The 'sweet spot'....

[Mr_Mark1]

I've seen several posters indicate that they have found the 'sweet spot' with their engines.

How do you know?

My thoughts are that the sweet spot is traveling in the highest gear at the lowest speed?

How do you determine your 'sweet spot' for your engine?

Thanks,
MM.

[gemert]

bluwtr49 wrote:
As has been said the "sweet spot" correlates to peak torque rpm. In my case the cat C12 is around 1250 or so and is fairly flat to ~1500 so that range is what I consider my sweet spot. ~1450 give me 65 mph so it all comes together nicely.

Find the torque curve for you particular engine and that should help.

If you are a member of BAC there is a retired CAT mechanic that will look up your serial number and tell you the testing data when new. If you already know this sorry for repeating. Being a member won't be an issue for him he is a very helpful guy. Happy Camping!

[bluwtr49]

As has been said the "sweet spot" correlates to peak torque rpm. In my case the cat C12 is around 1250 or so and is fairly flat to ~1500 so that range is what I consider my sweet spot. ~1450 give me 65 mph so it all comes together nicely.

Find the torque curve for you particular engine and that should help.

[gemert]

wolfe10 wrote:

gemert wrote:
I am no expert but I do know one! I have a CAT engine. He researched the serial number and gave me the original test data. The way he explained it to me, with my limited understanding is that at 1200 RPM my engine is at max torque burning least amount of fuel. for me that would be somewhere around 57 in 6th gear. You are probably cussing at me now because I didn't answer in a way that most of us would look at it. I'm pretty sure that with a new coach of the quality of yours that it would be relatively easy for you to get the same data from either your coach maker or the engine maker. I'm guessing that they are all going to be a similar speed. For me the question is how fast can I go before the mileage just completely goes to "you know what?" For my coach that seems to be around 62 mph. I determined that by watching the engine data readout for fuel mileage.

What Cat engine you have???

It's in my Sig. Brett, it's a C-12 505 HP 1550 TQ. As knowledgeable as you are you probably know the guy that looked it up. Name is Dave on the BAC.
Happy Camping.

The Caterpillar 3126 and C7 have peak torque at 1,440 RPM. C9 or larger, yes, at lower RPM.

Just do not want to mislead anyone with a different engine than you have.

[wa8yxm]

Mostly by watching the MPG display on the dash.. If it hangs in at say 10 from say 45 to 55 mph, but as you pass 60 and climbing it starts to make liek Bo Derrek getting older (9-8-7-6) you found your sweet spot (55-60 in the example)

I had a car once where peak MPG was 82MPH. I fixed it (needed new carb).

[wolfe10]

Reality is the "best MPG speed" varies with coach, gearing and terrain.

On flat ground, the lowest RPM you can hold high gear is the best "reasonable" speed. That will be right at peak torque RPM for your diesel.

When I say "reasonable" in most coaches peak torque RPM in 5th gear (MUCH less aerodynamic drag at lower speed) will give better MPG, but is excruciatingly slow!

But, that speed (peak torque RPM, 6th gear) in rolling hills will cause a downshift on each hill-- so a little higher speed that will allow you to stay in high gear is better.

[prstlk]

My 07 Hr ambassador with 330 cummins and Alison 6 likes 1600 rpm around 62 mph. Like posted above if I have more than 3 sometimes just 1 if I'm on hill I pull over. Coach seems happy, and last tank I got 7.5 mpg. With a toad.

[wolfe10]

gemert wrote:
I am no expert but I do know one! I have a CAT engine. He researched the serial number and gave me the original test data. The way he explained it to me, with my limited understanding is that at 1200 RPM my engine is at max torque burning least amount of fuel. for me that would be somewhere around 57 in 6th gear. You are probably cussing at me now because I didn't answer in a way that most of us would look at it. I'm pretty sure that with a new coach of the quality of yours that it would be relatively easy for you to get the same data from either your coach maker or the engine maker. I'm guessing that they are all going to be a similar speed. For me the question is how fast can I go before the mileage just completely goes to "you know what?" For my coach that seems to be around 62 mph. I determined that by watching the engine data readout for fuel mileage.

What Cat engine you have???

The Caterpillar 3126 and C7 have peak torque at 1,440 RPM. C9 or larger, yes, at lower RPM.

Just do not want to mislead anyone with a different engine than you have.

[DanTheRVMan]

I thought it was the rpm highest gear that you got the best mileage. And then people LIED about where their sweet spot was so that they could justify driving fast.:B

My Phaeton seems to hold 6th gear down to 44 mph in economy, but I have no idea what the mileage is there but I imagine pretty good as wind resistance eats a LOT of HP.

I drive 61-63 mph when I am in no hurry which I wish was all the time. When I am speeding I drive 65 mph if faced with constraints like long days mileage, etc.

I like to know my mpg, but I like driving at a speed that will make for low stress travel not too fast and not excessively slow. But slow enough most traffic is passing my rig.

[gemert]

I am no expert but I do know one! I have a CAT engine. He researched the serial number and gave me the original test data. The way he explained it to me, with my limited understanding is that at 1200 RPM my engine is at max torque burning least amount of fuel. for me that would be somewhere around 57 in 6th gear. You are probably cussing at me now because I didn't answer in a way that most of us would look at it. I'm pretty sure that with a new coach of the quality of yours that it would be relatively easy for you to get the same data from either your coach maker or the engine maker. I'm guessing that they are all going to be a similar speed. For me the question is how fast can I go before the mileage just completely goes to "you know what?" For my coach that seems to be around 62 mph. I determined that by watching the engine data readout for fuel mileage.

[Cloud_Dancer]

I don't call it a sweet spot. I call it the speed that I like, for the conditions that prevail. I trust my experience and my senses.

[the_bear_II]

Some folks consider the sweet spot is the high gear RPM where they get the best mileage. Others feel it's where their vehicle feels the best.

For example My Toyota Avalon loves 75 MPH. It is real smooth, gets the best gas mileage and still has enough power to pass or hill climb.

My old truck towing the 5th wheel was 60 to 62 MPH. My new truck towing the 5th wheel feels best at 65 MPH but I get the best mileage at 55 MPH. So I'm usually at 60 to 62 in the new truck.

So the best method is to try to maintain a specific speed from one gas fill up to the next and then see what MPG you got. For the next leg of the trip add 5 MPH and measure the MPG again. Keep doing this until you hit the speed with the best MPG. Of course the road conditions and weather need to be similar for each leg to get a true picture.

Or you can take it to a place with a dyno big enough for your MH and they can tell you where your engine performs the best.

[jplante4]

Actually, it's more than just engine related. There's usually a speed where the whole coach just feels right. I know it's a subjective thing, but after you've driven a rig at various speeds for a while, you'll know where that speed is. It's a combination of engine RPM and torque, drive train vibrations and how comfortable you are at the speed. And it may be more than one speed depending on road conditions and type of road. I like to run the Sahara at 64 on the highway. Some 2 lane roads in Texas have a 75 MPH speed limit, but I really don't like to run 64 on 2 lane roads unless I can see way in front of me.

On these roads, I keep an eye on the rear view. If it looks like I'm holding up traffic and there's a Bambi afraid to pass, I'll pull over.

Also, company 18 wheelers are governed, usually around 65. These guys are working, and I'm not, so I'll pull to the side, slow down and let them slide by.

If you go only by mileage, then it's all physics; the speed you get in the highest gear at the lowest RPM. However, you won't make any friends running 50 on a 75 mph road.

[J-Rooster]

MM, mine is 59 MPH in my Winnebago. I thru the years owning this coach kept track of fuel mileage and I get the best mileage at 59 MPH. My tach is at 1900 RPM's slightly lugging, which causes the engine to run quieter (engine not roaring or making a loud sound). And for some strange reason the coach rides better and, I don't get the bumpy ride in my coach as when I'm driving 65 MPH. That's how I determine my sweet spot in my RV. This is one of the more interesting questions posted on RV.Net! Thanks Mark, Outstanding effort!! I'm still trying to find my sweet spot when golfing? That's another story, my golf ball spends more time in the trees than the squirrels. My golfing does get better golfing in the desert!