Technical CIS operation & Cam Timing w/ CIS Discussion

[klatinn]

Hi,

The changes were +- ~0.4 AFR at different RPMs. Datalogged with an LM-1 on the same stretch of road on the same day. Made 2 runs before to see run-to-run variations (< 0.2 AFR) and after.
The old boot was pretty hard.

Regards,
Klaus

[911sTarga]

LM-1's are nice, glad to see you were using some good equip!

Maintaining the CIS is very important and your post shows exactly how important! Sounds like an inexpensive way to keep a CIS running correctly.

Thanks!

[MotoSook]

If you race the engine from idle, you should be able to see the boot flex...top of it will move down. But as the RPM increases at a steady rate (under WOT), I doubt the boot will do much to dampen any pulsation beyond what the mild cams can create.

The flow path downstream of the meter is also terribly tortuous....I really don't think the meter itself is a problem. If you really want to take the meter resistance out of the equation...re-engineer the fuel distributor so that the plunger does not have to act against the system pressure...use a booster between the fuel dist. and the injectors to get the fuel pass the injectors. The WUR would also have to be modified so in the end, you control pressure and your system pressure are very very low...but the injectors are still seeing the same DP and flow...

or put a TPS sensor on the lever..and a solenoid on the plunger...(shaking head)...

Better yet...redesign a plenum/manifold, move the throttle body to the center of a larger plenum... (shaking head again) ..wait..that's kinda like a 3.2 right

there are lots of things you can do, but again, all of it seems hardly worth the effort when you can just build a hotrodded engine with EFI or simple carbs. Sure I like my CIS, but somethings are just not worth the effort....glory..sure...there's plenty of glory in solving a problem that now one else has ventured to do...but by the time you are done dicking around with re-engineering a CIS engine, one could have built plenty of monsters.

Go back to the cams....that's where your going to get the most return on your effort.

[klatinn]

LM-1's are nice, glad to see you were using some good equip!

Thanx, I designed them.
I posted this before, so those guys who read it, forgive me. But it works really well.
On the CIS Lambda you can use an LM-1 or LC-1 to make it programmable. Disconnect the WOT switch wire from the throttle body switch. This makes the Lambda computer stay in closed loop all the time. Then program the second analog out of the LM-1/LC-1 to simulate a NBO2 sensor with a switchpoint at 12.8 AFR or whatever. Use the WOT switch to drive a relay that switches the O2 sensor input of the Lambda computer between analog out1 (regular O2 sensor simulation, 14.7 AFR) and the second output. The lambda computer then regulates at WOT to the programmed AFR.

I did NOT use that when checking the boot effects

Regards,
Klaus

[911sTarga]

Souk,

I agree with you. Cam design is going to be the biggest power maker, but I do plan on making small changes to the plenum size (maybe a simple spacer) and improving the exhaust flow at the same time.

I honestly would have thought that there would be more interest/information on this subject!

[randywebb]

"even managed to piss off Randy..."

- huh - well, I don't remember it... (just don't call me no engineer...)

I agree to focus on the issues and not personalities -- I'll often say something like "the poster above" rather than use a name for just that reason.

- I am curious if 'Souk' is in any way related to the famous Souk carpets.

[patkeefe]

Why are SC piston domes shaped the way they are?

[island911]

Originally posted by randywebb
the mass of the plate and its associated parts + friction will always limit the responsiveness of these cars

I would NOT try to experiment with hotter cams, etc. -- unless the engine belonged to an enemy...

FWIW - I am a known CIS-hater. Death to the CIS!

So what you are saying is; CIS is slow responding . . . but dont use hotter cams because CIS is too sensitive ...too quick to respond and will pick-up the cam pusling.

. . is that right?

That does sound like limited responsiveness.

[klatinn]

Why are SC piston domes shaped the way they are?

I think I remember that from a Porsche tech document a long time ago (the mind is the second thing to go, and I can't remember the first):

CIS injects continuously. During ~3/4 of an engine cycle the intake valve is closed and the fuel cloud just sits there. When it gets sucked in, a lot of turbulence is needed to mix properly during the intake and compression stroke. The dome-shaped pistons increase turbulence. Specifically a CIS problem during low load conditions.

Regards,
Klaus

[island911]

Shape; it's a balance between mixing(air/fuel - toroidal swirl) and burning (flame propogation)

[randywebb]

Isl - I'm saying dont use hotter cams because the pistons used in CIS engines will likely hit if you do.

Also someone asked for a source for that -- IIRC it's in BA's book.

[randywebb]

PS - I think it's great if somebody wants to put in the work to see how much lift/ramp angle you can do on a cam w/o having to change the pistons. It might be a poor use of time, but that has never stopped anybody else with a 911 - including me...

[island911]

Originally posted by randywebb
Isl - I'm saying dont use hotter cams because the pistons used in CIS engines will likely hit if you do.

Also someone asked for a source for that -- IIRC it's in BA's book.

Ah. sure.

fwiw, the conventional wisdom is that CIS is to reactionary to the pulses of hotter cam (over-lap, not lift)

[patkeefe]

Klaus,
I agree that the mind is the second thing to go. I previously answered a question with a question on the domes; the mix needs the turbulence to atomize in order to burn efficiently. Everything through to the early SC cars is pointed towards the reduction of emissions...the pistons, the K-Jetronic (which has proved it's durability and simplicity in years of VW use).

The camshaft profiles and timing are made to work with the "low response" CIS and provide enough power to get by, but not much more. The big picture becomes "how to increase the volumetric efficiency and raise the compression ratio", which have the biggest impact on power production. The proposition eloquently put forth by 911sTarga seems to be "what is the limitation of the K-Jetronic?". Difficult to quantify, especially when the cam options are either A, B or C....exactly why there is a PRACTICAL limitation of the CIS system. I have pondered this myself, and said "this looks easy to screw around with", but as Souk and others with a heck of a lot more experience than me have replied, it's really not. So, it ends up a matter of atomization and cam timing.

It's why I have a set of (plan B) Webers in the garage.
Pat

[Driver Ed]

For us shade-tree mechanics following along, when y'all talk about "reversion" what does that mean exactly? Does it refer to the tendency of the air sensor to oscillate due to the suction variations as the valves open and close? I'm more of an electronics guy than an automotive guy, so it makes me want to get out a o'scope and watch the air flow sensor waveform. Anybody got one they can post?

[Carrera3.5L]

Originally posted by Souk
Charlie...I just got some 964 cams in...maybe I should have gone with 20/21..but would the 20/21 pass emissions? I still have another set of cams I can send out for regrind.

Quick Jay...get to work on the cam! The fuel is simple...just get me some wild cams to work with my DME 3.2 Carrera! And still pass emissions!

Souk, 20/21's will pass emissions here in smoggy Southern California, so I'm sure you would be fine in Chi-town. These are the "wildest" CIS/DME cams that I know of that still will pass emissions, but there may be other grinds out there that do as well.

My 3.5L twin-plug motor recently passed with Steve's chip, 20/21 cams AND headers (but just barely). Throwing the heat exchangers and cat back on it would pass with flying colors.

Somebody asked for 964 cam specs, this is what I have in my notes from Andial's cam doctor:

964's are timed at 1.25mm. Intake is .470" lift and 240° duration and exhaust is .430" lift and 230° duration. I don't have the lobe centers (so shoot me!) but it will be on the wider side obviously.

As a comparison, these are the 20/21 specs according to Web-Cam:

Timed between 1.9mm-2.2mm with .10mm lash

Valve Lash intake: .004
Valve Lash exhaust: .004

Valve lift intake: .485"
Valve lift exhaust: .452"

Duration: 258° intake and 246° exhaust
Duration at .050": 238° intake and 226° exhaust
Lobe Center: 113° intake and 112° exhaust

Intake opens 6° BTDC and closes 52° ABDC
Exhaust opens 45° BBDC and closes 1° ATDC

Valve timing is checked with zero valve lash at .050" of valve lift

Minimum piston-to-valve clearance is .050" intake and .080" exhaust. I used the RSR style piston for my twin-plug application rather than the CIS/DME piston so I had PLENTY of room. I think I measured something like 2.5mm for the intake and exhaust with these pistons!

Y'all go back to analyzing CIS, I'm enjoying a beer and sitting back in my chair while perusing this enjoyable thread.

Ralph

[randywebb]

reversion:

Both the intake and the exhaust resemble 6 oboes (or something) next to each other so close that they interact. If one was isolated off by itself you could plot a sine wave for its pressure vs. time. Since they are near each other, one low pressure region will interact with the pressure from the tube next in the firing order (right before it, and right after it). For an exhaust system, this is usually called scavenging and for race engines, the post-WWII engine builder will try to enhance it at a favored rpm. The low pressure from one exhaust pipe helps pull the exhaust out of the one next to it. Reversion usually is used w.r.t. the intakes - on some engines a cloud of vapor particles will actually hang in the air above the intakes.

- There is also the tuning of each pipe for a given frequency...

- not exactly (for which you'd write the equations) but should be close enough - I hope it makes sense - not sure i did such a good job... wave physics sucks.

[ianc]

For us shade-tree mechanics following along, when y'all talk about "reversion" what does that mean exactly? Does it refer to the tendency of the air sensor to oscillate due to the suction variations as the valves open and close?

Yes. My understanding is this:

To enhance high RPM performance, it helps to have the intake valve start to open before the piston reaches TDC on the exhaust stroke to begin filling the cylinder with fuel air mixture sooner. If the intake and exhaust valves are open at the same time, this is called valve overlap. You can see that if the intake valve is open while the piston is still travelling out, this can cause a pressure reversal, or reversion, in the intake. When this momentary pressure pulse travels back up the intake, it can cause the sensor plate to 'flutter' up and down, which plays hell with the mixture and causes power loss due to the momentary lean condition induced.

Clear as mud?

ianc

[emcon5]

The biggest limitation of CIS is the single plenum.

There is a discussion of cam selection with single plenum intake here, (on page 2): http://forums.pelicanparts.com/showthread.php?threadid=161071

Another discussion of CIS cam selections, with a chart here: 3.0 CIS cam grinds, 964 vs. web 20/21

Another problem is fuel delivery, if you modify too far beyond the design parameters of the engine. Past results have shown CIS to tolerate power increases fairly well, with Dane's 3.4, My and many other short stroke 3.2s, Noah's C3 making 200+ HP with a 2.7 fuel distributor, a few aftermarket turbo kits, among others.

The cams chosen for the CIS 911s were designed to make the engine produce decent power, nice smooth idle, acceptable fuel economy, and low emissions. It met these goals quite well, especially if you consider the competition when they were built. Porsche got 180 HP out of a 3 litre engine, the early '80s Corvette only had 190, and that was from 5.7 litres.

With these design parameters in mind, the guys at Porsche sat down with the guys from Bosch, and put together a fuel injection system to meet these needs. Porsche, like every German car builder at the time used whatever the current Bosch fuel injection system was. CIS can provide as much or as little fuel the engine designers required, to meet the expected needs of the engine.

If you want to stick with CIS, make more power, and drive the car on the street, you are really limited on what you can do.

Displacement: for a 3.0, 98mm pistons/cylinders = 3.2L, add a 84-89 crank = 3.4L
Slightly hotter cam: conventional wisdom is Web 20/21 is as good as it gets. If you go a lot hotter, low speed drivablilty will suffer (see the discussion in the first link above).
Higher compression: depending on fuel availability, this may require twin plugs.
Better exhaust: If you want heat, SSIs are pretty much it.


rdanes's 3.4 CIS is a great example of how far you can go with CIS if you are willing to spend the cash. He got ~220 at the rear wheels. His engine is also for sale: FS 3.4 twin plug

My SC makes ~175 HP at the wheels, with 964 cams, 9.8:1 98mm pistons and SSIs, with CIS. My engine has the smaller late intake runners, which hurt top end, but boosted my torque to 186ft lbs @ 4200. My car has as much torque at the wheels as a Motronic 3.2 has at the crank. My dyno results and discussion are here:

Dyno results

Another discussions of hot-rodding CIS engines:

CIS Gurus - Help me Safely Inject my Hot Rod 3.0

Don't neglect your ports.

Tom

[MotoSook]

Originally posted by 911sTarga
Souk,

I agree with you. Cam design is going to be the biggest power maker, but I do plan on making small changes to the plenum size (maybe a simple spacer) and improving the exhaust flow at the same time.

I honestly would have thought that there would be more interest/information on this subject!

Jay, there is a lot of interest on the subject, but I think prudent action is to live with the system (those that understand and accept it) or go to carbs or EFI. And if one is going to go to the large effort (time and $$) one might as well build a better engine from all angles. Cost benefit is a popular factor on this board, IMO. If you can make a cam with more duration without stretching the overlap too far, you'll have the next best thing for a CIS/DME engine! Like I said...concentrate on that.

I would graph the 964 (or 20/21) cam profile on a chart with the piston to cylinder clearing on that same chart, that'll give you a good visual and a nice graphical tool to plan your work. I'm not a cam guy and I haven't the time to play with it, but I would really like to see a graph like I described for the various cam profiles.

Randy...Souk is all things and everywhere

Ralph...thanks for the post on the 20/21. I think I'm going to have a 964 cam set for sale very soon I'm actually working a deal to build a better engine than the one I set out to do..one with no EPA limits..just a cash limit, but I promise, it'll be a hell of an engine in NA form... this is the stuff I LOVE!