[quote]Originally posted by over1g:
<strong>I just took my car out, 100mph comes at 6krpm, and redline (approx. 6450) is around 118mph indicated. Stock size tires, stock gearing 86 turbo...
Ahmet</strong><hr></blockquote>

Your speedometer is definitely off, as is mine.

Here is some speed/rpm information for a small diameter and large diameter tire using the US spec 951 R&P gearbox (I used tire rack's revs per mile data for speed calculations):

Yokohama AVSi 245/45-16 841 revs per mile
6000 rpm 90.6 mph
6500 rpm 98.1 mph
7000 rpm 105.7 mph

Yokohama A032R 275/40-17 804 revs per mile
6000 rpm 94.8 mph
6500 rpm 102.7 mph
7000 rpm 110.6 mph

Ahmet, what size tires do you have? I'd love to participate in this test, but it is kind of hopeless with speedometer/tachometer error. Perhaps someone has a GPS we could all borrow.

 

I've hit over 100 in my 951 in 3rd... his speedo isnt off...

 

i dont know if your speedo being off would actually make any difference, 1-3 mph difference in calibration isnt a whole lot cause actually we're looking at a specific time to travel between 70 and 100 and not how fast it really is. the mph number is just a reference point the answer is the time between the 2 points.

 

This thread caused me to go buy a stopwatch, will post my times tomorrow, still a few more miles of break in, with my "minor" mods times should be respectable at 10psi boost

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[quote]Originally posted by ElonDooberhead:
<strong>I've hit over 100 in my 951 in 3rd...</strong><hr></blockquote>
Was this indicated on your speedometer? Have you calibrated it against anything else? What size rear tires do you have? I have seem 100 mph on my speedometer in 3rd gear in my 951 below redline. Additionally, I have hit what I have calculated to be 100 mph in 3rd gear, but it definitely was not at 6000 rpm.

[quote]Originally posted by ElonDooberhead:
<strong> his speedo isnt off...</strong><hr></blockquote>
Wow, you must be VERY familiar with his speedometer and his tires. I never said that he didn't hit 100 mph in third gear, I just wanted to point out that 100 mph in third gear at 6000 rpm in a US spec 951 is far from reality with our standard tire sizes. You would need 27.4" diameter tires to hit 100 mph at 6000 rpm. Tires used on our 951's are usually between 24.5"-25.7" in diameter. Smaller diameter tires will likely provide an acceleration improvement in this test, but not to the extent that the speedometer would lead you to believe.

Assuming that the Ahmet's speedometer error is like mine, and increases by a standard percentage over the actual speed, it appears that 70 mph equates to 4200 rpm. Assuming that he is using the common 245/45-16 rear tire size, his speed is actually 63.4 mph at 70 mph indicated, and 90.6 mph at 100 mph indicated. This would obviously make Ahmet's numbers more favorable compared to actual speeds because there is frictional drag at lower speeds, our cars tend to run out of steam at higher rpms with stock turbos, and he is only accelerating 27.2 mph instead of 30 mph.

Of course, there is slight possibility that his speedometer is accurate and his tachometer is wildly inaccurate, but that seems much less likely.

Still, I'll give it a shot tomorrow using the indicated as well as what I believe to be the accurate numbers.

 

[quote]Originally posted by PorscheG96:
<strong>Nope, my 3rd tops out at 100 mph as well [6500 RPM].</strong><hr></blockquote>

Of course, you get your speed off of the ABS sensors instead of the gearbox. I suspect that your speedometer is much closer to reality than ours. How far are your tire sizes from stock specs?

 

I have 255 zr-17's in the rear. Maybe i hit 100 mph close to 6500. I think I'll go test it just to be sure

 

"You would need 27.4" diameter tires to hit 100 mph at 6000 rpm. "

Well, you may be actually be traveling 100mph, but your speedo will still be indicating 94mph@6000rpm. That relationship won't change no matter what size tires you have.

Now there must be a calibration issue of some sort with my car because I've seen 115mph on my speedo in 3rd gear before. The best I can place it is that the speedo is optimistic by 5mph. Because 7000rpm in 3rd gear is really only 110mph.

 

[quote]Originally posted by Danno:
<strong>Well, you may be actually be traveling 100mph, but your speedo will still be indicating 94mph@6000rpm.</strong><hr></blockquote>
What you meant to say is that the speedometer won't have a different reading with the same rpm and gear, but different tire diameters. But, that doesn't mean that with the stock diameter it was actually accurate.

[quote]
<strong>That relationship won't change no matter what size tires you have.</strong><hr></blockquote>
I completely agree, unless of course you use one of those speedometer correction boxes or a different speed sensor drive gear.

[quote]
<strong>Now there must be a calibration issue of some sort with my car because I've seen 115mph on my speedo in 3rd gear before. The best I can place it is that the speedo is optimistic by 5mph. Because 7000rpm in 3rd gear is really only 110mph.</strong><hr></blockquote>

Long ago I had a friend time mile markers on the side of the freeway several times at both 60 mph indicated and 90 mph indicated. I pretty much came to the conclusion that my speedometer was about 10% optimistic (54 actual at 60 indicated, 81 actual at 90 indicated). That number pretty much jives with the 3rd gear, 6000 rpm, 100 mph indicated with 245/45-16 tires actually being 90.6 mph that I mentioned earlier.

When I test it tomorrow I'm going to try from 70 indicated to 100 indicated and also from the corrected numbers, which should work out to about 77 indicated and 110 indicated.

I see about 150 mph indicated at 6500 rpm in fourth gear, when I know the actual is around 137 mph.

What I don't know is how much of a factor tire stretch is in all of this. I suspect with radial tires it is neglible.

Oh well, time to party! <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" /> <img src="graemlins/drink.gif" border="0" alt="[cherrsagai]" />

 

Well My S2 with an '86 turbo gearbox seems to hit the old rev limiter of 6400 I think right around 105 indicated. My rev limiter is now 7000 but I don't take it up that high very often because I'm not sure how well my 150k mile rod bearing will handle it. I may try this test and see my times, obviously going to be much slower than you turbo guys but, just for comparison purposes.

 

[quote]Originally posted by Steve Lavigne:
<strong>Of course, you get your speed off of the ABS sensors instead of the gearbox. I suspect that your speedometer is much closer to reality than ours. How far are your tire sizes from stock specs?</strong><hr></blockquote>
I've seen 100 mph with the following front tire sizes. Since my speed is measured from the front wheels instead of the rear wheels like a 944...

stock tires 205/55-16: OD 24.87795275590551"
race tires 225/50-16: OD 24.858267716535433"
current tires 225/45-17: OD 24.97244094488189"

They all indicated right around 100 mph at the top of 3rd gear with a late model 86 951 gearbox [used in MY 87] and it's got the cooler loop, open diff.

 

ok, so what kind of error can I expect to see with 275's in the rear?

 

[quote]ok, so what kind of error can I expect to see with 275's in the rear <hr></blockquote>


<a href="http://www.4lo.com" target="_blank">www.4lo.com</a> good tire calculator here

 

I have stock sized 225/50/16s in the rear. I top out 3rd around 117 indicated. I do know that the speedo is within 1mph @ 50 checked with radar.
Ahmet

 

I did some tests on my 1991 S2 a while back and these are the results, car has an AMD custom chip , K & N panel filter and St.St. exhaust. Can't do 100mph in 3rd so I did it in 4th, but I was mainly testing the 50-70mph times. This is used in a lot of road test here in the UK and is a good measure of the cars overtaking ability.
3rd Gear
50-70mph - 3.4secs

4th Gear
50-70mph - 5.2secs
70-100mph - 7.8secs

Stock car figures according to Autocar tests in 1989 are:-

3rd Gear
50-70mph - 3.7secs

4th gear
50-70mph - 5.5secs

I'd be interested to hear how quick the Turbos can do 70-100mph in 4th so I can get a direct comparison, or as direct as you can get with the different gearing.

Andy

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