The Combine Forum banner
1 - 19 of 19 Posts

·
Registered
Joined
·
1,246 Posts
Discussion Starter · #1 ·
My "new to me", 75' w900 is short on top end. The truck is an immaculate 1975 needle nose, daycab. It has the 855 290 small cam, with a 9 speed transmission. This trucks top end is about 65mph but seems to have adequate power while pulling a loaded 40' hopper trailer in the hills. I'm not sure what gears it has in the rear ends but its got to be in the mid 4's, because at 65mph its showing 1900rpm. I'd enjoy driving the truck more if it would do at least 70-75mph and turn only about 1500-1600rpm. I don't have as much truck driving expireance as some on here but it seems that this low gear ratio makes it a little harder to down shift. What ratio of rear ends would you guys think are in this truck and can I achieve the faster speed at lower rpm's by just swapping rears? The truck has 22.5" brand new rubber so I'd hate to change up to 24.5's. TIA
 

·
Registered
Joined
·
3,616 Posts
I believe this had been talked about a while back with wanting a faster top gear and keep in mind if you did swap in some fast ( low numerically ) rear ends, you will loose your good bottom end gearing for lifting the load, maneuvering around and that means reverse as well.

If everything about the truck is original perhaps a serial number check can determine all answers for you, diff and transmission ratios but that is a long time ago and just not sure if records like that were kept as it was long before the computer age.

So if it were me wondering about doing changes to a truck and had no clue what the ratios of diff or trans were, I'd first look for the tag plate on the trans and get the numbers off of that to find out all of its ratios, reverse also. Then do the simple rolling test of the truck to count the revolutions/fraction of a revolution of the drive shaft. It takes two people to do it properly as the truck is parked on a flat smooth area with room to drive ahead, mark the sidewall of a drive tire at its bottom and the drive shaft & diff housing with chalk. Then one person drives slowly ahead straight while the other observes the revolutions of the shaft and stops you when the chalk mark on the tire has completed one revolution. Just make sure the person doing the counting is to the side and not in danger of being driven over !. To make the counting more accurate it can be rolled ahead two tire revolutions and then divide the driveshaft turns by 2. As its going to be a fraction such as 4.1 .. 4.66 as examples, note the direction the shaft turns and then will have to crawl under after the brakes are set to estimate how much of a revolution past 4.0 ( four turns as my example ) that the driveshaft turned.

Just from knowing what rear end ratio it has, and the engine RPM at a given ground speed, it can most likely be determined as to if your trans is a direct or overdrive model. To be honest I don't know if there is such a thing as an overdrive 9 speed back then, someone on here well versed with transmissions will be able to tell you that I would expect.

This way you do all the figuring on paper before one finger is lifted to modify anything and it may turn out that a 13 or 15 speed dropped in of the proper ratio ( having overdrive ) may be the best solution and not the diffs.
 

·
Premium Member
Joined
·
5,233 Posts
I'd enjoy driving the truck more if it would do at least 70-75mph and turn only about 1500-1600rpm.
The engine in that truck is not designed to operate under load at that low of RPM. You need to keep the RPM up on those older engines. They are designed to pull between 18 to 2100. Not being rude, just kind of blunt and honest, what you want or would like is irrelevant, you need to drive that engine as it was designed to be operated. You will kill off your engine if you continuously run it at low RPM under load. It is not like the newer engines that are designed to be run at low RPM for economy. Ya its an 855 alright, but it is not an N14;)

If yours does 65 at 1900, that is about how most of them were setup. If you want to spend the money to put longer legs under it, its simple, you have two choices, rear ends or transmission....one or both need to change...but I fail to understand why the need for speed? Why would you need a 75 MPH truck anyway? If you need a bit more speed for passing or such, you can push back the high end RPM limit to 2400 for intermittent use for that purpose. But as you pass 2100 RPM, fuel usage climbs fast for just those extra few hundred RPM if you run it continuously.
 

·
Registered
Joined
·
3,616 Posts
Glad you pointed that out AB, I knew the older Cummins didn't take to low RPM operation under a high load but I didn't realize it was quite that touchy.

As to power requirements to move a truck and trailer ( more so if its a taller grain trailer ) down the road and that's even empty, the power needed to maintain 75 is much higher then say 60/65, there is a reason why the trucking companies that are paying attention to the bottom line run their trucks at 55 or 60 because fuel and tire wear go up with high speeds and really the whole driveline and suspension just gets worked more if one hammers down the road fast.

You mentioned hard shifting, its pretty hard to sit here and say its mechanical or driver driving style or expectations in how it should shift but the diff ratio should have zero effect on how the transmission downshifts, operating RPM and technique will determine most of it and lets say by chance someone put in GL5 rated gear oil rather then the transmission specific oil, that certainly would have a detrimental effect to its shifting since the syncros so I am told do not like that extreme pressure additive.

I still encourage you to go through the motions to check out the ratios of the trans and diffs, at least you will know what you have under you if nothing else.
 

·
Registered
Joined
·
1,246 Posts
Discussion Starter · #6 ·
The engine in that truck is not designed to operate under load at that low of RPM. You need to keep the RPM up on those older engines. They are designed to pull between 18 to 2100. Not being rude, just kind of blunt and honest, what you want or would like is irrelevant, you need to drive that engine as it was designed to be operated. You will kill off your engine if you continuously run it at low RPM under load. It is not like the newer engines that are designed to be run at low RPM for economy. Ya its an 855 alright, but it is not an N14;)

If yours does 65 at 1900, that is about how most of them were setup. If you want to spend the money to put longer legs under it, its simple, you have two choices, rear ends or transmission....one or both need to change...but I fail to understand why the need for speed? Why would you need a 75 MPH truck anyway? If you need a bit more speed for passing or such, you can push back the high end RPM limit to 2400 for intermittent use for that purpose. But as you pass 2100 RPM, fuel usage climbs fast for just those extra few hundred RPM if you run it continuously.
Running and maintaining the suggested rpm's was something I was unaware of. I'll do the things that you, northern farmer, and Mr. Boles suggest. What is a safe max rpm limit for this engine? Like I said it is a day cab and has straight pipes so its pretty loud and it sounds like its wound up as tight as a banjo string at 2000rpm at just above 65mph.
 

·
Registered
Joined
·
149 Posts
We had an 87 t-600 with a 400 some horse cummins. Single straight pipe on passenger side. Scared the heck out of many truck drivers as I passed then at 2200 rpm and that truck screaming. I loved that truck. I'd say your plenty safe to run at 2100 rpm all day. Buy some ear plugs
 

·
Premium Member
Joined
·
5,233 Posts
Running and maintaining the suggested rpm's was something I was unaware of. I'll do the things that you, northern farmer, and Mr. Boles suggest. What is a safe max rpm limit for this engine? Like I said it is a day cab and has straight pipes so its pretty loud and it sounds like its wound up as tight as a banjo string at 2000rpm at just above 65mph.
Just so you understand, it is usually the block that will fail if you lug those older 855s too much. They can't take the strain and will crack the webbing in the bottom end. Cranks will crack and break as well along with other failures. many of these deficiencies were corrected and improvements made over time with the introduction of the Big Cam models leading upto the N14, which is designed to operate and produces it's torque at lower RPM.

As for high end RPM speed, 21-2200 is about the limit of sustained operation. For momentary needs they can safely be wound out to 2400. Empty can be sustained for increased road speed, but same not recommended under load. Do not engage the Jakes over 2200 either. Noticeable increase in fuel is going to occur at higher RPM. But being able to access those extra few hundred RPM can be a great help in hills or when you just need that bit of boost to pull out and pass or such in traffic.

My '72 HD 900 with a 350 has been setup like that for many many years and was worked very hard in heavy oilfield hauling both on and off road. It is also setup well over spec as for power and such as well. It required attention and respect when I was working it, while it was capable of pushing power and speed well beyond the limits, I only ever used them briefly when it was balls to the wall and I needed them;)
 

·
Registered
Joined
·
3,616 Posts
At least the ground speed aspect can be checked out pretty easy with a basic hand held GPS or ones that have phones with that ability.

The engine RPM isn't so easy unless someone had a trick I wasn't aware of as the hand held sensor type guns is what I presume can be used by placing a sticker on the front pulley. Otherwise if one knows all the gear ratios ( diff and top gear in trans ) and tire size, it can be theorized as to what the RPM should be and if your tach comes close to that number.

I had to laugh at Lester's ear plug comment because that is exactly what I do in noisy trucks or equipment, stick in the ear plugs and drive. At least with a noisy truck the wild life might notice and run to the side before you get there. If you thought the Cummins sounded fast, just drive a screamin Jimmy for a while and then it won't seem so bad anymore ;)
 

·
Registered
Joined
·
3,933 Posts
Put 4 feet of auger flighting in the straight pipe. If it a little big stretch it out. A tack weld in the bottom. Pull it in with a chain fall so its tight. Same performance, just quieter. Drive it like a two stroke. Pinch your hand in the door when you get in and you are good to go. And wind resistance and braking are like spray pressure. Double your speed and your braking distance, and wind resistance go up 4 times.
 

·
Registered
Joined
·
1,246 Posts
Discussion Starter · #12 ·
At least the ground speed aspect can be checked out pretty easy with a basic hand held GPS or ones that have phones with that ability.

The engine RPM isn't so easy unless someone had a trick I wasn't aware of as the hand held sensor type guns is what I presume can be used by placing a sticker on the front pulley. Otherwise if one knows all the gear ratios ( diff and top gear in trans ) and tire size, it can be theorized as to what the RPM should be and if your tach comes close to that number.

I had to laugh at Lester's ear plug comment because that is exactly what I do in noisy trucks or equipment, stick in the ear plugs and drive. At least with a noisy truck the wild life might notice and run to the side before you get there. If you thought the Cummins sounded fast, just drive a screamin Jimmy for a while and then it won't seem so bad anymore ;)
Yeah, I drove one of those Jimmy's before, had the 2 stroke supercharged V8 Detroit. I was about 19 years old and had never drove a jimmy or been around one. As I was opening the door of the truck, I asked the guy if it had any quirks that I need to know about, his brother quickly cut him off and stated "All you'll expireance is quirks and if you make it to the elevator, that will be a quirk in itself.
 

·
Registered
Joined
·
3,933 Posts
"If you thought the Cummins sounded fast, just drive a screamin Jimmy for a while and then it won't seem so bad anymore "

In the early 60's Cummins had a supercharger on a motor. Supposed to be more distinctive than a Detroit sound.
 

·
Registered
Joined
·
3,516 Posts
All original truck... Don't touch it..

The small can will not hold up to the horsepower you would need to run 75 mph.. And the money you need to put into it.... Buy another truck.. Or turn it into a glider kit.
 

·
Registered
Joined
·
931 Posts
I just came across a 76 W900A on the weekend with a 290 in it. Going to look into it and see what the guy wants for it. I also have a 1964 White Freightliner cabover with a 220 in it 6" straight pipe no turbo so it really barks. I run that one at 2100rpm when loaded.
 

·
Registered
Joined
·
9,738 Posts
Just cause it's old don't mean it needs to be driven like a screaming demon as it is not two stroke, I drive some of these old trucks, they like to be a little higher in rpm but it will not cause it harm to drive like a white man. I would consider putting a 13 speed double over in it. It probably won't quite get the speed you are looking for but it will make it a real treat to drive when you get the hang of it
 

·
Registered
Joined
·
1,093 Posts
If its as pristene as you say, I would have a rough time changing anything. I like clean and original stuff.

If some things have been changed around already, I would look into a trans swap.

See what you have now, first gear and top gear wise, and look into putting an 18 speed in. this should allow you to keep a nice low first, and give you two OD ratios.

a used takout early 18 speed will be getting sorta affordable (not nearly as affordable as an old RTO 9513. :) )

Keep us posted!
 
1 - 19 of 19 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.
Top