GMC Western States

 Tech Center Number 32 - March 1, 2001

Emery Stora

From our Technical Vice President

I hope that everyone had a great holiday season and have a lot of great plans for the New Year. If one of your plans is to attend the Western States GMC rally at Palm Desert, CA 3/18 - 3/23, I'll look forward to seeing old friends there and to meeting many Western States members that I haven't had the pleasure to know yet.

For those using their computers, be sure to go to the website to take a look at the newly posted write-up by Bill Harvey on retrofit fuel injection. "Electronic Fuel Injection: The Answer to Modern Fuel?" Look under the Tech Info tab. Those that have made this type of modification are very pleased with it. California, however, does not look kindly upon any modifications having to do with emissions even if it actually improves emissions. So, if you are registered in California or certain other states, you will have to take it off and replace it with your carburetor whenever you need inspection. Either this, or wait until this modification is allowed (I believe you are excepted from the law in California if your vehicle is over 25 year old - check the latest code before doing the modification). If you don't use a computer for "surfing the net", let me know and I'll get a copy of the article to you.

We have had only one suggestion during the last quarter for topics or ideas for the Tech Center. That was for cornering lights that I'll try to cover for the June issue. If you have any subject that you would like covered, please let me know.

Not having any specific input from members I will cover a few things that I think might be of general interest.

First I want to tell you that horsepower is not important to a GMC owner. Torque is important. We constantly hear and read about these numbers, but what exactly are they? What are those numbers at various rpm (rotations per minute) really representing and how do you know what is going to be right for your application?

A quick and easy answer is that torque, measured in pound-feet (lb-ft), is the measurement of the twisting force that is produced within the engine. It is responsible for getting your vehicle moving, and big numbers are preferred in the lower rpm range and for hill climbing with a heavy GMC motorhome. Horsepower is a function of torque and rpm. It can loosely be described as the amount of work it takes to keep your motorhome moving and how fast this work gets accomplished. Everyone generally likes to see bigger horsepower numbers, but in the case of the GMC motorhome, we like to see the maximum torque at or near the rpm where we usually cruise down the highway. Maximum horsepower is not going to be reached until the engine is at a much higher rpm, say 4000-4400, which is a much higher rpm than the GMC will reach at legal cruising speeds. For maximum economy you want to run within about 400 rpm of the peak torque rpm (2400-2800). The maximum torque and the rpm at which it is reached will vary from engine to engine depending upon a number of factors. Charts won't help much. The only way to know the actual number is to have a dynamometer test of your GMC. This can be a very expensive test though, so let's just live with the general numbers that I have listed above.

Looking closer at torque, have you ever tried to remove a lug nut with a 3/8" ratchet? Definitely not an easy task. However, if you were to exert the same amount of force on a much longer breaker bar, you could easily accomplish the job. The longer wrench gives you increased torque and, therefore, makes it easier to remove the lug nut.

Torque = force x arm length

400 lb-ft = 200 pounds of force x 2 foot long torque arm
400 lb-ft = 100 pounds of force x 4 foot long torque arm

An interesting note is that it takes the same amount of force to move 1000 pounds one foot as it does to move 1 pound 1000 feet.

With that in mind, we use the term power to measure how fast the torque or force is producing work or outcome. For our purposes, it's how the downward force of the pistons on each power stroke spins the flywheel, which in turn moves the vehicle via the driveline. To keep matters simple, think about how much torque it takes to spin a tire one revolution, and then think about what it takes to move it faster and faster.

Horsepower is a function of torque and is actually more a rating that was derived to create a perceptible number to act as a common basis for comparing engines. In fact, you can't measure horsepower. It must be calculated from torque and rpm. It is actually a totally arbitrary number.
A Scottish engineer, James Watt, invented it. The amount of work that a horse could achieve within a certain time frame is where our "horsepower" numbers originated. His draft horse, in pumping water out of a coalmine, could pull a 12-foot lever around in a circle covering 75.398 feet with each circuit. His horse could do this 2.4 times per minute, meaning that it was covering about 181 feet per minute. Watt figured his horse was pulling with a force of about 180 pounds. So, 181 feet times 180 pounds = 32580 foot-pounds of work. He rounded it off to 33000 foot-pounds per minute (or 550 foot-pounds per second) and to this day this is what we refer to as one horsepower. As I said, it is entirely arbitrary. Bear in mind that if Watt had been using a Scottish pony or a Belgium workhorse we would have an entirely different number, which we call "one horsepower." All that matters, though, is that we now have a standard number to define "horsepower" and can compare one engine to another or compute the effect of modifications on an engine.

Horsepower = lb-ft per minute / 33000

These are the theorems that allow you to compute torque from horsepower numbers and vice versa. When using these theorems, remember that the measurements you come up with are going to be at the same rpm you use in the equation. This means you will not be getting your peak numbers in both cases. If you use your GMC 455 engine peak torque of approximately 480 lb-ft which occurs at approximately 2600 to 2800 rpm to get horsepower numbers (hp), the answer is 224 hp at 2800 rpm (hp = 2800 rpm x 480 lb-ft / 5252). This is certainly not the engine peak horsepower number. The peak is more like 330 hp at 4000 to 4400 rpm.

Horsepower = (rpm x torque) / 5252

Torque = (5252 x Horsepower) / rpm

Now, you are probably wondering where the 5252 came from. To get distance we must take into account the circular distance; and, as we know, all circular calculations involve the mathematical constant PI=3.14. In this case, mathematically, hp = (2 x PI x rpm x Torque) / 33000. This boils down to hp = (rpm x torque) / 5252.
The torque and power of our GMCs can also be affected in a variety of ways. If you have a 330 hp engine and switch from 30-inch tires to 31-inch tires, you're affecting the engine output. All of a sudden, the radius and circumference of the tire has grown and the engine has to produce more torque to spin the tire and more work to complete one revolution in the same amount of time. Fortunately there is a way to gain this torque back - through swapping the final drive to a numerically higher ratio (lower gear ratio).
If you wanted to keep the same torque you would have to change the gear ratio as follows:

New gear ratio = new tire dia. / old tire dia. x old gear ratio

Once you know your engine torque output, you can determine the torque at the axle by multiplying all of the gear ratios in whichever gear you chose. This will give you the forward force measured at the contact patch of the tire.

Torque @ axle = engine torque x transmission ratio x final drive ratio

For first gear @ 2800 rpm: 480 lb-ft x 2.48 x 3.07 = 3654 lb-ft at axle
For second gear @ 2800 rpm: 480 lb-ft x 1.48 x 3.07 = 2181 lb-ft at axle
For third gear @ 2800 rpm: 480 lb-ft x 1.00 x 3.07 = 1474 lb-ft at axle

With that information, you can plug in the different radius of your tires to see the difference in the amount of forward force being applied.

Force = 12 / tire radius in inches x axle torque

Using a 30 inch diameter tire: 2923 pounds = 12 / 30/2 x 3654 lb-ft
Using a 31 inch diameter tire: 2829 pounds = 12 / 31/2 x 3654 lb-ft

By stepping up to a taller tire you affect the torque and power output of the engine. The larger circumference takes more work to turn one complete revolution.

With the standard tire size of 8.75R16.5 the tire diameter is approximately 28.9 inches. Note that it changes with tire wear and also varies with the manufacturer. Changing to a LT225/75R16 (Alcoa 16" wheels) gives a diameter of about 29.3". For those putting on oversize tires such as LT245, the diameter is even larger.

Next time that you think about going to a larger tire size, or swapping engine parts such as intakes or cams, keep in mind when and where you want maximum torque and the points in between. Careful planning and researching with parts manufacturers and engine builders will help you choose the torque and horsepower curves to fit your application. And all of this will help your motor home perform better. It does no good, for example, to install a new intake manifold that doesn't really kick in benefits until you reach 5000 rpm as you'll never rev your engine that high. We aren't driving race cars!

I have talked to Chuck Stoddard of Caspro and he tells me that they designed their Caspro 3.50 final drive gear kit for the transmission to give maximum torque at about 65 miles per hour. With the standard 3.07 final drive and the 8.75x16.5 tires, the rpm would be approximately 2325 rpm at 65 mph. Changing to a 3.50 final drive and the same tires would give 2650 rpm at 65 mph. In my case, changing from the standard 3.07 to a 3.55 final drive unit and changing tires from the 8.75x16.5 to a LT225/75R16 changed my rpm at 65 mph from 2325 to 2685 and at 70 mph from 2500 rpm to 2850 rpm. I am now able to keep my rpm up much closer to the peak torque of the engine when cruising or when climbing hills. This is especially beneficial when pulling a "toad".

You can even get a 3.70 final drive now, but you'll reach the maximum torque of the engine at lower highway speeds. This may be desirable for those who would rather drive at 55-65 rather than at 65-75.


Many of us are now carrying our computers in the GMC when we travel mainly just to keep up on our email. I have four kids scattered across the country, and we find it great to keep in touch by e-mail. I also enjoy reading the postings to the GMCnet site. That site is a great mail list of GMC owners from across the country that post their questions for help. If anyone wants to sign up for it go to for information.

I had resisted getting a cell phone for several years but finally decided last September to sign up for one prior to going to the GMCMI Convention in Oregon. My wife, Michelle, decided that it would also be good to have on hand for roadside emergencies and so that the person watching our house could contact us if he had to. Also good for relatives and friends to catch up with us as we would be gone for over two months. One of my reasons for getting it was because I found out that I could connect my computer to it and access my e-mail while going down the road or parked on a mountain top or a peaceful valley camp somewhere without a telephone line!

Several providers are available. Two of the largest are Sprint and Verizon. I am using Verizon since they currently seem to have more geography available across the country. I chose a plan that allows up to a set number of minutes for a flat price with no roaming charges. I signed up for the 400 minutes per month plan at $55.00 per month. If you aren't traveling, you can call them and reduce your number of minutes for that month and then increase it later. They have lesser and more minutes at other pricing. I had to purchase a phone but also bought a computer connection cable, which has one end that plugs into the cell phone and the other end that plugs into the serial port of my laptop computer. Software comes with it that turns the cell phone into a wireless modem when dialing up a number with your computer. You are limited to areas where there is digital service (you can make telephone calls in analog areas but not data transmission). If you are close to a metropolitan area you'll have digital service. At Mt. Hood, Oregon, we got digital service from Seattle. On my most recent trip, Michigan for Thanksgiving and Florida for Christmas, we had good telephone service the whole trip.

You can use their Internet service with either a PC or with a MAC. You'll need a serial adapter for the Mac to change the cable end to a round connector. If you are going to the Palm Desert rally, I suggest that you see Al Chernoff for info on the Mac hookup and Gene Fisher for info on the PC hookup.

 By the way, if you only want a cell phone in your vehicle for emergency purposes, you can pick up a used cell phone at about any Salvation Army, Goodwill Industries or such other stores for just a few dollars. You don't even have to sign up for monthly service. All cell phones will connect with the local 911 number at no charge. If you had to make a call to someone else, you can dial it and you'll get an operator or a message telling you that your cell phone is not signed up for service. You can then request to make a credit card call through the cell phone or reverse the charges or bill it to your home phone. You might have some added fees to the regular long distance charge from the local providers for doing this but in an emergency, it would be worth it.


Last summer I found that when we opened the front vent windows, we got a foul smell into the driver's cockpit area. After checking around I found that the drainpipe from the kitchen sink drops down into the top of the sewage tank just under the bottom of the cabinet across from the side-wet bath. I decided to remove my holding tank. If I had known just how the kitchen pipe attachment was fastened in, I would not have dropped the tank. The repair could have been made from the top.

In my 1977 GMC the pipe is behind a plastic cover in the air compressor cabinet just across from the side bath. On older models there may be a slanted wood board covering this area. After removing the cover, the easiest way to gain access is to take out the compressor and to remove the floor of the closet. Then, the easiest way to disassemble the pipes is to cut the pipe just below the top elbow. The fitting to the tank is a sink drain trap type collar. You might need a basin wrench to take it off; however, in my case, the metal ring had corroded and the rubber seal had deteriorated and the pipe pulled out. I suspect in most cases it can just be pulled up and out.

Mine had a steel collar screwed to the floor. Apparently once there was some sort of trim or flashing under this to close off the hole in the floor, but it had deteriorated and disappeared. I pried off the collar and discarded it. I used a 1-1/2" pipe thread to ABS pipe fitting for the tank, a 3-1/4" length of 1-1/2" pipe, a 1-1/2" rubber coupler, a 1-1/2" street elbow, a 4" piece of 1-1/2" pipe and then a 1-1/2" to 2" rubber reducing coupler. The 2" end of the coupler fits fine over the outside of the top elbow. It was not necessary to have a cleanout tee since the rubber couplings can easily be disconnected if necessary to clean out the pipe. These parts are easily found at Home Depot.

Pictures of this assembly and the material list are on the Internet at my PhotoPoint site:
Look under Kitchen Sink/Holding Tank. If anyone needs this information but does not have Internet access, let me know and I can send you the pictures and parts list.

If you are getting a lot of sewage fumes into the interior of the GMC, this connection might be the thing to check out. This corrected my fume problem and the summer/fall trip to Oregon was without fumes.

Now for the rest of the story -
On my recent trip to Florida the fumes returned - big time. I could not even use my Fantastic exhaust fan at night as it would pull horrible fumes into the bedroom area. I checked the connection I had made the previous summer, and it was intact. I then removed the toilet and found that the holding tank was cracked on the top right by the large pipe going up to the toilet. When going down the road, the sewage would splash on top of the tank and then be pulled into the interior of the motorhome, either by the low pressure created by the open front driver and passenger vents when going down the road or by the ceiling fan when parked. Jim Bounds of the GMC Co-op in Orlando, FL did an air pressure test for me and found that I had major air leaks at the seam where the plywood floor board meets the bottom of the wheel well liner all across the area of the rear wheel wells on both sides. He applied some silicone caulking at that seam and it stopped the entry of the fumes. It also stopped the wet floor problem I have had for some time when driving in a rainstorm!

I am now in the process of installing a new holding tank, which I purchased from one of our Western States GMC members, Duane Simmons. Duane has excellent prices on the GMC water and holding tanks, and I trust that this will take care of the fume problem.

If you just have a minor fume problem, it could simply be that the water has sloshed out of the sink traps or the shower floor trap. You may find it necessary to put some water into the drains before driving. A rubber plug in the shower floor and in the bathroom sink may also help.

 This material is based on my personal experience and the personal experience of other club members. It is our viewpoint and does not represent authorized data pertaining to the GMC Motorhome. It is the responsibility of the readers to make their own judgment as to the validity of this material in relation to any repairs and/or modifications to their own vehicles.

 3128 Vista Sandia, Santa Fe, NM  87501-8526

 (505) 989-8157

 Please send your comments and ideas for the Tech Center to:
Emery Stora, Technical Vice President,
3128 Vista Sandia, Santa Fe, NM  87501-8526
Phone: (505) 989-8157



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