CIS and lumpy cam - interesting stuff
 

I found this in Dave Walker's monthly column in Cars and Car Conversions - a UK magazine which focusses on club racing, track days and UK performance cars. He is the technical editor and the column each month covers off some of the cars going through his workshop. His website is:

http://members.aol.com/emeraldm3d/

The interesting stuff is in relation to a 1.8 litre Rover K-Series engine with a "165hp" engine conversion - it would either make good power and idle poorly or idle well and make lower power, depending on the timing of the inlet cam. The interesting quote is:

"The problem is one that I have come across on a number of occasions. Big cams and plenum chambers do not work together, it would be nice if they did, but they don't.

Anything much more than a fast road cam results in an erratic idle and a severe jerking on light throttles on the road. I had a Caterham making 195bhp on a plenum and it drove smooth as silk - from small throttle openings up to full bore. But it jerked like a bastard on very light throttle and idled like a jelly on a plate. Within a week the owner fitted throttle bodies to get around the problem....

...Years ago we used to avoid cams with K-Jetronic injection systems like the plague, thinking that it was the fuel metering flap getting upset by the overlap. With hindsight it was the long branch plenum chamber that was causing the problem."

So, does this mean there may be a future for an enterprising CIS engined owner who is doing a rebuild (and can swap pistons and cams for, say, S cams). Or is it going to be impossible to have a small enough plenum chamber (presumably the length of the runners? Or am I showing my ignorance (again) about how this all works!!!

Anyway, I thought it was interesting.

I'm not 100% clear on this? What was the solution then?

-Wayne

It sounds like the problem is in the size and length of the intake runners, and the size of the air box. Unless you are going to change the air box and intake runners, then you will have trouble with aggressive cams. My 20/21 cams don't like to idle through parking lots, and they're as aggressive as you should go, with CIS...they say. If you switch to carbs, then you don't have to worry about too-aggressive cams.

Wow Cam; that brings back memories. I used to buy C&CC all the time when I raced my FWD Mazda 323. There is little if any information about race tuning for that car, but I found C&CC invaluable because they covered low budget prep for just about anything. I always liked Dave's column because there would never be any hype, just a "let me show you what I've learned..." tone about it. I suspect that C&CC (30+/- years old now at least) was the inspiration for the folks who started "Grass Roots Motorsports" here in the US. Both magazines share the same qualities, but C&CC seems to cover the most eclectic range of cars (even an occasional 911!)

I've book-marked that page since a mapped ignitition system is in the plan for my car when I get to that point.

As far as your question about the plenum versus the air flow sensor, an interesting group of people to check would be people running under the "Stock Injection manifold" rule in the SCCA's Production classes. Essentially, a recent (last few years) change to the rules allows cars with factory injection systems to use the stock manifold and throttle body and a free computer rather then a manditory change to carbs. This often means that they pitch the air flow sensor and use a mapped system such as the M3D. The cams are essentially free. I saw a guy running a 944 with this set-up at an SCCA race I went to in the fall. I suspect that there are folks out there with experience.

I thought that one of the reasons CIS didn't work well with large cams was that the airflow sensor plate would essentially flutter with the pulses in the induction manifolds due to the longer durations and overlap... THis then causing the mixture to change rapidly and causing lumpy running. (Besides the obvious valve-piston clearance issues)

I would also surmize that a hot-wire setup would then help eliminate this, but I don't know if an SC injection can take one of these. Since I've never seen them adverstised, I'm guessing it can't.

I understood (not really the right word) what Chris and Super say.

What I think Dave Walker might be saying is that with shorter runners (ie less intake) then the pulses won't be as pronounced and the problem won't be as severe.

I might do some research later on - for a start plenty of VW four cylinders used CIS.

Unfortunately even if this is the problem, a cure involving a new fabricated manifold is probably impossible (I can't see how the "spider" manifold could be much smaller).

What I take he is saying is. That with a long plenum you have a larger airmass contained in the plenum that must be moved. That weighs more and requires more suction to move. With the larger cams you have poorer coupling to the airmass which results in a dissruption of the velocity of the airmass at idle.

Bobby

Coupling Airmass?

Disruption of Velocity?

Man, you guys are far to smart... I'm still noodling on how cam overlap works...

I'm okay now. It's just that this is the first time I've ever done Crack. I'll get a handle on it.

Bobby