3126 Governed RPM

P51bombay · Feb 14, 2009

Is there somewhere I can find general specs (seems to be very little on cat's site) and/or does anyone know if all 3126 models have the same max governed rpm? Just setting up a Gearmaster and I'm not sure if they are all set for 2400. I know some say waste of money but having never driven a 10 speed, I look at it this way: I can spend $ and get some lessons or spend $ on this and have something that provides other benefits afterwards. While I'm already getting the hang of things (without instruction or the GM) I won't have much more than a few days time to learn before putting the truck in service.

P51bombay · Feb 14, 2009

forgot to add this is a 2003 model, T300 w/10spd fuller

also, what his the low and high end of the sweet spot for this combo?

bluejaybee · Feb 15, 2009

Having had 3 of the 3126 Cat's (and have one now)(and also happen to like them), I have found that on top the valve cover, there is a sticker with your Max RPM's at different HP's. re: 195HP Max RPM 2450,,,250HP Max RPM 2250. Now while all of mine have been 1999 and 2000 year models, I assume they still put that sticker on the newer engines. This is just something that I have observed and may or may not help you. Presently, I have a 9 spd and am not real fond of it. For some reason, I seem to rake the gears about 30% of the time. While I seem to do better with no clutch, sometimes I have to go back to double clutching to convince myself that this thing will shift.

I also might add, that I have looked on several ocassions for technical info on Cat engines and it is just not out there. I can find offers to sell manuals to me, but they are always high in price and not worth the price for what I may be wanting to know.

Are you installing the Gearmaster yourself?

P51bombay · Feb 15, 2009

bluejaybee said:
Having had 3 of the 3126 Cat's (and have one now)(and also happen to like them), I have found that on top the valve cover, there is a sticker with your Max RPM's at different HP's. re: 195HP Max RPM 2450,,,250HP Max RPM 2250. Now while all of mine have been 1999 and 2000 year models, I assume they still put that sticker on the newer engines. This is just something that I have observed and may or may not help you. Presently, I have a 9 spd and am not real fond of it. For some reason, I seem to rake the gears about 30% of the time. While I seem to do better with no clutch, sometimes I have to go back to double clutching to convince myself that this thing will shift.

Good idea, don't know why I didn't think of that. Can you give me any hints as to the sweet spot or rpm range it likes to be shifted at? So far it seems to like upshifts somewhere around 2000-2200 but I'm only managed one downshift (by guess and by golly) so I really don't know where the lower end is and I'm sure I'm only in the ballpark on the top end. Maybe one of these gearmaster things is your answer - I'll be posting comments once its calibrated and I've run it some.

Bruno · Feb 15, 2009

You can have KW set the truck up to get the best MPG if you call them. I have 2 trucks with the 3126 Cat and I love them. Mine are 6 speeds with a 373 rear end. at 2200 on the rpm the truck goes about 67 mph.

P51bombay · Feb 15, 2009

MIleage should be fine, I'd have to look up the RE ratio but in 10th it turns about 1700 @ 62mph. Coming out of a 6 speed truck( also a 3126) I'm looking to get the basic parameters with a non syncho box to set up the gearmaster - the 6 speed is easy, just pick where you want. I'm sure I'd figure it out eventually but wont have that much time to play with it ahead of time.

bluejaybee · Feb 15, 2009

Mine seems to be same as yours. Upshifts work best for me at around 2000-2200 RPM. If I know I am going to have to downshift, I do it around 1500-1600. According to an owners manual that came with a 2001 Sterling, the Cat 3126 will take lugging. It said that if you think you can pull a hill at low RPM's, to let the engine lug you over. Within reason of course. Don't let it get down to where it starts shaking and jerking your drive train. But I always let mine lug on over if it will. Mine will get down to about 1200-1300 without screaming "shift me fool". Plus the manual said better fuel mileage would be noticed doing this. Anyway, keep us posted.

I came out of a 6 spd also. Haven't drove an unsynced transmission in many years. Last one I drove was in the Army and that was 39 years ago. I been thinking if I tore this up, I would go back with a 7 spd. I don't know my drive axle ratio, but I run 65 at 2250 and that is max rpm for me. It will run on up to 70 at 2400 rpm, but that is over according to sticker. I need to do something to get them down a little. At 55, mine runs 1900 rpm and it sounds so good there. Wish I could get it to run 70 at 1900. Guess I could if I had the money. Ha!

P51bombay · Feb 15, 2009

That is good info and sounds about right, so I think for now I will set it up at 1600 to 2100. I know the 6 speed I have is about 2000 @62 which I think is a tad high, it runs smoother at 1900 yet for the most part falls on its face much at or slightly below 1500 unless the ground is flat and or the load is light. That said I can stay in 5th and let it fall to 1100ish provided I don't hit any hills or have to slow further.

bluejaybee · Feb 15, 2009

All my life, I have thought the whole idea behind a diesel engine was run them at low RPM's. But every truck I have owned with a Cat 3126 diesel, was or is, set up to run right there at max rpm on top end. So guess I don't know as much as I thought.
Good luck to you. Keep us posted. It's my bed time.

Moot · Feb 15, 2009

In a mid-range gear run the rpm's up until the governor kicks in. That should tell you the max rpm setting. You don't want to run it to the limit every shift, especially in the first couple of gears so back it off a little from the maximum governed rpm.

piper1 · Feb 15, 2009

bluejaybee said:
All my life, I have thought the whole idea behind a diesel engine was run them at low RPM's. But every truck I have owned with a Cat 3126 diesel, was or is, set up to run right there at max rpm on top end. So guess I don't know as much as I thought.
Good luck to you. Keep us posted. It's my bed time.

Mid range diesels (of which the 3126 is) are designed to run usually in the 1900 to 2200 rpm range. The recommended spot for a 3126 was 2000 rpm at 62 mph. If you have an International DT466E it was 2200 rpm.

Big engine rules don't always apply to theses little guys. The sweet spot is all about volumetric efficiency, in other words, at what rpm does the engine use the least amount of energy pumping air and exhaust and still have enough HP to fall back on to do the job (and a wee bit of torque rise left too).

Above 30 MPH, if you shift a 3126 so it isn't below 1600 when you first engage the next gear (after you shift) you will likely do well. Below 30 mph you can shift earlier as you are not requiring as much horsepower.

Now before anyone chimes in with "I can shift my truck at 1100". If you can up shift earlier....great, it simply means you are not requiring a lot of horsepower for the conditions...or you have an engine with more horsepower than required. A 410hp engine in a straight truck for example doesn't ever have to really work hard except at full cruise speed, and even then it would need a big hill!

bluejaybee · Feb 15, 2009

piper1, I am so glad you chimed in here on this subject. You may not know a single thing you say, but, I for one, believe everything you say. The thing about "you" is you use good gramar and spelling, and lay it out where even a not so experineced person can understand. Your info on this subject has been most helpful to me. Now I know why all these trucks run high RPM's. And it makes sense what you say. Thanks. I am sure p51 can use some of it also.