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Rev Limiter

 
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numbers  
Guest





PostPosted: Sat Aug 18, 2001 6:32 am    Post subject: Reply with quote

Guys, Larso is always talking about a 6600 RPM rev limiter, and I know that my '80 hits the limit about about 62 or 63 hundred on the factory tach. I'd like to get a discussion started about how the rev limiter works, and what can be done to modify it to get a few more R's.
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Zuffen  



Joined: 31 Jul 2001
Posts: 1424
Location: Owasso, Oklahoma 74055

PostPosted: Sat Aug 18, 2001 1:41 pm    Post subject: Reply with quote

From what I've been told from the tech chairman for PCA is that the 931 rev limiter is on the circuit board in the control box. I was even asked at one time , with my slim to nothing electronic experience to see if I could locate the component and work out a way to change it. It has been implied that the 2.0 liter engine has about 3K more reliable rpm, but the higher you go and for how long the shorter the life. though.

_________________
Bob Dodd - 924turbo@cox.net
931 1982, 944 1982 euro, 924S 1988SE, 93 968 tip 06 Silver Cayenne S, 06 Black Cayenne S

I have Way too many cars, parts for the 931,944 and 951
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wdb  
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PostPosted: Sat Aug 18, 2001 9:44 pm    Post subject: Reply with quote

what method does it use to lower rev,cut off fuel or spark.maybe you could cut the wires if they are only used for rev limiter,or maybe add a diode(only allows current flow in one dirrection,or resistor to limit how high current will reach so it does not trigger the device that limits rpm.has anyone had a high prm failure,what usally fails,crank journal caps,timing belt ? . reinforcing the weak link would be wise.could you remove ecu by installing a carb.
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larso  
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PostPosted: Sun Aug 19, 2001 4:47 am    Post subject: Reply with quote

Yes in the road and track tests, an early version of a 1980 US 931 went up to 6600 RPM, later in 80 a 931S that road and track tested cut off at 6400 RPM.


There is also that distrubutor rotor spring thing that you should watch for.

Replace your rotor with a normal 924 NA one, see if that helps...if not then we know for sure that you have an electronic limiter on your car.

I popped open a 80 930 ignition box...very simple inside...tons of diodes, but nothing fancy showing (but i forget if there was some thing under black carbon stuff that i couldn't see). I am not sure if mine had the rev limiter inside it electronically, because there is that rotor thing too, so i never know if my car had just the rotor limiter, both the rotor and the electronic, just electronic...BAH! too confusing. If mine did have the electronic rev limiter in the ign. box, the circutry is simple, there are no "changeable" chips in it (can't remember if there was non changeable ones under that black stuff that they put over), but definately a lot of diodes and capacitors in there.
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John H  
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PostPosted: Mon Aug 20, 2001 11:37 am    Post subject: Reply with quote

Larso is correct in that the 931 up to 1980 had a rev limit of 6600 +/- 130rpm. This was set in the distributor - not sure how but assume by means of the springs etc in the rotor.
For the later units with the siemens digital control unit the rev limit was set by the control box and unless you know which chip to play with or which wire to cut you're stuck with the factory limit.
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cs  
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PostPosted: Tue Aug 21, 2001 1:03 am    Post subject: Reply with quote

has anyone examined the wiring diagram out of the factory manual just in case there is one wire that runs from the control unit that just has to be it.
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larso  
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PostPosted: Tue Aug 21, 2001 4:28 am    Post subject: Reply with quote

actually it was an early 80 and a mid-late 80 that contained the differences of the RPM limit. Maybe it was just as simple as a crappy rotor in the test car, and all the 1980s did not have an ignition module with this system. Spring maybe was stiffer or looser, however it works, in one test car maybe.
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numbers  
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PostPosted: Tue Aug 21, 2001 5:57 am    Post subject: Reply with quote

Larso, do you still have the distributor from your '80? What does the spring thingy look like. Can you get pix? For the guys with the '81 and '82 models, if anyone can get a discription of how the circuit works in the ECU and a schematic, I can probably figure out a mod. In a previous life I was an electronics tech.
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larso  
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PostPosted: Tue Aug 21, 2001 7:11 am    Post subject: Reply with quote

it's in the rotor... the one you buy...

I haven't bought one yet, I probably will and crack it open with an ice pick and hammer or something.



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wdb  
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PostPosted: Tue Aug 21, 2001 8:45 am    Post subject: Reply with quote

IF your determined to find the upper limit of these motors,Ill add my 2 cents.I forget where but I have seen a motor that had its main jounal caps combined into one unit shaped like a ladder,and bolted to the outer edges of the block as well as to the main journal seats.that would make the bottom end bullit proof. I suspect a machine shop could make one up for 2 hundred $ or so. maybe a 4 inch, toothed super charger belt and pullies could be adapted for use as a timming belt,they come in graduated sizes so they can be spun faster than engine rpm's ,that would prevent the belt from failing at 9 grand. that leaves valve float and the possibility of bending valves at high revs.new springs would help but not garranty long life.some thing will have to be done about the interference valve,whether its cutting a notch in the lip of the deep dish pistons to clear the valves, or maybe a copper head gasket 3/8 of an inch thick and flat top pistons or high compression pistons from the N/A would make up the difference,but what effect would that have on compression,maybe the turbo would minimize the effect of lowered compression. I dont know enough about this motor to know if this is feasable.whadeya think?

[ This Message was edited by: wdb on 2001-08-21 09:23 ]
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numbers  
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PostPosted: Sat Aug 25, 2001 8:17 am    Post subject: Reply with quote

I have an old rotor and a cap. I will take a look at them this weekend, but I believe that they just look like standard parts.
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larso  
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PostPosted: Mon Aug 27, 2001 5:03 am    Post subject: Reply with quote

well it all depends what you bought Dave, there are two rotors, the 931 one and the 924 one, two different part numbers, if you bought a 924 NA one, which you probably did since its half the price, then it isn't going to have the spring in it.
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