LM-1 Installation in Early 911- MFI

[merbesfield]

John, any update to this thread?

[304065]

Mark, I still have that muffler for testing. I had Ben make me an M&K with Oxygen Sensor bungs in the muffler inlets in the same location. Our cars run so rich that the sensors are very easily fouled, so it's a good idea to get them very hot indeed for testing purposes.

Other than that I have a whole bunch of test data from practice and actual races. One of my threads is here: One Lap of Lime Rock through the eyes of an Oxygen Sensor

I am really convinced that to properly tune the MFI system, the car should be set up on a chassis dyno, so that the load and operating conditions can be very tightly controlled.

[cmcfaul]

I have found my car runs best and has real performance gains when running in the 13 to high 14's.

Specifically, at idle, when engine is warm 11-12 (a bit rich but ideal situation for throttle response). AFR will lean to 14-15 as it approaches redline.

I have a lean spot at half throttle when the engine is warming up. Goes away for the most part when the engine is warm.

I timed how long it takes to get to redline with different settings and lean is definitely fast. The adage that MFI cars like to run rich is not true any longer.

Another data point. lean out the mixture at idle and the idle RPM's will increase. A clear sign of better performance.

Chris
73 911 E 2.4 MFI

[callard]

We got the best wheel HP on the chassis dyno running at 13.1 with a 2.4E and MFI. Judicious sanding on the space cam allowed us to get a flat fuel curve throughout the RPM band 5000 to 7000 (slightly richer at lower RPMs). Note that this is for an SCCA race car.
Chuck

[Jeff Higgins]

Originally Posted by cmcfaul

I have found my car runs best and has real performance gains when running in the 13 to high 14's.

Specifically, at idle, when engine is warm 11-12 (a bit rich but ideal situation for throttle response). AFR will lean to 14-15 as it approaches redline.

I have a lean spot at half throttle when the engine is warming up. Goes away for the most part when the engine is warm.

I timed how long it takes to get to redline with different settings and lean is definitely fast. The adage that MFI cars like to run rich is not true any longer.

Another data point. lean out the mixture at idle and the idle RPM's will increase. A clear sign of better performance.

Chris
73 911 E 2.4 MFI

Be very, very careful with this, Chris. At 14-15:1 the motor will certainly feel more responsive, like it has more snap, but that is dangerously lean. If you continue to operate the car at higher rpm's (like on a race track) at that mix, it will burn a piston, or detonate and potentially ruin the top rod bearings, or worse. You may (appear) to get away with this on a street driven motor that only occasionally goes to redline, and only does so for very short periods of time, but there is still a great deal of potential for expensive damage. Chuck's 13:1 is far, far safer. Myself, I won't let mine go leaner than 12.5:1 under load.

[BSNMOE]

Anything less than 13.1 WOT my 2.4S not only has a slight, lean issue when hot(thermostat leans out). It also does not start and pull away very well when warm.

My first 911 was a 72T with MFI. I purchased it from the Service Mgr of a local Porsche Dealer in 1976. He was a Factory trained and brought over from Germany. After purchassing, I had what turned out to be a pluged/dirty vacuum retard line. I brought it in and after his free replacement of the vacuum lines he took me out for a ride in my car. After a few hard miles he pulled up to the dealer, let the car idle, opened up the rear lid and opened up the throttle. I remember that the car puffed a little smoke when he opened the throttle and he then said that's how he likes them to run. Rich! He also said he likes it when the tail pipe is a little sooty and that you may always have a slight backfire.

[cmcfaul]

With the sensor in the tail pipe (as close to the muffler as possible) I am told that outside air can reach the sensor giving a slightly leaner reading than actual. Regardless, I notice a leaner reading as the outside air temps have dropped. I added a bit of richness and now she runs in the 13's all the time.

Chris
73 911 E