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931 has rev limiter function?

 
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John Brown  
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PostPosted: Fri Aug 16, 2002 6:11 am    Post subject: Reply with quote

It would appear that there MAY be a rev limiter function built into the 931 fuel pump relay. That would explain why the 931 uses a different part number than the 924 even for those years which share the same basic ignition and fuel wiring and the significant price difference.

Several people 'vaguely' recall something about a rev limiter function. But no one can find any reference in any technical literature to same. For example, the 944/924S tech ref lists a rev limit at 6500rpm. This is NOT the same as the redline listings. But the 924 booklet and factory maintenance vol are silent about any rev limiter.

Anyone find or have documentation of this function for the 931?

Hummm?
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924 turbo  
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PostPosted: Fri Aug 16, 2002 7:10 pm    Post subject: Reply with quote

I think it's built into the ignition, possibly into the O2 sensor system. I had a non-931 fuel pump relay in my car and the rev limiter worked. Oops! Too much RPM!
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larso  
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PostPosted: Sun Aug 18, 2002 7:13 pm    Post subject: Reply with quote

Isn't it in the ignition box. On the 80 modules in that silver aluminum bosch box, and on the 81-82 models, in the box behind the radio under the center console?

I guess there's only one way to tell, get someone to rev the F*** out of your engine, and sit back with a multimeter at the fuel pump. I'd be interested to know, as it may be hackable with a couple of capacitors, potentiometers etc. It appaeras that the rev limiter isn't like one on a honda, since in the 931 the car cuts out for a llonger period of time, whereas the hondas go blip blip blip...does anyone know what teh honda uses? It think I recall hearking that it cuts out every second cylinder, rotating, to somoothing and reduce the cut out effect and hence the blip blip blip feeling and noise.
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larso  
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PostPosted: Sun Aug 18, 2002 7:16 pm    Post subject: Reply with quote

Did you find an additional function in the 931 relay, because there is that fuel cut off for the overboost built in the relay ....
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924 turbo  
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PostPosted: Sun Aug 18, 2002 7:38 pm    Post subject: Reply with quote

The '81-'82 cars still use the Bosch aluminum O2 sensor box. It's a different part number, but it's there.

When I had a 924 fuel pump relay, the overboost switch still stalled the car. I don't think that's built into the 931 relay specifically either.

I'm still guessing it's built into the O2 sensor box (or relay?) and not into the DITC system. It may shut off the fuel pump, but I think it's controlled by the O2 box.

_________________
Jon Furst
'80 924 (daily driver)
'81 931 (never-ending project)

[ This Message was edited by: 924 turbo on 2002-08-18 20:05 ]
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TroyDest  
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PostPosted: Mon Aug 19, 2002 4:10 am    Post subject: Reply with quote

I don't know about a rev limiter on the later 931. I drive the crap out of my '82 and have never hit a rev limiter. I autocrossed it about a month ago, was holding it wide open in second gear most of the time, and still no rev limiter. That was at 15psi boost and revving about 6500. It doesn't seem to want to rev any higher but I think it's just breathing restrictions. It certainly doesn't cut out like when it's overboosted, I experienced that before I relocated my overboost switch. If it does have a rev limiter, I'm not gonna bother with it since it's way up there and the turbo stops making power at 5500 rpm anyway. -Troy
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larso  
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PostPosted: Mon Aug 19, 2002 6:48 am    Post subject: Reply with quote

Hmm well in the wiring diagrams, all the boost switch does is cut off power to the relay. so a 924 relay would still work, because it cuts power to the relay. But, maybe the 931 relay has some sort of special circuit that helps the car out, when it's cut such as pulsing the relay, or something akin.

I was talking about that bosh ignition box actually Jon, but the fuel box might be where it is too (the one under the steering column). I guess I'll have to rev the engine up and go back with a multimeter to the fuel pump and check for rapid on and off of power ..if anyone else wants to try it, do it!

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924 turbo  
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PostPosted: Mon Aug 19, 2002 7:22 am    Post subject: Reply with quote

Troy: mine definately has one.
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John Brown  
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PostPosted: Tue Aug 20, 2002 1:27 am    Post subject: Reply with quote

To clarify:

I am speaking specifically of the fuel pump relay. It is quite possible for there to be a rev limit function inside the relay. Open one (relay) up and you will find not just a solenoid and contact; but 2 small circuit boards stuffed with discrete devices and a couple of ICs.

I am ready to believe Porsche may have put a rev limit function in the relay. Cutting the fuel is a better way to accomplish than using the ignition. With CIS, a fuel pump interruption is very effective but still 'softer' than an ignition cut. And the complexity of the relay components is excessive.

The boost limit is a completely seperate function external to the relay and not what is suggested in this discussion. Also, the ignition control module and the relay both 'look at' ignition primary pulses. Green wires from top of the distributor. The ignition module does not control the fuel pump.
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wdb  
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PostPosted: Tue Aug 20, 2002 4:22 am    Post subject: Reply with quote

can you monitor the voltage coming out of the relay and bring the car up to 7000 rpm's and see if there is a drop off of current to varify that the relay has a duel function , or measure the fuel pressure to see if it drops at or near 7000 . or use a timing light to see if the #1 plug stops firing at 7000 rpm's
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John H  
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PostPosted: Tue Aug 20, 2002 4:11 pm    Post subject: Reply with quote

If you have alook under Rev Limiter there addiditional info - but risk of repating myself
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.
The wire from the coil John Brown's talking about most likely cuts the fuel relay as well to give the "softer" cut off?
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AndyUK931  
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PostPosted: Tue Aug 20, 2002 10:29 pm    Post subject: Reply with quote

My '80 931 has the rev limiter built into the rotor arm, one of those spring loaded thingy's. The '81 is controlled by the Digital Control behind the radio.
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John Brown  
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PostPosted: Wed Aug 21, 2002 5:32 am    Post subject: Reply with quote

John H, thanks. Now we're cooking. Where is that in the factory manual please?
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John H  
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PostPosted: Wed Aug 21, 2002 10:45 am    Post subject: Reply with quote

If you look at page 28-1 for the non digital turbo this gives the rev limit - this is set by the use of a spring loaded rotor arm.

Page 28 -21 is for the Digital and by means of ommission of rev limit it implies the control is inthe control unit. Also one of the factory bulletins I have on the Carrera GT mentions the rev limit is set in the control box.

Also the fuel pump relay for the turbo pulsates - ie switches on and off - I was never sure why it did this or whether mine was faulty but the additional circuitry inthe relay woudl account for this.
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dwak  
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PostPosted: Wed Aug 21, 2002 11:15 am    Post subject: Reply with quote

Has anyone noticed an improvement with a DITC
ignition control system vs the distributor type system on the pre-'81 931's? Sounds like it is easier to tamper with the rev-limiter on the earlier ones.

dwak
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