Rear end gear change from 5.13 to 4.33 on 8.3 Cummins u270

Topic: Rear end gear change from 5.13 to 4.33 on 8.3 Cummins u270 (Read 5317 times) previous topic - next topic

Rear end gear change from 5.13 to 4.33 on 8.3 Cummins u270

October 12, 2014, 11:06:50 pm

I need a torque and HP curve for 1997 C8.3 325hp RV. The best thing I can find on the internet is this link: Cummins C325-20 Diesel Engine for Vehicle---Diesel Engine, Diesel Engine Parts,

I'm planning on changing my rear end gearing and want to see where I need to have my RPM be for a nice 65-70mph cruise on the flat.

Re: 8.3 325hp mechanical torque and hp curve

Reply #1 – October 13, 2014, 12:02:04 am

Quote from: krush – October 12, 2014, 11:06:50 pm

I'm planning on changing my rear end gearing and want to see where I need to have my RPM be for a nice 65-70mph cruise on the flat.
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Krush

Your 8.3 Cummins probably turns about 100 RPM more than our 9 liter Detroit. With 295/75s, we top out at 82 MPH and cruise nicely at your desired speeds. We have a 3.07 ratio. There will either a removable tag under a third member nut or a tag tacked on the housing, probably on the right side viewed from the rear with the ratio and number of teeth on the R&P.

4th is 1:1 on our Allison four speed

Pierce

Re: 8.3 325hp mechanical torque and hp curve

Reply #2 – October 13, 2014, 12:18:22 am

Pierce, I wrote about this in that old thread: Rear vibration 65-70mph+

My coach has 5.13

Re: 8.3 325hp mechanical torque and hp curve

Reply #3 – October 13, 2014, 01:59:50 am

Krush,

For some reason, I was thinking you had an 1989 U280 with a four speed. With your 1998, you will have a 6 speed so should have an O.D. of .65. If you have the same diameter tires and switched to a 4.63, this would give you about the same final drive and revolutions per mile as we have with our 3.07 ratio in our U300. With a slightly smaller engine, the six speed will still give you a pretty good first gear start on most any hill or altitude. Between the 4.63 and the 4.33, I would think a primary west coast rig would be better with the 4.63 and a mid-west/east coast rig would be good with a 4.33. With your length, you have to knock off a couple of thousand pounds so the 4.33 might end up the best choice, especially with the six speed.

My ex-Greyhound 4107 had a much lower rear end ratio than our U300 and about 125 HP less with the non-turbo 8V-71. It would eventually get to about 90 mph and did get a couple of MPG better. Same length but 96" and lighter. With a VW Rabbit for a toad, the worst I every got was 9.3 MPG in the Rockies. Was sure slow on the grades.

Pierce

Re: 8.3 325hp mechanical torque and hp curve

Reply #4 – October 13, 2014, 11:20:45 am

I would investigate to see if gale banks still readjusts mechanical 8.3's.

More power can be geared higher. Normally improves mileage btw.

Turned up a lot of mechanical 8.3's.

More is better

Re: 8.3 325hp mechanical torque and hp curve

Reply #5 – October 13, 2014, 11:35:56 am

I don't have a power problem. There is plenty of power. I rarely tow, and if I do it's light. I'm around 28,000-29,000. And I haven't fully gone over everything to insure it is in tip-top shape (valves, fuel pressure, etc).

The gearing is horrible on this coach. I don't know what they were thinking. 65 is ~1900 in 6th. 70mph is about 2050. 75mph is 2200. 2400 is max governed RPM.

In my mind, it should be geared so 1:1 gear (4th) is used around 50-55mph. A regular overdrive shouldn't be used until above 60 (5th). and a double overdrive should be for high speed flat cruising. There is nothing wrong with dropping down a gear for grades.

The only downside is I need to watch what RPM will be around 50-55mph to ensure I'm around the power band and don't end up in a bad spot between gears.

I also need to make sure 1st gear doesn't get too high...but I'm not making a significant change, so that won't be a problem.

I need to do this before I go out west and have long stretches of flat nothing. The east has more hills than people think.

Re: 8.3 325hp mechanical torque and hp curve

Reply #6 – October 13, 2014, 11:55:17 am

at what RPM is Max torque achieved in your engine?Â Max torque is best efficiency.Â Most FT I have had have all been geared so that transmission is in top gear, and at Max torque RPM @ 65MPH

Is Max torque for your engine at 1900 RPM, or lower?

Re: 8.3 325hp mechanical torque and hp curve

Reply #7 – October 13, 2014, 12:10:23 pm

Krush,

You won't have a problem in first as it is MUCH higher numerically than first in our four speed Allison. That is the reason I have to back up our driveway rather than just heading up it. When the OD fifth and sixth are balanced against a 4.33 or 4.65 rear ratio, you will have far better gear selections than we do with our four speed. Fuel cutoff at 2110 RPM give us about 57 MPH in third so not the end of the world on grades. It does make "hunting" between third and fourth annoying if you don't just punch in third and relax.

You and your six speed will be happy with either of those ratios. Don't think you will find any bad spots between gears.

Yes, the east does have a lot of grades but they are shorter and much lower altitude but with a lot of rude drivers and a couple of low spots in each lane from overloaded trucks.

Third member from a RV wrecking yard seems a better choice than having a shop replace the R&P. Probably cheaper too. A couple of guys and a jack...but hey, you know the drill.

Pierce

Re: 8.3 325hp mechanical torque and hp curve

Reply #8 – October 13, 2014, 12:16:50 pm

Quote from: Tim Fiedler – October 13, 2014, 11:55:17 am

Most FT I have had have all been geared so that transmission is in top gear, and at Max torque RPM @ 65MPH

Is Max torque for your engine at 1900 RPM, or lower?

Are you sure you mean "Max Torque" RPM? On my engine, Torque Peak (lb-ft) is listed as: 820 @ 1300 RPM. At 65 mph in 6th gear, I show about 1900 RPM - well above the Torque Peak.

Re: 8.3 325hp mechanical torque and hp curve

Reply #9 – October 13, 2014, 12:22:36 pm

Yes, I mean Peak Torque - what speed are you going at 1300 RPM again?Â My recollection from a Cummins Technical bulletin was that Diesel engines have the greatest efficiency at their peak torque RMP - in mine that is 65MPH at 1500 RPM as I recall.Â Thought it was similar in my ISM on my 1999 U-270 as well.

Or I could just be getting m ore senile.....

:-)

Re: 8.3 325hp mechanical torque and hp curve

Reply #10 – October 13, 2014, 12:38:08 pm

I'll have to try it out next time we drive the coach - I don't think I could get down to 1300 RPM in 6th gear, without the trans downshifting. But I could be wrong!

If it is possible, and IF it is a direct numerical relationship, then if 1900 RPM (in 6th) = 65 mph, 1300 RPM (in 6th) would = 44 mph.

Re: 8.3 325hp mechanical torque and hp curve

Reply #11 – October 13, 2014, 12:45:57 pm

Our 6V-92TA generates it's peak torque of 1020 ft lbs at 1200 RPM and 350HP at 2110 RPM. That's why they got away with a 4.xx first gear with the 746 Allison. A 8.3 Cummins is only slightly smaller in displacement so should have plenty of torque for a 1500 RPM cruise at 65 MPH in sixth gear. Any higher RPM at 65 means more piston speed/travel per mile and that usually translates to less MPG.

We could easily pull a rear end ratio down in the high 2.xx and still have plenty of power. Would give us a good third gear grade speed.

Pierce

Re: 8.3 325hp mechanical torque and hp curve

Reply #12 – October 13, 2014, 12:46:59 pm

Quote from: Tim Fiedler – October 13, 2014, 11:55:17 am

at what RPM is Max torque achieved in your engine?Â Max torque is best efficiency.Â Most FT I have had have all been geared so that transmission is in top gear, and at Max torque RPM @ 65MPH

Is Max torque for your engine at 1900 RPM, or lower?

That was the purpose for starting this thread-----to see if anybody has any hp/tq curves. I pasted one in the first message. It's the best i can find.

If you gear for max torque with out ability to ever get to rated hp RPM, you will be underpowered and overloading the engine. It's not about having torque to cruise at 65mph, it's about ensuring the engine makes enough HP at that RPM. BUT we have ANOTHER thread for that discussion!!!!!! HP vs Torque discussion (split from Overheat on Long Grade)

I found take out rear ends for ~$300 plus shipping. Just pull the axles and bolt it right in. Just heavy, but I've done worse.

The only thing I've heard is that RV's have "quiet ride" gears that are polished. I can get a rear from another RV that has quiet ride, but they are 4.63. I can also try to polish up the gears myself. But really, I don't think it's a huge deal as the rear is back by the engine making all that noise anyway.

Re: 8.3 325hp mechanical torque and hp curve

Reply #13 – October 13, 2014, 01:02:40 pm

Krush,

Good price for the gears assuming no moisture has been in the rear axle assembly (reddish cast to most everything) . As far as noise goes, if the mesh pattern is at the correct spot on the ring gear, they will "polish" themselves in short time when new. Only have heard noise out of straight cut gears or gears with improper mesh pattern. The right bearing preload, lash, mesh pattern mean a quiet rear end. Nice to have an old timer "lead" (as in lead paint) the gears, rock the ring gear back and forth a few teeth and give his opinion.

Rear ends that are quiet while slowing, make a slight clunk when you accelerate and then whine have incorrect lash and mesh pattern.

Pierce

Re: 8.3 325hp mechanical torque and hp curve

Reply #14 – October 13, 2014, 01:10:49 pm

They call it "QuietRide and noise sensitive bevel gear technology for quiet and smooth operation".

ear Axle: Meritor (Rockwell) RS-21-145 Quiet ride Motorhome Gearing 4.88 Ratio Manual: Renegade custom built motorhomes and toterhomes

I spoke breify with James T at FOT and he said it may be noiser because they use Quiet Ride gears. But, I haven't found specifically what the difference is. I bet, they just spend a few more minutes getting tolerances closer.

Re: 8.3 325hp mechanical torque and hp curve

Reply #15 – October 13, 2014, 02:22:52 pm

Decent article (geared more towards OTR trucks) on fuel mileage - lots of info - see page 23 for gearing discussion and RPM on ISM (Mine is right there 1500 - 1600 RPM at 65MPH. - which is max torque

other factors:

Speed

Aerodynamics

Cooling Fan drag Fan

Tires (age and pressure)

Lubricants

and a host of other factors

Re: 8.3 325hp mechanical torque and hp curve

Reply #16 – October 13, 2014, 02:46:28 pm

I had a friend ( he has since passed away Replace the rear end in a U270 only to find that the WHINE was more than he could bear, he sold it soon after and I have no idea who has it know. He also related that he should have gone with RV REAR END. I also believe that a RV rear is set up more precisely than a truck would be.
Gary B

Re: 8.3 325hp mechanical torque and hp curve

Reply #17 – October 13, 2014, 02:55:17 pm

Tim,
Which way are they playing the power factor, leading or lagging to achieve the 60% reduction of LRA ? Would guess lagging but with Generac who knows the game plan, maybe a secret ?
Dave M

Re: 8.3 325hp mechanical torque and hp curve

Reply #18 – October 13, 2014, 04:01:59 pm

will send you a PM

Re: 8.3 325hp mechanical torque and hp curve

Reply #19 – October 13, 2014, 05:07:43 pm

100 LRA single phase will be 40 or less after install of Sure Start

Re: 8.3 325hp mechanical torque and hp curve

Reply #20 – October 13, 2014, 05:07:47 pm

One More - Cummins FMCA 2014 presentation .ppt

http://cumminsengines.com/uploads/docs/FMCA-presentation-August-2014.pdf

Re: 8.3 325hp mechanical torque and hp curve

Reply #21 – October 13, 2014, 05:07:51 pm

sorry, forgot to add document link - here it is

http://cumminsengines.com/uploads/docs/cummins_secrets_of_better_fuel_economy.pdf

Re: 8.3 325hp mechanical torque and hp curve

Reply #22 – October 13, 2014, 05:28:23 pm

The first image I posted have brake specific fuel consumption. You'll see this is lowest around 1300-1400rpm. But, as we go back to the same topic again. Running where torque curve is maximum is pointless if your engine isn't making enough HP to maintain vehicle speed.

Re: 8.3 325hp mechanical torque and hp curve

Reply #23 – October 13, 2014, 05:33:44 pm

Give MERITOR technical dept., a call and run your idea past them before you go to the trouble and expense of this change (please.) Then put your calculator to work and determine the expense of your idea divided by planned mileage.

Unless you plan on going a distance, I'd bet the expense isn't worth your effort. pc

Re: 8.3 325hp mechanical torque and hp curve

Reply #24 – October 13, 2014, 05:45:54 pm

A couple hundred bucks to prevent a well known $3000 transmission repair on the p3 output bearing?

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