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Excellent post, thanks! Boy that guys a gearhead! (archive)

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Posted by Steve Borsse on June 11, 1999 at 12:01:34:

In Reply to: More bad news for the Boxster? posted by Tim Milligan on June 11, 1999 at 11:12:31:

: I Found this on the Boxster message board. I know nothing of its authenticity, but it doesn't look good, does it?

: --- Start of Quote ---

: Here's an interesting editorial from 'Das Blinkin' Blatt', the newsletter of the Porsche Club of America, Germany Region. The author is Stef Scheepers. In the article, Stef mentions his Alpha GTV, but his P-car is an RS-America - very basic, very nice, and very fast.

: "Boxsters & Missing Ponies by Stef Scheepers

: Before we start the real story, let me first explain to you a few things about dynos and horsepower. A dynamometer (dyno) is a device that is used to measure the power output of a vehicle. There are generally two types of dynos in use; the engine dyno, which measures horsepower (hp) directly at the flywheel, and the chassis dyno, which measures horsepower at the rear wheels (which is flywheel hp minus transmission losses). To hook up to an engine dyno, the engine first have to be removed from the vehicle and then bolted to the dyno test bench. This is obviously an expensive and time-consuming operation, therefore most people go for the second option. To hook up to a chassis dyno, you just drive your rr wheels (if it's a Porsche) or your front wheels (if it's Jap-crap) onto a set of rollers, floor the throttle and take your reading. Unfortunately dynos have an inherent problem with accuracy. Dynos can reliably show power increases (before- and after comparison), but they have to be calibrated if you want reliable horsepower readings. However, it is extremely difficult to find a reliable constant power source of say 100 horsepower as a reference for your calibration. There are further issues that affect the hp readings: air pressure, ambient air temperature and tire slippage on a chassis dyno. To summarise: with a dyno it is reliable to measure (compare) that your new free-flow exhaust brought 5% improvement in power, but it is not-so-reliable to measure that your power went up from 100 hp to 105 hp. Next time somebody tells you that he had his engine tuned, then dyno'd and now it makes say 300 horsepower, take it with a pinch of salt. He most probably had it on a chassis dyno, which can give chronically unreliable horsepower readings. Furthermore, it is very easy to cheat the hp readings. To try and compare apples with apples, all dyno readings are "corrected" to a hp value at standard air temperature and -air pressure. If the dyno operator heats up the ambient air temperature sensor (by sticking it in his underpants for example) and then "correct" the hp, you will see a dramatic rise in power. 300 horses is not always 300 horses. Now that you know a little more about dynos, lets talk about horsepower. I don't want to bore you with the scientific definition of hp, let's just say it is simply the ability of an engine to perform work. To complicate things slightly, there are two types of horsepower: SAE horsepower (Society of Automotive Engineers) as used by the Americans, and DIN horsepower (Deutsche Industrie Norm), also called PS (Pferdestärke), as used by the Germans. For all purposes they are about the same, but you cannot make a direct comparison, since they are measured in totally different ways with not one common "fudge factor" between them. In the mean time the world have gone metric and we should actually talk about Kilowatt (kW). 1 kW is 1.360 PS, or 1 PS is 0.735 kW. If you hear about brake horsepower (bhp), that is just hp measured on a dyno (also called a brake), it is not a different type of hp. When comparing vehicles, most people stare themselves blind at the engine's rated peak bhp and forget the really important thing: the torque values. To simplify things, torque is the ability to turn (twist) a shaft (crankshaft). Torque is a static force (the shaft don't have to turn, you just measure how much it wants to turn) while horsepower is a dynamic force (the shaft must turn). Hp is basically torque times revolutions, which means that if you double the revolutions at the same torque, you double the hp. Rev your 911 to 17 000 rpm and it will make as much power as a F1 McLaren (Don't try it! On paper it could make that much power, but in real life a C2's motor tends to part company with it's inlet valves at anything over 8500 rpm) The reason why I mentioned torque is that my opening paragraph is not 100% correct: a dyno is used to measure the torque output of a vehicle and not the horsepower, but since the torque is measured at pre-determined rpm levels, the end result is horsepower. Now we can start the real story: I recently read an "interesting" article in which a tuning company insinuated that Porsche fiddled the Boxster's bhp figures: Company XYZ wanted to do some engine development work on a Boxster, but first a visit was made to the dyno. In baseline testing the stock Boxster put out a mere 136 bhp at the rear wheels on a Dynojet chassis dyno. The Boxster was re-tested several times and also taken to a Mustang chassis dyno, where it registered very similar numbers. Still scratching their heads, they then tested a different Boxster and got almost the same numbers. Typically, a manual trans-mission will show about 15% difference between rated flywheel bhp and tested rr wheel bhp on a Dynojet chassis dyno, but the Boxster showed a 32% difference. This would indicate that either Porsche overrates the engine by approximately 40 bhp, or the Audi-built transaxle have an unnatural appetite for horsepower. Reverse working the equation, 136 bhp at the rear wheels would be generally consistent with an engine developing about 160 bhp at the flywheel. Even though I have a copy of the dyno graph, this does not mean that I should blindly believe their results. I do not have too much seat time in Boxsters and do not profess to be an expert on the virtues (or lack of -) of a Boxster, but the article did start me wondering.... Stepping out of my RS into a Boxster Tiptronic, I thought it was as quick as a slug. The manual version was only slightly better. Furthermore, coming back from Le Mans I raced a 205 bhp Boxster with my 150 bhp Alfa GTV and I have never seen two vehicles that were more evenly matched in acceleration and top speed. Believe me, the Boxster driver caned it for everything that it had. At the time I thought that the Boxster must have some "less than optimal" aerodynamics that used up it's 55 bhp power advantage over the Alfa., but now I'm not so sure anymore. The Boxster have a drag coefficient value (Cd) of 0.31, a frontal area (A) of 1.94 m2 and a rated top speed of 240 km/h, which equates to a velocity (v) of 66.667 m/s. he air density factor (q) is 1.202 kg/m3. Using P(watt) = 0.5 x q x Cw x A x v3, I calculated that a Boxster would need 107 kW (146 bhp) at the rear wheels to do 240 km/h. Adding another 15% for estimated transmission losses brings you to 168 bhp at the flywheel. I don't know what to make of that: If I'm wrong then Porsche will hate me, if I'm right then Porsche will hate me even more. This potato is a little too hot for my fingers (and I have no desire to get sued either) . What I'm not afraid to say is that I don't believe the "official" engine power and torque curves as shown in the owner's handbook. I have spent a substantial part of my engineering life designing engines and I can recognise a "fixed" power curve when I see one. But then Porsche is in good company: we all "fix 'em" when they look bad.

: Maybe one of our Boxster owners can verify his car's top speed. There is no way on earth that you can reliably trust the speedometer to give you an accurate speed reading, but when you look at the Boxster's gear chart then the motor must turn 6350 rpm to do 240 km/h. Alternatively you can use a stopwatch: 15 seconds over a flying kilometer equals 240 km/h."

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