Optimum speed

[Pairajays]

I have a, new to me, 1997 Beaver Marquis with a Cat C-12 425 hp engine. How do I determine the peak torque point. I want to know the optimum speed for the highest mpg. Or maybe someone knows the RPM at this point in 6th gear.

Jim E

[VanDiemen23]

should be a torque/HP curve on the cat website or in your manual.

[Skip426]

I have a manual for my 99 3126B , and it gives 1450, but your bigger motor may be lower. At 58 mph I'm turning 1750 and the economy is good. Economical cruise Rpm is the HOLY GRAIL of MH owners. MHO. if you track your rpm at torque convertor lock up in sixth gear, then add 200 rpm to lessen the chance of kick down. Then check your speed against the limit. If your within " ticket tollerance " of the speed limt this is the Rpm. to use. Other info on achieving the best economy is in a PDF file on another thread. The Cat info on "understanding RV Diesel Engine Performance " is a must read for any one concerened with fuel mileage. I'll track it down and re-post that link to the PDF.

[Skip426]

Try
http://www.catrvclub.org/Understanding Perf.pdf
If the link woun't work from here, go to the Thread "3126b MPG", in the Cat forum. To post #44 by WOODYDEL and access it from there.

[Pairajays]

Quote:

Originally Posted by VanDiemen23 should be a torque/HP curve on the cat website or in your manual.

Can't find any C-12 engine information on the Caterpillar web site. No torque/HP info in owner's manual.

Jim E

[Pairajays]

Quote:

Originally Posted by VanDiemen23 should be a torque/HP curve on the cat website or in your manual.

Can't find any C-12 engine information on the Caterpillar web site. No torque/HP info in owner's manual.

Jim E

[Pairajays]

Quote:

Originally Posted by Skip426 Try http://www.catrvclub.org/Understanding Perf.pdf If the link woun't work from here, go to the Thread "3126b MPG", in the Cat forum. To post #44 by WOODYDEL and access it from there.

Link doesn't work. How do you "go to the Thread "3126b MPG", in the Cat forum. To post #44 by WOODYDEL and access it from there.? I am on the Cat forum.

Jim E

[Skip426]

Hold your cursor on forums,in the greenish bar below the iRV2 header, do not click,
when the drop box opens move cursor down to ; Powertrain Garage,do not click
When the side box opens , move cursor over to ; Catapilar Engines; and click.
When the thread page list opens scroll down the list and find " 3126 mpg" by bigdog and click. when the list of posts opens click page #4; the black bar along the top of each post has the time date on the left and the post # on the right.
Scoll down the page of posts to #44 by WOOYDEL and click to open.
Then click on the link contained in the post.
The pdf file will down load to your computor and can be opened directly, saved to a file or printed out. I prefer print as I refere to it often and easier to access when the DW is on the computor doing her stuff.

[Skip426]

I'll try the link here again; I think I see what I did wrong.
http://www.catrvclub.org/Understanding_Perf.pdf

[Skip426]

Just tested it from here ; it worked ! I'd left out an underscore in the Link when I posted it the first time .
Cat changed there web site when they stopped producing On-highway engines, so this pdf, and much more info is not available.

[Pairajays]

Talked to a Cat dealer today. He said to bring him my engine serial number and he would give me all I need to know about my engine.

Jim E

[spike45]

Quote:

Originally Posted by Pairajays I have a, new to me, 1997 Beaver Marquis with a Cat C-12 425 hp engine. How do I determine the peak torque point. I want to know the optimum speed for the highest mpg. Or maybe someone knows the RPM at this point in 6th gear. Jim E

Maximum fuel economy RPM will be shown on a BSFC curve on Cat engine data sheet. Brake Specific Fuel Consumption. Typically, the BSFC RPM will be above the peak torque RPM. It will not occur when pushing near the HP peak RPM for certain.

[Pairajays]

Maybe you all are aware of this but, visited the local Cat dealer with my engine serial number. He gave me more engine specific info than I could digest. When the opportunity presents it self, give it a try.

Jim E

[spike45]

Quote:

Originally Posted by Pairajays I have a, new to me, 1997 Beaver Marquis with a Cat C-12 425 hp engine. How do I determine the peak torque point. I want to know the optimum speed for the highest mpg. Or maybe someone knows the RPM at this point in 6th gear. Jim E

Go to your Caterpillar dealer and request the equivalent of an engine performance data sheet or curves. The curves will show the peak torque RPM as should the data point also be listed in the data sheet. Also, in my former life as a Cat guy, there used to be published curves for BSFC, Brake Specific Fuel Consumption. The bottom of the curve is the most fuel efficient RPM. That data was used to guide customers who were ordering trucks with CAT engines to get the tire size and gearing to achieve the cruise speed that puts them near the bottom of the BSFC curve which will be near and above the peak torque RPM. If the truck (MH) is geared right for a cruise speed that is just above the peak torque RPM, then as the load comes on the torque rise is so dramatic that downshifting (manual trans) was often not necessary. With an automatic transmission, from what I have been reading, the Allison trans does not down shift often (or enough). Instead it will stay in cruising gear and let the engine speed drop as the torque is rising. If you are unable to get the BSFC Curve bottom RPM, use peak torque RPM as the cruise speed, if you can stand to go that slow.

With as much frontal area as a MH has, cruising at 55 MPH will allow for better fuel economy as speeds above that cause an exponential increase in the wind drag HP and increased fuel consumption.