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Lizard  



Joined: 03 Nov 2002
Posts: 9379
Location: Abbotsford BC. Canada

PostPosted: Tue Sep 30, 2003 3:08 am    Post subject: Reply with quote

ok first time I have ever heard of that, thanx for correcting me Jon
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J1NX3D  



Joined: 06 Feb 2003
Posts: 1332
Location: New Zealand

PostPosted: Tue Sep 30, 2003 8:46 am    Post subject: Reply with quote

Lizard wrote:
the gearbox has DEFINATLY been changed, however IIRC there are 3 versions of the G31 .01, .02 and .03 all all 3 have different gear ratios, although, i dont hink you should be running at 2800rpm in 5th gear, J1NX3D, it could mean that someone has swapped out to a shorter diff, although 1 of the 3 versions might have had a short diff like you have,


i was wondering if my wheels may be putting my speedo out. i have 205/55/R16 front, 225/50/R16 rear. It kinda makes it less driveable doing an indicated 100 at 2800rpm as its only just starting to spool but not boosting so im not getting that urgent throttle response straight away.
i'll check the numbers on g/b one day.
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Khal  



Joined: 26 Sep 2003
Posts: 4859
Location: Hot and sunny interior BC, Canada

PostPosted: Tue Sep 30, 2003 10:46 am    Post subject: Wheels too big? Reply with quote

Yeah JINX3D, bigger wheels will almost certainly put your speedo out. Simple really: The bigger the wheel, the slower it revolves at a given road speed (say, 100km/h).

Don't know how much difference it will make i.e. will one-inch larger than standard wheels make a noticable difference to speedo reading?

But of course, you're running 50-profile tyres, too (at the rear, at least). If they're lower profile than standard (I don't know, I'm running 50-profile as well), then the overall circumference of the wheel/tyre might be the same or very similar to original spec. In which case, it'd make little or no difference to your speedo reading.

By the way, 'round here they call this a 'plus 1' wheel upgrade, where one-inch-larger wheels are fitted with lower profile tyres to maintain the circumference of the original wheel/tyre combo.
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J1NX3D  



Joined: 06 Feb 2003
Posts: 1332
Location: New Zealand

PostPosted: Tue Sep 30, 2003 12:01 pm    Post subject: Reply with quote

Ta, good read.
ive checked my haynes and the 16" fuchs had the same width and profile tyres for the carrera gt... ?? I sometimes get the feeling its reading lower than actual speed. I'll be doing the 50kph speed limit around town and i get people gaining and then trying to push me
i nearly got rear-ended the other day to on the motorway in the rain. theres a choke pint where one motorway joins another and it slows down abit. the guy behind me in a holden astra didnt have as much room to brake and he swerved onto the median strip to my left. his bumpercame up to my passneger door!
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Khal  



Joined: 26 Sep 2003
Posts: 4859
Location: Hot and sunny interior BC, Canada

PostPosted: Tue Sep 30, 2003 12:34 pm    Post subject: Wheels too big? Reply with quote

Lizard,

As noted on an earlier post, my speedo definitely reads fast. It reads about 8km/h fast at 100km/h i.e. I'm actually doing 92km/h. I checked this with a handheld GPS, which are apparently quite accurate when reporting speed.

If the difference is the same through the whole speed range (not sure about this, never checked), then at an indicated 50km/h I'd actually be doing 42km/h - no doubt buggin' the hell out the people behind me! Sounds like the trouble you're having.
I generally try to just go with the flow - whatever feels comfortable at the time.

As also mentioned in an earlier post, I've been told a couple of times by different people that German speedo's are overly optimistic... they usually tell you you're going faster than you are. This is definitely the case with my one.

I'm fairly sure the standard 924 Turbo came out with 15-inch wheels but I don't know the tyre profile. And, of course, Porsche may well have changed the wheel/tyre combo AND gearing many times over the production life of the 924 Turbo (as seems to be the case according to Lizard). Seems like you might need to do some research into your particular car to get the real story.
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Khal  



Joined: 26 Sep 2003
Posts: 4859
Location: Hot and sunny interior BC, Canada

PostPosted: Tue Sep 30, 2003 12:53 pm    Post subject: Last Post Reply with quote

Sorry, I meant to address the post to JINX3D.

Also JINX3D, the Carrera GT may have come out with bigger wheels (16-inch instead of 15-inch) and lower rpofile tyres because it was initially a racing special.
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Khal  



Joined: 26 Sep 2003
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Location: Hot and sunny interior BC, Canada

PostPosted: Tue Sep 30, 2003 5:55 pm    Post subject: Wheel too big? Reply with quote

And sorry JINX3D and all for going on, but to the best of my knowledge, car speedo's generally indicate faster speed than actual road speed. It's just that German cars (I've had a few) seem to be further out than normal - especially compared to newer cars i.e. most new cars appear to read only about 2km/h fast whereas the German cars I've had tended to read about 5km/h fast (or about 8km/h for my current one!).
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-nick  



Joined: 16 Nov 2002
Posts: 2700
Location: Cambridge, MA

PostPosted: Wed Oct 01, 2003 3:14 am    Post subject: Reply with quote

J1NX3D- the overall circumfrence of those tire sizes is the same as the stock sizes (fyi- the 16" wheel option on the 931 used 205/55/16's). Your speedo/rpm sounds fine too, us yanks were talking numbers in mph. People always tail me closely too, I think they want a good look at what's in front of them

General fyi- there were 3 different gearings for the G31 dogleg tranny-. the US/CAN spec for 1980, RoW spec '80-'82, and the Carrera GT. Porsche tire sizes have the same overall diameter across all they models- at least from the '76 924 on up to the latest and greastest.

Khal- 8kph sounds like a lot for speedo error. Are you using stock size tires?

-nick
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Khal  



Joined: 26 Sep 2003
Posts: 4859
Location: Hot and sunny interior BC, Canada

PostPosted: Wed Oct 01, 2003 10:14 am    Post subject: Tyres too small? Reply with quote

Actually -nick, to be honest I don't know. But I doubt it. I've got 'Carrera Cup' (I think that's right) wheels on my car. They're Porsche wheels, not third-party. They're 15-inch by 7-inch on the front and 15-inch by 9-inch on the rear. They're wearing 205/50/R15 on the front and 225/50/R15 on the rear. I'm guessing the 924 Turbo's with 15-inch wheels would've come out with 60-profile tyres originally?

But, as mentioned, I've checked the speedo with a GPS (I hope they're as accurate as everyone tells me!), so at least I KNOW my speedo's wrong and can compensate accordingly.
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Khal  



Joined: 26 Sep 2003
Posts: 4859
Location: Hot and sunny interior BC, Canada

PostPosted: Wed Oct 01, 2003 12:59 pm    Post subject: Tyres too small? Reply with quote

Oops! My wheels are 'Cookie-Cutters' not 'Carrera Cup'. And come to think of it, I reckon they're 8-inch on the rear, not 9-inch as stated. Sorry about that...

But they are 15-inch diameter front and rear. And the quoted tyre sizes are correct.
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Gregarious  



Joined: 13 Sep 2003
Posts: 86
Location: Los Angeles

PostPosted: Sat Oct 04, 2003 9:19 am    Post subject: Music Reply with quote

Neil, I began playing harp music, with flute, and Kitaro. I need to be calm and cool, not intense, while driving my missile.
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Fasteddie313  



Joined: 29 Sep 2013
Posts: 1418
Location: Not Detriot - NMI

PostPosted: Thu Aug 04, 2016 5:07 am    Post subject: Reply with quote

porschetroy wrote:
Linear, yes. But, there is not enough power to reach redline in 5th gear either due to wind drag or just not breathing enough air. My '82 931 will reach 135 after a while; it reaches 120 pretty quickly. I try not to drive over 110 very often! -Troy


How much power and how fast?

North American G31 (G31.02) as follows:
0.6000 ratio
2.8286 overall
4.7143 final
5th gear: 35/21
R&P: 7/33

24.9 tire diameter..

Right about 180mph @ 7,000 RPM

How much power??

19.224344 sq/ft frontal area
.35CD

Using your vehicle Speed of 180 MPH takes 267.88 HP to overcome air drag.
Rolling resistance is 15.60
For a total of 283.48 HP to run 180 MPH

So you need 290 hp @ 7k RPM

7k RPM looks to be about 15% less than peak power on dyno charts so 290X1.15= 340 peak HP will get you there..

Now is that wheel HP or crank HP? Lets call it wheel and add another 10% so 350X1.1= 385 HP..
How much boost to make 385 CHP?


Shurick wrote:
Fasteddie313 wrote:
That is sick!! great work

At 1.4 Bar/20psi you have to be getting around 300 crank hp you think? Maybe a bit more?

According to fuel/air consumption from EFI datalog it must be 320-330 hp.


Sounds like 25-27 psi would do it, surely 2Bar would get you there..
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Rasta Monsta  



Joined: 12 Jul 2006
Posts: 11332
Location: PacNW

PostPosted: Thu Aug 04, 2016 5:52 am    Post subject: Reply with quote




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