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WHATS BETTER: CIS, Motronic or Carbs?

 
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WHATS BETTER: CIS, Motronic or Carbs?
Carb(s)
0%
 0%  [ 0 ]
Motronic
60%
 60%  [ 6 ]
CIS
40%
 40%  [ 4 ]
Total Votes : 10

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Noahs944  



Joined: 08 Dec 2015
Posts: 726
Location: Calgary, Alberta, Canada

PostPosted: Mon Jun 06, 2016 8:50 am    Post subject: WHATS BETTER: CIS, Motronic or Carbs? Reply with quote

What's your preference and why?
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Noahs944  



Joined: 08 Dec 2015
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Location: Calgary, Alberta, Canada

PostPosted: Mon Jun 06, 2016 8:52 am    Post subject: Reply with quote

How much horsepower can a stock early CIS system support?
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nickthompson  



Joined: 26 Mar 2013
Posts: 859
Location: Central Georgia

PostPosted: Mon Jun 06, 2016 11:27 am    Post subject: Reply with quote

CIS is archaic it should only be used on the rarest of collector cars. The idea of having to make air move a heavy metering plate in order to make fuel flow is counter intuitive. If you look at an engine as basically an air pump then it makes very little sense. CIS is better at fuel management and atomization than carbs but at air flow? Not so much. EFI is obviously better than both but ot comes with its own headaches. That's just my two cents.
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Noahs944  



Joined: 08 Dec 2015
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Location: Calgary, Alberta, Canada

PostPosted: Mon Jun 06, 2016 12:02 pm    Post subject: Reply with quote

Pondering the possibility of running a larger Porsche engine in a 924 that originally came with the 2.0 CIS engine. The thought occurred to me today that I might be able to adapt the 2.0 CIS to operate the 2.5 litre sans ecm.

Last edited by Noahs944 on Tue Jun 07, 2016 12:19 am; edited 1 time in total
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morghen  



Joined: 21 Jan 2005
Posts: 7887
Location: Romania

PostPosted: Mon Jun 06, 2016 6:16 pm    Post subject: Reply with quote

Motronic is obviosly the best...i say the best because no hot/cold start problems easier maintainance...not because of better mileage or better performance.

CIS might be arhaic but its still up to the task for almost every application.
People still run CIS in the city, on the highway and even racing with fuel consumption and power delivery that combined to the 924 body still gives performance figures that are in the ballpark of modern cars.

CIS on a 2.5L is a very interesting idea....i would love to see one just for the fun
The NA cis can handle 160hp as i read somewhere.

PS: its not a VW engine, the vw engine made 75HP, the Porsche engine made 125hp. How about you import the bits needed to convert your 924 to 125hp spec?
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Noahs944  



Joined: 08 Dec 2015
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Location: Calgary, Alberta, Canada

PostPosted: Tue Jun 07, 2016 12:38 am    Post subject: Reply with quote

M,

I stand corrected. Porsche designed the head, did they designed some internals & external engine parts also? Probably! Ok, previous post corrected.

I noticed the metal panel above the driver's head in the 924 was stamps with a small Audi symbol. But I don't call the car an Audi. All parts used on the car passed Porsche's engineering desk & it's track performance was reviewed with a nod.

---------

I find CIS somewhat fascinating, but don't have enough experience with it to comment on it very much. I'm intrigued because I don't really like computers. And the bumpy roads I go on risk damaging the ecm internally .

The Motronic has been a pretty good system for me. But the idea occurred to me yesturday... Why can't you adapt CIS. All engines are designed to do 4 things primarily: suck, squeeze, bang, blow.

Re: 125 hp 2.0. It's not a bad idea. I also read in great interest the stroker thread of Ideolas. I've long been interested in a stroker concept.

I need to research more on the 2.0 CIS. I have questions, such as IGN; Can someone explain how the CIS spark ignition works?

I read that '82 had an ecm-is it still called CIS / Bosch KJetronic?

Is there anyway to get the n/a CIS system to work for a 200 hp engine?
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morghen  



Joined: 21 Jan 2005
Posts: 7887
Location: Romania

PostPosted: Tue Jun 07, 2016 12:55 am    Post subject: Reply with quote

Block design was done by Daimler for a turbo diesel earth mover.
Early blocks came with smaller journals, in 77 (Porsche?) increased the journal size, so even the block has had changes during the 924 production.
Pistons are obviously no the same as the VW engines....so whats left if the block was re-engineered, pistons, probably rods and head as well?
Production of all these parts was done where the original tools were...Audi/VW supply factories...so stamps are there also because of that.

Anyway, CIS on a 2.5 is for me a very interesting idea...not really practical or whatever...but very interesting.
I would really be curious how it would work if you just strapped it to the 2.5L engine

Ignition is a separate system...you just need to replace some bits and you're done.

Giving the fact that Gramps is an early car and you want more power, more grunt out of him...i think you need to change the following:

-cam
-ignition dizzy
-spark amplifier
-pistons
-throttle body
-exhaust manifold (change to 4 branch headers)
+get a wideband sensor and gauge to correctly set the mixture for maximum performance.

Then use self adjusting rear drums from later 924s..some 15" wheels and all around new bushings and you should have a quick and nice car.
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Noahs944  



Joined: 08 Dec 2015
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Location: Calgary, Alberta, Canada

PostPosted: Tue Jun 07, 2016 1:21 am    Post subject: Reply with quote

morghen wrote:


Anyway, CIS on a 2.5 is for me a very interesting idea...not really practical or whatever...but very interesting.
I would really be curious how it would work if you just strapped it to the 2.5L engine

Ignition is a separate system...you just need to replace some bits and you're done.



FIRST OFF, thank you for enlightening me on the 2.0l... VERY interesting history. This also explains why I've had a bugger of a time recognizing engine parts in the VeeDub world (those guys have such a cool yet insane world-love it!).

So, can you go into more detail on the ign side of things. Specifically... conceptually, what would be involved with adapting the ign over to the 2.5 What triggers the spark? How does it know when to advance/retard the timing (vaccum only?) On the Porsche 2.5 8 valve there is a sensor on the flywheel housing that serves as an input to the ecm to indicate crank position. Does KJetronic not have something like this?
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morghen  



Joined: 21 Jan 2005
Posts: 7887
Location: Romania

PostPosted: Tue Jun 07, 2016 3:00 am    Post subject: Reply with quote

No, no crank sensor for the old system, just align the motor at TDC and align the dizzy to that.
One would need to mod the 2L dizzy to fit the 2.5...crazy thing to do to be honest but i'd like to see it done just for the heck of it

I guess the old system could make less power than the motronic but not sure by what amount...and how the 2.5L would respond to that...maybe the dizzy would need to be dialed in to provide the propper vaccum advance but i'm guessing it would work right off the bat to a certain degree.

If you had the time and would be forced to do this it would be an interesting experience...but if you want more grunt..my proposals are the following:

scenario 1: get the needed stuff to get Gramps to european 125HP standard if not more with an aftermarket cam to bring the power where you want and gain a bit of hp.
side thing to scenario 1: get a stroker kit and bump the old 2L to 2.2L or even 2.3L to further upp the power.
I guess 150hp is piece of cake starting with a 125hp spec engine, increasing the cc to 2.2 or 2.3L and using a custom cam.

scenario 2: get a 2.5L engine, rip out the 2L, cut and weld the needed stuff on the frame rails to use the 2.5L engine and do a propper 2.5L transplant.

scenario 3: sell gramps and get a 924S if you really like the narrow body or just get another 944...or just sell gramps and use wilson.
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daniel  



Joined: 18 Jun 2009
Posts: 613
Location: Sydney, Australia

PostPosted: Tue Jun 07, 2016 12:28 pm    Post subject: Reply with quote

CIS is a very good system, Porsche used it so they could sell a car with injection that could be serviced by normal garages which was an issue with the electronic injection 914's. Also CIS dosn't do very much to air flow at all. Bigger is not always better, a matched system is always the best and clever obstructions in the air flow can accelerate air speed. Try this, open your mouth and breath in. Now purse your lips and breath in the same way, notice how much faster the air moves? Yes there is less volume but if the total volume is still more than you need (which it is in the case of the 924 and CIS) than the obstruction had provided and advantage.
The draw back of CIS is tuning. Air flow into an engine is not one continuous flow of air, the air moves in pulses. The amplitude and duration of these pulses is effected by almost everything in the top of the engine and the shape of the CIS cone and plate diameter and weight have to match these pulses inorder to give correct fueling at all engine speed/load combinations.
I am running a NA application with a turbo head and a mild race cam through CIS and I don't think I can modify much further due to the fact that my systems leans out from 2000-4000 rpm then richens back up.
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ideola  



Joined: 01 Oct 2004
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Location: Woodstock IL

PostPosted: Tue Jun 07, 2016 1:23 pm    Post subject: Reply with quote

Best for what??? General use? High performance? Economy? Power? You need to better define the use case you are building for.

Any way you slice it, changing from CIS to any other form of engine management is going to cost at least $1000, and probably much more.

With that in mind, assuming money is no object, the BEST approach for either optimal power or optimal economy is full blown stand alone engine management, which is not one of the options in your poll. The ability to precisely tune and control both fuel delivery and ignition as they relate to each other across a wide range of running conditions is the best engine management solution.

What too many people fail to realize is that repairing the CIS system is not that difficult, and frankly, no more expensive than a carb or EFI conversion ... and in many case, much LESS expensive. So best from an ROI perspective could very well be fix the damn CIS system.

I am routinely getting between 27-32MPG on a couple of my 931 high performance builds using nothing but bone stock CIS and ignition systems. You will be hard pressed to do better than that even with stand alone EMS, and certainly won't be able to get there with carbs. Unmodified OEM CIS is also quite capable of significant power adders in both NA and forced induction applications. If you're going to do something crazy like attempt to double the boost or HP, you might hit CIS limits, but in my experience, it has no trouble keeping up with ~50% increase in power. I am convinced that there is A LOT left on the table (performance wise) with the highly conservative stock ignition curves.

So given the options in your poll, my vote is for CIS.
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Rasta Monsta  



Joined: 12 Jul 2006
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Location: PacNW

PostPosted: Tue Jun 07, 2016 2:11 pm    Post subject: Reply with quote

ideola wrote:
You need to better define the use case you are building for.

+10

In general terms, CIS is better for power, efficiency, and fuel economy than carbs. In general terms, Motronic is better than mechanical fuel injection because it uses a greater number of variables to make intelligent decisions about ignition timing and fuel delivery.

But, Dan is right. In specific terms, to your purposes, why are you asking, and what are you contemplating?
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