 years, turbochargers have been essential in winning thousands of races with the diversity of Formula One, Indy Cars, and even the great endurance contests of Le Mans, Daytona and Sebring.

Is the disparity between the abilities of the turbo and the SC that large? Yes it is, and here's why.

Shape of the torque curve:
The centrifugal supercharger is fabled to offer huge low end torque advantages over the turbo. That is flat wrong, never had an inkling of truth. With a size compressor selected for, say 8 psi, the CS must turn at some specific speed at the engine redline to flow the air needed to produce that boost. It is necessary to understand that flow through these types of compressors varies with the cube of the shaft speed. In other words, doubling the speed of the shaft will produce 23, or 8 times the flow. Turn that around and clearly, the flow is 1/8 at half the engine speed that it would be at the redline. Basically, that means you have 1/8 the boost at half the redline. And that is about where it really falls, 1 psi boost at about 3300 rpm.

Please understand, that is what you get with the centrifugal blower, but we strongly suspect it is not what you either want or think you are getting with a supercharger. If you already have one, I'll bet 100 to 1 you are slightly peeved at the complete void of boost in the first half of the rev range.

Wouldn't it be a hoot if someone invented a way to let the same compressor wheel speed up relative to the engine so more low engine speed boost could be produced? Such a device exists and it is the turbo. The key to this great turbo performance benefit is that it can change speeds independently of the engine.

The net result is this: The turbocharger applied to the BMW M3 can produce all of it's boost before the centrifugal blower can produce a single psi.

That leads directly to the relatively stupid question of: what would produce more torque at 3000 engine rpm; a non-intercooled centrifugal blower with no boost, or an intercooled turbo at 8 psi boost? The answer to that question is the magnitude of the enormous difference between the low and mid-range torque of the turbo M3 and the centrifugal M3.


I hope I have answered the question of which type of system will produce the most low speed torque.

The fuss of owning a CS
All of the arguments of this paper are collectively the specific reasons why an engineer and an informed performance enthusiast will chose the turbo. History offers 95% of the proof of that.

As a category of parts, centrifugal blowers have proven to be noisy and non-existent below half engine redline speeds. The added vibrations to what surely is one of the world's smoothest engines, have not been a welcome addition. Further, the constant drive load requirements of the blower give the M3 the feeling of having the A/C half on all the time. Coupled with the ever present belt drive problems of a supercharger, it has often proven difficult to find a willing second time buyer of a centrifugal supercharger. The turbo fixes all this nonsense and creates no new problems peculiar to itself.

These are precisely the reasons there have been no OEM centrifugally supercharged cars produced by even a small manufacturer since 1931. Why? All the same reasons we rejected it. As did Rolls Royce, Porsche, Volvo & etc....

Power
There is a very simple equation for calculating the approximate power output of a normally aspirated engine after a forced induction mechanism is applied. There is no need to solve anything here, but it is quite revealing to understand what the equation says. Five factors are involved:

Power = Po x PR x DC x Evol Ratio x PL

Where:
Po is the original rated horsepower: say 240 bhp

Boost + 14.7
PR is the pressure ratio, or ---------------------, at 7 psi this is 1.49
14.7

DC is the density correction due to heating of the air charge. This is directly proportional to the absolute temperature of the ambient air to the boost air entering the engine. At 7 psi these corrections are about .85 without an intercooler, and about .96 with the IC.

Evol is the volumetric efficiency ratio of the blower to the engine. Since the CS and the turbo have the same Vol Eff, and the engine is the same, this factor can drop out.

PL is the power loss correction due to the necessary power taken from the crankshaft to drive the blower. Here the CS takes about 5% of the engine power, whereas the turbo only robs about 1.5%. The reason for the difference is that the turbo is largely powered by the heat energy in the exhaust gas. Keep in mind that the heat energy lost out the tailpipe is about the same number of horses as the engine makes. Remember, of the fuel burned, 1/3 goes to power, 1/3 to heat in the cooling system, and 1/3 out the exhaust. Therefore the lost exhaust energy and the engine power are about the same. When was the last time you saw a 240 hp fan? That is what is made available to the turbo for a driving force without taping off the crank. We don't need it all, but that's what's theoretically available. Enormous, eh?

So, plug the numbers in and see what they yield:

New Power:
Non-intercooled Centrifugal:

P = 240 x 1.49 x .85 x (1-.05) = 289 bhp

Intercooled Turbo:

P = 240 x 1.49 x .96 x (1 - .015) = 338 bhp

If you can do a back to back test on two real cars, those are very close to the numbers you will get. One wants to ask all the obvious questions of all the CS claims of 340 bhp at 5 psi without an intercooler. They will claim our numbers are silly. Ask in return, if they can prove it to you, show it to you on a real car, or guarantee it you. Go do it, it might and/or will prove fun. Confrontational, but fun.