Potential Class C MPG with cam'd 454 instead of stock 400

[MudiGGer25]

Ok first off i know its a MH and i know its probably gonna get crappy MPG regardless, we just have a 2300 mile trip coming up in 5 weeks and I am trying to plan out stops for fuel since the tank only holds 33 US gallons...

Quick info on the rig
1981 25' Class C Fleetwood Tioga
454/TH400 4.10 14FF 8.75-16.5LT's
11 ish feet tall and about 7ish feet wide.

Dry weight around 7,800, GVWR 10,500 and will be pretty dang close to it when loaded for the trip.

Towing a 1988 V20 suburban 4-down weighing 5,000lbs on its own and will have a lawn tractor and its trailer in the back of it.

The 454 was cam'd, and presumably cleaned at the time based on how clean the internals are when i pulled the intake off to inspect it.

Has a double roller chain and steel top gear. Absolutely NO specs on the cam whatsoever, was built for a mudding truck so probably a RV cam grind which would be fine anyways.


Our 1977 midas class C which was almost identical in every aspect would get 12-14MPG under 60mph not towing and 10-13MPG towing another camper behind it at 55MPH or less. It actually seemed to like having the trailer back there i think due to the wind rolling off the RV down across the trailer. the only difference was the 77 had a 400ci stock, and all i did was increase the timing as far as was safe to do, and usually ran premium fuel.


Assuming a mild cam grind with peak TQ ending up somewhere near the hwy cruise speed what do ya'll think I can expect for MPG? I would assume that by having more power than stock, the throttle can be closed more and thereby decrease fuel consumption (if that is even plausible on a 454 lol)

here are my approximate RPM at each given speed on the interstate given some TC slippage

2100RPM @45 mph
2300RPM @50 mph
2500RPM @55 mph
2800RPM @60 mph

and just for the Sh*** and giggles because i wont be sustaining these speeds in a RV except to pass or merge

3000RPM @65 mph
3200RPM @70 mph
3500RPM @75 mph
3700RPM @80 mph

ANNNDDD

4200RPM @90 MPH



Ok now that the #'s are there, If i install a "econo" meter on my dash (a vacuum gauge hooked directly to the carb), From what I have read, once I reach my "crusing" speed the goal is to keep my Vac reading as high numerically as possible without losing momentum, is that correct?

Also at my peak TQ/shift point, the Vac guage will jump up just a little???

If i understand this correctly I will be adding one to the RV to avoid unnecessary fuel use on the highway.


The toad tires will be inflated to 75PSI for minimum rolling resistance.

One more thing, but not until probably next spring, Could i change the ring and pinion for example to 3.73 or 3.4X to lower the RPM once cruising, i know it would cause a little more TC slip in town but in town driving would be maybe 5% as opposed to hwy.

Other than making me a little slower off the line, I dont see how its any different than having an O/D lower my RPM on the highway to save fuel

[OLYLEN]

So you have a class C towing a heavy load. 6.5-7 mpg. Fuel every 200 miles and carry a 5 gallon Jerry can. If you get better great.

LEN

[George Schweikle]

I put 97,000 miles on a 23 Ft. 1976 Midas Mini with the small block 400, and never saw anything near the mileage numbers you quoted. I learned to drive with a feather foot on the gas and was sure to have the thing well tuned by a commercial shop that knew how to set timing for peak performance with regular gas. Also had a cold air intake. Maybe your using premium fuel really made the difference or, in spite of my attempts at economical driving, our driving styles were significantly different. And, I don't think the octane rating of today's fuels are as high as the 70's so your new engine can't have as aggressive ignition timing. I know when regular gas went from 89 to 87 octane I had to back off the advance of our 1983 carburated 454 Winnebago, and the mileage suffered.

Whatever, keep us posted on your adventures with the big-engined Tioga. It sounds like a fun project

Originally Posted by MudiGGer25

....Our 1977 midas class C which was almost identical in every aspect would get 12-14MPG under 60mph not towing and 10-13MPG towing another camper behind it at 55MPH or less. It actually seemed to like having the trailer back there I think due to the wind rolling off the RV down across the trailer. the only difference was the 77 had a 400ci stock, and all i did was increase the timing as far as was safe to do, and usually ran premium fuel....

[lanerd]

I think that Sub will weight more than 5000 lbs....have you ever weighed it? And adding a lawn mower plus trailer in the back....

Your Class C will only have a MAX tow rating of 5000 lbs...maybe less. Do you know what it is off hand?

Loading the C to max (say around 10K lbs) plus the (at least) 5500 lb toad, I suspect that 454 will only get around 6 or 7 at the most.

I think you're gonna be making a lot (and frequent) fuel stops.

Good luck

Ron

[jacwjames]

We had a Class C Ford 460, good power but fuel economy was usually 7 mpg. If we were on back roads ~45-50 mph we could do better, maybe 8.5 mpg but that was it. We did have a +50 gallon fuel tank but stopped for fuel ~250 miles, more frequently if fuel stations were sparse.

[twinboat]

You talk about rear end gear ratios to slow down the engine RPMs to save fuel.

I specific weight, moving at a specfic speed, will need a specific amount of horse power to do that.

An engine need a certain amount of fuel to create that horse power.

Yes, slightly higher RPMs will burn more fuel, but probably not enough to measure, by the tank full method.

Unless your engine RPMs are way above your hp range, there will be little or no gain by slowing it down. Too slow and it will be lugging and waisting fuel or shifting constantly.

[MudiGGer25]

Originally Posted by OLYLEN

So you have a class C towing a heavy load. 6.5-7 mpg. Fuel every 200 miles and carry a 5 gallon Jerry can. If you get better great.

LEN

I think this is what i will be seeing most likely since i intend to use the low octane fuel with the low compression, and only god knows what cam they put in lol

Originally Posted by George Schweikle

I put 97,000 miles on a 23 Ft. 1976 Midas Mini with the small block 400, and never saw anything near the mileage numbers you quoted. I learned to drive with a feather foot on the gas and was sure to have the thing well tuned by a commercial shop that knew how to set timing for peak performance with regular gas. Also had a cold air intake. Maybe your using premium fuel really made the difference or, in spite of my attempts at economical driving, our driving styles were significantly different. And, I don't think the octane rating of today's fuels are as high as the 70's so your new engine can't have as aggressive ignition timing. I know when regular gas went from 89 to 87 octane I had to back off the advance of our 1983 carburated 454 Winnebago, and the mileage suffered.

Whatever, keep us posted on your adventures with the big-engined Tioga. It sounds like a fun project

Thanks, and i will say that for the RV, because i was told (most likely faulty information) that the 76-79 400 sbc had a 9.5:1 compression ratio for the RV's i have never to this day found documentation to actually back that claim up, however the person that told me had rebuilt and built several for the local dirt track so i believed him anyway I took his word for it and always put in the 91 Octane and made sure the pump did not have the 10% ethanol ( they are only required to sell a minimum % of ethanol blended fuel, and so long as they meet that requirement they can continue to supply straight gasoline also) then I also cleaned it out with seafoam and lucas, then full sythetic 5w40 and a qt of lucas full synthetic oil stabilizer, and 16oz of lucas transmission treatment. after all that I brough the timing as aggressive and with as much base as humanly possible without a. ripping the starter off, and b. minimal to no pinging under light load when it was cool out (denser air mixture) I was rewarded with the mileage above and much much better throttle response as well as a bit more actual power as well im sure.


I am not sure what timing this cam will like, i will probably set everything with the nice gas, 91/no corn, and then put in some el-cheapo 87/10%corn and see if it pings or not. my 87 v10/v20 burb that we will be towing, has a noticeable difference in power and fuel economy between the different fuels. I can go 0-60 in about 7 seconds usually on 91 and can reach 5000-5500 rpms with ease, but lately i have been a cheapscate and been using the 87/89 w/ 10% corn and my in town mpg dropped easily 2MPG and my 0-60 times is near 9 seconds now, and i can get past 4000rpm before it is pinging and the computer retards the timing.

Originally Posted by lanerd

I think that Sub will weight more than 5000 lbs....have you ever weighed it? And adding a lawn mower plus trailer in the back....

I have weighed it atleast 30 times this year alone at the junkyard, gravel yard, and city baler. with me and the wife (500lbs) and a pile of tools and a spare tire and usually a full tank of gas he weighed in at 6500, i have seen it as low as 5800 with just me and some stuff out. I will be strippin it down for the trip. i.e. seats will be removed, and other than the mower and a small trailer (about 300lbs at most) i should be able to have it darn close to 5,000lbs

Your Class C will only have a MAX tow rating of 5000 lbs...maybe less. Do you know what it is off hand?


no listed tow rating anywhere on the rig because it is not a standard receiver style hitch, but when i worked at a diesel repair shop, I have both help design and install unconventional hitches on anything from a midsize class A up to a concrete pumper truck capable of towing equipment in excess of 10,000lbs so im quite confident I can reinforce it adequately, I havent crawled under yet to see if fleetwodd stretched the frame or anything.

Also fwiw, I have flat towed a class A motorhome for weighing over 11,000 lbs using a upgraded HF 5,000lbs towbar and a 3500lb bumper hitch from walmart made by reese. the ratings are based on a trailer. I have never seen a rating for towing a vehicle flat where there is little to no tongue weight, and depending on the state laws may or may not have its own braking or a supplemental braking system. oh and that was with a 1/2 ton dodge ram with the v6 ( i either had to move it or lose it, so i took the risk)

Loading the C to max (say around 10K lbs) plus the (at least) 5500 lb toad, I suspect that 454 will only get around 6 or 7 at the most.


like i said above, most likely thats what i will be seeing. I will take a test" run once the engine is up and running for about 65 miles and see how im doing without the toad


I think you're gonna be making a lot (and frequent) fuel stops.

Good luck

Thanks, im probably gonna need it

Ron

Originally Posted by jacwjames

We had a Class C Ford 460, good power but fuel economy was usually 7 mpg. If we were on back roads ~45-50 mph we could do better, maybe 8.5 mpg but that was it. We did have a +50 gallon fuel tank but stopped for fuel ~250 miles, more frequently if fuel stations were sparse.

im hoping to make it 240 miles on a tankful, but i have been planning on topping off every 40-60 when we stop to see a landmark or bathroom or food break lol

Originally Posted by twinboat

You talk about rear end gear ratios to slow down the engine RPMs to save fuel.

I specific weight, moving at a specfic speed, will need a specific amount of horse power to do that.

An engine need a certain amount of fuel to create that horse power.

Yes, slightly higher RPMs will burn more fuel, but probably not enough to measure, by the tank full method.

Unless your engine RPMs are way above your hp range, there will be little or no gain by slowing it down. Too slow and it will be lugging and waisting fuel or shifting constantly.

I would have to disagree with that since all modern vehicles including 3/4 ton and 1ton trucks have atleast 1 overdrive or even 2 in the case of some manuals and the engines for the most part have remained producing the tq and hp in the same ranges over the years. i.e. lets take 3 identical 3/4 ton chevy work trucks, they all have the same vortec 7400 (454), #1 lets say for arguments sake that the o/d is locked out for towing purposes by the owner. #2 uses the overdrive 4L80E once up to highway speeds. #3 is using the new 6spd automatic 6L80E, I can tell you from personal experience that #3 will get the best MPG by utilizing the 2nd overdrive, and #2 in the group with still do better than #1 because of the overdrive. now I have towed with the 4L80E, and the 6L80E, and I can honestly say that if you are towing over 6,000 lbs in western north dakota that you will end up using 5th or even 4th to get up those really steep grades, however once it is back out on the level it does just fine in 5th and 6th most of the time i think it was the 2500HD with the 6.0L rated : 300 hp (224 kW) @ 4400 RPM 360 lb·ft (488 N·m) @ 4000 RPM, and i can quite confidently say that the 454 with a little tuning would be able to very easily exceed that, the cam i have sitting on my desk can bring out 400+hp and almost 500ft-lbs out of a bone stock 454 on a dyno.

I do know that i am much larger so wind drag is a factor, hence why we all try to stay under 60mph unless its a hurry lol however i am quite sure i have more than enough tq to move 15K-17K, and considering brand new off the line class C's are only being offered with 305HP in a gas engine, I will be exceeding that, and yet they have been coming stock with O/D since the mid 90's that i have seen. even those puny stock engines are capable of running OD all the time, so im sure I could safely lower the ratio, although if i had the $3000 i would buy a gearvendors lol, that would give me 6 forward gears and 2 reverse


The 454 should be able to produce more than enough power by 2000rpm without lugging. now on the other hand if this engine was bone stock from a car or truck , then I would be skeptical but im pretty confident it was pushing some good power considering it was built for mudding

And to everyone sorry for taking so long to reply, for some reason i havent been getting notifications

[twinboat]

Most of the 5 and 6 speed transmissions add gears between 1st gear and O/D.

Most top O/D ratios are between .71 to 1 and .65 to 1, a slight difference. This is done to keep the engine in its optimum power band and not lug, running up, thru gears.

The more gears between 1st and O/D, the less work needed by the engine to accelerate. This is to increase mpg in local driving.

I stand by my statement that you need a set amount of power, to move a set amount of weight, at a set speed. That will use a set amount of fuel, at any rpm, in the power band, of the engine.

[MudiGGer25]

Ok

Originally Posted by twinboat

Most of the 5 and 6 speed transmissions add gears between 1st gear and O/D.


actually the 6l80e whoih is most appropriate for my application because my rv is chevrolet only adds a single gear at the beginning for power to get moving.

4.027 2.364 1.532 1.152 0.852 0.667

Vs my th400

2.48. 1.48. 1.00

It adds 2 seperate overdrive gears. Both 5 and 6 are overdrive.

I agree that ot will always take x HP to move X weight against X wind at X speed. However thats not what i disagree with. What im trying to say is that if. And only if my 454 is producing enough HP and Tq by 2000RPM to be able to move my X load at 55-60mph wihout lugging. I should see a substantial mpg oncrease by lowering RPM by 600-800 because that is roughly a 22%-25% decrease in engine speed.

That being said i can attest that the 6.0l vortec while towing roughly 7200lbs in a trailer that was very resistant to wind above 60mph i switched from od to drive and watched the instant mpg and when the engine rpm jumped up it was almost always accompainied by a 2-5mpg drop unless it was going up a steep grade in which case sometimes it was more effencint to use d and gain 3 mpg going up the hill. If od did not increase mpg by lowering rpm then they would not be onstalling them into motorhomes or anything else. I am just wondering about a 300 ring and pinion swap to gain the lower rpm vs the 3000 for a 4l80e or a gear vendors. If the engine is not lugging in od or at the lower rpm then i believe it would increase mpg. Remember these old 454s once improved can walk all over the engines the RV s use nowadays.


Pardon the spelling. My phone autocorrect iant working




.
Most top O/D ratios are between .71 to 1 and .65 to 1, a slight difference. This is done to keep the

engine in its optimum power band and not lug, running up, thru gears.

The more gears between 1st and O/D, the less work needed by the engine to accelerate. This is to increase mpg in local driving.

I stand by my statement that you need a set amount of power, to move a set amount of weight, at a set speed. That will use a set amount of fuel, at any rpm, in the power band, of the engine.

[MudiGGer25]

My last post was supposed to reference the 6l80e not the 4l80e

[MudiGGer25]

Sorry for no updates in a while, lost our house to a storm and broke my heel the day of our trip so everything went south really fast, on the other hand i did find the pic of the other RV we had pulling a 7000lb trailer and still getting 14-15mpg on the highway at 55-60mph with a 400 sbc.


i intend to have it running and driving within a month once i am healed and i wil post an update with numbers

[George Schweikle]

Sorry to hear of your misfortune. But reading about your Tioga brought back more memories of our 1976 Midas. A co-worker had a friend who was a GM automotive engineer and towed a hydroplane trailer with the same motorhome as ours. I asked my friend to query this guy for some advice, and the response was to "get a complete gauge package". I did this, and saw transmission temperatures around 250 degrees - while NOT towing. Adding a secondary transmission cooler brought this down to below the engine coolant temperature.

And, BIG ISSUE. We towed a small sports car on a single axle trailer and, after a few years, the rear "frame" broke on the motorhome. What appeared to be a continuation of the main chassis frame rails backward from the rear leaf spring mountings were just two extensions butt welded to a crossways brace. This looked like the main frame continued, but the joint had limited strength. The modest trailer tongue weight broke the butt welds loose, and the entire coach body sagged sufficient to result in cracks at the back compartment doors. Since my Midas motorhome was built about the same time as yours, my advice is to check your frame very carefully.

Originally Posted by MudiGGer25

Sorry for no updates in a while, lost our house to a storm and broke my heel the day of our trip so everything went south really fast, on the other hand i did find the pic of the other RV we had pulling a 7000lb trailer and still getting 14-15mpg on the highway at 55-60mph with a 400 sbc.


i intend to have it running and driving within a month once i am healed and i wil post an update with numbers

[MudiGGer25]

Originally Posted by George Schweikle

Sorry to hear of your misfortune. But reading about your Tioga brought back more memories of our 1976 Midas. A co-worker had a friend who was a GM automotive engineer and towed a hydroplane trailer with the same motorhome as ours. I asked my friend to query this guy for some advice, and the response was to "get a complete gauge package". I did this, and saw transmission temperatures around 250 degrees - while NOT towing. Adding a secondary transmission cooler brought this down to below the engine coolant temperature.

And, BIG ISSUE. We towed a small sports car on a single axle trailer and, after a few years, the rear "frame" broke on the motorhome. What appeared to be a continuation of the main chassis frame rails backward from the rear leaf spring mountings were just two extensions butt welded to a crossways brace. This looked like the main frame continued, but the joint had limited strength. The modest trailer tongue weight broke the butt welds loose, and the entire coach body sagged sufficient to result in cracks at the back compartment doors. Since my Midas motorhome was built about the same time as yours, my advice is to check your frame very carefully.

Oh yes, i know all about the hot transmission temperatures, it has to do with all the TC slippage moving such a large amount of weight with such a underpowered (for the time) 400ci. I always add the largest aux cooler i can get my fingers on, usually a 26,000GVWR unit. I adjusted my 400 i had in my midas in the picture above and tweaked the airflow and timing to produce peak power when running 91 octane, just by re-tuning the engine i dropped the transmission temps 30F-50F most days and i was plenty happy with that. Even though the 454 will have way more power than needed to move such a small RV i will still be getting an AUX cooler, the PO did upgrade the system with a new larger radiator than stock with a built in oil and transmission fluid cooler.

I have attached pictures of the frame "extension" on this model, i cannot recall what it looked like on the midas we owned though. I do say that the arms are quite heft on this fleetwood model, and in my experience fleetwood usually doesnt cut many corners on most models, where as no-name manufacturers will do anything to make a few extra bucks imo.'


Let me know if that looks like what you had,I will be re-inforcing it anyway but flat towing for the time being has less than 75lbs of tongue weight.

I would say with the hitch setup currently i would not exceed 5,000lbs anyway since it is only a 2" ball, but it can easily be reinforced to support the capactiy of a 2 5/16" ball, provided that i keep the tongue weight reasonable

[oldsman41]

if you dont burn the trans up you might get 6mpg but i bet you wont even get that.lol