I realize that this is a long thread but if you are pondering the Gasoline versus a Diesel based on power consideration or are wondering about adding an after market performance enhancing kit to a Gasoline engine to close the power equation it might be worth a read. This thread is intended to be an addendum to the First Time Buyer... sticky.
The question of a Gasoline versus Diesel is a topic that seems to have no correct answer. But one that comes up frequently Here is some food for thought, after you have done some looking post questions about specifics and you will get plenty of feedback.
Diesel can have more torque and HP but that does not always translate to better performance and higher MPG. There are some configurations of Gasoline powered Motorhomes that will out perform some Diesel coaches due to weight to power ratio, gearing, rear axle ratio. There are some diesel powered configuration where power is not an issue with any grade or load.
In terms of reliability Diesel engines are constructed to have a longer life span but for most folks and I mean most, the reliability and longevity is sort of a non issue due to the amount of mileage and the length of ownership. Diesel's will be somewhat more costly to maintain.
The longer and heavier a coach is the more need there is for power. There is also the issue of carrying capability. So you will want to focus in on what size motorhome then look at the weight carrying abilities of your choices.
The modern Gasoline power trains and chassis have evolved tremendously in the past several years and the expectation that they will continue. If there is a Gasoline powered motorhome that meets your fancy it is worth a long hard look.
There are some benefits to going diesel powered other than fuel mileage ( which is not usually anything to write home about) A DP pusher will generally be quieter in the cockpit area when underway and will have the benefit of air suspension and Brakes.
Another minor benefit of a DP is that in many cases A DP will have a much longer range between fuel stops due to the size of the fuel tanks.
If you are looking at a length of the 35 or less then I would be looking at A Gasoline and over that you are in sort of a mixed bag up to about 38 feet then you really want to be leaning in the direction of a Diesel simply because of the torque to handle the weight.
You will need to sort out what you are looking for in terms of length and CCC plus the price range you are wanting to be at. A diesel will be a significant increase in price. Drive both and then figure out if the added cost is worth it to you.
So what does this all mean?
Some gasoline powered coaches will be challenged going up some steep or long grades but then so will some diesel powered coaches. The real question is so what? How much time will one spend going up steep grades?
There are aftermarket kits that can enhance performance such as the Banks System or the gear Vendors add on to the none Allison transmissions that adds extra gears. The Banks after market performance enhancements are available for both Gasoline and Diesel powered units. I have installed both in the past and still have a F350 4x4 that has both the Banks system and the Gear Vendors. The addition of a Banks or a competitive alternative will yield additional available power both in terms of Horsepower and Torque. The question of whether the additional cost of these systems will yield enough performance enhancement of warrant the cost.
In looking at the performance curves relative to the 8.1 on the Banks web site it should be noted that for an 8.1 the optimum gain is at a fairly high RPM which should be taken into account relative to cockpit noise. Using the most optimistic gain estimates you could be looking of 16 to 23% depending on rpm's in terms of torque. The RPM range will be between 3800 and 4800. The most material gain will be on acceleration especially on on ramps and passing.
In terms of pulling power up grades there will be improvement there as well. But lets compare getting 25,000 pounds up a grade with a banks enhanced gasoline 8.1 with getting 30,000 pounds up the grade with a Cat 330 powered diesel. Looking at the chart below the 8.1 with the Banks system will have to move 45 pounds for every foot pound of torque versus the Cat which will have to move 35 pounds for every foot pound even at the heavier weight. You can also see on the chart below that at the same weight the diesel will have even a greater advantage. However that is not the end of the story.
The W24 chassis comes with a 5.86 rear ratio which is nominally in the range of 20% higher than the typical rear ratio of a cat 330 equipped chassis so the 29 percent more weight per foot pound of torque available with the cat 330 is down to about a 9 percent difference in power to move the weight provided the gasoline unit is 5,000 pounds lighter. If the same weight then the cat will have about a 25% advantage in the ratio of weight to torque to move it even with the performance enhancement on the Gasoline engine. How much difference will this make to you? The only way to answer is to drive the same course with the two different power trains and see for your self.
Keep in mind that you will more than likely be testing a stock Gasoline engine and that adding a Banks or similar performance enhancing package will yield somewhere in the neighborhood of a 20 % improvement. What this means is that the performance enhancement kit will offset the weight that you are going down the road with towing a dinghy and with all your stuff on board. So if the performance suits you and you like the rest of the coach the Gasoline powered motorhome ought to be on your short list since it is possible to add the performance enhancement kit. The added expense of a performance enhancement kit will be far less that going the diesel route. The diesel as tested will be just about as good as it will be so keep in mind that the added wieght of towing a dinghy and carrying all your stuff may impact the performance you experienced during the test drive without the additional weight.
So the answer is yes these after market systems will help off set the power differential between a Gasoline Powered versus a Diesel Powered unit.The difference in actual performance getting that weight up the grade while not be as much as the chart would indicate since the Gasoline powered coach will have a higher rear end ratio and the higher revving gasoline engine to be able to take the advantage of the higher rear gearing.
One other issue to take into account with a normally aspirated engine (non turbo) is that the air density will have an impact on the power generated by the engine. "On a hot day, or at high altitude, or on a moist day, the air is less dense. A reduction in air density reduces the engine horsepower." Link So on those hot muggy days a non turbo engine will have less power than on a cool dry day. Adding altitude to the equation will also rob power on non turbo assisted engines. Virtually all modern diesels in today's motorhome will have a turbo as standard equipment. The power loss in most situations is not really significant. The loss is in the order of 1 % per thousand feet up to three thousand feet then about 3 % per thousand feet there after.
There is one advantage to a diesel worth considering and that is the auxiliary braking that is available via either an exhaust brake or an engine brake. To my way of thinking the availability of auxiliary braking is a far more important advantage than any extra power. Having said that, the perspective I have is that at times it is convenient and others it is a margin of safety that I really appreciate. The thing to keep in focus that it is possible to adjust ones driving pattern to proceed in a safe manner without the need or desire of an auxiliary brake.
A diesel powered coach with a power to weight ratio that is superior to a gasoline powered unit will be able to pull grades at a faster rate and descend down grades more aggressively if equipped with an auxiliary brake. An engine brake being superior to an exhaust brake. But again the question is so what? Just what percentage of the on the road time will be spent going up and down grades where this is really a material issue?
There are Gasoline coaches that will out perform some diesel coaches in terms of pulling grades. As I mentioned the longer and heavier a coach gets the more that the need of a diesel comes into play. In this case the word need is a euphemism for desirable. Feeding that desire does have a price tag. The issue is determining the need based on the size coach and then working through the coast versus the benefit.
The only way to know is to figure out what you want in terms of size and floor plan which will include the added weight of sliders. If you are a driver instead of a parker and you want a large coach then a diesel may be the better choice if you are a parker then it may make more sense to go with a Gasoline powered unit in terms of price.
Our needs were such that a 40 footer was our choice which got us into a Diesel. For our traveling partner a 32 footer Gasoline unit is the perfect size he is as happy with his choice as we are with ours. We have traveled together extensively in mountainous country in all but the steepest of grades he is able to maintain the same speed with his vortec 8.1 as we are with our ISL 400. While I think he would like to have a larger fuel tank and would benefit from an auxiliary brake he is as happy with his coach as were are with ours. Our ride may be a little smother due to the air ride and the cockpit noise level lower when climbing grades but to him those issues are not enough to justify the price differential of his coach versus ours.
With the innovations in chassis design of gasoline powered coaches and the addition of 5 and 6 speed transmissions given the price differential of going the diesel route I would look long and hard at the gasoline powered coaches. The only way to know is to pick your size and floor plan then do a test drive.
One thing that you will find is for the most part all of us like what we have but there is always something "better". I would discount those who disparage one type versus the other... Or makes extravagant claims not supported by any logic of the physics involved in moving weight. But I would pay attention to those that take the time to go beyond this is bad and that is good.
I also think that we at times get lost in generalities so I would suggest that you get in the ball park with generalities but focus on specifics when you are in decision mode.
In general a diesel pusher will be quieter in the cockpit area, will offer a smooth ride due to air suspension. Some diesel pushers will offer pass through storage. Most diesel pushers will have some form of auxiliary braking either in terms of an exhaust or and engine brake. (Diesel Engine auxiliary Brakes)
In general At some price point the interior fit and finish of a Diesel Pusher will be a step up... But for the upper end of the Gasoline powered units and the entry level diesel and up to some point along the DP price curve I do not believe there is much if any difference.
In General the more expensive DP's will offer additional amenities beyond what is available on Gasoline powered motorhomes.
In General Diesel pushers will have a longer range due to larger fuel tanks.
I'm a little surprised that other members haven't replied to this thread, but that is probably because you did such a good job of writing it. You really didn't leave any stone unturned. I read it and was saying to myself, yeah, yeah, right, right, yep, and on and on! I drove the Freightliner "FRED" today and I think it fits somewhere in between the gasser and DP. You'll be able to buy a class A from 34-39' on the Fred and all should have between 2-4 slides. Kevin Cook (AAFD) and I are interested in the Allegro Bay 37QDB/FRED. We're (Joanne & I)leaving for Grand Junction, Colorado a week from now and like you mentioned, we should have no problem traversing the mountains on I-70 with our Banks equipped W-22. I feel that both gasoline powered and diesel powered class A's have continued to evolve as a FIRST class way to travel and vacation. I know that my wife and I are eagerly looking forward to our 5,100 mile southwestern loop that will last (19) days. I wish I could write as well as JohnnyT. Sincerely, Chuck/Bagman.
Well stated JohnnyT. I also agree with all of your points though I couldn't write them as well. I could add that the Cummins ISB 5.9 engine falls at about 550 ft-lbs of torque just where the Banks equipped Chevrolet 8.1 engine is in your chart. The chart seems to dispel a lot of smoke about various engines since it is torque that you need to get you down the road and up the hills. It doesn't necessarily get you off the line as fast since you are not in the torque band if you have a diesel.
Great job at pointing out the facts as opposed to the hot air & egotistical replies that I have seen touting their own personal performance. . .bottom line is that no one size fits all be it engine, transmission, length or color and there is some thing for everyone out there.
I just cringe some times when a newbie asks for recommendations and only gets one side of the story without facts for him/her to make a decision by.
You are a fair person & we are lucky to have you as a moderator.
If I might add to your thoughts, it would be that diesel engines and chasses, being heavier built, last longer. Hence, the best value in used coaches is in DPs: a 100K mile DP, if it was a quality coach when new, will last longer than the 2nd and 3rd owners if properly maintained.
I think you've got a problem with defining the issue in terms of torque, despite what some may say. First, horsepower is defined as lifting 33000 lb-feet per minute which dimensionally is equal to moving a weight up a hill at some speed. So horsepower is whatmoves the weight. The other fact which seems to escape some people is that there is gearing in between the engine and the rear wheels, which multiplies torque. So, for example, if I am driving a diesel with 1000 lb-ft of torque at 1000 RPM, I apply that through a set of gears such that I turn the rear wheels at 60 MPH. Now, if I have a gasser that delivers 500 lb-ft at 2000 RPM, the gearing will be such that the rear wheels also turn at 60 MPH. Assuming the same wheels and tires, the gearing will be twice that of the diesel, and the torque will be multiplied by a factor of two, thus giving the SAME torque at the rear wheels for both the gasser and the diesel. (Note that because HP is torque times RPM, both engines have the same HP at the defined engine speeds). Why do people prefer the diesel then?- because it's running at half the speed and thus doesn't seem to be working as hard. It's even more obvious if you've got a gasser running flat out at 4500 RPM right under your feet. It's also obvious that if you have a 500 HP diesel it will climb better than a 300 HP gasser.
It is good reading, and I know torque is important for driveability, but the point I often make is that an 8.1 has 400 ft lbs of torque (455 peak) from about 1500 rpm to 4000 rpm. Put a 2:1 gear reducer at tailshaft of engine and you have minimum of 800 ft lbs from 750 rpm to 2000 rpm. Now you have a power plant that mimics the C-7,with max torque of 920 ft lbs. I know fat torque curves make for easy driving and minimum shifting, but I still look at horsepower if you want to judge hill climbing ability, acceleration, etc. Everyone says it's torque that gets you up the hill, but I say it is horsepower and proper gearing. I have a 36' unit and tow and am at maximum total weight of 26000#. I have been all over the US and Canada and have never had a problem. I have gotten down to 35 mph at altitude on steep grades a couple of times, but I didn't see any diesels whipping up these grades either. I also will be the first to say that if I went any larger, I would get a diesel (at least 370 horsepower) for many of the reasons you mention. But I think there are a lot of folks out there who drive diesels who would be better off with gassers and vice versa. You make many good points in your post--great reading.
2007 Allegro Bus QSP
06 Volkswagen Jetta Diesel