The engine (460) in my '90 C-350 is still basically stock, and runs pretty well at lower speeds (up to 45-50), but turns into a bit of a dog above that. It'll get up to 80 mph (all the faster I've tried to get it), but it takes a fair bit longer than it should to get there. No, I have not done every last tuneup item yet but that is not the point here.


The elevation where I live is about 9000 feet. This means the engine is likely to run rich. This sort of problem is usually handled by increasing air flow, or cutting back on fuel flow. The manufacturers I'm familiar with opt to use the fuel solution, cutting back on power in the process. Every 1000 feet in elevation decreases air pressure by 3%, so at 9000 you have a pressure decrease of 27%! Up on the passes at 13000 feet it is extreme at nearly 40%.


My old jeep had an altitude compensating 2150 carb on it. All it did special was create a massive vacuum leak at higher altitudes. I find that approach much more intelligent, even if more complicated. How to do this on a EFI engine?


Well, it occurs to me that a larger throttle body accomplishes this in an even better way by being more linear with throttle position. It's not perfect as it would be ideal for only one single altitude, but it would be better than the smaller, stock TB across the entire range.


The obvious choice for a larger TB is the BBK 61mm aftermarket unit. I've already bought a factory reconditioned TB ($262 to my door) and a spare upper intake as the intake must be port-matched to the TB and I don't want the truck down for more than a few hours.


So, am I all wet here? It seems like a relatively easy solution to an ongoing problem at high altitude.


Thanks for any comments.

 

A '90 460 would be EFI. That has an oxygen sensor in the exhaust that the computer uses to control the air-fuel ratio. It works WAY better than any altitude-compensating carb.

The thin air does mean that you won't make as much power, but that has nothing to do with air fuel ratio. A turbo will get the air pressure up so you can make the same power at high altitude as you do at low. Otherwise you've just got to accept that you'll lose power as you go up.

 

O2 sensors of that vintage only say "lean" or "rich", but not how lean or rich. Additionally, they generally have about a 10% range, as does the closed-loop mode in the computer systems. Most electronic systems are designed to run from sea-level to about 4000 foot in elevation.


In other words I don't trust the stock truck system to handle it completely at these altitudes. Please convince me otherwise.

 

As I understand it, the MAP sensor also samples altitude/air density information to the ecu. Hence, the "density" in the "Speed density" term.

 

Exactly. .....MAP= Manifold Absolute Pressure.

 

That is the case for all primary O2 sensors of any vintage, but since the computer is constantly polling the O2 to see if it has switched from one state to the other it can and will zone in on the ideal 14.7:1 ratio when operating in closed loop.
But there is another part to the programming that ignores the O2 called open loop, the computer switches to that strategy when engine load is above a certain level such as when climbing grades, towing or accelerating. In this mode the computer provides fuel based on pre programmed maps which are tuned for the particular engine it is controlling . This mode is always richer than closed loop.. can be anywhere from 8:1 to 12:1, so for a vehicle operating at altitude more fuel isn't going to be the problem. There may have been some high altitude PCM versions made back when most vehicles were speed density but if anything these would have been calibrated leaner than a sea level PCM.

 

All these comments are true, but some are "relative". A MAP sensor does measure manifold vacuum, but it is relative to outside pressure. In other words, if you've only got 9 psi outside (instead of the 14.2 at sea level), the MAP sensor is never going to read more than about 13 inches of mercury of vacuum, and the engine will be trying to run more rich because of it.


It's also true that the ECU cannot compensate for more than about a 10% change in air/fuel. They are not infinitely variable. It is quite possible that the system never even goes into closed loop at this elevation. It isn't definitive, but I can completely unplug my O2 sensor and the MAL light does not come on.


I had not considered the special ECU for high altitude use, but it sounds quite plausible, and I'll check into that. Remember the old Dura-Spark ignition box? There is a version of it with an extra wire that, when grounded, advances the timing by some number of degrees to "compensate" for additional altitude, up to about 4000 feet.


Back to the topic at hand: if the error between what the TPS is reading and what the MAP and O2 are reading is too big, the system may never go into closed loop operation and, regardless of anything else, even if the system worked perfectly and stayed in an ideal state of tune at the elevation, there is still a 27% loss of air pressure here, and more at higher elevations. I don't think I can change anything at full throttle conditions (there's only so much air pressure available), but I think it's likely to make a nice difference at part throttle conditions.

 

Back in my dark years, when I was an engineer at General Motors (although I still drove a Ford truck), the MAP sensors sensed Manifold Absolute Pressure, not Manifold Relative Pressure. Given that Ford calls them MAP sensors as well I'm pretty sure they're measuring the same thing.

So if you're at higher elevation and there's only 9 psi absolute pressure, the highest the MAP sensor will read is 9 psi. It will give that reading at WOT, but it will also give it when you first key on. The GM ECMs back in the '80s took that signal to figure out what elevation you were at so the ECM could know how much fuel to throw at it while you were open loop. It leaned out the open loop mixture when it sensed a higher elevation. I'm not 100% sure if Ford does this same thing, but I'd be pretty surprised if they were stupid enough not to. It really makes engines start better at higher elevations.

As to a low MAP reading giving rich mixtures, why in the world would it do that? The whole point of taking a MAP reading it to help figure out how much air the engine is getting so it can figure out how much fuel to give it. If it gave more fuel at lower MAP readings, then it would give more fuel as the throttle was closed and less as it was opened. They don't do that.

Factory EFI might not give perfect air-fuel mixtures under all conditions. But they are so much better than the altitude-compensating carbs that it's not even funny.

 

You have some misunderstanding of how a MAP sensor works. The MAP is a manifold absolute pressure sensor. It measures the absolute atmospheric pressure, which is in reference to a near perfect vacuum.
The Ford EEC-IV can actually add/subtract around 30% change fuel, but it doesn't need to for high altitude use, which I will explain below
No need, which again, I will explain why:

Your truck uses speed-density fuel injection. The computer infers the amount of air going into the engine by the manifold pressure and the engine speed. As far as the engine is concerned, being at a high altitude is functionally equivalent to just not opening the throttle as much. The throttle position itself plays very little role in the fuel control.

The computer mainly only cares about the engine speed, and manifold pressure with O2 sensor feedback for minor correction. So the computer just checks the manifold pressure, checks the engine speed, calculates the air-mass going into the engine, and then injects the according amount of fuel. It doesn't really care that you're at high altitude. It just sees a lower manifold pressure, and injects less fuel accordingly. The computer also has high-altitude compensation tables for spark control, so no need for any adjustments there, either. The computer checks the barometric pressure using the MAP sensor when the key is first turned on, before the engine starts spinning.

"the more intelligent air-leak method" of fueling the engine at altitude doesn't actually make more power than the EFI method. Carbs fuel based on the velocity of air going through the venturi, but the combustion itself cares about the amount of air mass going into the engine. So at high altitudes, there is less air mass going into the engine, but the velocity of the air is still the same, so the carb is just dumping as much fuel as it normally would. The altitude compensation effectively lowers the air velocity through the venturi by bypassing air around it, making the carb fuel less. So it's not really adding extra air to the engine, it's really just making it fuel less.

The amount of power your engine makes is limited purely by the mass of air going into the engine. At high altitude there is less air mass per given volume of air, so there is less power to be had. The computer in your truck will maintain the correct 14.7:1 air fuel ratio no matter what. You could take the truck right up to the edge of space, and it would still inject the proper amount of fuel.

An aftermarket throttle-body will not do anything to your air-fuel mixture, an will not likely add any extra power. At 9,000 feet your 460 is down to around ~170 HP at the crank, which is not going to make your truck go anywhere in a hurry.

 

Yes, I mis-wrote the stuff about the map sensor. Thanks for pointing that out. I don't want to confuse anybody that might be reading this in the future.-


I'm sure we would all agree that the salient point is that, as pointed out, driving at high elevation is like driving at part-throttle all the time.


Allow me to come back to this in a bit.
Thanks.