MFI - from altitude to sea level

[stownsen914]

Might as well rebuild the engine too while you're at it, haha. It might be worn out.

Have to agree with Jeff here. Adjust a couple clicks and see how it runs.

[nigel911]

My pump was completely rebuilt 'as new' when I purchased the car, so a mere 24 years old... never had any issues with it, used frequently but not high mileage on it. Car was running like a champ at elevation, and as previously mentioned, fully serviced roughly a month before I left by a very reputable Porsche shop.

I'll start with a few clicks richer before going into doomsday mode, new plugs on the way as well.

[rowingone]

Call the shop that you trust and ask them .. they know how many clicks to start with as they do this all the time.

[nickelplated5s]

Knowing what I know now my advice is drive it this summer and get your ducks in a row for rebuilds. Counting the dyno work perhaps 3 months down time. Depending on how much time you put in could be <6k.

I'm also wondering and Jeff should start another thread. "How I put a 3.0 with MFI into a 72." Curious minds want to know.

[cmcfaul]

The answer to every question here is adding a AFR (Air, Fuel Ratio) meter. it takes all the guess work and magic out of setting up MFI to run optimally. Traditionally, MFI would be set up to run rich. This was a motor safe solution. With an AFR meter, set it up to run at peak HP without detonation. Easily 10+ hp gain.

Just weld a bung onto the exhaust for the sensor and run the wires to the dash or glove box for the meter.

Chris
73 911 E

[nickelplated5s]

Already brought that up CMC!

[GH85Carrera]

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.

I went to the Boulder, Co area this weekend in my 85 3.2 Carrera. The car ran OK, but it felt like the handbrake was on or I was pulling a trailer. Air pressure up there is much lower. It was nice to get back to my 2,100 Ft. above sea level air pressure.

At sea level, you should notice a real increase in power once things are adjusted right.

[Jeff Higgins]

Originally Posted by nickelplated5s

I'm also wondering and Jeff should start another thread. "How I put a 3.0 with MFI into a 72." Curious minds want to know.

My 3.0 MFI build has been extensively documented on this forum. Right down to how I re-profiled the space cam (from a 2.4 "T" pump) to accommodate its differences in fuel delivery requirements.

Originally Posted by cmcfaul

The answer to every question here is adding a AFR (Air, Fuel Ratio) meter. it takes all the guess work and magic out of setting up MFI to run optimally. Traditionally, MFI would be set up to run rich. This was a motor safe solution. With an AFR meter, set it up to run at peak HP without detonation. Easily 10+ hp gain.

Just weld a bung onto the exhaust for the sensor and run the wires to the dash or glove box for the meter.

Chris
73 911 E

Absolutely. A wide band O2 sensor with a way to record A/F over the full spectrum of operation is a must for any real serious MFI tuning. I used the old Innovate Motorsports LM1 and found it invaluable. I run thick flange SSI's, which come with the bung already in place. Lacking that bung, a tailpipe clamp can also be used.

But, tuning for an altitude change? I don't think doing that level of deep dive is necessary.

[nickelplated5s]

Originally Posted by Jeff Higgins

My 3.0 MFI build has been extensively documented on this forum. Right down to how I re-profiled the space cam (from a 2.4 "T" pump) to accommodate its differences in fuel delivery requirements.

Throw a link in Jeff.

[Jeff Higgins]

Search on my username, set it to "threads started by", "search titles only", with the topic "MFI", and restrict it to the 911 tech forum. That should capture everything.

[Billiam 911 2.8]

I remember getting re-educated on MFI tuning from Jeff's posts. I first experienced the art of MFI tuning when I was going to college and spent summer weekends helping out on the VonK Turbo which ran a 2.5 and later 3.0 turbo. This yellow brick was the fastest 914 at 700 h.p.; it had some spectacular failures as well.

[Billiam 911 2.8]

I remember getting re-educated on MFI tuning from Jeff's posts. I first experienced the art of MFI tuning when I was going to college and spent summer weekends helping out on the VonK Turbo which ran a 2.5 and later 3.0 turbo. This yellow brick was the fastest 914 at 700 h.p.; it had some spectacular failures as well.

[nigel911]

Update - five clicks richer on the main rack, single click richer on the idle, tank of fresh fuel, reset the idle / balanced the air intakes using the CMA process, new sparkies and voila - back to normal.

Pulling cleanly throughout the range, idling perfectly, no pops or bangs - will take her on a good run tomorrow and then check the plugs.

Many thanks to all who contributed.

[tobluforu]

Don't waste your time reading the plugs, because if you do, the next thing you know you are back to square one with it running lean as hell.

[nigel911]

Going to check them for soot, not creamed coffee... also have an appointment at a shop with an AFR so I can dial in the correct ratio - not too lean, not too rich.

May have to lean it out a little for the Spanish emissions test, than back to whatever - aiming for 12-12.5 at WOT.

[r lane]

I have 3 motors with MFI, 2 track engines and one on a street 72 2.7 with a 2.7 RS space cam, 4.5 S cams. Those are the extent of my MFI indulgence so my overall exposure to that system is limited. Anything mechanical will wear out, but my experience is that those pumps are very robust and tolerance issues from wear are rare excluding a poor oiling issue or extremely high miles. I installed the 2.7 space cam and a used set of S stacks and throttles 30 years ago.

[r lane]

I drive it regularly. Rock solid idle, great performance in town and on the highway. I am amazed that the throttle bodies have held up as well as they have but they have. If a mixture adjustment doesn't get you going, ( assuming ignition is all well), put the pump on a dyno and run through the delivery chart just to see if your CCs are correct. Bob

[Jeff Higgins]

Originally Posted by nigel911

Going to check them for soot, not creamed coffee... also have an appointment at a shop with an AFR so I can dial in the correct ratio - not too lean, not too rich.

May have to lean it out a little for the Spanish emissions test, than back to whatever - aiming for 12-12.5 at WOT.

Does the shop have a dyno, or do they test out on the road? I've done it both ways, both are equally effective. On the dyno tends to attract a little less unwanted attention...

Glad to hear this was just an easy adjustment. You knew it was going in, of course, but you did manage to get us all going on a fun little conversation.

[nigel911]

Dyno... thankfully - I'm sure my Colorado plates will attract enough attention as it is!

Was pretty confident my car and fuel system was in good shape - looking forward to having just a little more power now I'm back at sea level.

[tobluforu]

Did u have to smog that car in Colorado?
And I hit 4k posts, even though I probably have 6 as I had a different name. Wooo, hoooo.
R-lane. My 72 has the org. pump rebuilt and recalibrated by Gus (twice) back in 92. Could I benefit from having them rebuilt or swapped to S stacks, of course. But I don't see the point as I have my car dialed in.
Famous last words.