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Supercharged N/A with CIS
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Joined: 18 Sep 2013
Posts: 129
Location: Scotland

PostPosted: Wed Apr 22, 2015 3:44 am    Post subject: Reply with quote

Good point about the injectors, hadn't considered that.
The overall OD of the injector isn't that big... about 3/4" perhaps.
( 3.14 x 0.75^2 ) / 4 = 0.44 square inches.
At a boost level of say 10PSI, that's 4.4 pounds of force trying to push each injector out. Double it for a factor of safety and you've got 8.8 pounds of force. I think some high temperature glue should be able to solve this problem.

If the N/A CIS is good to flow 200BHP worth of fuel, the air plate would never rise more than about half-way during the operation of a stock 924 N/A engine. I'm not sure if this is realistic, and I will definitely need to test it (or hear from someone who already has tested it).

I do remember Probst writing that the CIS is designed to be able to flow twice as much as the stock application requires including the 924 but this sounds rather flippant to me.

You're absolutely right about keeping it simple. For me the fun of the 924 is in it's low cost and simplicity - I'd rather continue that theme with modifications than detract from it.
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morghen  



Joined: 21 Jan 2005
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Location: Romania

PostPosted: Wed Apr 22, 2015 4:02 am    Post subject: Reply with quote

Well you can alter the amount of fuel per inclination. It's that Allen head screw. Then the WUR from the S1 turbo should enrichen even further(on the same scale inclination) if boost is present. Just do a test, I'm sure there is a flow limit at the factory pressure but you can also shim the distributor return valve to keep higher pressures.
I would not use glue on the injectors, maybe larger prints will be enough or some sort of brackets to hold them in.
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Joined: 18 Sep 2013
Posts: 129
Location: Scotland

PostPosted: Wed Apr 22, 2015 4:10 am    Post subject: Reply with quote

My speculations:

If you alter the fuel per inclination, you'd also have to alter the control pressure to ensure the plate doesn't rise as fast otherwise you'd have a super-rich condition everywhere.

According to Haynes the 924 and 924 Turbo have the same control pressures from the WUR (excluding the boost sensing feature) so you'd need a kind of adjustable WUR (too expensive) to set the control pressure higher if you wanted to use the allen head screw to increase fueling.

Either the N/A distributor has a huge amount of headroom, or the Turbo distributor is substantially different internally and I will need to use one.
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morghen  



Joined: 21 Jan 2005
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PostPosted: Wed Apr 22, 2015 4:57 pm    Post subject: Reply with quote

I dont see your point. The fuel/inclination is just a theoretically linear change. With the pressure fixed, you alter the amount of fuel / inclination.
Thats it...tha means that everywhere in the range of the A/F scale you get more fuel...idle all the way to redline. I dont think you will have to alter the scale at all if you use an S1 wur.

Just get an S1 WUR that is know to be working and calibrated well and that will allow you to keep a stoich idle and will only raise the pressure to the injectors while on boost.
If its good enough for the 931 its good enough for a lightly boosted NA.

If you further need to adjust how much compensation the WUR puts in, you can modify that WUR.
Mine is actually spot on, warm engine i get 13.7AFR at idle, 0.6bar gives 12.5AFR, 1bar gives 11AFR, 1.1bar gives 10AFR
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Joined: 18 Sep 2013
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PostPosted: Thu Apr 23, 2015 12:49 am    Post subject: Reply with quote

I've been thinking hard about the idea of increasing the system pressure substantially to cope with the extra fuel requirement and came to the conclusion it's not much use.

The allen screw for the mixture is an "additive" adjustment.
If you adjust the allen screw, you might have 5CFM worth of fuel richer than you did before throughout the flow range.
So at what would have been 50 CFM, you'd have fuel for 50+5 = 55CFM.
At what would have been 100CFM, you'd have fuel for 100+5 = 105CFM.

The system pressure is a "multiplicative" adjustment.
So if you increase it 10% you'll have 10% more fuel throughout the flow range.
At what would have been 50 CFM, you'd have fuel for 50 x 1.1 = 55 CFM.
But at what would have been 100 CFM, you'd have fuel for 100 x 1.1 = 110CFM.

So it's not possible to adjust the system pressure to give more fuel without screwing up the mixture for most amounts of flow.
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Fasteddie313  



Joined: 29 Sep 2013
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PostPosted: Thu Apr 23, 2015 2:23 am    Post subject: Reply with quote

Higher system pressure will increase the amount of fuel flow through the injectors at a given metering plate position.

Raising control pressure will provide more back force to the metering plate via back side of the piston in the distributor. Therefore the same cfm of airflow will move the metering plate less.

Then you get more fuel vs plate position but it will take more cfm of air to move the pate the same amount.

Theoretically raise both pressures 20%. You would get something like 100% of the fuel at 80% plate vs 100% plate before due to higher system pressure. And you will get 80% plate movement at the same cfm that moved the plate 100% before due to control pressure. But then you have 20% more plate movement after the cfm that previously maxed it out, and fuel to go along with it.

Now your system can meter and provide fuel for 20% more intake air at maximum than before.

I dont think that 20% 20% magic is going to work though, probably going to be a bit of trial and error tuning, maybe raise both, test, then adjust system pressure accordingly because its much easier to get at.

But I would think that setting it for more resistance to the metering plate (control pressure) would cause an increase in your intake air restriction having a negative effect on performance, total system VE or whatever.

931 boost sensing WUR lowers control pressure when it senses boost, letting the metering plate move a bit further for the same given cfm, richening the mixture.

System pressure = fuel trying to get through the distributor piston metering slits and out your injectors.
Control pressure = force against the top of the distributor piston, which is what the metering plate actuates, fighting the metering plate against the incoming air trying to move it the opposite direction.

Disclaimer: This could all be completely wrong, just what I think would happen due to my understanding of K-jet..

Oh and the mixture screw on the distributor only adjusts the metering plate arm angle vs the piston.
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Joined: 18 Sep 2013
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PostPosted: Thu Apr 23, 2015 3:10 am    Post subject: Reply with quote

I agree with the theory above.

To get more fuel out of the CIS, you'd have to raise the system pressure (to get more fuel overall), raise the control pressure (to get the mixture right) and possibly adjust the mixture screw as well.

Raising the control pressure would require a custom WUR. For much less than the cost of that I could get a 931 fuel distributor which will do all the fueling I need (unless there's an easy way to adjust the output pressure of a WUR?).

I'll test my N/A's fuel delivery at maximum air-plate-openage and that should conclude the whole "which fuel distributor" question.
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Fasteddie313  



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PostPosted: Thu Apr 23, 2015 3:25 am    Post subject: Reply with quote

931 WUR will make the mixture richen at a given cfm by using up more metering plate travel actually reducing the amount of max cfm that can be metered.

Are you just trying to enrichen the mixture on boost for detonation safety within the bounds of oem max meterable cfm, or are you trying to exceed the total cfm the oem system can effectively meter?

edit: OH you said 931 distributor not WUR..
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Last edited by Fasteddie313 on Thu Apr 23, 2015 3:32 am; edited 1 time in total
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Joined: 18 Sep 2013
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PostPosted: Thu Apr 23, 2015 3:31 am    Post subject: Reply with quote

Definitely want boost enrichment, but 931 WUR will do that perfectly - that's all fine.

Possibly trying to exceed the total CFM the N/A system can effectively meter. I'm not sure, because I don't yet know what the maximum amount the N/A system can effectively meter IS.

Approximate stats below:

N/A Distributor Stats:
System Pressure 4.8 Bar
Warm C.Pressure 3.6 Bar

931S1 Distributor Stats:
System Pressure 6.2 Bar
Warm C.Pressure 3.7 Bar
Different air bowl shape.

No flow rate stats anywhere to be found (except for the fuel pumps).
Using the Turbo WUR with the N/A fuel distributor will give me a lower than expected control pressure resulting in a rich mixture. I can probably adjust that out using the mixture screw though.

Going by the system pressures alone, and assuming the metering heads are identical, and the 931 has a different air bowl which allows the plate to rise more slowly (to compensate for the higher system pressure), the N/A fuel distributor would be able to fuel around 70-75% of what the Turbo fuel distributor can fuel.

Given the highest CIS turbo HP that I know of is 250, that leaves us with a disappointing 175 - 190bhp and leaves me needing a turbo fuel distributor.

I'll flow test the N/A distributor anyway just to make sure.
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Joined: 18 Sep 2013
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PostPosted: Thu Apr 23, 2015 6:55 am    Post subject: Reply with quote

Flow measured at 800CC/minute with all injectors firing at maximum capacity.

Therefore it would lean out by 160hp and is entirely useless.

Well, that's what everyone's been saying since Page 1 but at least I learned a few things about the CIS in the process of verification.
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morghen  



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PostPosted: Thu Apr 23, 2015 3:33 pm    Post subject: Reply with quote

When do you plan to do the first tests?
Did you get the S1 WUR?
You need a WUR, and a wide band sensor & dial to start with. Then add your supercharger and plumbing and fire it away.
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Joined: 18 Sep 2013
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PostPosted: Sat Apr 25, 2015 7:22 am    Post subject: Reply with quote

morghen wrote:
When do you plan to do the first tests?
Did you get the S1 WUR?
You need a WUR, and a wide band sensor & dial to start with. Then add your supercharger and plumbing and fire it away.



I didn't get anything yet as I don't want to accidentally buy anything I can't use because I got the theory wrong. My time to spend on designing is practically unlimited but my money is very limited.

However it seems quite conclusive that a full 931 fuel and ignition system is the best option for the price.
And a wide band kit is universal, so will always be an asset (unlike 924 parts which can quickly become "junk").


The next thing to consider is the belt drive. My current idea is to leave the pulley on the supercharger alone, and control drive ratio with the crank pulley size.
The 924 crank pulley has an "unused" section of about 80mm OD between the accessory belt and the timing belt that would be just wide enough to accommodate a pulley for a 5PK belt.
So the plan would be to get a 5PK pulley of the right OD, bore out the inside to fit very neatly over the 80mm part of the 924's crank pulley, then slide it over and weld it on (or press fit it perhaps - I can do either).

Any known reasons this wouldn't work?
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chris79  



Joined: 29 Jan 2014
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PostPosted: Sat Apr 25, 2015 8:33 am    Post subject: Reply with quote

I'm not expert user, but do you know this article ?

http://www.benzworld.org/forums/w126-s-se-sec-sel-sd/1372083-warm-up-regulator-wur-calibration.html

The stardard wur of na 924 allow to set max warm control pressure and even cold control pressure.
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leadfoot  



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PostPosted: Sat Apr 25, 2015 12:59 pm    Post subject: Reply with quote

Fueling at injectors is proportional to system vs control pressure,
so higher system and lower control = more fuel at boost... the screw adjuster is only for idle setting on K-jetronic...
if you have a look at the fulcrum lever it is a set radius that actuates the control plunger, what your effectively doing is moving the level a step up the already set curve of this radius...
there is also an audi 5000 turbo WUR that can be used... or potentially a Volvo 240 turbo unit as this was also cis...
here's an example of a volvo 240T fuel dissy mated to a 924 afm body with only 4 of the six ports plumbed in... anythings possible
FWIW I'd stick with just adding the 931WUR and going from there... there's no way of knowing your sc efficiency or motor efficiency or combination of the two at this stage... but I guarantee it's less than you think... planning for fuel delivery on this basis is a waste of time.
http://e21.tricord.be/forum/view.php?view=23806

In respect to crank pulley, place it on the outside of the pulley not the inside, because of the OPRV with some grinding the max size you can run is 165mm... also if you ever have to change pulleys it's alot cheaper to do the sc as opposed to crank... It's also a PITA to have to remove the alt belt every time you change the sc belt, take it from me you will be doing this alot in the testing phase, in out in out in out.... one less thing to do...
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Joined: 18 Sep 2013
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PostPosted: Sun Apr 26, 2015 5:11 am    Post subject: Reply with quote

I'm wondering now; since the control pressure is a function of system pressure, would a system pressure increase result in a proportionate control pressure increase and cause the mixture to remain correct while being able to provide ultimately more fuel (air bowl shape aside)?

Just read Probst's book and while interesting I wish there were more technical data.
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