Hey all, I know this is a Cummins site, but with all the diesel engine gurus that are in here, I'm sure you can give me some info that I need. Heres what I am lookig at. I got a GMC 6500 3126 Cat w/ 6-speed behind it. It is a rollback (tow truck), but it isn't as strong a runner as we had thought it should be. Everything is stock that I know of, has 136k on in. We don't want to do anything as far as messing with the puter. I was considering chopping the muffler & replacing it w/ a 4" pipe. More flow equals more power, at least it worked on my 90' Cummins. What do you all think?

 

It'll be obnoxious, and it won't make a real big difference.

 

I would strongly consider running the overhead at 136k if it hasnt been done recently. The 3126's are notorious for dropping valves due to a loose overhead.

Other than opening the exhaust and a little more flow from the intake, there isnt much else you can do unless you want a rerate from CAT or put box on it.

 

Power difference will be negligible.

 

Exhaust will make absolutely ZERO difference. I have a 4" straight pipe on ours in our FL80, and it's still GUTLESS. I put a TS MP8 on it and it did make a difference, but not very much. And at $800+ to rerate it at CAT another 25hp, that's not even close to worth it!

Overall, I can't stand the little CAT. We've run the big ones for years, so I figured the 3126 would be decent...should've found one with an ISB Cummins, at least then I could put injectors in it The Mercedes in out M2 will eat the 3126 alive too...even with more load!

Basically, your stuck!
Chris

 

Fire, prolly gonna have to re-rate to get anything out of it. Is it REALLY $800 to rerate for another 25 hp???.. You need to have the configuration looked at to see what GMC has the engine rating set at. You can't always go by the nomeclature plate on the valve cover. There are times that the OEM actually has the engine defueled to protect the rest of the drivetrain. I agree, if you haven't ran the overhead, it NEEDS it.

 

Called CAT, gave them my ESN, and they gave me the run down on what it would take to up-rate it. I can go from 205 to 230 for just the $800 liscence, charge, and fees for the rerate (those would be the dealers own fee basically), to go farther than that I would have to change the harmonic balancer, injectors, turbo, etc.

As far as protecting the drivetrain...I've driven 330hp 3126's and they aren't any better than my 205hp...I think they're just a guttless engine for anything very heavy. Just the empty truck and trailer...about 20K gross it does OK, anything more, and it's a TURD! My 5.9's and the little Mercedes will eat it for lunch!

Chris

 

I'm with you on that. I drive a tandem axle straight truck with a 3126 rated at 300/860 and a 10 speed. It's just too much truck for that motor, we have them in single axle trucks (245/600 rating) and they are sorta OK. Loaded cement mixers can beat me from a light, and the motor just does not seem to have any guts in the hills. Oh well, can't expect too much from a Class 7 motor in a Class 8 truck. Of course, if they'd let me spec the truck it would have had a 550 HP ISX.

 

550hp...come on Dave, go with the big-un...Signature 600

A C Series Cummins, ISL, or ISM, or a C9 CAT would HAVE to be better...but I figured for sure this little CAT would do the job...it's almost unsafe!

Chris

 

Im not a big fan of ANY of the 3100 engines.... the fuel system on the mechanicals are a time consuming mess IMHO. I don't deal with the little engines much since customers in the industrial market typically need more HP, from 550hp to 5,000 hp. natural gas and diesel.
Sounds like its just not enough engine for the truck.