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difference between a 931 and 937

 
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Rosnik  
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PostPosted: Thu May 09, 2002 11:02 pm    Post subject: Reply with quote

Hi Guys ,
Is there somebody that my to explain to me the mechanical difference between a 931 ( 924 TURBO )
and a 937 ( 924 Carrera Gt ) the intercooler apart ?
regards
Nico

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Rick MacLaren  
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PostPosted: Thu May 09, 2002 11:45 pm    Post subject: Reply with quote

From memory and what I've read, it's the IC, compression bumped (via pistons) for smoother ride and digital ignition. I've also heard the car has numerous other differences that the pros know about. I am no expert on the 937, and you're best to ask John H from NZ, as he's built a replica (Money is on the way John, thanks).

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John H  
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PostPosted: Fri May 10, 2002 8:45 am    Post subject: Reply with quote

Very breifly the main mechanical differences off a 937 over the 931 (1981 model) are:

- Pistons are KS forger rather than Mahle cast pistons (same compression ratio)
- Installation of charge air cooler (inter cooler)
- Different Charge pressure control valve (Waste gate set to different pressure)
- Different Turbo charger – larger turbine housing – compressor side the same as a 931.
- Camshaft is clear chill casting (but same grind as 931)
- Tappets (cam followers) are of a different material than 924 /931
- Modified ignition curve on the 937
- Oil cooler mounted in opening in grille rather than where the 931 is mounted
- Exhaust system has shorter primary muffler and only two hangers
- Clutch is identical to 911’s (911 SC’s)
- 1st gear synchro is from a 911, 3rd, 4th and 5th gears have been wire shot blasted
- Front tie rods are different
- Coil springs are 10 mm shorter
- Ride height at rear is lower than 931
- Rear sway bar is 16 mm
- Axles are of different material
- Rear trailing arms have strenghtening plates around the bearing and the shock mounts
- Wheels – variety of sizes 7 and 8 X 116 inch
- Brakes – split circuit different and also different master cylinder
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numbers  
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PostPosted: Sat May 11, 2002 3:43 am    Post subject: Reply with quote

So, basically everything is beefed up or hardened for racing with a larger turbo for more air at top end.

John, any more information on the synchro's? I believe that the 937 used the snailshell, so these synchro's may be the key to solving the snailshell synchro problem. Any idea what modle 911 box the first gear synchro came from? Any indication that the other gear synchro's were changed. Who on this board has a 924 GT? What box are you running, and how do your synchro's hold up?
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John H  
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PostPosted: Mon May 13, 2002 8:22 am    Post subject: Reply with quote

Numbers.

I run the snail shell box (first gear down to the left the other gears in normal H pattern)
So far – I haven’t found a 911 synchro’s that are a straight fit. I had a custom dogtooth made for 1st gear in 1987 and this has lasted for over 100,000 miles. I tend not to do down shifts into first gear while moving and even when racing find I don’t need to use first after starting.

I’ve found the snail shell is a fairly strong box – in the 14 years of racing and driving I’ve only ever blown 1st gear. This was due to over exuberance and selecting 1st for a hairpin corner and dumping the clutch with about 600 rpm and carrying a speed of around 20 mph. As I exited the corner the gear decide it had had enough and stripped the dogteeth. Other than that there has been no problems. I had a slight notchiness going into 4th while racing so we pulled the box and swapped the 4th gear dogtooth for 5th. This worked a treat as 5th isn’t used as often as 4th.

I did get a replacement dog tooth from Aase Bros at the same time and put this into another 931 – Supposedly this is still working but as the car has gone through about 3 owners since I’m not sure what’s been done to it.
You can use a 930 dog tooth but in order to get it to fit you have to carry out some machining but I’m not sure where and how much. The guy I know who has done it won’t tell me, unless I pay him to do it to one of my gearboxes. I’ll be playing with my spare gearbox this year so I’ll find out what fits and how.
2nd gear dogtooth is another that seems to be unique to the 931 snail shell. There are at least two different dogteeth for second gear so one might be a 911.
3rd, 4th and 5th dogteeth are out of the 911 type 915 gearbox and should be readily available.
When replacing the dogteeth you must replace the brake bands and sliding sleeves otherwise the dogteeth gears will crap out almost as soon as you drive off done the road.

If anyone has the Porsche PET they should be able to look at the individual parts for the GT gearbox and see what is common with the 911. Some parts such as 1st gear are common to the 928 but parts for a manual 928 are as expensive and as hard to find as those for the snail shell.
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Rosnik  
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PostPosted: Mon May 13, 2002 10:19 pm    Post subject: Reply with quote

Hi ,
thank you John H. and the other guys for the Info .
A little question for John H. ; I have seen your site http://communities.msn.co.nz/924CarreraGTInfoSite/general.msnw
but I can not read the BOOST 1.doc .
regards
Nico



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numbers  
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PostPosted: Tue May 14, 2002 12:12 am    Post subject: Reply with quote

Thanks, John. I too believe that the snailshell is a strong box. The only problem is the Porsche designed synchro's. I have never had a problem with 3rd, 4th or 5th. I have replaced 1st once, and second twice (including a whole gearset the second time) and both are ready to be replaced again. The cost is not too bad, it is just the work of droping the box. I am considering some experimenting on the next rebuild.
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Rick MacLaren  
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PostPosted: Wed May 29, 2002 11:43 pm    Post subject: Reply with quote

Numbers: TRY double clutching! Honest, it feels better and safer for the car at speed, and it's really the way the car should be driven. Double clutching is cooler than granny shifting.
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numbers  
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PostPosted: Thu Jun 13, 2002 6:49 am    Post subject: Reply with quote

You know Rick, I have tried double clutching, since I grew up driving old farm trucks that didn't even have synchro's. But, in the Porsche, it just grinds worse. Don't know why. Anyway, as long as I do a slow 1, 2 upshift, I don't have a grinding problem. Of course, if I am in a heated contest with a ricer, it is hard to convince myself to shift slow.
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John H  
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PostPosted: Thu Jun 13, 2002 9:59 am    Post subject: Reply with quote

Quote:

On 2002-06-13 06:49, numbers wrote:
Anyway, as long as I do a slow 1, 2 upshift, I don't have a grinding problem. Of course, if I am in a heated contest with a ricer, it is hard to convince myself to shift slow.

You shoudl still be able to do a quick 1 to 2 shift without grinding - the trick is not to try to push the gearlever. The lever is naturally spring loaded to the 2 / 3 plane so all you have to do is gently but quickly move the lever forward and the spring loading will do the rest.
I can do really quick changes 1st to 2nd onthe track at the start and have been know to piss a few 911 guys off with the rapid starts I get (even without a LSD). In fact a few of the guys I used to race dreaded it when I was behind them on the grid as they knew that they woudl have to try and pass me aftere the first corner as 9 times out of 10 I dicked them on the start.
What you have to remember is don't change down to 1st unless the car's stopped. I've found the 931 is flexible enough to use 2nd at low speed. If you regularly shift to 1st while moving (even at walking pace) your box is going to destroy the 1st gear.
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larso  
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PostPosted: Mon Jun 17, 2002 9:28 pm    Post subject: Reply with quote

Double clutching may be a fallacy, fantasy, etc. The best way to shift, is to look at you tach. When the tach drops down to "A Certain" RPM, let the clutch out at this time. Letting the clutch out at this certain rpm, matches the rpms with the engine speed. If you get it exact, you can actually shift WITHOUT the clutch. Although, it requires you slam the gears more, and the shifter feels more notchy...But back to the point: Double clutching may be a fallacy, because double clutching just means you take longer to shift. This may end up matching your rpms, by accident. This makes the driver think that it is the double clutching that makes him shift better, when actually it is the matching of the rpms.

The way to tell what RPMs to throw the car in gear at, is to drive a long...and WATCH your tach when you put it in gear.

Let's take an example: driving along at 4500 RPM and shift at 4500 RPM. Say your tach drops to 2500RPM when you put it in gear. Next time you do this shift, let the rpms DROP to 2600-2550 RPM, BEFORE you throw it in gear (allow some extra RPMS for momentum, time for the human body to react). If you match your RPMs, the clutch friction material is spinning at the same speed as the material it is contacting with..causing less gronch.

The other thing to do, is a get a clutch plate with slots in it, instead of the smooth PORSCHE disc. Slots in the friction material "cut" the vacuum more, when spinning. And of course, you can adjust your clutch pedal...so that the clutch rides higher, to help.

Don't be fooled though, you probably have a problem. Doing all these things listed above will HELP, but are not intended for youto do it you have a more serious problem and are just trying to cover that problem up by doing silly little improvements. Just like when we add that "stop smoke" stuff to our engines, when the engine actually needs a rebuild.
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924RACR  



Joined: 29 Jul 2001
Posts: 8242
Location: Royal Oak, MI, USA

PostPosted: Tue Jun 18, 2002 12:23 am    Post subject: Reply with quote

Jeeze, wander off the topic much?

It's already been pretty well explained, but the complete exhaustive detail of differences can be found here:
http://www.924.org/models/carreraGTbrochure.htm

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'79 924 #77 ITB racecar
'82 931 Plat. Silver
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