Keeping RPM up...?

Fireman450

Fireman450

Daring to be diffrent!!
Joined
Nov 24, 2007
Messages
1,606
Ive been tryn to figure out how to sustain my rpm out on the track. Last season I was leaving the line at around 4500 and dropping to around 3500 then gaining a little back while going down the track.
This year I have been leaving around 5k-5200 and still dropping way down to around 4k then not gaining to much.
I am currently running 4.63 gears and pulling in 2nd (2wd) w/nv4500. @ 45psi in my tires (MT-ATZ +s)
Ive watched alot of trucks that can keep there rpm up off the line and not pull down like i do, but I do notice that my truck dead hooks off the line where others tend to blow the tires off.

Any suggestions? Thinking about going back to a 4.88 or trying a 5.13 gear but dont want to throw my final off.

Thanks
Lynn
 
id try shifting your weight forward just a little bit. if you wanna keep your rpms up you'll have to get those tires spinning some to let the rpms come back up goin down the track. i dont see it taking much of a weight shift either. but a little will be needed
 
Whats your exhaust temps and timing?
 
Last edited:
Do not give up traction if you already have it! Run a lower final if you want it to spin more rpm. But one question for you is will the motor actually make any power at that rpm? That is the first thing you need to find out is where the motor makes power and how much then gear accordingly. Good luck.
 
Thanks for the tips guys. Im seeing about 17-1800 EGT and im on 28 deg of timing.

Lynn
 
Lynn, Is it snuffing the motor out at the end? Or are you looking to turn more rpm to gain some ground speed going down the track?
 
Power.


Looking at a dyno graph from a load cell and noting the power at the various rpm values you've listed in this thread would make this pretty obvious IMO.

Go hit a load cell dyno and make a few pulls out to 5200 or so rpm. A load cell because it will give your pulling engine plenty of time to make power, and the actual number you get isn't important anyway, just where it makes it.
 
not used to 2 wheelers but your timing is WAY low compared to what we run. but what class are you pullin in.
 
Were the engine makes power really has nothing to do with it on a p-pump were you can over come a low gear with rpms. My truck makes the most power at 3500 rpm but i turn it 5500 and it performs 100 times better at that rpm then 3500. Alot has to do with gearing go down in gear and up in rpms and wheel speed. Also your timing is way low as smokinrod said compared to alot of others including myself.
 
bubba2400 said:
Lynn, Is it snuffing the motor out at the end? Or are you looking to turn more rpm to gain some ground speed going down the track?
Click to expand...

Thats what im looking for is a little more wheel/ ground speed. Im not snuffing out at the end I am spinning out.


smokinrod said:
not used to 2 wheelers but your timing is WAY low compared to what we run. but what class are you pullin in.
Click to expand...

Im pulling in the 2.6 class . Im gonna try bumping my timing up a few degres and see what that does. Ive got an adj gear here just havnt tore it back apart to put it on yet.

Lynn
 
Lynn, in the higher rpm trucks most are running between 34-40 degrees.
 
DISTURBED said:
Were the engine makes power really has nothing to do with it on a p-pump were you can over come a low gear with rpms. My truck makes the most power at 3500 rpm but i turn it 5500 and it performs 100 times better at that rpm then 3500. Alot has to do with gearing go down in gear and up in rpms and wheel speed. Also your timing is way low as smokinrod said compared to alot of others including myself.
Click to expand...

There is no way you will pull the same load farther, and faster without operating the engine in such a way that the average power for the duration of the pull is higher, or you find better traction to apply the power you already have.

Period.


A smart man would dyno the truck on a load cell where the rev gain most closely approximated what you see at the track and then choose his gearing based on reviewing his datalogs and choosing gearing that maximized average power over the course of the run. Especially the second half of the run when the weight starts to hit.

Because for any given available traction, the truck will pull farthest when the average power is highest. There's nothing complex about that. The complexity is only in finding the best gear to accomplish that with the infinitely variable track conditions you face.


And I agree that the timing seems soft for much anything over 3000rpm.
 
mmmm - if you can get your hands on one of the many gear/wheelspeed/rpm calculaotrs and plug your factors into it you will see that just changing gearing and rpm up will not necessarily increase your wheelspeed appreciably - what stiffer gearing will do is allow you to run higher rpm and keep it there - but to what advantage, if the stiffer gearing keeps the wheelspeed intrinsically the same

You said you dont wish to change your final ratio so the following applies

As i dont know your gear ratios I cant give an exact example but lets say

3500 rpm x your current gear = 24 mph wheelspeed
5500 rpm times new stiffer gear ratio still = 24 mph wheelspeed

so in order for you to increase wheel speed you would need appreciably more rpm to make up the gear ratio difference.

The other thing to keep in mind is that wheelspeed is "not" ground speed.

If you are totally hooked at all times going down the track, except the spinout at the very end, I would say you have a perfect setup now, and just lack some HP to pick up the rpm/groundspeed

If you think a stiffer gear will solve your problem bolt on a shorter set of tires and give that a whirl - much cheaper and easier than a gear change, especially if it doesnt pick the truck up in performance and you need to change back

I would take your "hooked" setup anyday over a wheelspinning/rpm setup
 
Lower final drive ratio will get you started easier and not spin the tires as soon. This can give you better ground speed if you can make power in the higher rpms through the middle of the track.
 
I ran 26* of timing last year, leaving the line at 5k and turning right at 4k down the track and it worked very well.
 
So what would yall advise to bump my timing up to? Im at 28 right now. What is too high for a 12 valve? Im thinking 34-35 degrees. Opinions?

Lynn
 
Out of curiosity, how long is an injection event in a cummins? Meaning, how long (in measure of time or crank angle degrees) does it take to inject a given shot of fuel from the time the injector starts opening, til it closes & is no longer spraying?

What dictates that time? Shot size? Something else?
 
Glen" Lynn has got the old cam out of DESTROKED.Lynn I still think you need to be running 5.13 are the 4.89 JMO.:evil

Dale
 
You must log in or register to reply here.
Back
Top