How much elevation change before jet changes?

[squirrel]

Re: How much elevation change before jet changes?

There is probably a small benefit to be had changing jetting with change in elevation....but I've never bothered and I haven't noticed any driveability problems because of it. Jetting for mid level elevation would probably be a good idea if you're planning on spending some time at both sea level and high up in the mountains.

I am agreeing with A/Fuel that carbs are good at compensating for changes in elevation, although I also agree with Scott (dieselgeek) that there will be a slight change in jetting requirements. Is it enough to worry about? that might depend on how picky you are, and how well you set up the carb in the first place.

Might want to look into a wideband O2 sensor so you can see how it's doing...but then when you see the numbers, you'll freak and want to run EFI, right Scott?

[dieselgeek]

Re: How much elevation change before jet changes?

they may not "freak" but like I've said other times, you'd be surprised at how many "perfectly tuned" carb setups I've plugged a wideband into and watched them idlling at 10.5:1 AFR.


-scott

[squirrel]

Re: How much elevation change before jet changes?

mine probably idles at somethign like that, but it also keeps running. Leaning it out more makes it stall.

I don't claim my carbs are anywhere near perfectly tuned...but the car runs ok, doesn't foul plugs, doesn't burn pistons, etc. It does put some black smoke out at the top end, and probably would be a bit quicker with less fuel in the secondaries.

Also the fuel cutoff thing with EFI won't help highway mileage, right? but it could help in town mileage a little bit. How much fuel goes thru a carb with the throttle closed, anyways?

[dieselgeek]

Re: How much elevation change before jet changes?

mine probably idles at somethign like that, but it also keeps running. Leaning it out more makes it stall.

I don't claim my carbs are anywhere near perfectly tuned...but the car runs ok, doesn't foul plugs, doesn't burn pistons, etc. It does put some black smoke out at the top end, and probably would be a bit quicker with less fuel in the secondaries.

Also the fuel cutoff thing with EFI won't help highway mileage, right? but it could help in town mileage a little bit. How much fuel goes thru a carb with the throttle closed, anyways?

ehh? the fuel cutoff thing is a HUGE advantage. FOr the guys who have logged it, the least I've ever heard reported was a 10% improvement in economy. Think abotu it, every time you coast - be it city or highway - the fuel turns off.

The amount of fuel that goes through a carb during decel is going to be, at minimum, the amount of fuel that it idles on, no? possibly even more than that because there's more vacuum during decel? (not a carb guru here so correct me where I am wrong). Think about all the time during driving when you're coasting, but off the throttle... that the fuel could be shut completely off. That's a decent amount of driving time. Equal to the amount of time the car is accellerating, yes? and for long downhill runs, it should be even better.


One guy whose car I tune, who drives it and logs fuel economy religiously (not per tank, but over hundreds of gallons of fuel used) and uses a GPS for his mileage: it's a second gen Ford Probe with a turbocharged 2.5l v6 (did all the work himself) and makes about 400whp at 20psi on our mustang dyno. He drives it to work whenever it's not raining or snowing, has a perfectly flat 20 mile commute, and right after we enabled the decel fuel cut his mileage jumped up 15%. 24mpg to about 28mpg. That was the only thing we changed last spring and he's been happy with it.

Some other things he's tried, driving on e85 and shown that with his combo (admittedly, too low compression to be a good e85 motor) it's cheaper to run on premium pump gas without ethanol than it is on e85. And that the 89 octane with 10% ethanol costs more to drive on than plain old 87 octane with no ethanol. Benefit of low compression turbo motor is, if you stay out of boost, you can run crappy gas all week long.


And finally, this thread makes me sound like an EFI snob. I'm not, I'm totally into cars with carbs that don't need EFI or when the owners want something with "character" - so I am in no way trash talking carbs.

-scott

[squirrel]

Re: How much elevation change before jet changes?

Interesting....I think that the results would depend a lot on gearing, though. How about a car with a 6 speed (double OD) transmission? My guess is that the Probe is turning a bunch of rpm at highway speeds. But I also consider highway driving to involve endless miles of steady crusing, so there is very little acceleration/deceleration. This is not the case in daily commuting on big city highways.

[dieselgeek]

Re: How much elevation change before jet changes?

Interesting....I think that the results would depend a lot on gearing, though. How about a car with a 6 speed (double OD) transmission? My guess is that the Probe is turning a bunch of rpm at highway speeds. But I also consider highway driving to involve endless miles of steady crusing, so there is very little acceleration/deceleration. This is not the case in daily commuting on big city highways.

Looking at a few of his datalogs I have here, seems he cruises around 2800-3000rpm on his commute. So you are right, that's turning some RPMs (remember, this is a tiny little v6). And his commute is technically "highway cruising" but I see 8-10 major "coast downs" where the decel cut activated, on a typical cruise. You're right, his commute is more "big city highway" than "Interstate"

[Mr4Speed]

Re: How much elevation change before jet changes?

Vacuum is much higher on decel than at idle. When you see a circle track/road race car belch fire on corner entry that's excess fuel on throttle closing. When you slam the throttle shut it will momentarily pull fuel out of the main metering circuit and the idle circuit when it doesn't need any at all. As I understand it that was something the EPA was wanting cleaned up as that's one point where carbs are really "dirty", including the shift points on manual trans cars.

Electronic fuel management was what allowed the use of steep overdrives and low cruise RPM since it's really hard to manage the fuel with a carb when the vacuum signal is so weak. I'm still astounded that my EFI full size pickup gets 19-19.5 mpg every tank, but I've also been amazed at the mileage I've gotten out of some of my older pre-emissions "stone age" cars. I think that sometimes the tuning for economy ,emissions, and driveability are at odds with one another.

[squirrel]

Re: How much elevation change before jet changes?

Yeah, we get about the same mileage from the 70 camaro with a stock 6 cyl as we get with the late model full size 5.3 chevy truck.

But I think that electronic fuel management was necessary for many reasons, including meeting emission system warranty requirements.

[cantvalve16]

Re: How much elevation change before jet changes?

Some real good experienced knowledge here gentlemen. Thank you. I am 26 and have been driving for 10 years now. First half with carbs. Second half with EFI. I miss the carbs and the packages they came in. Thanks for helping me with the thoughts for my next vehicle.

[squirrel]

Re: How much elevation change before jet changes?

I suggest you base your carb/efi decision on a lot of factors...how old the car is, how easy it is to set up an EFI fuel system (this is much easier on cars that came with EFI, or some oddballs like the Monza that had an in tank pump and a carb), how much you like messing with electronic stuff, how much you like nice looking engine compartments (all that wiring is hard to make look nice), etc.

[cantvalve16]

Re: How much elevation change before jet changes?

Ya, I have been leaning toward something pre-75 so I don't have to smog here in Cali. It will be our second car, so it's perfection is not that big a deal. Just want something old and fun. Oh and cheap. I find old cheap cars easier to maintain than newer cheap cars that have to smog. Plus, they are more fun for me to modify and restore. Like you said, Squirrel, hard to make all that wiring look good. I just appreciate simplicity.