What is the problem with CIS and aggessive cams?
 

Hmm, I have been thinking...
CIS uses mild cams with little overlap and pistons with a special dome shapes right?
I guessing becouse of the piston shape the lift (or is it duration, I'm far from a cam expert) is limited?

A while ago I read (here) that some people used the 2.7 HC RS pistons still using CIS (for 2.7l engine).
So therefor could you not use cams with higher lift / longer duration with the extra clearence from the RS pistons?

Also after reading "Bosch Fuel Injection and Engine Management" by Charles O. Probst, about the limitations of CIS. He writes that the limitations are that it cant handle overlap very good at low rpms, and you might have to raise idle to above 2000.

Could you use CIS with hotter cams (maybe not early S) and accept rough idle. I meen the pulsing that fools the metering plate cant be an issue at higher rpms, or am I wrong?

Mangus

I have a pair of freshly ground 2.7 CIS cams with a 3.0SC CIS grind.

These cams are in excellent condition, still in the sealed box from the shop and ready to install.

These cams have the nut style front snout and fit early Porsche 911 2.7 litre CIS type engines. These cams should provide an easy 15 to 25 additional horsepower to a stock engine and still allow your car to pass emission testing.

The Intake duration @ 1mm is 236 degrees valve lift is .450".

The Exhaust duration @ 1mm is 224 degrees valve lift is .395"

Set up at 1.4-1.7 mm

A very popular upgrade for the 911 2.7 S engine with CIS injection.

The casting numbers are 911 105 143 0R left side and 911 105 144 0R right side.

Price is $290 but all offers and trades are considered.

The info on this is in the Engine Rebuild book. Briefly, the hotter cams produce those vibrations that affect the metering of the CIS sensor plate. It's also a throttle-body based system which generates cross-talk across several cylinders. Using hotter cams is just not the right thing for a street car, as it would make the car undriveable at lower RPM. I'm not sure about a race car - I suppose it could work for certain circumstances, but at that point, you'd surely want to go to carbs or some other individual throttle-body setup...

-Wayne

It seems that in our situation the CIS metering device itself doesn't impact things much more than the single throttle body with all runners behind it, since people don't really run any "hotter" of a cam in a 3.2 carrera than in an SC, they both will do a 964 or 20/21.

I would love to hear examples of CIS or DME shared manifold 3.0s or 3.2s with hotter than 964 or 20/21 cams, and how they performed. Lower RPM throttle response is at stake.

Reminder: the added displacement mitigates a lot of this negative effect...

-Wayne

because the weight of the car stays the same, and the increased displacement fills in the low end torque?

What about this: is there any reason why a 3.6 will run well with a hotter cam than a 3.2, assuming they both have shared plenum single throttle body manifolds? I ask because people use things like the super cup cams in 3.6s and they don't seem to go bigger than 964 or 20/21 in a 3.2.

EDIT: looking at John's awesome spec sheet, the 3.8 super cup cams really aren't that much more agressive than a 964 cam, and it seems they could be used with a 3.2 with shared manifold just fine, as long as the valve clearance was there (e.g. with JEs or mahle sport). I'm not sure I know of a cam that gets used in street driven 3.6s that isn't suitable for a 3.2 manifold setup.

I'm not being very specific here, but I'm taking about both 3.2s and 3.6s with stock type intake manifolds and taking valve clearance out of the equation.

Andy;

Of the cam variables (lift, duration, acceleration, lobe angle, overlap, etc), it would appear that engines with a common plenum and throttle body suffer the most from anything more then a little overlap. As long as you keep the overlap low, you can run as big of a cam as you can fit without interfereing with the pistons. At some point the duration will be so long though that you'll go "over the top" on the intake closeing point and exhaust opening point and start to lose HP. Where this point is is best determined on a dyno and can be influenced some by the CR.

In the case of the 3.8 Supercup cam, the overlap works out to about 11 degrees, which appears to be at the upper limit for engines with a common throttle body and plenum. I seem to remember Steve Weiner posting something in the past about some of the Supercup engines had poor part-throttle driveability largely due to the common throttle body and plenum.

I believe this is a myth...

It probably started when some shop started to lobby for their own ITB's and put the word out that nothing but their (very expensive) ITB will ever work on common-plenum car.

Certain members of P-car community accepted this as universal truth, and it somehow propagated into a cult of "intake reversion".

930, for example, has CIS-system and very low C/R pistons. Path-length from throttle body to CIS metering-plate is very long and passes intercooler cavity, lot's of tubing and compressor vanes spinning in opposite direction. Chances of any "shock waves" disturbing the CIS are zero, zilch.

I have dyno papers from 4-cylinder 2v turbocharged engine using CIS that has been tested with four differenc cam profiles. It responded to hotter cams just as eny other engine should.

So to begin with, all this gospel about CIS being disturbed by hot cams is 100% not true with turbocharged engines.

Also, Juan Ruiz has pretty potent engine (understatement of the week) running with plain stock Carrera 3.2 plenum and throttle and some very hot cams. So there goes "intaker reversion will ruin your life"-theory as well...

There are millions of cars (well, I believe 99.99% of produced cars have common plenum with single thottle) with hot cams happilly rolling along.

I haven't seen any proof of this famed revesion ever taking place. I've seen few people claim it (who might heard it from couple of other guys who heard it from somebody ... who owns a shop?)

While there might be some truth about problems with hot cams on SC's due to their unusual piston shape and small piston/valve clearance, this propaganda propagated far beyond beliavable.

I'm willing to accept that SC engine itself might have problems w/ hot cams due to above reasons and that CIS plate maybe is little too close...but this became such blanket-statement that it's time to torroughly debug it.


jluetjen, Wayne:
I would be glad to hear your explanation on how CIS air-flapper is disturbed by vibrations on 930. I'm also very interested to find out how those vibrations go past compressor vanes. Tolerances inside compressor are in order of 0.1mm and vanes are spinning at speeds beyond 60000 RPM, connected to shaft with lot's of intertia.

Also, I would like to know how's that that Porsche plenum is somehow very suspectible to "intake reversion" when most other cars are not? Any dyno results? Sources?

Goran: Thats what I mean. Lots of references to people saying CIS and hot cams are a big "No, No", but no one who has tried and failed.
Or has anyone tried and failed?

There is some discussion of cam choices/limitations with single plenum here, including the discussion of the 993 Cup cars John mentioned. To summarize:

Tom

mmkaay...so all those tens of millions guys who run hot cams on single plenum are actually wrong? All Hondas, Miatas, Alfa Romeos etc. should run to Rennsport and buy some hot ITB's? :-)

I mean, most long duration-narrow lobe cams will worsen low-range torque ...that's the usual tradeoff with hot cams otherwise everyone would be installing them, right?

I'm offended by very general statements seen here. They actually default to things like:

"CIS cars won't do hot cams beacuse CIS-flapper will get upset by vibrations" and "Single-throttle plenum doesn't work with hot cams".

I have sold a the Super-C2 grind for 3.0 litre engines with cis in the past. I warn the customer about problems with cis and a cam with a lot of overlap. One shop owner told me " Dont worry, I will tweak it, it will work". He did get it to work and the customer eventually
complained about the lack of low speed response (under 3000 rpms).
I agree the problem is not with the cis sytem. Using an agressive camshaft in an early SC with barely 8-5-1 compression and big ports would kill driveabilty with or without the cis.

Thanx John! nice to see somebody with hands-on expirience posting in this thread.

That's approximately what I was trying to say: hot cam is a compromise and using one means you agreed to trade low-range driveability for top-end power. Always has been no matter what FI was used. There are probably some minor improvements to be won by using ITB's but it's a loooong way from "hot cams don't work with single throttle body/CIS".

Beepbeep - I was discussing non-turbo applications. I don't have any comment on using "wild" cams in a turbo CIS application since I don't have any data. My apologies if you thought that I suggested otherwise.

The other thing to consider since most CIS cars still require emissions testing in many states is can you get it to pass?

No probs.

But as we all can see, the blanket-statement of "hot cams don't work with CIS and/or single-throttle plenum" is shrinking towards something resembling "hot cams have quite bad low-RPM response when fitted CIS-equipped SC with low-comp pistons". That's what I was trying to point out.

I believe most engines will suffer from worsening emissions if fitted with hotter cams. Lot's of overlap is there to "pre-emtively" open the valves when things are spinning at high revs and air intake-velocities begin to make a huge influence. Unfortunately, overlap at lower revs will also create lot's of "draught" trough cylinder...spitting out some of unburnt fuel. Not good for emissions, I'm afraid.

Further on, I believe most turbocharged engines equipped with CIS are immune to "flapper vibrations" for reasons mentioned above.

That leaves us to final question:

Is there anybody who dynoed his (SC) engine with CIS and carbs/ITB's using same (hot) cams?

No. Hotter cams with a stock CIS system will cause the rattling of the CIS sensor plate. This is common knowledge with CIS systems and is the reason why power is often down when moving to a CIS system. Case in point 1979/1980 Ferrari 308s. Carb vs. CIS. The 1980 version with the CIS has milder cams installed in order to "cooperate" with the CIS system. To gain back the power, you need to swap the cams, machine the pistons (if needed), and move to a throttle body system. These cars are designed from the factory like this. CIS is not a high-performance fuel injection system.

As for the argument of throttle-boddies versus separate intake bodies, you should talk with Rick Clewett - he's the expert on the TWM systems and the TEC-3. You can run hotter cams with a fully programmable EFI system that uses the Motronic manifolds. However, these manifolds are not the greatest design, and will suffer from cross-talk or fuel reversion with certain camshafts.

For the record, the 930 has relatively mild camshafts. In fact, a good Turbo upgrade is to install the 911SC cams...

-Wayne

Why couldn't you run higher compression say 10.5.1 with 964 cams.

this would get around the agressive cam issue whilst still increasing the HP and thus drivability across the range.

Michael

You could do that, if you ran twin-plug or higher octane gas.

I stopped by my buddy Rick Clewett's house tonite (picking something up). He reminded me of some things I had forgotten.

Yes, 3.2 Motronic manifold intake systems with hot cams have lots of problems at lower RPM. He has direct experience with these - they are fine at RPMs above 3000, but you wouldn't want to drive them. Part of the problem is the charge momentum. When the valve closes, the air hits the back of the valve and bounces back up, like a rubber ball. This charge reversal then intereferes with the proper mixing of fuel (particularly on the CIS systems where the fuel insertion is not sequential - meaning in time with the opening and closing of the valves). This fuel reversion effect is mitigated by taller manifolds and high butterflys on MFI cars.

The bottomline is that this is not a myth. Yes, you can run hot cams on a CIS car and / or an 3.2 manifold car, but you will have problems at lower RPMs...

-Wayne

Another thing to keep in mind here is that a "hot cam" for an average car is probably not the same as a "hot cam" for a 911. In other words, I'm not seeing a comparison between durations, lobe centers, and lift. I suspect that in some cases, 911 stock cams may be closer to the aftermarket cams put in some other models.

Well, I am offended that you question the integrity of one of the most well respected people in the Porsche racing/performance community.

Just in case you aren't aware, BTDT is an acronym for Been There, Done That.