MFI - from altitude to sea level

[nigel911]

1972 911T 2.4 with MFi

Recently relocated from Colorado (5300ft) to sea level - had the car tuned and setup about a month before I left.

Shop that did the work was aware I was relocating, and said adjust a couple of clicks on both main and idle enrichment when I get to sea level, and that should give a reasonable baseline.

Car runs okay on it's current setting, but pickup is not as clean as it was at elevation - my question - assume I need to enrich rather than lean? More air, more fuel, therefore richer?

[Jeff Higgins]

Yup.

[porschedude996]

If I recall correctly, the MFI on my ‘72 911T had a Atmospheric Barometric Android that compensated for altitude. The adjustment accessed through the static engine fan was for idle richness only, otherwise the system is based on effective mechanical map, the altitude, rpm, throttle angle, and a stack of Belleville washer stack, being heated from the left heat exchanger. My experience is that you need two hoses, one within the other, that runs between the pump and the heat exchanger.

[Jeff Higgins]

In my experience across several cars fitted with MFI, the barometric cell has a pretty limited range of compensation. I have yet to see one that can compensate for a 5,300 foot change in elevation. Oh, the car will run across that variation in altitude, but not optimally. It's best to readjust the pump, especially if we have moved from a high altitude home to a sea level home, or vis versa.

We do that by adjusting the main rack, not the idle mix adjustment. I probably wouldn't even bother with the latter, it gets completely overwhelmed by the main rack adjustment.

The main rack adjustment is behind the allen bolt to the lower right of the warm up solenoid. It is best accessed with the speciality tools available for this, the long t-handle allen wrench and the long, very fine tip flat blade screwdriver. Snake them in under the coil, to the left of the fan shroud.

The warmup solenoid is unaffected by changes in altitude. Don't even bother looking at that.

[porschedude996]

Understand that, I had a problem meeting CA Smog Checks back in the ‘80s. I sent my pump and injectors to a service shop in South San Francisco. He worked for Bosch in Germany when Porsche was still building cars with Pump type MFI. He had two Pump/Injector test stands. At that time there were three places in the US that worked on the pumps. New Jersey, South San Francisco California, and somewhere in between. I picked up the pump in person and he was a very personable guy. He walked me through the whole process of overhaul and calibration. I wish I could recall his name. A very German name.

I would have thought that with all the MFI equipped cars driving around the Alps to the sea shore it would be close. But the Barometric aneroid is probably past its design life.

At the time, his tune worked for 100k+ and 10 years without a hint of a problem.

I didn’t have enough experience to make adjustments to anything on the rack other than the idle mixture. I imagine that with familiarity one could wing-it. I guess the adjustment would be pretty linear.

What I found amazing, was the throttle response.

Originally Posted by Jeff Higgins

In my experience across several cars fitted with MFI, the barometric cell has a pretty limited range of compensation. I have yet to see one that can compensate for a 5,300 foot change in elevation. Oh, the car will run across that variation in altitude, but not optimally. It's best to readjust the pump, especially if we have moved from a high altitude home to a sea level home, or vis versa.

We do that by adjusting the main rack, not the idle mix adjustment. I probably wouldn't even bother with the latter, it gets completely overwhelmed by the main rack adjustment.

The main rack adjustment is behind the allen bolt to the lower right of the warm up solenoid. It is best accessed with the speciality tools available for this, the long t-handle allen wrench and the long, very fine tip flat blade screwdriver. Snake them in under the coil, to the left of the fan shroud.

The warmup solenoid is unaffected by changes in altitude. Don't even bother looking at that.

[nickelplated5s]

EuroMetrix on the east coast. Bad thing is a two + month turnaround. Good thing is mines ahead of you. Then you get into throttle bodies, another two months at a different shop but mine are there too. Then you get into dyno tuning.
Jeff, in WWII they ran MFI motors above 30K altitude. MFI was a precursor to EFI and computers.

[Jeff Higgins]

That "very German name" was probably Gus Phister of Pacific Fuel Injection. One of the acknowledged masters.

I am aware of the use of MFI on the ME 109. I've even read the flight manual. Absolutely different system insofar as altitude compensation. In short, strictly manual control from the cockpit, by the pilot. As were carbureted aircraft of that period. They all had manual mixture control. Completely irrelevant to this discussion.

[nickelplated5s]

The point was and you acknowledged that there are very few people who really know these. My shop won't touch them and there is only so much I'm willing to do. I pretty much lost my summer driving but my fault for not doing this last fall. On the other hand the car was warm and dry in my shop.

I've flown air cooled flat 6's to 23K so I know how they work. Even with speed brakes it takes time to descend without shock cooling the motor. Clocked 396 knot ground speed once but that's a different story.

[Jeff Higgins]

Hmm... Forgive me, but I'm still struggling to find the relevance of anything you guys have added. All that poor Nigel wanted to know was if he was correct in assuming that he had to adjust his pump to run richer with his drop of 5,300 feet of altitude. The short answer was "yup". And it sounds like he knows how to that. Everything after that just muddied the waters, and some of it is inaccurate to boot. Not sure why any of it was necessary, especially the stuff about aircraft. Maybe I missed something, I dunno...

[porschedude996]

It’s a healthy conversation. Different points of view were discussed and first hand experiences. I guess if the OP was only looking for a simple “Yup”, then he should have set it up as a vote. “Yup or Nope” If one “Yup” was good enough for him, then why did you come back to the thread?

Originally Posted by Jeff Higgins

Hmm... Forgive me, but I'm still struggling to find the relevance of anything you guys have added. All that poor Nigel wanted to know was if he was correct in assuming that he had to adjust his pump to run richer with his drop of 5,300 feet of altitude. The short answer was "yup". And it sounds like he knows how to that. Everything after that just muddied the waters, and some of it is inaccurate to boot. Not sure why any of it was necessary, especially the stuff about aircraft. Maybe I missed something, I dunno...

[Jeff Higgins]

Originally Posted by porschedude996

If one “Yup” was good enough for him, then why did you come back to the thread?

To correct the misleading and irrelevant information you provided in your first post. You were leading him way off track.

[nigel911]

Just to recap - 18 years ago I relocated my car from FL to CO - took it to a well known Denver shop who tickled and leaned out the main rack to suit the elevation. The barometric cell works fine but as others have noted, is fairly limited in it's range - and as I rarely drove out of state it made sense to have the 'baseline' adjusted for majority use / the elevation. My car would mainly be used between 5,000 and 10,000 feet in a very dry climate.

Now the car is 500 feet from the beach in Spain and will very rarely see elevation, so again it makes sense to adjust to suit - and Jeff was entirely correct, I pretty much wanted to verify it should be set richer - and perhaps see if anyone with experience would suggest two clicks / four clicks etc.

Which brings us to today... I have adjusted the idle a single click richer, and the main rack two clicks richer - the car is still backfiring occasionally and not revving as cleanly as it was previously - so I'm assuming I may need to add a couple more clicks to the main rack?

Appreciate everyone's input.

[rowingone]

I would pull a plug and see if it shows you are too rich or too lean . . . if the plugs are black start with new ones and proceed with adjustment in the right direction. You need to pull the plug without letting the car idle as this alone will blacken the plugs.

[rowingone]

I would pull a plug and see if it shows you are too rich or too lean . . . if the plugs are black start with new ones and proceed with adjustment in the right direction. You need to pull the plug without letting the car idle as this alone will blacken the plugs.

[Jeff Higgins]

Two clicks on the main rack is a pretty minimal change. There are twelve clicks per full turn of the adjustment screw. Try at least half a turn, six clicks out. Too rich won't hurt these things, it will just suck fuel and be down on power. I would much rather err on the side of too rich than too lean.

Edit - about those spark plugs. "Reading" spark plugs on an MFI inducted motor is far different than on others. Every one that I have ever seen that is tuned properly will have sooty black spark plugs. Trying to achieve those nice light sand beige plugs will see these things running dangerously lean. Oh, and run something with a nice, big, fat electrode (I like the NGK BP 6 and 7). The fancy multi electrode plug die a quick death in this environment.

[nigel911]

Agree - having owned this car for almost 25 years I'm used to 'sootier' plugs, and am running BP7's.

Cheers Jeff, I'll give her a few more clicks and go from there, easy enough to back off once the occasional backfire subsides.

[nickelplated5s]

I tried to write above so anyone searching MFI could read between the lines. I guess not so here I go. Nigel has a 72 pump with an issue. Nowhere is there any mention of AFR, injector or air flow testing. The only thing we got was a guesstimate on the rack. Perhaps the space cam is out of spec? Who knows?

My mechanic shook his head and asked why don't go EFI, cheaper and easier. I'm a purist so went the whole rebuild route. That includes dyno testing. These are, mine included, over 50 year old pumps. Things wear out.

Nigel, you will always get a backfire if you drive hard.

[Jeff Higgins]

Let's not over complicate matters. All Nigel did was move. In doing so, he dropped 5,300 feet in elevation. That's it, that's all. He never mentioned anything about the car running poorly before he moved. He correctly diagnosed the change in its behavior to his change in elevation. He's on the right track.

I've never had a properly tuned MFI car backfire. There are two of them in my family, my '72 with its twin plugged, substantially modified 3.0 liter, and my son's '68, which is running the 2.4 that was in my '72. Beyond that, I have tuned several cars belonging to my R Gruppe buddies, ranging from 2.0 and 2.2 S motors to 2.7's and both short and long stroke 2.8's. Not a single one of them backfires. And we run them hard, both on the street and on the racetrack. If you are getting backfires from this system, something is wrong.

[nickelplated5s]

You're still missing the point Jeff. Nigel has a 53 year old system with nothing other than rack adjustments presumably. My advice is to go back to square one which is what I'm doing albeit it at expense. BTW, no backfire just a bit of flame when the exhaust hits the tip for the atmospheric O2. Unburnt fuel as those motors run rich.

[Jeff Higgins]

My advice vs. your advice - a few clicks richer on the main rack adjustment vs. a complete overhaul of the pump and throttle bodies. Five minutes with a couple of tools vs. waiting through several years of specialists' backlogs. Free vs. thousands of dollars...

I bet most of us know where he will start. And end.