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rev limiter?

 
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LJD@FSU  
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PostPosted: Thu Apr 11, 2002 12:36 pm    Post subject: Reply with quote

anybody (or should i just ask bob directly?) know much about the 931's rev limiter? what rpm? ability to bypass the circuit?

-- not that i want to rev my 931 into pieces, just wondering.
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John H  
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PostPosted: Thu Apr 11, 2002 1:03 pm    Post subject: Reply with quote

If you're car is a digital ignition unit it's not easy to by pass the rev limiter. It's set in the engine management box. Basically these cars ran a system that was the first generation of chip type controls and as such it's not recommended to play with them.
Besides there is no real benefit from taking the 931 above the rev limiter. the rev limit should be around 6300 to 6500 depending upon the accuracy of your rev counter.
To tel if you've got the digital ignition the simplest method is to look at the throttle body - if it's oval it's not digital and you can play with the rev limiter

[ This Message was edited by: John H on 2002-04-11 12:04 ]
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numbers  
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PostPosted: Fri Apr 12, 2002 8:11 am    Post subject: Reply with quote

We have had a long discussion of the rev limiter before on this forum. However, no one seems to know exactly how the rev limiter works on the stage 1. Lars thought it was a "spring thingy," built into the rotor in the distributer, but I have cheked my rotor, and it is just the standard brass strip on a plastic base. I too would like to play with my rev limiter, not to eliminate it, but to get another 500 revs out of the engine before I hit the limit. There is always a large safety margin built into factory limits for warrenty purposes. And, with the high boost that I run, I will hit the rev limiter in first almost as soon as the turbo kicks in. An extra 500 revs would allow me to get a good one two up shift in without hitting the limiter.
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wdb  
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PostPosted: Fri Apr 12, 2002 9:25 am    Post subject: Reply with quote

how does the rev limiter slow the RPM's cut off fuel or spark. does it close a valve to cut off fuel,open a circuit to shut off injectors ? or is it not yet known how it works.
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Vince Ponz  
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PostPosted: Fri Apr 12, 2002 9:57 am    Post subject: Reply with quote

When I bought my very original 931 it ran poorly and the first thing I did was to replace the distributor cap and rotor. The rotor looked odd and the rotor they gave me was a standard rotor very plain. The one I took out had a spring in it and when it spun the outer part pulled away from the center. I think it was a rev limiter of sorts. Friends told me that the same rotor was on 914s.
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LJD@FSU  
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PostPosted: Sat Apr 13, 2002 1:19 am    Post subject: Reply with quote

thanks for the info guys, i was just wondering how porsche made rev limiters 20+ years ago. i still have a lot to do on the car before i worry about stealing a few more revs out of the engine. vince, that's really interesting about the rotor spring device, i'll have to look at mine.
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numbers  
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PostPosted: Sat Apr 13, 2002 9:40 am    Post subject: Reply with quote

Well, no spring on mine, but revs are still limited to 6200 on the Tach. I want to build or by some kind of a rev counter that will control a relay at around 3000 Revs. Anybody got any ideas?
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John H  
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PostPosted: Mon Apr 15, 2002 10:01 am    Post subject: Reply with quote

I'll repeat what I've said in the second posting for those of you didn't understand my comments.
In the 931's controlled by the digital engine managemnt (post 1980)the rev limit is set by the engine control unit under the radio in the centre console. It cannot be altered unless you have the facility of down loading all the information out of the "chip" and then recreating the logic with a higher rev limit. The limiter interrupts the circuit to the coil. If you can figure out which wire to cut /by pass then you can override your rev limiter.
The early cars pre 1981 the rev limiter in the distributor.
I run 1 1981 turbo so I don't have access to the early car to see how it's a controlled but my N/A unit with similar ignition to the early turbo's doesn't have a rev limit once I changed the rotor with the spare rotor from my digitaly controlled turbo.
The factory manual for the early (pre digital units) states the distributor is
"With double pressure box (idle: vacuum:, full load: charge pressure), without points, with speed limit at 6600 +/-130rpm"
Hence the rev limit is set in the distributor for early cars - by means of the rotor arm.



[ This Message was edited by: John H on 2002-04-15 09:04 ]
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larso  
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PostPosted: Wed Apr 24, 2002 9:34 pm    Post subject: Reply with quote

There is supposed to be two rev limiters in some of the 931s. I've yet to buy one at the dealer, but the rotor for the 931 is a different part number. There is supposed to be a rev limiter in the ignition box, but I am wondering if the earlier 931s did not have this, and just used the rotor. How many have an 80 931 that they have replaced the rotor with a 924 NA one? Results? Whatever the case, the later 931s cut out sometimes at 6100.

The 80 931 i had didn't cut out until 6600. The road and track did a test on an early 80 931, and it cut out a 6600.

They did a test on a later 80, and it cut out at 6100. On the road and track results, this makes a huge difference. I know once the turbo is spooling up, that extra 400-500 RPM will shave HUGE numbers off accel times. Of course this puts more wear on your engine, and it makes book values look nice, but on track I still see it helping a lot. I find the extra 400 RPM up in the 6000 RPM range makes the biggest difference in the world, simply because this is a turbocharged car. On an NA it would not make as huge of a difference since the power curve usually cuts off or evens out anyway. With the 0-60 times, it was also very important since this extra 400 on the dog leg tranny made the car go to 60 in second gear. The audi trannies only go to about 50-55mph in second.

When I am coasting at 4000 RPM in second or third gear, I find the car loses TONS of momentum when you shift at 6000 RPM. Just that extra 500 RPM makes you feel so much faster if you pay attention. The only thing I am worried about is wrecking up the engine, so for now I like the rev limiter at 6100. I'll show some discipline :wink


[ This Message was edited by: larso on 2002-04-24 20:36 ]
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