MFI vs CIS

[regency]

So, as I'm looking at early 911 cars for sale, ebay, Pelican & CL, I'll run across ads where the car, a 73.5 911 is for sale and the ad will say something like, "the 73.5 has the much inproved CIS fuel injection" or "Vastly improved CIS".

Seems to me that most people seek the MFI cars, Am I wrong??

Steve

73 911 T MFI Coupe, Aubergine

[72doug2,2S]

MFI -more desirable

It's akin to a quartz wrist watch or a mechanical Rolex.

[onlycafe]

cis = more environmentally friendly, better mileage, easier to dial in, etc.

mfi = better performance, more fun.

[T77911S]

MFI is a better performance system but from what little experience i have with it, there is a "3d" spacial cam (SP?) in the pump that gets a groove worn in it from the repeated driving conditions. once this groove is worn in, the system will never work right. i dont know how or where to get the injection pumps repaired or rebuilt.

CIS is a very reliable but much less of a performance system.

[sketchers356]

I have CIS, very much want MFI.

With CIS you can't run an aggressive cam, you need MFI or an EFI to do that. Therefore the CIS cars are down on power.

On the plus side CIS is bulletproof as long as you put a popoff valve in to prevent backfires from blowing up the airbox.

[speeder]

Originally Posted by regency

So, as I'm looking at early 911 cars for sale, ebay, Pelican & CL, I'll run across ads where the car, a 73.5 911 is for sale and the ad will say something like, "the 73.5 has the much inproved CIS fuel injection" or "Vastly improved CIS".

Seems to me that most people seek the MFI cars, Am I wrong??

Steve

73 911 T MFI Coupe, Aubergine

That's like saying that a Nissan 280Z is much more desirable than a 911 because they are less hassle, cheaper parts, etc...

It's pretty hilarious to read *car for sale* ads. Private party sellers make Harry's Used Cars look like the American Red Cross.

[island911]

MFI is not measuring air flow, whereas CIS does. This makes the CIS perform better from fuel/air metering POV, But... on higher output engines, the cam timing has overlap . . which induces bad flow at certain rpm ...which confounds the air flow measurement. -a modal thing.

So, yes; "much inproved CIS fuel injection" . . but at a cost of a more broadly rev-range tuned motor.

[Flieger]

I disagree that CIS is more environmentally friendly. CIS injects fuel into all cylinders all the time no matter what. It just varies the amounts. MFI injects fuel into only the cylinders that want it, and only when the driver is on the throttle. There is a microswitch to shut off fuel when off throttle and above 1500 rpm. It is a high pressure injection, too, so the power you get out of the fuel I would say could be double the CIS.

The difference is that CIS cars are slow and slow cars are not going to need as much energy to go slow. MFI cars are fast so they need more energy. If you made a CIS car as fast as an MFI car the MFI car would get better mileage.

It is also the tuning. You can make either one rich or lean.

MFI was actually adopted for the 911T in 1969 because it was the only way to meet the emissions regulations.

[Flieger]

Originally Posted by island911

MFI is not measuring air flow, whereas CIS does. This makes the CIS perform better from fuel/air metering POV, But... on higher output engines, the cam timing has overlap . . which induces bad flow at certain rpm ...which confounds the air flow measurement. -a modal thing.

So, yes; "much inproved CIS fuel injection" . . but at a cost of a more broadly rev-range tuned motor.

MFI still compensates for altitude/pressure and has a warm-up circuit, so there shouldn't be much difference between the calculated airflow and actual airflow. If anything it would be lean because the air leaks in worn throttle bodies let more air in.

[Superman]

CIS and MFI are at polar opposite ends of the throttle-response spectrum. If you autocross a CIS 911, you will come to understand the throttle response problem. You actually have to plan ahead for power. At least one second, maybe two. Like a turbine engine. The power builds slowly.

With MFI, power happens NOW. Full power. You feel as if your right foot is pushing the car. If your brain and right foot are connected properly, you'll feel like you're thinking the car to acceleration. By the time you finish thinking it, it's already happened.

I really wish I had MFI.

[boba]

MFI pump repair and calibration - Pacific Fuel Injection

Throttle bodies - Eurometrix

MFI has to be set correctly which takes attention to detail. When set it is bulletproof and has great throttle response.

Keep in mind that for these systems to work like new they need to be in good condition. They are at least 38 years old now. The items that need attention are the throttle bodies, throttle linkage, ball and socket connectors, bushings, and the pump. Not cheap to get all corrected if everything is worn and needing rebuilding but good for the next 20 years when done correctly.

OH, and the fun factor is priceless.

[boba]

Originally Posted by Flieger

MFI was actually adopted for the 911T in 1969 because it was the only way to meet the emissions regulations.

Minor detail 1969 E & S had MFI the T was still Carbs. 1972 all were MFI in the US.

[Flieger]

OK, couldn't remember which year it was that they did that.

[island911]

Originally Posted by Flieger

MFI still compensates for altitude/pressure and has a warm-up circuit, so there shouldn't be much difference between the calculated airflow and actual airflow. If anything it would be lean because the air leaks in worn throttle bodies let more air in.

Yes, I know there are various compensations for the MFI system - which btw, often get discarded/disabled in effort to make them run better.

The thing is, we are talking open loop vs closed-loop feedback. Don't get me wrong, as a Mechanical affectionado/egr the MFI system is very cool. ...but do note, that injection systems will bring more and more precision, as the open-loop gets closed.

[Flieger]

I think that is more than balanced out by CIS dumping fuel into all cylinders like a garden hose.

[Flieger]

I do have headers so have my pump set to permanently warmed up condition. It is too lean to start easily but after a couple starts it keeps running and once up to temp it runs great. I have the baro cell and the microswitch. I know some people don't like the microswitch but I like it. No more backfires when engine braking, and there is more engine braking as well. It sounds crisper and cleaner, too.

[boba]

Originally Posted by island911

btw, of course any system can be made to run lean. With MFI, however, lean for regular driving will become TOO lean for high loads. (read; burned pistons)

Not quite accurate, there is not a linear ramp of fuel delivery. This is why there is a specific pump cam, to match the engine configuration (T,E,S,RS), plus the click adjustments for fine tuning. Again why these systems need to adjusted correctly.

When shops did not know how to adjust correctly they would blame the system and sell the owner a carb setup.

The old, "blame the tool not the operator" mentality.

[pwd72s]

Originally Posted by boba

Minor detail 1969 E & S had MFI the T was still Carbs. 1970 all were MFI.

Still better bump up the year a bit...my then new 1970 911T had carbs..

[boba]

Originally Posted by pwd72s

Still better bump up the year a bit...my then new 1970 911T had carbs..

You are correct it was 1972 for the T to get MFI. I will correct my post so as not to confuse.

[javadog]

Originally Posted by boba

You are correct it was 1972 for the T to get MFI...

... in the US. Euro models kept the carbs.

JR