GMC Western States

 Tech Center Number 15 - June 1996

by Frank Condos

By now most of us have had some experience with the newly mandated reformulated gas and we may or may not have noticed a difference. I'll try to limit this report to an explanation of the reported technical information and what it means and leave the politics to others. Most of the information bas been obtained from sources such as the EPA Office of Mobile Sources, The American Petroleum Institute and fuels consultants.
Reformulated gasoline (RFG) is blend of fuels designed to reduce Volatile Organic Compounds (VOC) that result from the evaporation of gas, the emissions of carbon monoxide (CO) and nitrous oxide (NOX). Summer evaporation is reduced by reducing the Reid Vapor Pressure, or the measure of volatility, during the summer months by about 15-20% (1 psi). This change reduces the amount of evaporation under ambient temperatures and also reduces tendency to form a tree vapor lock (more on this later).
CO is reduced in any engine by the introduction of oxygenates to the formula. These are compounds that carry an extra oxygen atom that is released during burning to help convert CO to CO2. As most people now know, the oxygenates are either ethel alcohol or Methyl Tertiary-butyl Ether (MTBE). The alcohol addition is used more often by refiners because of cost. On the average, about 6% by volume alcohol is used as compared to the earlier gasohol mixture of 10%. 11% MBTE is used. The NOX emissions are reduce by closely controlling the aromatics, sulfur and benzene in the fuel.
RFG is mandated by the EPA in most major metropolitan areas of the northeast, middle Atlantic states, the industrial areas of the mid-west and southern California. Other areas have also opted to join. Oxygenated fuels are required during the winter along the Rocky mountain area. In short, one third to one half of the fuel supply will be RFG or oxygenated as supply sources convert.
The effects of RFG on our GMCs can be divided into the following categories: performance, drive ability, and materials. Since RFG contains a bit less energy per gallon, fuel economy is off very slightly. On the order of 2-3%, probably not noticeable in light of all the other variables such as wind and temperature. There is an additional loss caused by the shift to a leaner bum in the combustion chamber by the introduction of additional oxygen. Assuming the engine operates at the optimum air/fuel ratio of 14.7:1 the ratio is shifted to about 15.1. Again with a slight loss in power, but I would guess most of our coaches operate on the rich side of 14.7 under load because of wear on the metering rods and jets. According to Joe Mondello, one could change the jet metering rod combination to off set this leaning by the new fuel, and he plans to experiment on the right combination.
Drive ability is probably a bigger factor for us than the performance loss. The specifications for RFG reduce the vapor pressure about 1 psi from conventional gas during the summer. This change actually reduces the likelihood of vapor lock which is generally defined as a vapor bubble in the fuel line or fuel pump on the suction side. The bigger problem comes from what is more correctly called percolation, or boiling of the fuel in the carburetor bowel. RFG lowers the boiling point at which 50% of the fuel will boil away by 5-50 degrees F and the 90% point by 30 degrees on the average. It is this change, which is also true for the older gasohol, that causes the problem. The writer has observed a coach at the Denver elevation that suffered fuel starvation on a moderately warm day even with the electric booster pump on. Inspection with the carburetor top off and the engine hot, indicated that the fuel fed by booster pump boiled so vigorously that the bowl would not fill. This percolation problem is exacerbated by RFG.
If fuel starvation is a problem with your coach there are several solutions. First try different brands of gas since there are differences in formulation that can affect driveability. A flow through booster pump or full time electric pump near the tanks will eliminate the vapor lock issue. For those who prefer not to use electric fuel pumps, use some form of insulation or shielding of the lines from opposite the exhaust manifolds to the mechanical pump. The more serious percolation problem can be helped by installing laminated insulating gaskets between the carburetor and the manifold These gaskets are more effective at reducing the heat transfer to the carburetor than the stock spacer and can be obtained from stores specializing in speed equipment. Ducting cool air directly on to the carburetor or into the air cleaner could also help, Finally, one can plug the heat riser in the intake manifold but this will adversely affect cool weather driveability and fuel economy.
Some elastomeric materials are affected by the additives, particularly the alcohol. Since gasohol has been around for about ten years, most replacement
parts have compatible materials but some original equipment may have problems. Areas to watch for leakage include the fuel lines, the transfer valve and the carburetor float needle.
Whether or not one agrees with all the EPA policies, Reformulated Gas is here to stay and it is not just a California phenomena. With a little bit of attention, the GMC should be able to digest RFG.


by Bill Harvey and Frank Condos

Air bag extenders are devices to raise the air suspension bags approximately 3 inches above the stock position by extending the vertical suspension lever arm. The purpose of this article is to discuss the possible advantages and any down sides to such a change.
The original idea was developed by Clarence Buskirk when he determined that the additional weight of his lengthened coach required higher air pressure to maintain the ride height. His approach was to weld additional length onto the vertical portion of the bogy ann. Since then, someone determined that the arm extension could be fabricated separately and bolted on using the original air bag attach holes. Several GMCers have taken the bolt on approach and there is at least one commercial version available.
What does the extender accomplish? The bogy arm is really a lever pivoting around the pivot pin. lengthening the air bag end of the lever means that less force is required at the air bag end to hold up the standard coach, or in Clarence's case, nearly the same force to hold up a heavier coach. Current designs extend the arm about 3 inches or 25% longer. Remember, force is related directly to the pressure in the air bag therefore bag pressure is either reduced or remains close to stock, for a 25% increase in the coach weight.
But what other effect does extending the arms have? we will examine some claims and the physics of the change.
Ride quality is affected by the change in the following way. The reduced pressure on a stock coach results in a reduced spring rate. While this yields a sorer ride it also means more swaying or wallowing and an increase in the tendency of the coach to under steer, both bad handling characteristics. Remember, performance cars stiffen the suspension and add rear sway bars to improve handling and move steering toward neutral or over steer. The addition of a rear sway bar may help but probably won't overcome a 25% reduction in spring rate.
What about the heavier coach, doesn't the near stock bag pressure maintain the original ride quality and handling? No, the physics say one needs to increase the spring rate proportional to the weight increase. For air suspension, this means higher pressure. Now most of us drive conservatively and may not notice the difference in handling until we are forced to make some evasive maneuver or are on a poorly designed or maintained road.
Running at a lower pressure will extend the bag life won't it? An examination of the geometry of the bogy arm indicates the bag end lever and the wheel end lever are approximately equal for the stock units. Consequently any displacement of the wheel by the road is translated into the nearly equal movement on the bag as the coach tries to absorb the uneven road. If the lever arm is lengthened 25%, the bag will be deflected or flexed 25% more for the same road conditions. The total effect is analogous to the increased side wall deflections on an under inflated tire. the effect can be increased heat build up and faster fatigue. Taking the analogy further, tires seldom blow out on a parked car. Likewise air bags life is not simply affected by the static pressure. Failure results when they have been weakened through flexing or some external damage or some combination with old age. Damage and old age may be the major failure factors, with flexing a marginal one, but the change to extenders is clearly in the wrong direction for increased life.
Some other factors to consider are the reduced leveling capability since the longer lever now means 25% less height before the bag "hot-dogs" On the other hand, fittings are more accessible and servicing is easier when the bags are lifted.
Are the over all effects of changing the arm lengths a significant improvement or a detriment? Probably neither. If one has overloaded the coach it is a way of keeping the pressure within design limits. If you decide the change is for you, details of an inexpensive do it your self design follows.


by Bill Harvey

BAR ASSEMBLY The extender bar can be either 1/2" or 3/4" steel bar stock 2 1/2" wide by 6" long. The side bars should be 1/2" square bar stock. These are welded to the fiat bar along the outer edge and ends. It is recommended that inner welds on alternate pairs be made only along that portion of the extender where the airbag will be bolted - not the suspension arm. The reason for this becomes apparent during installation. (The purpose of these 1/2" bars is not to stiffen the extender. Rather the bar keeps the extender from rotating about either the arm bolt or the airbag by fitting close to the outer edge of the arm.)
Slightly oversize holes (25/32") are drilled as shown to accommodate the airbag fitting and the 3/4" bolt to the arm. Note: These holes are not centered on the extender. As an additional precaution, it may be worthwhile to measure or use a paper pattern to verify that the arm will fit between the hole and the 1/2" bar before drilling. The pattern shown appears to provide adequate clearance, however.
ASSEMBLY The head of the 3/4" bolt must be ground off along one edge to BAG fit within the web of the airbag cone. It should be ground off enough to permit the arm to be rotated from side to side when snugly bolted to the cone. With this in mind, the extenders should be bolted to the airbag with the 1/2" bars to the outside for the installation on the coach.
Both extenders should be attached firmly, but not tightened, with the bars to the outside, and the air inlet fitting to the right on both bags, with the original star washers and 3/4" nuts. To be sure that the extenders are mime with each other, it is recommended that the bag with extenders be stood up on the extenders on a flat surface. If the assembly rocks, the airbag nuts should be loosened and the extenders rotated so that both stand fiat on the surface. (This is why it is necessary to grind the bolt heads down so that they can be swung from side to side within the cone web.)



At this point it is only necessary to reinstall the assembly into the suspension arms with the 3/4" bolts in the arm holes and secured with the 3/4" nuts and star washers. It has been recommended that these nuts be tightened well beyond the specs for the airbag fittings of 50 to 70 ft lbs; however, this seems extreme since the 1/2" bar will keep the assembly from rotating. It is hard to get much more torque than this in that area. It is recommended that these nuts be tightened as tight as practical without rupturing either yourself or the airbag cones. (Only aluminum airbag cones should be used with extenders, but everyone is probably using aluminum anyway.) Alignment of the extenders can be checked while tightening everything to spec. (This alignment is mentioned since it seems a good idea not to twist the bag and add any torque to the bond of the airbag to its fittings.)
TOOLS Unless airbag isolator valves have been installed, the only tool required for this is a 1 1/8" socket with drive for the nuts. However, either vise grip or water pump pliers may be necessary to hold the bag cone when removing the nuts originally. A little WD-40 on the nuts before starting will help also.
COST Fabrication by pros may be beneficial unless one has a halfway well equipped machine shop. Most welding shops should have the steel stock and can cut, drill and weld accordingly for $25-$30. Bolts, nuts and washers arc going to cost another $5-$10 so the total, not counting installation time, should be less than $40.
DISCLAIMER What you do to your coach is your business and no responsibility is assumed for the results, good, bad or otherwise relative to this project.


Materials List

4 pcs 0.50" x 2.50" x 6.00" steel stock
4 pcs 0.50" x 0.50" x 4.50" steel bar
4 ea. Grade 5 0.75" dia. x 1 1/2" SAE bolts
4 ea Grade 5 0.75" SAE hex nuts
4 ea Grade 5 0.75" star lockwashers


 Tech Info