Truck Avenger 470 Jetting for (Really) High Altitude

[CincoBravo]

This past summer I attempted to take my '77 with a relatively stock 302 over Corkscrew Gulch, an old mining road in Southwest Colorado. I never made it over the top. For those not familiar, this road climbs to a max elevation of approx. 12,000-12,500 feet. I have a Weiand low-rise dual plane intake and a Holley Truck Avenger 470 cfm carb. Everything else is stock/stock equivalent. The carb came with #55 primary jets and runs fine at low altitudes. For my trek across the mountain I started out with #51's and didn't get too far before it ran horribly and couldn't pull the hills. Then I switched to #49's and got a little farther, but still encountered lots of sputtering and dying. I was all set to try it with #47's, but then had a trip ending mechanical failure on something unrelated to carbs and jetting (so I never got to find out if the #47's would've gotten me over the top). I want to try it again this upcoming summer and I'm curious about how small of a jet others have used in the 470 Truck Avenger. As long as I check the spark plugs and see no sign of running lean, is there any limit on how small I can go?

 

[jckkys]

The jets are only part of what needs to be done to calibrate a carb to run well at a given altitude. The power valve feeds a second set of "jets" called power valve channel restrictions. These are referred to as PVCRs. The restrictions and vacuum value of the power valve are set for sea level. So at high altitude the power valve is open consistently. It's a little difficult to reduce the size of PVCRs without modifying the metering block to use select-able PVCRs. This isn't practical for temporary operation at altitude. A lower value power valve that won't open as soon would be a way to avoid the over rich mixture at altitude. A vacuum gauge is very helpful to see what's needed at 12000ft. The idle mixture needs to be set for altitude also and the choke plate has to be fully open. The Holley is simply built for sea level and it's not practical to make all the changes needed for 12000.

 

[CincoBravo]

Thanks JCKKYS.

 

[WheelHorse]

Cinco...install a wideband...makes tuning much more scientific as opposed to guess.

My carb runs great and I always know the AFR.

 

[jckkys]

It's a lot easier in the middle of the Midwest where everything for hundreds of miles is 500 ft. in altitude. For all practical purposes that's sea level, where the carb was calibrated to run. The fact remains that some things like the idle feed restrictions and PVCRs are not easy to access, let alone modify to achieve the correct A/F mixture. For 12000 ft. both have to be reduced in size. These are semi permanent changes and not a good idea unless the PO lives in Leadville Co.

 

[CincoBravo]

Cinco...install a wideband...makes tuning much more scientific as opposed to guess.

My carb runs great and I always know the AFR.

WheelHorse— thanks for the reply. I’ve considered the wideband. Can you tell me which one you use and if you’d buy it again?

 

[CincoBravo]

It's a lot easier in the middle of the Midwest where everything for hundreds of miles is 500 ft. in altitude. For all practical purposes that's sea level, where the carb was calibrated to run. The fact remains that some things like the idle feed restrictions and PVCRs are not easy to access, let alone modify to achieve the correct A/F mixture. For 12000 ft. both have to be reduced in size. These are semi permanent changes and not a good idea unless the PO lives in Leadville Co.

JCKKYS: I actually live in Mississippi at a whopping 505 ft above sea level. To your point, the carb works great here. But I’m bound and determined to get over that mountain in Colorado! All that said, what are the things you would do knowing my situation. I gathered that you believe changing the power valve would help, and I of course would change the primary jets. Are there any other bits and pieces you would change or would it just be turning screws (and reading a vacuum gauge) after that?

 

[WheelHorse]

WheelHorse— thanks for the reply. I’ve considered the wideband. Can you tell me which one you use and if you’d buy it again?

Innovate mtx-l... Made it super simple to dial in afr at idle and wot. I even adjusted my power valve down to a 6.5"

My wideband thread

 

[CincoBravo]

Innovate mtx-l... Made it super simple to dial in afr at idle and wot. I even adjusted my power valve down to a 6.5"

My wideband thread

Thank you, sir. I just read your wideband thread. Very informative and shows me I have so much to learn!

 

[jckkys]

I have and a wide band too, but that won't make the changes to your Holley that you need. I run a vacuum gauge next to the wide band to see when the power valve opens, and what effect it has when it does open. A vacuum gauge is a must have for this kind of tuning. At 12000 ft. an open power valve can easily make the mixture too rich. Single stage power valves flow more fuel than the PVCRs. So the PVCRs need to be smaller to get a lean enough mixture. You could try a 2 stage power valve where the first stage has a small orifice.
The 500 ft. altitude I was talking about, is Chicago where WheelHorse lives and I lived. In the Mid West, altitude is constant and there's no need to factor it into carb tuning. In Col. it's a huge factor.

 

[jckkys]

Thinking back to my trips to Col. I remember driving to the top of Mt. Evans a little over 14.000 ft. in 1981 with an '81 Bronco. It smoked and was gutless but made the drive. In 2018 I drove an EFI equipped 1999 Crown Vic from Durango to Silverton on a road that got to 11,000 ft. It didn't do much better than the carb. equipped '81 Bronco. In both cases they made it on paved roads. I wonder if there isn't some other reason your '77 can't. I drove my '77 with a 302 to Mt Lemmon at 9200 ft. many times, and once to Estes Park. Never had a hard time making it. Out side of extreme angularity the Truck Avengers are not as good as the average 4160 Holley. I use 2 4100s one for sea level to 5000 ft. on one for more than 5000 ft. on hunting trips. For day trips, the low altitude carb works well. I would just set up an altitude carb for your '77. With a 2 to 4 bbl adapter you could even run an altitude compensated 2150.

 

[CincoBravo]

Thinking back to my trips to Col. I remember driving to the top of Mt. Evans a little over 14.000 ft. in 1981 with an '81 Bronco. It smoked and was gutless but made the drive. In 2018 I drove an EFI equipped 1999 Crown Vic from Durango to Silverton on a road that got to 11,000 ft. It didn't do much better than the carb. equipped '81 Bronco. In both cases they made it on paved roads. I wonder if there isn't some other reason your '77 can't. I drove my '77 with a 302 to Mt Lemmon at 9200 ft. many times, and once to Estes Park. Never had a hard time making it. Out side of extreme angularity the Truck Avengers are not as good as the average 4160 Holley. I use 2 4100s one for sea level to 5000 ft. on one for more than 5000 ft. on hunting trips. For day trips, the low altitude carb works well. I would just set up an altitude carb for your '77. With a 2 to 4 bbl adapter you could even run an altitude compensated 2150.

Excellent idea, sir. Thank you!