CIS Performance mods......

[fred cook]

Other than retrofitting to 4R (early SC) intake runners and maybe using a low restriction air filter (K&N, etc.) is there anything else that can be done to a CIS system to increase airflow? How about changing the fuel distributor to one for a Turbo motor for additional fuel under load? I'm looking for any and all tricks that have been used on CIS in the past.

Thanks,

[Dr J]

Here are a couple on the fuel side:

1) Increase system pressure by shimming the regulator
2) Get a higher performance fuel pump (lots written on the 044 pump)
3) Get an adjustable WUR- decreasing control pressure yields more fuel at the injectors
4) Have the fuel distributor adjusted to flow more (search Larry I think its cistech)
5) Go for a Euro fuel distributor

Other than #1, the rest will require $$.

[Bob Kontak]

The throttle body can be bored out. There may be a fancy air box out there that adds some flow.

But as Dr J says $$.

You can adjust the cam timing for higher top end but that is dependent on where it is set currently.

[jluetjen]

Extra fuel only helps if you're running lean. The trick is to get more airflow. I've heard of some people flow-benching the meters and tweaking the passage to flow more air. I heard that on the 934's Porsche added a cone (on the down-wind side of the plate?) to improve the flow of air around the plate. I think that this is the sort of stuff that you need to be looking at.

[fred cook]

Going to install an AFR gauge to monitor the mixture being fed thru the engine with the current bit and pieces. The engine in my '80 SC is stock (I think) unless one of the 4 previous owners changed something inside. When I got the car about 14 years ago it already had a pop off valve and Carrera tensioners installed. I haven't had any reason to go inside to see if the cams, etc were stock. I do know that it runs quite well and pulls strong up to 7k. In Wayne's 911 engine rebuild book he mentions a 3.7 liter engine that was built using CIS that made an estimated 280 - 300 hp. The paragraph also mentions "tweaks" done to enhance the performance but doesn't say what they were. I believe this engine was built by Andial.

Not real sure what changing the fuel pump would achieve as the system pumps more fuel than the engine can use now with the extra fuel going back to the tank via the return line. One way to boost fuel pressure might be to put a restrictor in the return line but don't know if anyone has done that. Going to a digital and/or adjustable WUR would open some windows for trial and error. Seems that I have read that the 930 CIS fuel distributor was set up to provide extra fuel at wide open throttle. Need to look into that. I also like the idea of improving the airflow around the plate. Not sure what a flow shaping cone would be made of as it would be less than desireable to have bits come off and run thru the engine! I might need to get my aerospace employed son to work on that one.

Great replies, just what I was looking for, a fun venture into virtual enhancements for the CIS system!!

[Joe Bob]

Modify the intake all ya wants....it has to poop.....

3.0s like to exhale/breath. Early HEs or SSIs. 964 or 20/21 cams.

[fred cook]

The engine already has the SSI conversion with a 2 in/1 out muffler. Also, the CDI box has been replaced with an MSD 6A unit. I have the timing very slightly advanced and the O2 sensor unplugged. The plug wires are Magnacors. As far as I know, everything else is stock.

I was just looking at the path that air has to take to get into the engine. Very convoluted! Just started looking to see if there was any way to straighten out the airflow some. This little "what if" session could wind up with a system that doesn't look at all like the stock system! I think this is going to call for some serious airflow analysis!

[kiwiokie]

Independent throttle bodies would clean up the airflow

[davidbir]

Fred, I posted a similar question a couple of years ago and this is the thread that developed:
250 bhp from CIS ?

Oh, I see you already posted on that thread...

[fred cook]

That ITBs or PMOs would be better, but I have this curiosity about what could be done with CIS. Curiosity killed the cat, but it also led to a better mousetrap!

[Joe Bob]

The cam upgrade REALLY woke up my 3.0.......

[fred cook]

I have a set of twin plug 3.2 Carrera heads that have been ported by Ollies, 100mm Mahle pistons at 10.3:1 compression ratio, LN cylinders, XDi twin plug ignition and an early SC intake setup with the 38mm runners. I will have to port match the runners to the heads (heads are 42mm intake ports). Oh, yes, 964 cams. This will give me a 3.3 (almost 3.4) ltr engine with around a 10.1:1 compression ratio to work with the 964 cams. I will probably set the cam advance towards more top end hp as the added displacement should take care of low/mid range torque. Exhaust will be via SSIs and a 2 in/ 1 out muffler. All of this along with an adjustable WUR and maybe a ROW fuel distributor should make and interesting engine. The chassis it is going into has had about 200 - 250 lbs removed and is now somewhere around 2500 lbs with a half tank of gas. If I am not satisfied with the engine's performance, I can pretty easily change out the CIS for a set of 46mm PMOs. However, I was blessed with a virtually trouble free CIS system and will retain it if possible. I do want to get as much performance out of it as I can.

Head



Intake port

[T77911S]

Originally Posted by Dr J

Here are a couple on the fuel side:

1) Increase system pressure by shimming the regulator
2) Get a higher performance fuel pump (lots written on the 044 pump)
3) Get an adjustable WUR- decreasing control pressure yields more fuel at the injectors
4) Have the fuel distributor adjusted to flow more (search Larry I think its cistech)
5) Go for a Euro fuel distributor

Other than #1, the rest will require $$.

you would be better off going with an after market system. with all that money you have in the rest of the motor, why restrict it with CIS?
air flowand lack of throttle response is the biggest problem with CIS.


adjusting the control pressue does not change the amount of fuel at the injectors.
if you lower the pressure, then you will just lean the mixture back out with the allen screw to obtain the mixture you had before you changed the pressure.

the air flow meter (AFM) or bowl has 3 sections to it. the lowest is the idle section. then the mid RPM section for a leaner burn condition and then the upper RPM section that richens the mixture up for higher loads.
the factory has figured out what control pressure keeps the sensor plate in what section for given RPM ranges. IE, the transition from idle to mid RPM is set to happen at a certain RPM and the transition from mid RPM to upper RPM's is set to happen at a certain RPM. so changing the control pressure, (unless other engine mods are done such as compression ratio, cams, displacement increase), will change when these transitions happen and can make for a bad driving car.
if you make major engine mods such as the ones listed, you will want to raise the control pressure because now the engine is requiring more air for a given RPM.


increasing fuel distributor flow is a waste of money....unless you have made enough engine mods that the CIS is at its limit. there are others here that know more about that limit.
trying to put more fuel at the injectors only works if enough air can get there for the correct mixture.


raising the system pressure? perhaps better atomization of the fuel, but i dont know how the higher pressure would effect the rest of the system.

for those that have the 2.7, you can do as i did. i put the throttle body from a 3.0 on my 2.7.
for those with a 3.0, perhaps look into fitting a 930 throttle body, i dont know if it will work or not.

as much as i love CIS, i would not waste the money on modding it. its always a nice thought since it is so reliable and easy.


sounds like you will have a fun car.

[Ron.G]

You may have non stock cams already if your car pulls strong all the way to 7K.
stock cams drop off steep around 5600.

My 1980 euro motor with big intakes and fuel head start to really pull around 3200 RPMs. then at around 5400-5600 it start's dropping off (pull wise). there is not much left after that.

Isn't 6500 red line with your car?

[JoeyD911]

Modify the intake all ya wants....it has to poop.....


Joe Bob, as always you have a way with words!

[fred cook]

I finally got around to doing my engine build (3.3SS) and have been doing some thinking and reading on the CIS system. My plan is to keep the CIS and see how well it can be made to perform on the new engine. I like the simplicity (and fuel mileage) of the system as well as the fact that it is paid for and a set of carbs or ITBs are not!! So, that said, here is where I am at the moment.

1) I built a hybrid CIS system using the large runner box and tubes but using the throttle body, WUR, AAR and fuel distributor from the original system.

2) I have eliminated the two AAVs and their associated plumbing which cleans things up greatly. The hybrid system (FrankenCIS?) has all new gaskets, seal and intake boots. I have matched the openings on the end of the intake runners to the intake port size in the Carrera heads for a nice clean transition and flow.

3) Pulled the injectors and cleaned them up, added new seals and installed them in the large intake runners. The injectors were new about 4-5 years ago so I don't expect to have any problems with them.

Now we come to the interesting part. I would like to have a WOT (wide open throttle) enrichening device since this will be a 3.3 ltr engine and think that I have found an easy
solution. I am considering adding a contact switch to the throttle lever along with a plate for it to contact to create a temporary ground for the Cold Start Valve. Since it is actuated when the Themo switch is in a cold condition and creates a ground, why not add another switch and ground wire? This way, the system would run as normal unless the throttle goes to the wide open position, at which time the contact switch would complete the ground and allow the Cold Start Valve to add fuel for high speed running. Anyone know of a reason that this would not work? I also plan on adding a wideband AFR gauge so that I should be able to see the system go to work when it kicks into operation. Some pictures..

Mount switch in round hole in throttle lever.


Add L shaped plate for a contact surface.


Airflow enhancements will be another consideration!

[RarlyL8]

Adding a 7th injector that dumps in the plenum will not distribute evenly. You will have some cylinders lean and some rich.
We can modify the fuel head for more flow and also the WUR to be adjustable. Those things will give you control to smooth and tune the fuel curve.

[fanaudical]

In addition to the question about air/fuel distribution, remember that the cold start injector has fixed flow. I believe that fixed flow is set to enrichen the mixture at near-idle conditions. I'm guessing that you'll see negligible enrichment at WOT (although it may be worth testing).

[T77911S]

use a WUR from a 76-79. they have a vacuum port that will richen the mixture at WOT.
it can also be adjusted for the amount of vacuum needed (actually not needed) to go rich.

that sounds like a REALLY nice motror.
a shop here just built i think the same thing. except is might have been a 3.4. have not heard how it runs yet.

[schumicat]

adding more fuel doesn't really add power unless more air is being added (and extracted) too. You can dump fuel in with a firehose but that doesn't really add power. the mixture should already be richening at WOT so making it richer won't help. the turbo needs more fuel because it has more air molecules (pressurize/denser air) and because boosted systems need to be rich to avoid detonation.