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CIS pressures

 
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Raize  



Joined: 18 Sep 2013
Posts: 131
Location: Scotland

PostPosted: Fri Jun 21, 2019 6:50 am    Post subject: CIS pressures Reply with quote

I did a bit of pressure and flow testing on my CIS with the following results:

924 measurements:
20C control pressure 0.5bar
hot control pressure 3.4bar
system pressure 4.7bar
system pressure full load 4.4bar
fuel delivery full load 600g/minute (820mL/minute)
individual injector delivery at three different plate positions:
16/18/19/19g
37/39/40/40g
74/76/77/77g (max deflection)
system pressure 20 minutes after pump off: 2.5bar, no dribbling.

Most of this is OK according to spec but I was wondering if anyone can comment on:
Is the injector balance reasonable?
Is it worth adjusting anything apart from the cold control pressure?
Is a system pressure drop at max load normal or indicative of a weak pump?
_________________
1981 N/A daily driver with:
Headlight relays. Vacuum gauge. Cold start switch. Kill switch. Ported cylinder head, intake and 944 exhaust. Brake booster venturi delete. 53/35mm throttle body. M62 supercharger mounted but not connected
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Fasteddie313  



Joined: 29 Sep 2013
Posts: 1992
Location: MI

PostPosted: Sun Jun 23, 2019 3:44 am    Post subject: Re: CIS pressures Reply with quote

Did you swap the injectors to other injector lines to see if the flow differences followed the injector or the line?

The % difference between them is much smaller at high load than low load so that is a good thing.
If they all have nice spray patterns and the differences are acceptable to you then I think you should put the leanest injectors toward the front of the engine in order from front to back..

Does it idle good enough for you?
I think NA injectors are pretty cheap to get anyway aren't they? If you want to change them..

I think this is for your car..

ideola wrote:

from the Porsche 924 Technical Specifications Booklet
1976-1984 Porsche 924 NA Fuel System Specifications
(all markets except as noted below)
Electric fuel pump Delivery rate: Min. 750 cm3/30 s
System pressure test value: 4.5...5.2 bar
Adjust, value: 4.7...4.9 bar
Control pressure cold (approx. 20 C): 1.3...1.7 bar
Control pressure warm: 3.4...3.8 bar
Leak test
Min. pressure after 10 min.: 1.7 bar
Min. pressure after 20 min.: 1.5 bar
Opening pressure of inject valves: 2.5...3.6 bar


It's crazy your cold control pressure is so low.. Has your WUR been messed with? How are the cold starts?

You could bump your system pressure up a smidge with a shim and that would also bump all of your other pressures up a smidge, but I don't know if it's necessary..

Being an NA car I think as long as it runs good then you are probably good. You aren't on the ragged edge of an engine meltdown being a bit off here and there on your fuel like a turbo is..

I might mess with adjusting that cold control pressure if it causes bad symptoms.. It might make your rings wear a bit faster if the cyl walls are getting washed clean on every cold start if it is indeed way too super rich from that..
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80 Turbo - Slightly Modified
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safe  



Joined: 18 Mar 2017
Posts: 166
Location: Sweden

PostPosted: Mon Jun 24, 2019 6:18 pm    Post subject: Reply with quote

I would't touch any of the pressures. System preassure is not worth messing with, warm control pressure is fine.
I would change the injectors if they are the reason for the differences in fuel. A nice spray pattern can also make big difference in how the car runs an idles, I don't think you will notice any power increase.
Would be a good idea to change around the injectors to see if the "low injectors" follows.

If your fuel pump cant deliver the specified amount it could be worn or the in tank fuel pump (if you have one, -80 and later) maybe isn't working.
Could also be weak wiring, oxidation at the cables and terminals, oxidations at the relay and fuses etc. Check voltage at the fuel pump with the car running, then check at the fuse, then at the battery to se how much voltage drop you have to the pump.
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Raize  



Joined: 18 Sep 2013
Posts: 131
Location: Scotland

PostPosted: Fri Dec 06, 2019 1:55 am    Post subject: Re: CIS pressures Reply with quote

Fasteddie313 wrote:

You could bump your system pressure up a smidge with a shim and that would also bump all of your other pressures up a smidge, but I don't know if it's necessary..


Going back to this topic after taking the whole engine apart to port the head and so on...


I know the generally accepted theory is that increasing system pressure will increase fuel delivery for given airflow will richen the mixture. This makes sense in isolation.

However, it does make sense too that increasing the system pressure will also affect the control pressure. Is it possible that the overall effect of increased system pressure would be a LEANER fuel mixture because the increased potential flow is outweighed by the increased resistance of the control plunger?

I would especially appreciate if anyone has any empirical experience with this, ideally having measured the AFR with a wide-band before and after the system pressure adjustment, and not having made any adjustments at the WUR itself. I can't find anything conclusive searching here or elsewhere.
_________________
1981 N/A daily driver with:
Headlight relays. Vacuum gauge. Cold start switch. Kill switch. Ported cylinder head, intake and 944 exhaust. Brake booster venturi delete. 53/35mm throttle body. M62 supercharger mounted but not connected
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