What's the limiting factor on squeezing more fuel flow out of CIS systems?
 

Read frequently you guys state the max fuel CIS can flow for a given hp ceiling and have always wondered what exactly is limiting this.

Please explain.

With EFI, you can (up to a point of diminishing returns I assume) simply add additional fuel pressure and easily supplement the fuel needed with smaller injectors. So it would be the same case with mechanical as they operate similarly. Playing devils advocate with myself, I understand fuel pressure dictates all range of operation, so I assume that is the reason you don't just "jack up the fuel pressure?"

Am I on track? :)

Hydraulic control pressure pushing down on the top on the top of the metering piston in the fuel head against the force of the air flow meter linkage pushing up on the bottom of it is the main limiting factor against more fuel going to the injectors with CIS.
Lower control pressure alot and you can make a lot more power with CIS if you open up the intake manifold holes, injector blocks, and intake ports and then raise boost so you get enough air going in with the extra fuel.

Want to experiment with lowering the control pressure a whole bunch to get a richer mixture to go with way more boost?
With the motor and everything fully warmed up to operating temperature just unplug the 12 volt supply plug on the WUR/CPR.

Then drive for 15 to 20 minutes and watch your AFR gauge gradually go richer and and your onboard control pressure gauge drop from around 60psi down to 40 psi if you have one. At that point the motor will be getting more fuel than it can deal with, but if you had enough boost and intake air flow the motor would make more power.
Try it. All you gotta do is open the engine lid, unplug the 12volt plug on the WUR, and go back to driving. Then plug it back in and it goes back to the way it was in a few minutes.

You could make it adjustable from inside the car with a wire wound potentiometer tapped into the 12 volt supply wire going to the WUR.
Let the cold start aux air valve keep getting 12 volts all the time so the idle speed doesn't go up at the same time.

Hmmm, not certain if you meant to say diminishing returns or demolishing returns, Tip, as either could, at times anyway, be applicable ...SmileWavy

Geez, what the hell was I writing?! :p

I'm EFI, just curious.

The limiting factor is the factory settings. Change those and CIS can support twice the power it was designed for.

Hugh? I'd like to understand that more.

JFairman,

I've been planning on adding a potentiometer to the WUR circuit for some time now. Could you please give us additional details on which potentiometer you used in your setup?

Thanks for any info you can provide!

Could it be possible to use 964 dimmer switch.. it's a porsche part and it can be wired up to the dash for a stock look... I might just do that

Why would you want that action to be manual? It needs to remain automatic and consistent. If the fuel head is adjusted for the flow needed and the WUR is then adjusted to match that flow spec all is good.

It wouldn't be completely manual. More so having the ability to tune/adjust:

- Length of time it takes the WUR to change from cold to warm pressures. Sometimes I'd like my WUR to not warm up as quickly as it does.

- Ability to richen warm AFR on the fly. This could be convenient when the outside temperatures are changing.

If these things are not happening your WUR is likely worn out. Compensation for temp changes is part of normal function. Ambient temp under the hood has some affect on pressure control.

With all due respect, I actually think you are cutting this a bit too short.

A stock car is rarely ever absolutely optimized, it is within a safe range at stock performance level. However, we tend to take this a little further. Furthermore, today's AFR gauges allows us to see in real-time if the AFR is not completely optimum. I can see a difference between a cool morning, and a hot 2pm. A pot-meter may allow us to slightly lower WCP lower AFRs on cools mornings etc.

At least in principal, is seems interesting to pursue, if sticking with CIS.

Optimizing I understand but sometimes the persuit of perfection can create hypersensitivity. If your car runs just fine but then you install a guage that tells you the AFR is 0.1 point off, your car still runs fine. Different strokes but I'd prefer to spend what precious little time I have with my car driving, not fiddling with gizmos. Now get you're car back on the road and enjoy it's foibles, you've obviously got toooo much time on your hands, ha! ;)

I don't know anything about the 964 dimmer switch or how much wattage it can take or how it would handle the heat being used like this.
I suggest a wire wound potentiometer that can handle some heat. Resistance creates heat so a wire wound potentiometer like a passive 8 ohm L pad for speakers that can handle 100 watts RMS or more controlling a woofer speaker level from the back of a speaker cabinet.

I really wouldn't want anyone to try using a cheap carbon trace potentiometer or other low wattage unit that could overheat quickly and become a big problem. I don't think speaker fader potentiometers made for cars would be able to handle the continuous 12 to 13.5 volt current for an application like this safely. You need something with some wattage headroom that won't get real hot.

A good quality wire wound potentiometer that can handle some wattage could be mounted on a metal bracket under the dash behind the knee pad so it's out of sight but still accessible from the drivers position.

Good wire wound potentiometers have the resistance wire wrapped around a ceramic ring inside a metal case that can handle the heat without melting or burning so they are safe for something like this. I wouldn't try a cheap one made of plastic.

The limits of CIS can be summarized by stating that system response is not linear across the range of engine speeds/loads. If you have the ideal AFR at 3000 rpm, for example, CIS will lean a full 1% or more by 6000 rpm for the reason that Fairman stated.
What is required is way to lower boost control pressure incrementally as rpm increases. Thierry25 developed a way to accomplish this but it's not the cleanest solution from a component standpoint.
The problem with using the bimetallic strip to accomplish this is the response time will be poor.

JFairman,
Thanks for the info on the potentiometers. I was thinking about this yesterday and starting wondering if a PWM (pulse width modulation) controller may also be a good answer:

MXA066 PWM DC Motor Speed Controller

12 Volt Speed Controller | Qkits

911nut,
I agree with what you're saying and I'm hoping the FrankenCIS project will accomplish this. In the mean time this WUR mod seems to allow a general pressure change that under certain circumstances would be helpful. Especially if it doesn't cost much and is easy to install/remove.

if you are talking about changing the voltage to the WUR to change the mixture, you will not get the response needed to make it worth while.
do this. get the engine good and hot. with gauges attached, unplug the power to the WUR and see how long it takes for it cool down on a hot engine to effect pressures.
i have left the power off the WUR before. i could not drive it much past what the normal warm up time was. i dont think i was a half mile down the road before i was connecting it back up.

listen to what rarlyl8 is trying to tell you in a nice way.

If we stopped short of trying something new, nothing would evolve. Your data seems to support your argument that this is not worth trying. However, as long as non of us are encouraging anything dangerous or otherwise destructive, there is no reason not to fiddle and have fun. The way I see it, that is what this community is about. Sometimes the experts can dispel suggestions off the bat, sometimes not. And sometimes they are even proved wrong. :)

Worst case, during this discussion, though it in itself does not lead anywhere, it educates the partakers.

T77911S . This would not work like an on and off switch, the WUR would work as designed, and by adjusting the voltage it would not work instantly because of the engine bay temps.....ideally you would have your WCP set to the high end of you WUR settings , something like 3.85 bar, at 12 volts or so, 12 + volts would be the starting point on the adjustable voltage reg and by lowering the voltage it drop the WCP much like doing it manually but from the comfort of your cockpit. But you can only lower the WCP which would make you car run richer... Just saying I'm gonna try it and get back to you all,