RV weight and engine size and does size matter?

[DaveTN]

Our rig is at the larger end of the Jayco C's but has the same engine (Triton 6.8L EFI V-10 305 HP) as the 26 foot Greyhawk. Would the 26 have lower RPM's when traveling at the same speed? Or does size not matter, just weight?

My Expedition rarely goes over 2500 RPM even at 80 but the Jayco is 2500-4000 during normal travel (55).

[Capn_Dave]

Well that is hard to answer. It all depends on the gear ratio in the rear end. As a rule, if every thing is the same as far as transmission and rear gear ratio, the weight of the vehicle really doesn't make any difference as to the number of RPMs the engine turns. The heaviest vehicle will just take longer to get to the same speed.

[Sparky1956]

Originally Posted by DaveTN

Our rig is at the larger end of the Jayco C's but has the same engine (Triton 6.8L EFI V-10 305 HP) as the 26 foot Greyhawk. Would the 26 have lower RPM's when traveling at the same speed? Or does size not matter, just weight?

My Expedition rarely goes over 2500 RPM even at 80 but the Jayco is 2500-4000 during normal travel (55).

Size DOES matter. The weight of a vehicle will require more power thus higher RPM's. The gearing of the vehicle is usually set up to compensate for weight, tire size and power needs. Put a road behind your MH and not the difference. The power curve programmed into an RV is different than that of a car, truck or SUV.

[DaveTN]

Originally Posted by Sparky1956

Size DOES matter. The weight of a vehicle will require more power thus higher RPM's. The gearing of the vehicle is usually set up to compensate for weight, tire size and power needs. Put a road behind your MH and not the difference. The power curve programmed into an RV is different than that of a car, truck or SUV.

So on my original question, would the smaller (I assume lighter) RV with the same chassis have a lower RPM at the same speed? I guess this boils down to being used to my truck never getting over 3000 and the RV frequently at 3000 plus. Is this bad for the RV engine?

[genesimmons]

i bet you have a higher ratio rearend in the jayco

[Sparky1956]

Originally Posted by DaveTN

So on my original question, would the smaller (I assume lighter) RV with the same chassis have a lower RPM at the same speed? I guess this boils down to being used to my truck never getting over 3000 and the RV frequently at 3000 plus. Is this bad for the RV engine?

The answer is yes if the "smaller" RV is lighter. Bottom line is the Ford V10 used in the 350 & 450 platform are geared for pulling hills. It is common to hit 4000 rpm going up hills. It is ok for that engine. It takes getting use to. Mine does not do it nearly as much nor do my RPM's go as high since I installed the 5-Star upgrade. Made a world of difference. I have a 31 foot Winnie on the E450 Ford chassis.

I try and change my trannie filter and fluids every 15,000 miles as a result. I have 65,000+ miles pulling a Jeep Wrangler for 50,000 of those miles. Trying to keep RPM's under 3,000 on hills or continuous inclines is hard to do and maintain 60 mph.

[Capn_Dave]

Sorry Sparky,

If you have two identical chassis and engine set ups, the only difference is the total weight, one with more load than the other. It will take the same RPM to achieve X MPH. The only thing that will change is the time it takes to get to that speed.

The way you get a smaller chassis design,to carry more weight, is to change the gear ratio. That is why there are transmissions that downshift and up shift.

[Sparky1956]

Originally Posted by Capn_Dave

Sorry Sparky,

If you have two identical chassis and engine set ups, the only difference is the total weight, one with more load than the other. It will take the same RPM to achieve X MPH. The only thing that will change is the time it takes to get to that speed.

The way you get a smaller chassis design,to carry more weight, is to change the gear ratio. That is why there are transmissions that downshift and up shift.

Sorry Capt Dave....

I guess cause I stayed a sgt I have this thing for being argumentative with brass. What you say is partially true. Where you miss step is the big picture which is often left out by the brass.

A larger / heavier coach will reach a higher rpm sooner and for a longer duration and require repeated increases in rpm sooner and for longer durations than a smaller and lighter coach. Which is the issue the OP was seeking to have answered. He was also comparing his SUV with a similar motor to his RV which most likely are geared differently.

You are correct mechanically speaking but not when applied to real life driving and experience.

Love you cap....

[Capn_Dave]

Sparky you are correct, however that was not what was asked.

"Would the 26 have lower RPM's when traveling at the same speed" Is the question asked.

Please note nothing was said about going up and down hills, starting and stopping.

Now if the poster did not mean what he asked, that is one thing. One thing I have learned is to be precise. When I ask How do I get to Rome. I will say How do I get to Rome Georgia. Kinda like when I was a FAC pilot. I was careful where I marked my targets. Someone's life depended on it.

Love ya brother

[frankdamp]

If the rear axle ratio is different, then the rpm for a particular speed will also be different. The transmission is probably the same in both.

[Steve Ownby]

Originally Posted by frankdamp

If the rear axle ratio is different, then the rpm for a particular speed will also be different. The transmission is probably the same in both.

This is the absolutely correct answer.

[Conquest102]

I would agree Rv is going to have a very differant ratio in the axle. Needs a lot more torque in the lower speed range to get the beast moving. So yes higher rpms less speed.

[Capn_Dave]

My point exactly

[DaveTN]

Originally Posted by Sparky1956

The answer is yes if the "smaller" RV is lighter. Bottom line is the Ford V10 used in the 350 & 450 platform are geared for pulling hills. It is common to hit 4000 rpm going up hills. It is ok for that engine. It takes getting use to. Mine does not do it nearly as much nor do my RPM's go as high since I installed the 5-Star upgrade. Made a world of difference. I have a 31 foot Winnie on the E450 Ford chassis.

I try and change my trannie filter and fluids every 15,000 miles as a result. I have 65,000+ miles pulling a Jeep Wrangler for 50,000 of those miles. Trying to keep RPM's under 3,000 on hills or continuous inclines is hard to do and maintain 60 mph.

This is what I was rolling around in my head, just that huge difference between my expedition rolling at 80mph @ only 2700 RPMs, then I get in the Jayco (ford 450) and I assume with the heavy weight I seem to always be at 3-4,000 RPM's even at 55 and it is hard to get used to and made me worry a bit. So I assumed the weight (expedition vs Jayco being 5,800 versus 15,000) was responsible for the difference.