[flash]

i accidentally dug up a formula i found decades ago that dealt with real world horsepower to speed, and thought it would be an interesting reality check for many of us



we all talk about horsepower increases and anticipated speeds - we have also seen people talk about top speeds the cars should be capable of with a given amount of horsepower - there is talk of the car going 200mph amongst some, and 175 amongst others - i thought i would take a minute and play with the formula



besides the fact that the speedometers in the 968 have a high error rate (indicated 139 is really only 130), it takes a lot more power to get to that speed than you would think to go fast



with all factors remaining constant (not possible, but fun for the math) given that the car tops out at 156 with 240hp:



160 would require 258hp - this would be an indicated 171

165 would require 283hp - this would be an indicated 176

170 would require 310hp - this would be an indicated 181

175 would require 338hp - this would be an indicated 187

180 would require 368hp - this would be an indicated 192

185 would require 400hp - this would be an indicated 197

190 would require 434hp - this would be an indicated 203

195 would require 468hp - this would be an indicated 208

200 would require 506hp - this would be an indicated 213



this does not take into account any aerodynamic changes that would need to be made to keep the car on the ground at high speed - the car is know to get very light over 160 - that's why they used the turbo s wing - but, anything adding downforce, also adds resistance, so the power needs would go up as well, and it is going to take a lot more power to reach 200, assuming you can overcome the lift, so this is where the formula falls down - but, i kept the numbers running anyway though so you could see how bad the speedo was



on that, and in more real world attainable terms for us:

an indicated 150 is only 140

an indicated 155 is only 145

an indicated 160 is only 150

an indicated 165 is only 154



so, if you have a chip/airbox, and a good cat-back you are probably topping out at right about 161mph, and your speedo should be reading 172



the numbers are even worse for us cab guys - it will take 306hp to hit 165 in a cab



and now i really need another cup of coffee

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[Renalicious]

On the subjects of math and formulas, this is something I did back a while back when I was noobishly (not really a word) comparing one car against another in terms of gearing ratios and the like:



968 gears

---------------------------

1) 3.1782 = 12.0072:1 ... 1.38806rev/s x3600 = 04997/h /518.1149 = 09.6kph / 1000RPM

2) 2.0000 = 07.5560:1 ... 2.20575rev/s x3600 = 07941/h /518.1149 = 15.3kph / 1000RPM

3) 1.4350 = 05.4214:1 ... 3.07424rev/s x3600 = 11067/h /518.1149 = 21.4kph / 1000RPM

4) 1.1110 = 04.1974:1 ... 3.97071rev/s x3600 = 14295/h /518.1149 = 27.6kph / 1000RPM

5) 0.9120 = 03.4455:1 ... 4.83723rev/s x3600 = 17414/h /518.1149 = 33.6kph / 1000RPM

6) 0.7780 = 02.9392:1 ... 5.67048rev/s x3600 = 20414/h /518.1149 = 39.4kph / 1000RPM

final) 3.7780



Formula @1000 RPM:

16.667 RPS / (gear ratio) = tire RPS

tire RPS x 3600 = tire RPH

tire RPH / <rotations per kilometer> = KM/H



Theoretically at 7000 rpm, we could be going about 275 km/h, or 170 mph. According to Flash's formula, we would need over 300 hp <img src="/forum/images/smilies/968/smile.gif" class="smilie" alt="" />



Later came across the 968 gear chart which more or less confirmed the math lol

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[flash]

we are such geeks

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[Anchorman]

Just addressing the point about 968 speedometer error - while this may be the case at high speeds, on the occasions when I have the chance to check the speedo reading against a radar gun (approaching highway toll booths, a few curves where the locals think cars should go slower than they (I) usually do, etc., I've found my speedo to be spot-on. For example, there is one place where I will engage cruise control at, say, 70 mph, and the radar readout will be 70 mph. Again, there may be deviation at higher speeds, but I can't run these little tests at those speeds.

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[flash]

i'll bet your tires are a bit bigger too - wouldn't take much - it is a percentage, so it would be less at lower speeds than at high ones



also, i am going from a test conducted by autocar magazine - the did it from a true 30 to 130 - the car had 17s on it with bridgestone expedias

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[Anchorman]

Careful - I try to avoid comparing sizes. <img src="/forum/images/smilies/968/wink.gif" class="smilie" alt="" />



Along those lines, though, and to digress (pardon me for this, folks, but I thought this is a cute story that I'll keep clean): A husband says to his wife, "Tell me something that will make me both happy and sad". She replies, "Your's is bigger than the UPS delivery man's."

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[Duckman]

Hey, guys. I hardly ever post anymore but thought I'd throw my two cents into this discussion. My car does have oversized tires and it has been stop-watch timed @ 60 mph when the speedometer registers 58. That's an error of 3.45%. When my speedo registers 72.5 mph, the car is traveling 75. On my car that registers as 3,100 rpms.



If the car lost no speed due to drag, which is doubtful, it would hit 150 mph @ 6,200 rpms, and attain a speed of 169 @ 7,000.



According to Porsche, the car is capable of reaching 156 mph @ 6,600 rpms. That is consistent with with my car's 75 mph @ 3,100 rpm when adjusted for the speedo error caused by my oversized tires.

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[Eric_K]

I doubt the error rate is the same between cars. There are too many variables - tires being the biggest one.

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[flash]

agreed that tires are a poetential issue, however, with a given diameter and its inherant revs per mile, and the fact that this is an electronic speedometer, and not a mechanical one, things should be reasonably consistent car to car



i have now ridden in or driven a number of cars, and they all seem to read a bit high - perhaps not as high as the test in the magazine article, but high nonetheless - it gets worse as the tires wear too



this seems to be verified when we hear people say they were doing 165 in a stock car - we know that is not possible, and pushing it in a car with a chip/airbox/cat-back



should be pretty easy to verify with the tach too - we know the gear ratios and miles per 1000 rpm - take it to a particular speed at a particular rev, in a given gear and then do the math

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[gryphon]

I guess I've been fortunate with my two 968s. Both speedo's have been within 2 mph to up to 100 checked by my hand-held GPS receiver.



Tom

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[flash]

i've been wondering about the gps things - i have one in the denali - it is not as accurate as i thought - over a distance it is, but it's actually off over short distances - i found that if i set the cruise, and then watch it, over a mile it will be off a touch, but over 10 miles, it's dead nuts

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[RPM]

My TomTom One nav system seems very accurate and confirms the speedometer at every speed I have checked it from 40 to 110 (I am such a geek - went out as soon as I got the thing). I do have to hold a specific speed steady for a few seconds for the system to update and re-compute (bit of an adventure at over 100 on a public road). It seems about as dead on as it can be with a speedometer without detailed gradations - clearly within a mile or two after the GPS catches up. Hard to image that both the speedo and the nav system would be inaccurate overall, but in sync with each other.



My tires are stock size on 16" rims.

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[flash]

i haven't tried checking my 968 speedo yet, since i don't have a gps in that car (yet), and i haven't checked to see just how little distance the gps in the denali needs, but i tend to think they all will behave pretty much the same

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[gryphon]

[quote name='RPM' post='31807' date='Mar 2 2007, 04:41 PM']I do have to hold a specific speed steady for a few seconds for the system to update and re-compute (bit of an adventure at over 100 on a public road).[/quote]



And, trust me on this, it's really easy to fly right by a cop while focusing on GPS speedometer testing. Fortunately he was busy fighting crime and didn't come after me.



Tom

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[SpeedForceRacing]

[quote name='flash' post='31774' date='Mar 2 2007, 09:30 AM']i accidentally dug up a formula i found decades ago that dealt with real world horsepower to speed, and thought it would be an interesting reality check for many of us



we all talk about horsepower increases and anticipated speeds - we have also seen people talk about top speeds the cars should be capable of with a given amount of horsepower - there is talk of the car going 200mph amongst some, and 175 amongst others - i thought i would take a minute and play with the formula



besides the fact that the speedometers in the 968 have a high error rate (indicated 139 is really only 130), it takes a lot more power to get to that speed than you would think to go fast



with all factors remaining constant (not possible, but fun for the math) given that the car tops out at 156 with 240hp:



160 would require 258hp - this would be an indicated 171

165 would require 283hp - this would be an indicated 176

170 would require 310hp - this would be an indicated 181

175 would require 338hp - this would be an indicated 187

180 would require 368hp - this would be an indicated 192

185 would require 400hp - this would be an indicated 197

190 would require 434hp - this would be an indicated 203

195 would require 468hp - this would be an indicated 208

200 would require 506hp - this would be an indicated 213



this does not take into account any aerodynamic changes that would need to be made to keep the car on the ground at high speed - the car is know to get very light over 160 - that's why they used the turbo s wing - but, anything adding downforce, also adds resistance, so the power needs would go up as well, and it is going to take a lot more power to reach 200, assuming you can overcome the lift, so this is where the formula falls down - but, i kept the numbers running anyway though so you could see how bad the speedo was



on that, and in more real world attainable terms for us:

an indicated 150 is only 140

an indicated 155 is only 145

an indicated 160 is only 150

an indicated 165 is only 154



so, if you have a chip/airbox, and a good cat-back you are probably topping out at right about 161mph, and your speedo should be reading 172



the numbers are even worse for us cab guys - it will take 306hp to hit 165 in a cab



and now i really need another cup of coffee[/quote]





Flash if you make it down one day,I will take you for a ride in my 200+mph daily driver <img src="/forum/images/smilies/968/cool.gif" class="smilie" alt="" /> Well according to your math anyways, it will attain 200+ with room to spare. My car has a better CD then the 968 too. Its not a Porsche though <img src="/forum/images/smilies/968/sad.gif" class="smilie" alt="" /> Unfrtunately it still has a speed govenor so I can only take you to 155 very quickly.LOL

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[Kim]

I have a GPS and also find that if you set the cruise at, say 110 km/h and reset the GPS, after about 5 minutes the average speed is about 100 km/h. I have always wondered if it was just the result of the PO replacing the US speedo with a "used" Canadian one, perhaps from the 944S2 that was also in the lot. Now I don't think so.

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[333pg333]

The same inaccuracy is on my 951. I have always allowed approx 10% error rate but is this fair? Does the inaccuracy just keep getting bigger the faster you go, or is it just sort of about 10mph out and stay there?

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[ether_joe]

well the good news, it would seem, is that a V8 conversion with custom gearing would get us pretty efficiently to these upper numbers. I mean, put a Z06 engine and there's 505 hp. . .



then another wallop of time and money modding the car to deal with THAT . . . if it's even possible. . .



but on the face of it a 350/400 hp V8 car would seem very doable, at least on paper. . . ?

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[Renalicious]

Well what about the supercharged / turbocharged engines that put out 400+ HP. I think you can make the car as powerful as you want it, but the stock gearbox will keep you from going any faster than 275kph, or so. Unless you want to drive well into the redline <img src="/forum/images/smilies/968/smile.gif" class="smilie" alt="" />



Personally I think 275 is plenty fast for North America. Besides, what can you do at that speed other than go straight? Sure it's exciting, but I'd rather be able to take corners better than other cars, and show them what a 968 can really do <img src="/forum/images/smilies/968/wink.gif" class="smilie" alt="" />

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[flash]

wow - lol - i was only playing around with a little math - this wasn't intended to become a debate about the merits of such speed - clearly the car was not designed for it, and will likely never be capable of it, no matter what is done, but i thought it was fun to look at and think about



tim - the formula presumes x hp to reach y speed, and calculates from there - so, depending on what was stock and how fast it went with that power, it can be calculated how fast it will go with additional power - 200 is one of those near impossible numbers for any car - it takes a LOT to make a car stay on the ground at that speed, so regardless of what power we see tuners getting, they are usually still top speed limited by the aerodynamics



333 - it's a percentage, so it's less at low speed and more at high speed



ren - yeah - the gearing is an issue - but, with any engine that puts out that kind of power, i would think you would have at least an 8k redline, so you'd get pretty close



have fun guys - it sure has me thinking

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