Tuning FI for altitude with Wideband O2

[Mike R.]

I have a stock 65 FI car that was tuned before my puchasing it by a "FI Expert" at sea level. The car runs fine but I am curious how much performance I could expect to gain if I re-tuned it to my elevation of 2100'. 2100' is about the lowest elevation around for hundreds of miles so I will rarely, if ever go lower. I understand that I would have to re-tune if I go to a lower elevation.

This is the only non-electonic vehicle I have, so I don't want to buy a wideband O2 sensor system if the gains will be negligable.

Thanks

Mike

[Dan H.]

Re: Tuning FI for altitude with Wideband O2

Mike, my 64 FI was set here in San Diego, has run up over 9000 ft passes on the way to Bend Oregon, no change noticed in performance per se. You might make sure your idle air screw likes the setting at your current area, turn it in til it's rough, turn out til it's rough and set it in between. It's just a matter of feel, wouldn't touch the mixture screws if car runs OK and plugs are acceptable.
Dan

[Duke W.]

Re: Tuning FI for altitude with Wideband O2

Like a carburetor the FI system meters fuel based on air flow volume, not mass, so it will richen as altitude increases, and there is a power loss associated with lower air density.

There's little point in leaning the mixture unless you do most of your driving at 3-4000 feet or more and don't venture down to sea level often.

Modern cars maintain sea level A/F ratios at higher altitudes, but they still lose power due to lower air density.

At best "retuning" the FI system for your altitude might get you a few percent better fuel economy, but it might be tough to measure.

Duke

[Mike R.]

Re: Tuning FI for altitude with Wideband O2

Thanks Dan, I concur that FI or a carb tuned for sea level will run adequately at altitudes that we will see on paved roads but it can't be optimium. As air density goes down these systems will run richer and richer. The affect is not super dramatic but it is there. This is why racers travel with instruments to measure air density and tune for the conditions of the day. I wouldn't go this far but would like to get all that I can out of my system under the conditions I operate under. Modern EFI has overcome these limitations with MAF sensors.

Mike

PS I think I might be deaf on arrival if I drove my side exhaust car from San Diego to Bend!

Mike, my 64 FI was set here in San Diego, has run up over 9000 ft passes on the way to Bend Oregon, no change noticed in performance per se. You might make sure your idle air screw likes the setting at your current area, turn it in til it's rough, turn out til it's rough and set it in between. It's just a matter of feel, wouldn't touch the mixture screws if car runs OK and plugs are acceptable.
Dan

[Jack H.]

Re: Tuning FI for altitude with Wideband O2

Agree with Duke... Most carbie guys fine tune jet selection based on each 1000 ft of altitude. At 2000 feet, you're not that far from optimum right now.

In my case, home at 7930 ft and frequent trips over the Continental Divide at 11,000 feet, tweaking my carb jets has a reasonably noticeable effect...

[Mike R.]

Re: Tuning FI for altitude with Wideband O2

Thanks Duke and Jack. You have saved me a lot of work and a couple hundred bucks!

Mike

Like a carburetor the FI system meters fuel based on air flow volume, not mass, so it will richen as altitude increases, and there is a power loss associated with lower air density.

There's little point in leaning the mixture unless you do most of your driving at 3-4000 feet or more and don't venture down to sea level often.

Modern cars maintain sea level A/F ratios at higher altitudes, but they still lose power due to lower air density.

At best "retuning" the FI system for your altitude might get you a few percent better fuel economy, but it might be tough to measure.

Duke

[John D.]

Re: Tuning FI for altitude with Wideband O2

Hi Mike, As mentioned you really don't have to be too concerned for several reasons.
One of the reasons is that you are not that high above sea level. 2100-2200 is nothing to be concerned about.
One time a 63 FI owner I knew lived about 5500ft + in CO. (this story about 30 years old). So I sent him a smaller set of nozzles. Never heard of him again. Not a thankyou or nothing as I recall. So that must of helped him.

I wouldn't jack with the screws on the fuel meter if your FI has been recalibrated.

Do what Holstein told you. Play with the "idle fuel screw" on the air meter instead. It has a hex head and looks like an idle fuel screw you would see on some carbs. Massaging that screw will make the car run leaner.
You don't need to pay anyone for this job. Adjust it by the seat of your pants.
Expect a rich operation in higher altitudes unless you can do something about increasing the air flow.
To increase the air flow a tad- throw away the original air cleaner element and buy nice paper one from Ford. Those cone shaped elements will give you a tad more fresh air than the POC ancient originals.

One more thing. If the FI is running OK as is don't mess with it. Make sure you have nice original ignition coil. Remove the distributor cover and put it on the shelf. Put some good spark plug wires on it and a nice set of AC 45 plugs. If possible try and get some good fuel in lieu of the ethanol. Happy motoring. You have the best FI that Rochester made in that era. JD

[John D.]

Re: Tuning FI for altitude with Wideband O2

Mike, Here is some more info for you fellows living in CO, etc altitudes with thin air as you climb.
Now this info came from a friend/customer so I don't know how scientific it is. Saying that to protect myself from a beating here by the talented gearheads. LOL
As you climb in altitude the engine vacuum drops. As the engine vacuum drops then the Fi tends to run a tad richer. Say you get up 11,000 as my friend does. He says he looses a ton of power but makes it thru by putting the pedal down and romping it. The higher up he goes the more idle issues he has.
So now you have two minus factors. Less engine vacuum and a lack of air. That in itself puts a hurt on the old FI's as they run on a vacuum principle and even at sea level they could use more CFMs.

See Jerry Gollnicks post on his race engine.

Mike, Ask your FI restorer if he used the correct 30-30 enrichment diaphragm spring. That's the short one-the hot rod spring we call it. JD

[Jerry G.]

Re: Tuning FI for altitude with Wideband O2

Like a carburetor the FI system meters fuel based on air flow volume, not mass, so it will richen as altitude increases, and there is a power loss associated with lower air density.

There's little point in leaning the mixture unless you do most of your driving at 3-4000 feet or more and don't venture down to sea level often.

Modern cars maintain sea level A/F ratios at higher altitudes, but they still lose power due to lower air density.

At best "retuning" the FI system for your altitude might get you a few percent better fuel economy, but it might be tough to measure.

Duke

I'm in my normal state....confused. The air meter is just a venturi meter governed by Bernoulli's equation. The velocity is affected by the density of the air. The delta P is affected by air density so I have assumed that the FI system will correct for higher air density and put in more fuel for lower altitudes. If this isn't the case I have a BIG problem. I'm heading for a race in Portland and I just spent 3 hours on a chassis dyno tuning the unit at 6000 feet elevation. I really don't want to burn pistons. Something just isn't making sense here. I understand a power loss for FI and carbs because there is less O2 but... There is also a 1957 SAE paper on FI design that talks about the design of the venturi minimizine density effects.

[Clem Z.]

Re: Tuning FI for altitude with Wideband O2

i don't think a mechanical FI like the chevy compensates for altitude like electronic FI do. it is more like a pressurize carb. JMHO

[Jack H.]

Re: Tuning FI for altitude with Wideband O2

Technically, air pressure is a measurement of the weight of a square inch of the atmosphere from where you stand, stacked all the way up to outer space.

There are several factors involved here...

First 'air' is a mixture of the various gasses native to the atmosphere; primarily nitrogen and oxygen, but with trace amounts of argon, helium, Etc. There's also a significant content of water vapor present.

Oxygen is slightly heavier than nitrogen (see periodic chart of the elements) and therefore has a tendency to 'sink' in the mixture, if we neglect air turbolence (which we can't). This means you can, in general, expect a greater concentration of O2 per cubic liter of air at lower altitudes.

Water vapor whose composition in the atmosphere varies with heating, weather, is MUCH lighter than either O2 or N2. That's why barometric pressure DROPS as a storm center approaches. There's an abnormally high concentration of water in the air making it lighter.

So, the altitude effect on combustion is more complex than just raw air density and the relative height above sea level. It's the absolute concentration of O2 molecules per cubic liter of air that's important to proper combustion and that varies with altitude as well as humidity...

[Duke W.]

Re: Tuning FI for altitude with Wideband O2

I'm in my normal state....confused. The air meter is just a venturi meter governed by Bernoulli's equation. The velocity is affected by the density of the air. The delta P is affected by air density so I have assumed that the FI system will correct for higher air density and put in more fuel for lower altitudes. If this isn't the case I have a BIG problem. I'm heading for a race in Portland and I just spent 3 hours on a chassis dyno tuning the unit at 6000 feet elevation. I really don't want to burn pistons. Something just isn't making sense here. I understand a power loss for FI and carbs because there is less O2 but... There is also a 1957 SAE paper on FI design that talks about the design of the venturi minimizine density effects.

Velocity through the venturi is not effected by air density. The air meter vacuum signal is based on volume flow, so, yes you will be lean at sea level.

You may need to go back to the dyno and develop a sensitivity curve for the economy and power stop settings. For example, how much more percent fuel will the system add with each quarter turn of the adjusting screws. That would be a real useful piece of information for any FI owner, and I'm sure it's been done, and maybe one the the guys here who has a lot of hands on FI tuning experience has an idea. I'd also suggest you contact Jerry Bramlett with the situation. and I'm sure he'll be able to give you some decent guidance.

Then figure how much denser the air will be at Portland and make the suitable adjustments. The good news is that once you have the sensitivity profile for the power and economy stop settings, you should be able to adjust the system for any air density conditions you meet.

You want the power stop A/F to be in the range of 13.5-12.5:1

Even better would be a wide band O2 or CO sensor in the exhaust. There is a near linear correlation between CO and A/F, with 6 percent CO associated with 12.5:1 and near zero equal to stoichiometric.

If you had a carburetor, you'd have the same issues - set it up at 6000 feet and you will be lean at sea level. At least with the FI system the adjustment is simple - no disassembling the carb to change jets - just the turn of a screw.

Duke

[Jerry G.]

Re: Tuning FI for altitude with Wideband O2

Well , I agree the venturi is measuring velocity but the velocity calculation is based on the difference between inlet and outlet pressure. This delta p is a calculation based on the absolute pressure at the site it's being measured, therefore the calculation will be affected by the value of the numbers. A denser fluid will give a greater boost or delta p. I'm not disagreeing that there may be a leaning but it's not as simple as a carburetor. I suspect it's a little more complicated interaction. Where are our fluid mechanics proffesors? Somebody has taken their FI cars to different elevations, what are thir expirences?
As far as developing curves on a dyno, if it is indeed affected by altitude then the data collected at a given altitude would not translate to a different altitude easily. This is not a linear function as I've found from hours on the chassis dyno.

[Dan H.]

Re: Tuning FI for altitude with Wideband O2

Jerry, used to drive my 59 with FI here in San Diego, always noticed when we went for meets up at Big Bear, 5000 ft. elev. it ran nice and smooth. I didn't know then it was 'richer' up there, does make a difference though. My 64 380 unit was set at .8" mercury at San Diego, maybe a little lean, ran ok at the high passes going to Bend. I recently set it again with my newly acquired manometer and it still read .8". I turned the economy stop about 1/4 turn and it went up to .9" mercury. A littlle change happens quickly. Tested it with my Heathkit o2 tail pipe sensor, read 14.3 at 2500 rpm, so was a little rich. Do you have a manometer to quick check your unit with at the various locations? Are you coming to the North Island races this year?
Dan

[Clem Z.]

Re: Tuning FI for altitude with Wideband O2

jerry buy a portable EGT exhaust gas temp unit and all you need is a 3/16" hole in one header pipe 1-1/2" from the the head. the one i have holds the highest reading so you can check after making a run. the model i have is a "computech" model 4005