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ZV
Joined: 27 Nov 2002
Posts: 297
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 Posted: Fri Mar 07, 2003 2:41 pm Post subject: Differential ratio questions, and, what RPM = 60 mph in 5th? |
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Anyone know anything about a 4.35 differential for the 924S/944?
I noticed when I got my 924S that it seemed to be running at a higher RPM than normal at cruise on the freeway (60 mph is 3,000 RPM, 80 mph is 4,000 RPM). At first I thought that the speedo might be off, but I had a friend follow me with a car that had a known-good speedometer and I found that the 924S's speedo was fine.
Working some calculations based on the circumference of a 195/65 R15 tire, I figured out that the overall ratio for fifth gear was 3.610:1. Since the gearbox ratio is 0.829:1, that means that the final drive ratio is 3.610/0.829 or roughly 4.35.
I've gone through the maintenance records and I cannot find anything about a differential, which means that it was probably done early in the car's life when maintenance records are few (after the first owner I have all sorts of records, but very few for the first 60,000 miles).
Are my calculations just way off? Or am I right to suspect that the differential was changed at some point? What RPM's are usual for 60 mph and 80 mph for all of you?
ZV
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Shifting is an art, learn it, love it, live it.
1976 Porsche 914 2.0 - Driving Restoration.
1987 Porsche 924S - Daily Driver |
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Capt EZ
Joined: 20 Feb 2003
Posts: 441
Location: Slidell, Louisiana (east on I-10 from New Orleans) 70458
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| Posted: Fri Mar 07, 2003 7:20 pm Post subject: |
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80Mph = 3200 rpm Approx.
That's about where she likes to run.
Anyone else?
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Margaritaville Airways
1987 924S |
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wdb
Joined: 02 Nov 2002
Posts: 2024
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| Posted: Sat Mar 08, 2003 12:53 am Post subject: |
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the parts interchange manual I have ,quotes manual trans 924s has a (3.88 ratio) ring and pinion gear that is interchangable with 83-88 non turbo 944 8 valve ,and 86 audi 5000 (front axle) CS ,MT.
the auto trans 924s gear ratio is (3.45 ratio ) and interchange with 83-88 944 auto trans and audi 5000 82-83 front axle turbo diesel a.t. |
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924 turbo
Joined: 02 Nov 2002
Posts: 1566
Location: Simi Valley, CA, USA
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| Posted: Sat Mar 08, 2003 2:11 am Post subject: |
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You've definately got something different in there. Also, if you're in North America, your 5th gear should be 0.729, not 0.829. That would put your differential ratio around 5! Yikes!
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Jon Furst |
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-nick
Joined: 16 Nov 2002
Posts: 2699
Location: Cambridge, MA
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| Posted: Sat Mar 08, 2003 2:48 am Post subject: |
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you're sure you're not in 4th gear? just had to ask 
the steepest final drive i've heard of in these audi boxes is a 3.89 (3.889 whatever). you must not spend much time in 1st.
-nick |
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Capt EZ
Joined: 20 Feb 2003
Posts: 441
Location: Slidell, Louisiana (east on I-10 from New Orleans) 70458
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| Posted: Sat Mar 08, 2003 3:41 am Post subject: |
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How do ya'll find all of the wonder numbers?
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Margaritaville Airways
1987 924S |
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ZV
Joined: 27 Nov 2002
Posts: 297
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| Posted: Sat Mar 08, 2003 4:21 am Post subject: |
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| Capt EZ wrote: | | How do ya'll find all of the wonder numbers? |
The numbers for the stock setup are in the back of the manual.
As far as calculating the numbers, that's a trifle more complicated. The first thing I had to do was figure out a rough circumference for the tires. To do that, I needed to find the sidewall height. Sidewall height is the width of the tire (in my case, 195mm) multiplied by the aspect ratio (in my case 65%). I then doubled the resulting figure (since there is sidewall on the top and bottom of the wheel). Then the figure was converted to inches and the diameter of the wheel was added. This gives the diameter of the tire when it's mounted on the wheel.
Since circumference is diameter multiplied by pi, you just multiply the number you got for the diameter by 3.14159265359 to get the circumference. That's how far the car travels for each revolution of the wheel. You convert that number into miles.
To get wheel RPMs at X mph, you divide X by the circumference of the wheel and tire combination (making sure that the circumference has been converted into miles). That's how many revolutions the tire has to make to go X miles.
You then take the total number of revolutions necessary to go X miles and you divide by 60 to get revolutions per minute necessary to go X miles in one hour.
That gives you wheel RPM at speed X miles per hour. To get engine RPM by looking at your tachometer.
The ratio of engine RPM to wheel RPM is your overall ratio. Since the overall ratio is also equal to the gearbox ratio multiplied by the final drive (aka differential) ratio, if you know either the gearbox ratio or the final drive ratio along with the overall ratio, you can use basic algebra to figure out the value of the unknown ratio.
Back to the topic though, I've also noticed that in snow and other slippery situations there seems to be a lot of wheel-hop unless I'm really careful with the throttle. Is that normal with these cars or not? I'd have expected the open differential to just spin the free tire without any hop.
ZV
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Shifting is an art, learn it, love it, live it.
1976 Porsche 914 2.0 - Driving Restoration.
1987 Porsche 924S - Daily Driver |
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beario
Joined: 03 Nov 2002
Posts: 139
Location: st albans/cornwall
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| Posted: Sat Mar 08, 2003 5:40 am Post subject: 924s to 944 turbo box |
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does anyone know the ratios for this box as was trying to work out 0-60 times or 1/4 mile times....she ways 2100 ibs and has a 250bhp tvr engine how do you work this out...mmmmmm? 
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Capt EZ
Joined: 20 Feb 2003
Posts: 441
Location: Slidell, Louisiana (east on I-10 from New Orleans) 70458
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| Posted: Sat Mar 08, 2003 6:06 am Post subject: |
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OK Z! that was a mouthful. I think I just failed "differential" equations. Thanks for the explanation, though. Next time I ask slap me silly and tell me that it doesn't matter on that issue....I don't think I'll be using it.
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live as if you'll die today. - James Dean
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Margaritaville Airways
1987 924S |
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ZV
Joined: 27 Nov 2002
Posts: 297
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| Posted: Sat Mar 08, 2003 3:28 pm Post subject: |
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| Capt EZ wrote: | | OK Z! that was a mouthful. I think I just failed "differential" equations. Thanks for the explanation, though. Next time I ask slap me silly and tell me that it doesn't matter on that issue....I don't think I'll be using it. |
Nothing quite like a resourceful geek, eh?  This is what happens when you pay too much attention in Geometry and Physics. The actual rolling circumference of a tire is actually slightly smaller than I calculated since the tire "squishes" at the contact point, but that's much too complicated for me to be able to figure out. The other stuff was just off the top of my head.
ZV
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Shifting is an art, learn it, love it, live it.
1976 Porsche 914 2.0 - Driving Restoration.
1987 Porsche 924S - Daily Driver |
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Dluney
Joined: 16 Nov 2002
Posts: 25
Location: Sugar Hill,GA.USA
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| Posted: Thu Apr 10, 2003 7:03 am Post subject: |
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There is a 4.1 ring and pinion available. It was standard equipment on the 88 or so Audi 5000 2wd. I installed one in my ITS 924S and it makes about 200 rpm difference from the standard 3.89. BTW the 924S come standard with a "short" fifth gear at .8286 Where 944's up to 87 have a .7297
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David Luney
87 924S SCCA ITS |
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78porsche924
Joined: 14 Dec 2002
Posts: 1217
Location: Newark, DE(near where DE,MD and PA meets)
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78porsche924
Joined: 14 Dec 2002
Posts: 1217
Location: Newark, DE(near where DE,MD and PA meets)
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| Posted: Thu Apr 10, 2003 7:47 am Post subject: |
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| The only trans listed on here that has a 5.0 diff is the the 016/Y or the 79 NA snailshell. |
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RageLtMan
Joined: 05 Mar 2003
Posts: 184
Location: Boston, Ma
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| Posted: Thu Apr 10, 2003 6:41 pm Post subject: |
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zv, i get the exact same rpm/speed ratio as you do, 60mph at exactly 3k and 80 at 4. however, i've had the car up to 140 before, and it was still under 6... so i dont know what the hell's going on.
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1987 924S 2.5L SOHC. |
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924 turbo
Joined: 02 Nov 2002
Posts: 1566
Location: Simi Valley, CA, USA
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| Posted: Thu Apr 10, 2003 8:58 pm Post subject: |
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Whoops! Dluney is right that 5th in the 924S is .829. Sorry for the misinformation.
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Jon Furst |
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