MFI vs CIS
 

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

MFI -more desirable

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

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

mfi = better performance, more fun.

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.

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.

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

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

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.

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.

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.

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.

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.

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

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

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.

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

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.

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.

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.

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

JR

I've highlighted everything that just isn't true. Your homework assignment is to go back and correct these things. :D

Have fun,
JR

I will extend my remarks.:D

CIS is more environmentally friendly than MFI and meets later, more stringent emissions stds. EFI is much better than either in that regard.

CIS is also gives much better fuel economy.

CIS was a significant advance when first introduced by Bosch for the technology extant at the time in terms of fuel mileage.

If (for some reason) you have an old Volvo and want to keep it stock (period correct) then CIS is the way to go.

CIS is not a good choice for a sports car* and I'm told it is more susceptible to EtOH fuel problems than MFI, EFI or carbs.

MFI is a period-correct, concoursmobile correct performance injection system.

Direct injection is top dawg right now, but for the best performance on our old air colled motors, just take a wad of $10,000 bills to Motec and PMO.


* yes, MFI is derived from a diesel tractor system, and yet somehow...

Nobody mentions one of the biggest feats of MFI:

MFI has the best throttle response money can buy in terms of classic 911 engines.

"money can buy" is pun intended, because it's more expensive to get a non or poor running mfi injection to work, if the pump is out of wack, it's probably going to get costly.. whereas CIS will probably work out cheaper...afterall, CIS was the mass production thing, it was not about performance, at all.

Why is this in OT?

It should be in PARF! The MFI people are religious fanatics!

reporting to the mods :mad:

Oh, and MFI sounds better then anything else.

CIS was good enough that Porsche used it for the 930s entire run and into the '90s, well after everything else in their lineup was using motronic.
That seems like a pretty good endorsement to me.

OK, 1972 was the year that the 911T got MFI in the US. So there. The rest is correct.

That logic is flawed. Porsche went through some tough financial times and the 911 was wanting to be phased out. Nothing new was going the 911s way until they realized that the market wanted 911s and not 911 replacements, so they brought in Motronic from the 956 and then finally upgraded to the 964.

Define "environmentally friendly". Someone.

I agree electronic, direct fuel injection is best.

or does it just mean they did not have the development dollars to put an improved system on a turbo (not that easy either AFAIK)

we know they were way short on $$ during that time period

Most of my statements were opinions, which are not provable or dis-provable therefore.

However, MFI injects fuel only into the cylinder that needs it. Have you ever studied the inner workings of an MFI pump? They have a cam like the cam actuating the intake and exhaust valves. These push plungers in the right sequence and at the right time to increase the pressure in the fuel lines to overcome the MFI injector's spring loaded valve and spray fuel just in front of the intake port.

Then there is the 3D space cam that works with the flyweights and throttle position and barometric compensator and the thermostat to change the fuel delivery based on load, engine speed, air pressure, etc.

= meets later, more stringent emissions stds, Flieger

improved fuel economy also translates directly into lower CO@ emissions, tho that danger was not well appreciated at the time

I thought you were an engineering student - has this not been covered in the curriculum anywhere?

My BMW is pretty dirty. It just squeaks by 1983 emissions but I am not allowed to improve the fuel injection by California law. I wouldn't say CIS is doing it any favors.

That is why I was talking about the speed of the cars. If you drive a car hard you will use more energy and so more fuel. CIS cars are tuned pretty anemic, so they can't go so fast (accelerate as hard). I venture that since MFI has much more precise fuel control as far as pulsed injection that MFI has lower brake specific fuel consumption.

I think the oil and stuff old cars burns is probably more polluting than the part of the exhaust that is actually combustion products.

the oil and stuff old cars burn = gross polluters

Ahh, not so fast, whippersnapper. Let's have a look...

Well, Porsche went to CIS beacuse it enabled them to meet economy and emissions standards that they didn't feel they could meet with MFI, so I guess they would disagree with your statement, as do I.

Yeah, I've studied MFI but this statement really makes no sense.

Double? Ahh, no. In fact, recall that the first CIS car, the 1973 T, made the same power with CIS as it did with MFI. The CISs car actually had a half a pound foot of torque more, or 3, depending on which numbers you believe, so...

This also makes no sense, so I'll go on.

No. ANybody that has spent much time with MFI realizes that those systems don't offer near as fine fuel metering control as CIS. They make great power and sound nice, but they aren't frugal with fuel. Not even close. The CIS cars were eventually developed to the point that they got fuel mileage an MFI car couldn't even dream about.

And, if you look at the BSFC curves, CIS cars yielded 20% better numbers, give or take.

Cheers,
JR

Whatever. I can't convince you to mend your CIS ways so I will continue having fun in the MFI 911 which is so much more fun than the CIS BMW. Idles better, better throttle response, doesn't stink like fuel.

In seriousness, though, you should check out how MFI works. It is basically EFI done mechanically. The space cam is the fuel map and there are various sensors that input into the MFI pump which can be seen as a sort of mechanical computer.

Actually, some EFI systems and ignition systems do not differentiate between cylinders like MFI does. Throttle body injection, wasted spark.

MFI is quite sophisticated.

My thoughts on the MFI to CIS transition was cost and ease of setup. I see no reason why properly tuned MFI needs to waste more fuel than CIS.

Coolness >>>>

http://forums.pelicanparts.com/porsche-911-technical-forum/386414-mfi-pump-open-heart-surgery.html

http://forums.pelicanparts.com/porsche-911-technical-forum/617039-mfi-open-heart-surgery-iii-reworking-space-cam.html

Why is this in the OT forum? the subject has been discussed ad infinitum on the 911 BBS since 1999. David E. Clark even consolidated the thread into one massive thread.

Come on, let's discuss something interesting like hookers and blow, legs and butts.