Diesel vs gas

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oldwildman

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Feb 17, 2024
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Butler PA
I've posted about this sort of thing before. It was decided that diesel is preferable. I get that. But, I'm having some issues as I shop around. The v-10 gas has around 360 hp. I've seen diesels 325, 350 and 400 hp. Looking in the 35-40 class a range. Opposing slides. Some more than 400 but they're usually the ones that are too large for us. We're planning on towing a car and dolly weighing around 4500 lbs max (3500 when we take her car). I see that the diesels are built heavier and ride better. But, we're not putting on a lot of miles. Planning on around 8-9000 the first year and probably 3000/yr after that.

So, if I go diesel, should I limit my search to 400 hp or more? 350 would be the equivalent to gas hp but isn't the diesel chassis heavier?

I understand that diesels have more torque but turn slower. So, if my math is correct, the net power to the wheels is the same. Or do I have that wrong? My main concern in this thread is the towing capacity.

We're looking in the 100k price range so it's going to be an older used one.
 
I agree, find a unit you like. As far as gas vs diesel power, horsepower is the product of torque x RPM. Gas engines make their maximum HP at very high RPMs so you have to discount that figure when comparing engines unless you plan on driving with the engine turning screaming fast. Large diesel engines turn slower so even though they may have less rated HP they make that power at more usable speeds as indicated by their higher torque values.
 
So, if I go diesel, should I limit my search to 400 hp or more? 350 would be the equivalent to gas hp but isn't the diesel chassis heavier?
You shouldn't limit it to any particular number. What you should focus on is having enough horsepower to do the job adequately. The rule of thumb for powering motorhomes is 1 hp per 100 lbs. That's "adequate" but not great performance. In comparison, your car or SUV probably has more like 18-25 hp per 100 lbs. When you are shopping you won't be sure what the ultimate weight will be as you travel, so use the motorhome GVWR as your weight measure. And if you will tow a car, add that in as well.

I understand that diesels have more torque but turn slower. So, if my math is correct, the net power to the wheels is the same. Or do I have that wrong? My main concern in this thread is the towing capacity.
Yes, horsepower is horsepower. Gas engines rev higher to produce the rated power, whereas diesel have it all on tap at relatively low rpms, so it's more readily usable. Accelerating and hill-climbing are the power-challenges for a heavy motorhome and that where a diesel's low end hp shines. But winding up a gas engine under those circumstances isn't a big deal either - just a brief lag in response.

Yes, diesel chassis are near-always a 2000 or lbs heavier than an equivalent gas chassis, but in most cases the diesel will also be heavier duty with a superior suspension, air brakes and other quality features. So typically they end up being quite a bit heavier (bigger GVWR) and thus warrant more horsepower to move them around. Again, buy enough horsepower to do the job. Maybe something like this one.

You should find a few diesel motorhomes that are essentially the same as a gas motorhome but with a diesel engine. These are called FREDs, FRont Engine Diesels. Class A FREDs aren't very popular because they offer only a small performance advantage over gas. You need to get into a diesel pusher (rear engine) with air suspension & brakes and a heavy duty chassis to begin to fully realize the comfort & capability of a diesel coach. The more popular FRED is the Super-C, a large class C motorhome built on a medium duty diesel truck chassis-cab. Take a look at some - maybe they will suit your needs. Maybe something like this.
 
My main concern in this thread is the towing capacity.
For me at least, the key was my financially secure budget. I bought a gas chassis class A for the very reason that I have driven small, fuel efficient cars rather than a Cadilliac, Lincoln, BMW, or Mercedes. My gas motorhome towed our automobiles well and it got us everywhere than even the most impressive diesel pushers could go, and probably a few that they couldn't because of the increased length. We may not have been first to the top of most mountains, but we were never more than a few minutes behind them and at far less cost. With the prices of fuel the gas chassis is also lower in fuel costs as well. We owned our big gasser for 14 years and were fulltime in it for 12 years. As nice as it may be to ride in a Lincoln, I'm going just as far in my VW at a lot less cost per mile, just was was true when we traveled in a class A. If you just want to tow a small car the gas chassis will be just fine but if you want to tow a 10,000# or more cargo trailer, look at getting a super C with a big diesel engine.
 
From post #4;
" The rule of thumb for powering motorhomes is 1 hp per 100 lbs. That's "adequate" but not great performance."

That math seems really off ...
A 8,000 lb. unit would need 80hp to be adequate ?
 
oldwildman, this is an excellent tutorial explaining horsepower and torque: What's the difference between torque and horsepower?
You mentioned the Ford V-10, which is a solid reliable engine; the new Ford V-8 is an entirely new engine and received good reviews.
A diesel engine is designed for heavy, continued work. If you aren't planning to drive 10,000 miles a year, IMO a gas engine would be the better choice today.
Most gas engine Class A MH chassis are rated to tow 5,000-8,000 lbs., with a Class C MH rated at 5,000 or less towing capacity.
A gas chassis will be supported by leaf springs unless the owner changes to Liquid Springs. It will have a harsher ride because it is actually a medium-duty truck chassis. This chassis is cheaper to maintain than a diesel pusher MH on air springs and with air brakes. For instance, a gas engine oil N filter change is at most $150. A diesel engine oil N filter change is $450-$550; both at commercial businesses.
A DP rides much smoother and is quieter due to the driver and passenger being over 30' from the engine.
Interior care/coach care is a wash for both chassis.

On the subject of towing, carefully consider using a tow-dolly vs 4-down towing. What will you do with the tow dolly at an RV park? Some have storage lots with a cost per day/week. If the park doesn't have a storage area, what then; well you must hand push it to the back of your site and place it so it's not in your way or blocking something.
If you stop for one night and want to go out for dinner, the car must be unloaded, dolly stored out of the way. Next morning you must re-load the car on the dolly. Yes towing 4-down also required unhooking, then hooking up next morning too, but no tow dolly to hook to the MH, then finding a place to load the car.
A tow dolly and a tow-bar+car base+plate(installed) are about equal in cost.
It's desicion time for you both.
 
@Gary RV_Wizard already mentioned it but 1hp per 100 lb is about right.

The max gross for my MH is 26k lbs and we tow a 4k-ish car. Our actual weight on a scale is 23,000 lb, so we have just under 30k lb going down the road with a 300 hp engine.

We've never had an issue merging onto a 70 mph interstate with a short ramp from a rest area. We've never been out west but it can handle the grades in the Smokies with no problem.

I think the "rule" scales rather well and you would be OK with 400 hp @ 40k lb. For a reference, we get 10 mpg which I think is very good for a DP.
 
@Gary RV_Wizard already mentioned it but 1hp per 100 lb is about
@Gary RV_Wizard already mentioned it but 1hp per 100 lb is about right.

The max gross for my MH is 26k lbs and we tow a 4k-ish car. Our actual weight on a scale is 23,000 lb, so we have just under 30k lb going down the road with a 300 hp engine.

We've never had an issue merging onto a 70 mph interstate with a short ramp from a rest area. We've never been out west but it can handle the grades in the Smokies with no problem.

I think the "rule" scales rather well and you would be OK with 400 hp @ 40k lb. For a reference, we get 10 mpg which I think is very good for a DP.
Thanks! Gary mentioned that formula in a previous thread and I'm glad it works. We're going for a somewhat longer model than yours but it looks like I won't turn my nose up at the lesser powered diesels. I'll have to give them a try.
 
oldwildman, this is an excellent tutorial explaining horsepower and torque: What's the difference between torque and horsepower?
You mentioned the Ford V-10, which is a solid reliable engine; the new Ford V-8 is an entirely new engine and received good reviews.
A diesel engine is designed for heavy, continued work. If you aren't planning to drive 10,000 miles a year, IMO a gas engine would be the better choice today.
Most gas engine Class A MH chassis are rated to tow 5,000-8,000 lbs., with a Class C MH rated at 5,000 or less towing capacity.
A gas chassis will be supported by leaf springs unless the owner changes to Liquid Springs. It will have a harsher ride because it is actually a medium-duty truck chassis. This chassis is cheaper to maintain than a diesel pusher MH on air springs and with air brakes. For instance, a gas engine oil N filter change is at most $150. A diesel engine oil N filter change is $450-$550; both at commercial businesses.
A DP rides much smoother and is quieter due to the driver and passenger being over 30' from the engine.
Interior care/coach care is a wash for both chassis.

On the subject of towing, carefully consider using a tow-dolly vs 4-down towing. What will you do with the tow dolly at an RV park? Some have storage lots with a cost per day/week. If the park doesn't have a storage area, what then; well you must hand push it to the back of your site and place it so it's not in your way or blocking something.
If you stop for one night and want to go out for dinner, the car must be unloaded, dolly stored out of the way. Next morning you must re-load the car on the dolly. Yes towing 4-down also required unhooking, then hooking up next morning too, but no tow dolly to hook to the MH, then finding a place to load the car.
A tow dolly and a tow-bar+car base+plate(installed) are about equal in cost.
It's desicion time for you both.
I thought I replied to this but the cyber gremlins seem to have eaten it. LOL

We've discussed 4 down towing. I'd prefer it. But, with us, that would require buying another car that would only be used a few months a year. A standard transmission is out, the boss has enough issues with an auto. LOL. And of the automatics that can be flat towed, none are what we would want to use full time. I have a hitch on mine and we'll be putting one on hers for the bike rack. That will help moving the dolly around. We can try it at the beginning of the adventure and we can always revisit the subject.

The harsher ride might be an issue for Deb. But I've been driving a contractors truck around all my working life. Don't think it's going to be an issue with me. We'll know more once we start test driving later this year.
 
Our first coach was a 38’ gasser with about 340 hp. Weighed about 25,000 lbs and we towed a car that weighed around 3,000 lbs.

Our current coach is a 44.25’ diesel pusher with about 410 hp. Weighs about 43,000 lbs and we a car that weighs about 4,300 lbs.

Using @Gary RV_Wizard ’s formula, you can easily tell our 1st rig was faster off the and line climbed hills faster. But both rigs always got us where we pointed them; but the diesel is a much more comfortable ride… quieter and smoother with engine in rear and air suspension.

My suggestion: if you’re up in the air between gas and diesel, go with diesel… DON’T worry about the hp and torque. It’s not a race. The goal isn’t to get up the hill first. The goal is only to get up the hill.
 
From post #4;
" The rule of thumb for powering motorhomes is 1 hp per 100 lbs. That's "adequate" but not great performance."

That math seems really off ...
A 8,000 lb. unit would need 80hp to be adequate ?
Probably not valid for that low a weight, but good enough for class A and larger C's. Say 20,000 lbs & up? Remember, it's a "rule of thumb", not an iron clad law.

I suspect 80 hp might actually move an 8000 lb vehicle sort-of ok. That's about the weight of a 3/4 ton pickup or a E350 van without a RV body. Wouldn't be exciting, but neither is a 30,000 lb coach with a 300 hp engine.
 
The new Ford 7.3 'Godzilla' V8 is an awesome engine for mid sized coaches. One of my friends has one in a large-ish class C and he has no problem going up hills. It doesn't rev as high as the V10s either.
 
The new Ford 7.3 'Godzilla' V8 is an awesome engine for mid sized coaches. One of my friends has one in a large-ish class C and he has no problem going up hills. It doesn't rev as high as the V10s either.
I've heard nothing but good things about it.
 
Probably not valid for that low a weight, but good enough for class A and larger C's. Say 20,000 lbs & up? Remember, it's a "rule of thumb", not an iron clad law.

I suspect 80 hp might actually move an 8000 lb vehicle sort-of ok. That's about the weight of a 3/4 ton pickup or a E350 van without a RV body. Wouldn't be exciting, but neither is a 30,000 lb coach with a 300 hp engine.
Just for giggles, I googled the hp and gross vehicle weight of a VW Beetle. I figure that would constitute 'adequate' power. Hp-43, GVW-4200. Looks like the formula works. :)
 
Just for giggles, I googled the hp and gross vehicle weight of a VW Beetle. I figure that would constitute 'adequate' power. Hp-43, GVW-4200. Looks like the formula works. :)
I’d question that GVW, 4200 seems a bit heavy for a big.
 

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