High engine revs at highway speed

[RSchwarzli]

Well, after many years of looking, I finally found my diesel truck. It's a 1990 W250 LE 3 spd. auto( 727) 4x4 reg cab and I think the rear ratio is 3. 07's. I am a new member as of last month and boy am I glad that I stumbled across this site. Talk about first class.

But, I do have a question. Did all these 1st gen diesels rev so high at highway speed? At 60 mph I'm well over 2000 rpm, which would probably be around 2500 rpm doing 70mph. How does anyone drive the interstates which post 70 mph speed limits without making this engine scream :{ for hours?

Thanks for any input.



Bernie

 

[DPKetchum]

You got it. Sorry. The early ones were that way. Can't go but around 70 or so wide open. Later ones have 4 speed auto.

 

[dan_gilson]

I can't speak to the high rev's.

Cummins says that any RPM under the

red line is safe to run with for long periods of time. The issue here is more economy and noise. The closer you are to the cross over between torque and HP the better economy you should have. If I remember correctly, a stock 1st gen's HP falls off after 2250RPM. So your fuel milage should suffer cruzing high than that.



Could the trans be slipping?

 

[RSchwarzli]

I do not believe that the trans is slipping. It only has 70,000 Miles on it and was pampered since new by an older diesel head. Besides, the 727 was one of the strongest auto trans ever made by Chrysler. This was used for the HEMI over the years. Thanks for the input.

Bernie

 

[DBF]

I sure hope I can get this table in correctly.





Speed vs. RPM for tire and gear:



This is the theoretical speed for a gear and RPM.

Torque converter slip is not taken into account.



This calculation is for a Chrysler A518, 3. 56 rear end,

and 235-85/R16 Load Range E tires (31. 73" dia). 1. 16



RPM 1st 2nd 3rd OD 3rd(3. 07)

1000 11 18 27 38 31. 31

1500 16 27 40 58 46. 38

2000 22 37 53 77 61. 46

2500 27 46 66 96 76. 53

3000 32 55 80 115 92. 77

3500 38 64 93 135 107. 84



AT Ratio: 2. 45 1. 45 1 0. 69



The first four columns in the table above are for my 92 with 3-speed auto and overdrive, assuming no slippage in the TC. The fifth column should be correct for a 3. 07 rear end with the same tires in direct gear (3rd). Since this is all just math, we can see the values even for RPM's our trucks can't make.



You can immediately see the benefit of the overdrive unit tacked on; that's about what an A518 is - a 727 with OD. If you're towing something heavy uphill, the OD isn't usable, but for making time on the flat, it's great.



If you're not somewhere close to the speed-vs-rpm for the 3. 07, let us know.



Regards, DBF

 

[RSchwarzli]

I would think that your numbers are pretty close. I will have to try this out in the morning.



Thanks. Bernie

 

[Joseph Donnelly]

Back then, Mopar used the 3 speed auto and 3. 07 gears to give you at least some highway speed. They used 3. 54 with the Getrag 5 speed manual that had an overdrive gear. A year or so later, they put in the overdrive 4 speed auto and went to 3. 54 gears.

 

[farmerzo]

I like the rpm vs. mph info. I was going to see if you could figure the rpms with 33 in tires. I have the 3 speed with 3. 07's and I've been over 100mph, just kinda curious what the rpms were. thanks, alonzo

 

[DBF]

Farmerzo, let's look at just one number in the table, the direct gear (3rd) on my truck at 2500 RPM, where it says the speed is 66mph.



The difference between my rear-end and yours is 1. 16, so 66 * 1. 16 = 77 mph (rounded).



The difference between a 31. 73" and a 33" tire is 1. 04, so 77 * 1. 04 = 80 mph.



Looks like a direct-gear (3rd) 727 at 2500 should get 80mph. Have you been tweaking the max RPM screw?



Folks, check me out on this stuff. I make mistakes fairly frequently. If you see where I've gone wrong, yell.



I do know that the RPM vs Speed shown is very close to what I get with my DTT'd A518.



Regards, DBF

 

[farmerzo]

thanks for that, no wonder I can't pass my math class. The max rpm screw has been played with a little I need to invest in a tach, for the interest of my engine life. Looks like I'm turning about 3150 rpm at 100mph. thanks again, alonzo