Choosing Gasoline or Diesel Powered?

[JohnnyT]

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.




Torque.........455.. 560... ..660.....860.. 1050.. 1200

Weight
20,000......... 44... 36........30........23......19......17
25,000......... 55... 45........38........29......24......21
30,000......... 66... 54........45........35......29......25
35,000......... 77... 63........53........41......33......29
40,000......... 88... 71........61........47......38......33
Pounds to move per increment of torque

455 = Stock 8.1 Torque
560 = Banks equipped 8.1 Torque
660 = Cummins ISB 300 Torque
860 = Cat 330 Torque
1050 = Cummins ISC 350 Torque
1200 = Cummins ISL400 Torque

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.

Here is a thread The power equation...Horsepower- Torque -Gear Ratio- Weight for those that want to ponder on this issue further.

JohnnyT

[RaeDar]

Thank you JohnnyT for taking the time. We currently own a TT and are seriously considering purchasing a motorhome. You insite and experience and the sharing of thereof, is greatly appreciated!

[RV_CONUS]

First of all, thanks for sharing the great knowledge and opinions. This thread has been extremely helpful...

We have always had gassers, because, basically, I don't know anything about diesel.

After reading, and reading, and reading some more, looks like we are going to take steps to go diesel.

Mainly what I do know mirrors MRUSA's thoughts, and what I do think I know. This whole thread has been extremely valuable. Thank You

Anyway, thanks for providing clear and concise information regarding diesel and gassers.

[mountainkowboy]

Great thread, although I thought that you left one thing out on the gasser side. Since you were using the 8.1 for your comparison the Allison trans has a grade brake that is simular to an aux brake on a DP. I have an 05 W22 with the grade brake and while it isnt an engine brake it does a very good job of mimicing one. I live in the mountains and drop 5000' in 18 miles when we go on an outting and the grade brake keeps our MH at 40-50mph without using the brake pedal at all.
We considered a DP when we bought our MH but couldnt justify the extra cost for our needs right now. Maybe when we retire, but if things continue to improve at the present rate it will be interesting to see what the RV companies come up with in 10 years!

[Boonedocks]

One other diesel benefit I would add is that generally the generator is up front, not loud and vibrating away under the bed.

[MRUSA]

There are numerous ways in which a Class A diesel is better that are unrelated to engine power or fuel economy. A diesel is capable of running many hundreds of thousands of miles. With good maintenance, a diesel engine will outlast several gas engines. A diesel chassis will usually ride smoother, handle better, be quieter and cooler up front. The diesel will generally have larger holding tanks, more basement storage, and higher carrying and towing capacities. It will likely have a more powerful aux generator and more or better amenities than a gas coach. So aside from the issues that have been discussed here of power, maintenance costs, fuel mileage etc., remember that comparing a diesel with a gasser is kind of like comparing a Cadillac with a Chevy. Both are good vehicles, but the Caddy offers more at a higher price.

[RangerPaulS]

I am SO glad I read this thread. My wife and I are beginning to look at MH's and I was sold on the diesels for all the usual reasons, but this gives me a lot to think about. One thing that comes to mind though is, while it's true that one may not spend a majority of one's driving going up steep grades, for every up, one must also come back down. I'd like to experience that part of the drive with the same amount of control as the drive up, so, do the newer gassers have some kind of supplemental braking? One poster eluded to that on a Ford using the tow/ haul mode, but I'd like some more information on that aspect of gasoline powered rigs in general.

[427435]

Some good information here although you put too much emphasis on the torque of an engine. It's been said before, but I'll say it again, its the hp that gets things up a mountain. You can have 10,000 ft-lbs of torque but, without rpm, you won't move. Once you do have both rpm and torque, you have hp.

That being said, a diesel with similar (and maybe even poorer) hp/weight ratio will likely climb a mountain better. That's due to 2 reasons:

1. The diesel is turbocharged and loses much less of its hp as it climbs over 3000 ft compared to a non-turbo gas engine.

2. A diesel's peak hp usually falls between 2000-2400 rpm----an rpm that owners are comfortable to run their engines at (especially when the noise is in the back). A gas engine's peak hp usually occurs over 4000 rpm---which gets noisy in a front engine MH (especially when the fan kicks in). As a result, many gas MH drivers don't keep their engines in their peak hp range while a diesel driver likely will----leading to the diesel (and it's turbo) climbing the mountain faster due to greater actual HORSEPOWER being delivered to the drivetrain.

[Farmerjones]

i think a guy can get lucky sometimes too: my 35' w/ford V10 must've been made on a Wed. The guy with the class C w/V10 that couldn't keep up? Gearing out @ 84? I don't know how fast i can go, i back out @ 90.
I drove ready mix trucks in a past life so i've dealt with a diesel most of my life. I chose not to take the ding for the diesel power, and i don't miss it. I do miss air brakes. I don't miss fuel gelling, water seperators freezing. There was a time when gas engines didn't have electronic fuel injection. There was a time when diesel's didn't have electronic controllers. There's never been a better time to pick either one. No matter what your fuel, these new engines run cleaner and less trouble. You can bet i wouldn't have owned a coach with a carburator. Heck, nowadays diesels don't regularly consume oil either. Let's argue something easy like 5ers vs. Class A MHs. 🙂

[markbento]

Johnny this was a great post but can I ask why just the chevy 8.1 and not also the ford v10? it would have been nice to see how it can out in this fight. and no I am not a chevy v ford guy as a matter of fact my cars are almost all GM a Pontiac grand am se GMC Envoy XUV Caddy DTS, Jeep Wrangler toad and the winne on the ford f53 with the v10. does anyone know the numbers on the ford to be added to that chart that Jonney has.

Thanks for the great post just would like to see the whole lot of numbers if poss.

Mark

[zoondoggie]

Thanks for taking the time to lay out the advantages & disadvantages of both gas & diesel. You also clearly describe the extra cost involved in the ownership of a diesel. Could you give the average ( ballpark ) added amount involved. Do we have statistics on that?

Thanks again:W

[Cousin_Eddy1]

Chuck,

I believe you are correct. It was the pedal position sensor that konked out. This is actually the suspected problem. The incident occurred over two weeks ago. The coach is still in the shop waiting to be looked at. (Another consideration with diesels) So it hasn't been confirmed. This is incident very scary to me. $120K for a coach, and this happens. I feel bad for my neighbor.

[chuck4788]

I think you mean throttle pedal position sensor and most new gas engines also use them, I have a 2001 car with that set up. The ECU controls the thottle opening (or fuel flow on a diesel), not what the operator selects with the throttle pedal. I like the setup in my car because it seems to be responsive to not only throttle position is but also how fast I move the throttle, and on a 290hp V8 the gradual take up from idle makes it as easy to drive in the snow as a car with a much smaller engine.

[Cousin_Eddy1]

I have thought a lot about diesel vs gas. This is a great post. The question I ponder on diesels is reliablity.

The diesel rigs seem much more complex. My neighbor just bought an '04 Monaco Diplomat under 10K miles. It konked out on the maiden voyage. Throttle position sensor died. Had to be towed. Over $1K tow bill.

I understand that you get more longevity, but what about general reliablity?

[stevelv]

Generally speaking, yes. It should be easier to sell and depreciation should be better BUT selling anything is fickle and there are no hard and fast rules. You just need the right buyer at the right time.

[woolfy]

Were in the process of making the decision of gas vs diesel in a 35' to 37' Allegro Bay. And one of our thoughts is re saleability would a diesel sale better than a gasser when the time comes.

Richard