Cummins Diesel Forum banner

Kinda sluggish in 4th...

774 Views 7 Replies 6 Participants Last post by  12vrichh1989
Perhaps it's because I am somewhat new to a manual transmission diesel truck, but I can't help but feel I'm shifting in completely the wrong spot. Typically I start in 2nd gear, rev out to about 2k, shift to third, roll through that to around 2k, and shift to 4th and the RPMs just nose dive and then whatever head of steam I had is lost... once the RPMs pick up to around 1500 it starts to move out again. I don't have any smoke which I guess isn't a bad thing, but everyplace says turn the starwheel until I see smoke... I've never seen any hahaha...

Am I shifting WAY too early? Do these motors want to rev out past 2k? Few quick details about the truck:

#11 Fuel Plate in stock position (prior owner), starwheel turned a bit (zero smoke on throttle tip in... maybe I need more turns?), AFC slid forward, and 5" straight pipe. I have a pyro gauge on the way, and the reason I haven't hardly futzed with it is because I simply don't want to burn things up while running around blind. Have new OFV, filter, prefilter, though I'm not sure if fuel pressure would have a big impact (got the parts for that too just have to get some free time to tap the banjo bolt).

Would I benefit from a few more RPM? I have 410s so the motor likes to rev out to begin with... thanks for the insight.

1 - 8 of 8 Posts
I dont have a stick but that sounds like a good spot to shift exspecially being the 410, my auto shifts around 2 as well. Are you on the highway when this happens or around town, I ask because sometimes my o/d kicks in and its doggy till the rpms get back up but if Im on it I dont really have any issues
Thanks for the input. On the highway I am already in 5th when I merge, so mostly around town. I agree it seems like a good place to shift, but it just seems that 2 to 3 puts the RPMs in a nice "pulling" range, then 3 to 4 drops me in the cellar and I almost have to lug it to get out of my own way... I tried bringing 3rd a few more RPM, but it just seems like more noise and no more get up and go (probably where a GSK comes into play, heh...).

And I know what you mean with the O/D, my Chevy used to do that... then wonder what gear it REALLY wanted to be in, then make dinner, and then do my taxes (damn Allison transmission is sometimes too smart...).

go ahead and wind it up more in third, you need to to hit 2400 in third (with 3:73's) to be at least 1500 in 4th. These trucks do a lot better with the 6-speed. The 5-speed is a solid transmission, but it leaves gaps between 3rd/4th and 4th/5th. You can shift early if you are not in a hurry, but for the fun starts you need to let it rev to at least 2500 before shifting. Keep in mind that if you are running 4:10's, look at your rpm at freeway speeds that you could run all day if youwant.... more than 2k, for sure.

The gaps between 3rd and 4th are noticably long, just get to 2400 rpm in 3rd and shift to 4th. You can run high rpms, just don't lug it too much.

BTW look into adding a 3 gsk it shouldn't get things too hot but will allow higher rpms.
i have a dualie w the same set up 411s and manual with that little bit of tweeking your truck should wanna run pretty hard truck is basically stock with a couple of things moved the truck up to 2400 to 2600 thru the gears and if 4th falls flat ck your fuel pressure ..Big truck will burn rubber in 1st and spin them in 2nd on cold pavement and pulls like a gasser in 4thalmost making u lite in the seat
Thanks for the input everyone. I will be setting up a FP gauge to monitor when I get my pillar in and see where I'm at. I gotta say compared to how the truck was with no mods versus now, it's better now, but I gotta think it can be better. I'm gonna check the valves and adjust when I get a free Saturday, and check my timing as well (not saying it's the cause of anything, but 233k miles and not sure the last time it was done... might as well check it).

hmm, my truck has 3:55's with 35's on it and i always like 1500 to 1800 rpms, might lug a couple seconds then shes turning right on. plus i like keeping my 19mpg city.
1 - 8 of 8 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.