Altitude is tough

[Bill]

Rocky Mtn Ray, our resident 14-er, has told us many times about the effect of altitude on the towing capacity of your tow vehicle. And I always understood it on an intellectual level. But last week, it really hit home in personal experience.

My wife and I were making our annual pilgrimage from Arizona to Maine. We try to vary the route each year, so it doesn't get boring. This year our route involved I-70 through Utah and over to Denver. We had done this route years ago, but without a trailer, and I remembered it as very high and very beautiful. Both adjectives still apply. The culmination, in many ways, is the areas around Vail Pass, Loveland Pass, and the Eisenhower Tunnel. They top out just over 11,000 feet.

Our 2720SL weighs right around 3500 pounds. Our tow vehicle is rated to tow 7300 pounds, so no problem, right? The Explorer has a small V-8 (4.6 liter), a 3.73 rear end, 5-speed automatic, the factory towing package, and we seldom, if ever, have any problem dragging our TM anywhere we want to go. But as we approached these passes, the engine just couldn't breathe in the thin thin air. Much to my surprise (sorry, Ray, you warned us), we had to downshift to 4th gear, and then to 3rd, and were barely able to maintain 35 mph for the last mile of the approach to each summit. All the way up, I kept thinking "What if I were trying to do this with a small tow vehicle?"

I don't mean to lecture, but there is a question that comes up again and again on this board. That question is "Can I tow a TM with such-and-such a tow vehicle?" As demonstrated on I-70 last week, the whole question is meaningless until you decide just WHERE you want to tow your TM. Crossing the farm country of Iowa is a lot different from approaching Loveland Pass. Before you choose a tow vehicle, you MUST know where you want to go.

I have some pix of these passes, which I will post as soon as I find them.

Bill

Crossing that area even without a trailer can be dangerous. Several years ago, we were going through Eisenhower Tunnel when our Dodge Caravan stalled and stopped dead in the middle of the tunnel. We thought we were living our last minutes when suddenly up ahead we saw a message board light up with the message, "Disabled vehicle - Use left lane". Another message told us to stay in the car help was on the way. And, sure enough, in about a minute a large truck with a push plate on the front came up behind us and pushed us out of the tunnel.

We were told that we had "vapor lock" which is a common occurrance in the upper elevations. We waited 5 or 10 minutes and the minivan started up again without a problem.

[RockyMtnRay]

Originally Posted by Bill

Rocky Mtn Ray, our resident 14-er, has told us many times about the effect of altitude on the towing capacity of your tow vehicle. And I always understood it on an intellectual level. But last week, it really hit home in personal experience.

My wife and I were making our annual pilgrimage from Arizona to Maine. We try to vary the route each year, so it doesn't get boring. This year our route involved I-70 through Utah and over to Denver. We had done this route years ago, but without a trailer, and I remembered it as very high and very beautiful. Both adjectives still apply. The culmination, in many ways, is the areas around Vail Pass, Loveland Pass, and the Eisenhower Tunnel. They top out just over 11,000 feet.

Our 2720SL weighs right around 3500 pounds. Our tow vehicle is rated to tow 7300 pounds, so no problem, right? The Explorer has a small V-8 (4.6 liter), a 3.73 rear end, 5-speed automatic, the factory towing package, and we seldom, if ever, have any problem dragging our TM anywhere we want to go. But as we approached these passes, the engine just couldn't breathe in the thin thin air. Much to my surprise (sorry, Ray, you warned us), we had to downshift to 4th gear, and then to 3rd, and were barely able to maintain 35 mph for the last mile of the approach to each summit. All the way up, I kept thinking "What if I were trying to do this with a small tow vehicle?"

I don't mean to lecture, but there is a moral here, and we have covered it many times on this board. The question is often asked "Can I tow a TM with such-and-such a tow vehicle?" As demonstrated on I-70 last week, the whole question is meaningless until you decide just WHERE you want to tow your TM. Crossing the farm country of Iowa is a lot different from approaching Loveland Pass. Before you choose a tow vehicle, you MUST know where you want to go.

I have some pix of these passes, which I will post as soon as I find them.

Bill

Ayup. The western approach to the Eisenhower tunnels (tunnel entrance is 11,100 feet above sea level) is an 8 mile long, steady 7 percent grade. The western approach to the top of Vail Pass (10,600) is a 10 mile long, 6 to 7 percent grade. Both are long, tough climbs (Subaru uses western approach to Eisenhower tunnel to test its supercharged cars for power) but they aren't the longest, steepest, or highest altiutude highway grades in Colorado. But, being on the Interstate, they do have the highest speed limits so vehicles are driven harder/faster and on a hot summer day, the roadside near the top of both passes is littered with overheated and broken down motor homes and tow vehicle/trailer rigs. Two other passes in particular...Monarch Pass on US50 (8 miles of 6 to 7 percent grades on both sides), and Rabbit Ears Pass on US40 (7 miles of at least a 7 percent grade on the western approach) are notorious for causing tow vehicles to overheat and/or blow their transmissions, in part because both have fairly high speed limits (45 mph to 55 mph) so most people push their vehicles hard to keep their speed. Other long steep climbs (e.g. Wolf Creek with 9 miles of 7% grades) aren't quite as hard on engines because the speed limit is lower (~30 mph).

I'm somewhat surprised your transmission stayed in 3rd gear...my truck (with a 4.7L V8 that puts out a very respectable 320 lbs-ft of torque and has 3.91 gears) will downshift to 2nd gear when I'm towing on those grades...but I also flog it pretty hard (4500 to 5000 RPM) to hold 60 to 65 mph on the climbs. Running the higher speed is safer as I'm staying with traffic...and I'm getting enough airflow through the radiator and transmission cooler that I don't have overheating issues. But even with it's excellent torque rating, because I'm revving the engine this high on a regular basis I'm seriously considering spending around $1200 to install JBA Racing's exhaust headers to wring an additional 10% of mid-range (3000 to 4000 RPM) power out of it.

So take Bill's and my comments to heart folks...a tow vehicle that has adequate, even good power in the midwest and eastern US will be woefully underpowered in the high Rockies. And even pretty powerful midsize V8s will need to be flogged hard to maintain traffic speed on Colorado's climbs with even a TM 2720SL in tow.

Footnote: Loveland Pass (which is on US 6 and serves as the hazardous cargo bypass for the Eisenhower tunnels on I70) is just shy of 12,000 feet (11,991 to be exact). The approaches on both sides involve around 10 miles of 6 percent grades and lots of hairpin switchbacks. Very scenic but also has some heartstopping 1000 ft drop offs along the road with no guard rails!

[Bill]

Originally Posted by RockyMtnRay

I'm somewhat surprised your transmission stayed in 3rd gear...my truck (with a 4.7L V8 that puts out a very respectable 320 lbs-ft of torque and has 3.91 gears) will downshift to 2nd gear when I'm towing on those grades...but I also flog it pretty hard (4500 to 5000 RPM) to hold 60 to 65 mph on the climbs.

Ray -

Remember, I have a 5-speed automatic, so I shifted down 2 gears to reach third. This would be more or less the same as a 4-speed shifting into second.

According to the tach, my 4.6L V-8 red lines at about 6200 RPM. That is AWFULLY fast. I have to admit that I let up on the gas to hold it to 4000 RPM, which corresponds to 35 mph in 3rd gear. I tried (once) running the RPM up to 4800 and then allowing it to upshift to 4th. But it couldn't hold speed in 4th on those grades, so I dropped back to 3rd and resumed 4000 RPM.

Bill

[RockyMtnRay]

Originally Posted by Bill

My 4.6L V-8 red lines at about 6200 RPM. That is AWFULLY fast. I have to admit that I let up on the gas to hold it to 4000 RPM, which corresponds to 35 mph in 3rd gear. I tried (once) running the RPM up to 4800 and then allowing it to upshift to 4th. But it couldn't hold speed in 4th on those grades, so I dropped back to 3rd and resumed 4000 RPM.

Bill

Hmmm...guess it kinda depends on how comfortable you feel with running sustained high RPMs. I've had great durability out of the overhead cam engined sports cars, sport sedans, and motorcycles I've owned despite regularly running them right at redline...especially those that had 4 (smaller, lighter) valves per cylinder and dual overhead cams so there were no rocker arms in the valve train. By contrast I've always been pretty hesitant about taking the pushrod engines (OHV) much beyond 4000 RPM because there was sooooo much mass/inertia in the valve train (and usually also hydraulic lifters that tend to pump up) that valve float was a serious concern.

The engine in my truck is a bored/stroked version of the 4.3L Lexus V8...dual overhead cams on each bank, 4 valves per cylinder, bucket/shim (no problematic hydraulic lifters) between cam and valve, and near perfect harmonic balance...that engine happily and extremely smoothly revs right to its 5200 RPM redline. It roars a lot over 4000 RPM (fan and exhaust) but there's just no vibration...unlike the 4L straight six in my Jeep that sounded horrible over 4000 RPM.

So given that it's a rev happy (and near bulletproof) engine, I happily rev it. When I get to the point that it won't hold a steady speed in 3rd and the drop into 2nd, accelerate the speed, upshift to 3rd, start losing speed cycle begins...I simply yank the shift lever into 2nd to kill the transmission's shift logic and then l just run the revs as needed. That turned out to be around 4500 to hold about 60 mph (5000 RPM gave me close to 70 but I decided I probably didn't need that much speed to stay with traffic).