Why such mild cams with CIS?

I believe that with CIS systems, the amount of fuel injected is directly proportional to the deflection of the air flow sensor plate in the air diffuser housing. The intake of air must be very consistent, and I think with aggressive cams the air flow "signal" to the fuel distributors would be erratic. I'd like to hear other more educated types chime in too.....

 

yup.

 

Yes, the CIS is a HUGE restriction. Simply replacing it with EFI or carbs on a QV or 328 will add about 30 hp with no other change. CIS makes the air make a lot of turns and there is that great big mass air plate in the flow path. bad, bad, bad......

 

As said before, the amount of fuel delivered by the CIS is dependant on the steady deflection of the air flow sense plate (aka 'barn door') in the fuel head unit. More 'aggressive cams' are not only about lift, but also duration - and with duration comes (usually) increased overlap which gives rise to 'reversion waves'. These reversion waves, while dependant on numerous parameters, can be most disturbing to the airflow column at lower rpm -- enough so that it affects the stability of the airflow sensor plate making the unit difficult to set up.

An previous FChatter from some years ago, "Kermit", actually did put a set of slightly hotter cams in a 2v CIS car with good results. However if I remember correctly, he had to raise the idle quite a bit to get it to idle. Without doing a search, I believe he had a 2v 3 liter Mondial with CIS up to 260 hp after porting, polishing the heads and cams. Quite an accomplishment in anyone's estimation. He also broke a lot of conventional wisdom about that these CIS systems simply would not run well - he really broke a lot of new ground.

The Porsche 911 2.7/3.0 and 924 guys also have CIS, and they have pushed the limits of what the CIS system can do as far as out put and cams. I read in the magazine "Excellence" once about what was the limit they discovered you could go on duration/overlap (lift is NOT a limiting factor as far as the CIS goes). They are VERY smart on CIS. Intrestingly, there is an entire market and company set up (PMO) that replaces CIS with Webers with terrific results, and is a recognized standard and reliable upgrade for CIS Porsches. They also always say that while you do get an immediate hp boost going to carbs, the biggest seat of the pants change comes in the much improved crispness in engine response. This has been my experience replacing CIS with Webers on a Ferrari motor as well.

In fact, just from observation, in a naturally aspirated set-up, the 308 CIS maxes out in delivering enough air/fuel for around 270 or so hp -- this was the most they ever got out of a 4v 3.2 liter with it, and just switching to carbs on a stock 3.2 engine with stock cams instantly adds 30+ hp -- so this shows that at about 270 hp is where the stock NA CIS K-jetronic runs out of air. Now obviously, you can super/turbo charge the system and push through more, but in a naturally aspirated set up, 270 or so is about it from the stock unit.

Hope this helps.

 

I wonder if there is an airflow or vacuum "buffer" that could smooth out the "reversion" feedback pulses to the air flow sensor plate? They do make those vacuum canisters for big lift/duration cams that allow you to continue to operate power brakes, etc. You can't really do anything to the free movement of the sensor plate as that would only make throttle response even more sluggish. So is there anything to equal out the ups and downs in air flow? I just had a wild thought, you know those resonators that go into modern car air intake systems to cancel out unwanted pulses...I think they call them Helmholtz Resonators.....is there a way to make one for the intake tract of a CIS system? It would take some design work, and some measurement work based on the cam profiles to determine what location and what shape this resonator would need.

But there will come a time where there will be diminishing returns upon your investment....a point where it would just be easier to remove the CIS system and run carbs or an aftermarket EFI. Never the less.....it's fun to shoot out ideas.....

 

that, would be an OUTSTANDING conversion ... !

 

I'm almost doing that right now. I'm using 12 individual throttle bodies mounted to a TR head, which will be mounted to a 400i block. I just finished designing the manifold, if you need one let me know.

If you wanted to go carb, the PMOs they sell for 911 applications are probably the way to go. You'd need to make intakes,but that's not a real big deal.
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itd be a crap conversion - why would anyone want to go for carbs these days when ITB's give the best of both worlds ?


(btw im not sure i understand your terminology - CIS = continuous injection system - yes ?)

 

Carbs work fine as long as the cams aren't crazy and for most people the installation is much easier than trying to write in an ECU. ITBs are a definite improvement, but carbs are still WAY batter the OEM CIS system at making hp.

(yes continuos injection system, its the oem mechanical FI system)

 

That's what I heard

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It's about 5%-10% if the carbs are sized small enough to aviod drivability issues

The CIS fuel pump is fine for EFI, so that's one thing at least. EFI is also much easier and faster to tune than carbs. Something like a haltech E11 will run a 12 just fine and cost under $2k with a flying lead harness, figure $2500 with sensors and such. The I'm using motorcycle TBs which can be had for about $200 pre set of 4, but you really need 4 sets to make them fit together properly and have 4 odd TBs left over, so $800.

I think the ITB conversion is actually cheaper than the carb conversion as 4 3 barrel PMOs are are $6k plus other parts. You might find used webers for 3/4 that I guess. Then you need to make the intake which will be about double the cost of the ITB intake because the carb bore spacings don't line stay up with the port bore spacings.

Yes, his 3.2 mondial

 

you say its a small difference HP wise - 5 - 10% is often quoted - so call it 7.5%.
on a 250bhp carb'd engine thats an extra 19bhp which isnt to be sniffed at.

plus you will get better midrange etc as the ITB's will be tuned for the whole rev range - not a "best fit" like carbs are.

swings and roundabouts on the costs really - you save in some areas and lose in others
however dont forget that ideally to do the carbs properly you would need to add some form of mappable ignition which negates a great deal of the ecu cost.

over here in the uk ITB's are getting really popular for the n/a modifiers - and can be fully installed for £1500 - 2000 on the 4 cylinder hot hatches (thats $3 - 4000 USD)

(again terminology - ITF being another term for ITB yes ?)

 

First off, while you can argue all day about some minimal percentage difference between ITB's and carbs, NOTHING on earth sounds like a carburator venturi, and 12 of them make a sound that is music to many peoples ears. I would easliy sacrifice a "possible" and perhaps nonexistant 5% power loss over ITB's, to have Webers on a 12 cylinder Ferrari, no matter the cost. IOW, if I was going to all the trouble to yank all the CIS BS off the car, there would be no other reason than to put carbs on it.

On the CIS cars, as Russ mentioned, Kermit was doing a lot of forward thinking. IIRC he ran early carb cams on a CIS car, perhaps the car Russ mentioned, and had it idling pretty good. I think he had to retard the intake cam a bit more than normal IIRC, and I dont know if it would pass emissions either. He also made a larger throttle butterfly, but those who tested it didnt bore out the manifold to match it, thus in a dyno test it actually made a bit "less" power, which would be understandable as the airflow was being disrupted. Kermit also did some flow work on the CIS aircleaner box and distributor housing and found some ways to improve airflow. Maybe one of us should try and find old Kermit and drag him back here.

 

I'll respectfully disagree very slightly based on what I've seen around FChat and suggest it is more accurate to say "EFI is easier and faster to make changes to than carbs".
You can get a Weber dialled in very straightforwardly while I've seen EFI guys here futz with their systems for months at a time.