top speed question
 

well i just saw the pic on the internet.i thought the 97-99 top speed was 149mph.
how is the boxster doing 150mph and still have room to go?


http://www.industryfigure.com/berkeley/Porsche/Germany/243kph.jpg

We are not allowed to talk about this! Sorry. :)

edit: whoops... never mind.. my fingers were faster than my brain...

Posts about ridiculous speeds on public roads is not encouraged by the owners, but this is a legitimate question to me. Just how fast will the 2.5 boxster go?

Hi,

Well, factor in the well-known Porsche Safety Factor (+3-5MPH) speedometer reading and the car is really going only 145-148MPH. Only the Climate Control display, in diagnostic mode, will display true speed (in KPH).

Add to this that the car has already reached it's Peak HP and it's not going to go any faster. The Fuel tank reads 3/4, and we don't know if there is a headwind/tailwind, an incline/decline, a passenger load, though the Top does appear to be UP, so 149MPH sounds about right under optimal conditions...

Happy Motoring!... Jim'99

To clarify:

Is this only not allowed if someone is talking about driving 149MPH on a road?

If we are talking about “how fast have you driven your Boxster on a track in controlled conditions during a legitimate PCA, PDA or NASA sponsored event,” are we OK here?

Also, keep in mind the car might be modded and not bone stock so the top speed may be slightly increased.

yeah, he might be running rocket fuel

Just because its at its peak horsepower doesnt mean it wont go any faster, unless the guy was flooring it and it stayed at "150" for like 10 seconds without the rpms going up any more (aerodynamicaly limited). Ive topped out my bmw under "controlled and track regulated conditions" and I only took my foot off the gas because I didnt want to run in the red for too long, and its peak torque was at like 4 grand. I guess all im saying is that just because it reached its peak torque/horsepower doesnt mean it wont continue to accelerate, it just wont do it as fast.

It's not unreasonable to think that he may have 15-20 or so hp more than a standard 2.5L and that raised his top speed about 2mph. :rolleyes:

Yeah, that's what this kid I know does with his Civic.

Hi,

We disagree, which, when discussing Torque, Horsepower and Top Speed isn't unusual because there is a huge variance in the knowledge base, how it applies, and the fact that when making such all-inclusive statements, you're really discussing the theoretical, which is rarely present. The following is an except from a pretty good article I have on the subject. I don't have the source handy, but I'll look for it and edit this post later.

There's been a certain amount of discussion about the concepts of horsepower and torque, how they relate to each other, and how they apply in terms of a car’s performance.

Looking at top speed, horsepower wins, in the sense that making more
torque at high rpm means you can use a taller gear for any given car speed,
and thus have more effective torque at the drive wheels.

Finally, operating at the power peak means you are doing the absolute best you can at any given car speed, measuring torque at the drive wheels. I know I said that acceleration follows the torque curve in any given gear, but if you factor in gearing versus car speed, the power peak is it.

An example, of the LT1 Corvette will illustrate this. If you take it up to its torque peak (3600 rpm) in a gear, it will generate some level of torque (340 foot-pounds times whatever overall gearing) at the drive wheels, which is the best it will do in that gear (meaning, that's where it is pulling hardest in that gear).

But, if you gear the car so it is operating at the power peak (5000 rpm) at the same car speed, it will deliver more torque to the drive wheels, because you'll need to gear it up by nearly 39% (5000/3600), while engine torque has only dropped by a little over 7% (315/340). You'll net a 29% gain in drive wheel torque at the power peak versus the torque peak, at a given car speed.

At any other rpm (other than the power peak) at a given car speed will net you a lower torque value at the drive wheels. This would be true of any car on the planet, so, theoretical "best" top speed will always occur when a given vehicle is operating at its power peak [sic] Horsepower.

Now, again, it's important to note that the author uses the term theoretical, because these are not the only factors. In fact, the variables are almost too uncontrolled to account for, or duplicate accurately every time such as; Air Density, Parasitic Losses, Frontal Area and Cd, etc. This is why two cars with the same Power/Torque values and curves can have very different performance.

We need an easy way to make some assumptions and so these are often not included, but make a huge difference in real life. Instead, we use HP/Torque curves from which to draw some conclusions.

But, even the value Horsepower is an arbitrary figure. James Watt determined this to be 33,000 ft.lbs. of work per minute, but he did so after observing some Draught Horses pulling coal from a mine and actually determining that they produced 22,000 Ft. Lbs. of work in one minute. He then threw in an arbitrary 50% Fudge Factor to come up with the 33,000 Ft. Lbs./min. which today we call 1 Horsepower

I did some rough calculating using a standard model (STP) and an estimate of the Boxster's Frontal Area, Cd, Parasitic Loss, Standard Gear Ratios, and such. The estimates are necessary because I lack such things as my own personal Wind Tunnel and Rolling Road.

Based on these reasonable estimates, the calculations showed that the Boxster will hit the wall (aerodynamically limited) at 176.2MPH (Coincidentally, I have never heard of a Natrually Aspirated, Gasoline fueled Boxster reaching this speed). The car would need to be modified to produce 532 HP (my theoretical limit of how much you can Bore and Stroke the motor and still use Natural Aspiration, Gasoline as a Fuel, and retain reasonable strengh).

I'm sure my calculation could be debated, but I'm equally sure that it has an accuracy within ±5MPH. Using these same factors, my calculations on the 2.5L Boxster show it's limit to be 148.8MPH (manual tranny) and 146.1MPH (Tiptronic S) which agrees pretty much with the published data.

Remember, it takes 4 times the power to go only twice as fast at a given speed. In the end, Drag wins over Torque, Horsepower, or Gearing everytime(again, except in theory)...

Happy Motoring!... Jim'99

Are you saying a boxster needs approx 532 hp do 176.2 mph? If it had a taller ratio surely it would be able to due more with that kind of power. Is the stock boxster redline limited at 176.2 mph? If not then what speed?

Hi,

Shifting at Red Line will not make the car any faster at all. It will just get you to the Top Speed marginally sooner.

Taller gearing is always theoretically possible, but there are limits such as the availability of higher ratios and maintaining some sort of drivability. If all you're after is a Bonneville Land Speed Racer, then that's different.

But, the power requirements increase relative to the square of the Speed being travelled and this limits what can be achieved tremendously. And, it's just not as simple as adding +1 Horsepower to the Math or Theory, because Drag and Stability issues will win in the end...

Happy Motoring!... Jim'99

Personally I wouldnt want to gear it so high I had to get a push start from every red light I hit, lol. I think 150 (149 whatever) is just fine. Although......... I would really like to go 170ish, on the autobahn of course :aherrmm: because ive "heard stories" that the white lines in the middle of the road turn into a solid stripe.

I can confirm the white line thing. I sustained 265 Kph in a fairly new 2000 Carrera for about 10 minutes on the autobahn out of Berlin years ago and noticed this right away. The exits pass you like lamp posts at "normal" freeway speeds too.

One's field of focus and concentration moves far further down the road as well. I noticed that I was looking for cars and road conditions three miles away instead of just a little ahead of the car when I was driving it 85 Mph.

Well, in the case of the boxster and boxster S we don't need more gearing to go faster we need more power. The boxster is drag limited not gearing limited. Just look at the original posters pic.....he at the 150 mph top speed of his 2.5L boxster but he has about a 1/2 grand left on the tach.

Hi,

Bingo!!! He has 400-500 rpm left, but he's already past peak HP (assuming the car is stock), so he's not going to make any more power with those extra spins. And, it's power, not spins, which beats the wind. Classic case really.

But don't lose sight of the fact that you cannot just continually add more power (except in theory). A Top Fuel Dragster making 6,000HP can't simply double it to go faster, there's a point where the trade-offs cease, trading off that is. More HP, stronger engine, more fuel , equals more weight, meaning even bigger engine, even more fuel, even more weight, and on and on.

There are engineering limits. These can, and have, been overcome, the Land Speed Record exceeding MACH1 for example, but these technological breakthroughs take years to solve and some never (at least to present) are...

Happy Motoring!... Jim'99