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

Re: 8.3 325hp mechanical torque and hp curve

Reply #25 – October 13, 2014, 05:48:42 pm

Had real noise problems in the 300 ORED rear ends starting in 1987. Howl under load.

Had to add gm positraction additive and b&m heavy weight rear end oil to make the noise inaudible brand new.

Oshkosh used the less expensive non polished gears. $40 difference oem cost.

40k miles later you could pull the pumpkin out and redo the backlash and the noise was reduced.

You could see the machining marks on the teeth. The quiet ones were shiny smooth.

Don't IMO.

Learned to max the power then regear. Did many different rigs.

Mechanical cummins if someone were inclined to do so can adjust the Bosch fuel pumps pressure with a screwdriver to turn the wheel under the hex nut. See gale banks as this has been done thousands of times safely.

Ten percent faster zero to,sixty at least. Screwdriver....

Re: 8.3 325hp mechanical torque and hp curve

Reply #26 – October 13, 2014, 06:46:22 pm

I assume at some point Foretravel switched from the mechanical to the electronic C8.3. If so, then what year?

Re: 8.3 325hp mechanical torque and hp curve

Reply #27 – October 14, 2014, 01:16:07 am

Running at peak torque under heavy load such as head wind or slight grade can easily build exhaust temp. Engine speed is our friend in these situations. Try pulling a gentle grade into a headwind at 65mph with cruise on. I've accepted 64 mph is the all around most efficient cruise speed on the flats and down shifting to get RPM up when the pyrometer starts to climb. Amazing how flooring after downshifting significantly drops coolant and exhaust temps uphill in the Rockys!
Several owner operator Semi drivers and a knowledgeable member of this Forum have insisted I implement this technique. Factory might have known what they were doing with our rear ends? The aerodynamics alone make 75 + cruising laborious no matter how you gear our 8.3's

Re: 8.3 325hp mechanical torque and hp curve

Reply #28 – October 14, 2014, 02:24:57 am

While our U300 makes 1020 ft lbs of torque at 1200 RPM, it does not taper off that much at the fuel cutoff of 2110 RPM. Sometimes we cruise at 55 or 60 but when in a hurry, I have gone several days at 82 MPH without a complaint. When it slows down on hills, I shift down but unless it wants to hunt between gears, I keep it petal to the metal letting up only when the engine temp gets to 200 and then dropping gears to keep it under the magic 200 mark. I have driven Detroits and Mercedes that way since the late 1960's with manual transmissions but exactly the same way without any kind of failure in several hundred thousand miles. Diesels whether Detroits, Mercedes, Cummins, CATs are built to produce max power as long as you don't overheat them or bog them down but the Allison takes care of the bog part and makes them pretty idiot proof as long as you watch the temperature gauge.

While I love my Detroit, I think Krush's Cummins will do the exact same thing day after day, mile after mile. It's the unskilled RV driver who thinks they can idle the engine all day while they have lunch or shuts it down to take photos after climbing a long grade without bringing the coolant and oil temperature down that damages the engine and shortens it's life.

Look at any big rig with over twice the load we have and probably pushing twice the air out of his or her way. You don't see them baby the engine. They shift down to keep the RPM up at the high end of the power band and move a lot more coolant at that RPM also while going down the road usually at much higher than the posted speed limit. The only thing that keeps a big rig at 65 is the price of diesel and not knowing who belongs to the headlights behind.

With the horsepower we have in our engines compared to marine use and the Allison automatic that won't let you lug the engine, even the newbie will have a hard time hurting the engine. Unless you have had a shop turn the fuel way up, EGTs are not going to be a problem with the Allison behind the engine.

We used to turn our engines at 2400 RPM in the fire service so the only worries I have at 2100 and 82 MPH is getting a ticket and paying for the extra fuel used. Cummins also makes great engines and being slightly smaller displacement (8.3 vs 9.0), allows it to turn at even higher RPM for extended periods also without any damage.

The biggest challenge for an RV driver is not going up the hills but coming down the grades in the correct gear and knowing how much to use the service brakes with the added bonus of a little retarder use if so equipped. The Allison takes most the thinking out of it going up but it can't do it all on the backside.

Pierce

Re: 8.3 325hp mechanical torque and hp curve

Reply #29 – October 14, 2014, 10:22:47 am

Quote from: TAS69 – October 14, 2014, 01:16:07 am

Running at peak torque under heavy load such as head wind or slight grade can easily build exhaust temp. Engine speed is our friend in these situations. ........... Amazing how flooring after downshifting significantly drops coolant and exhaust temps uphill in the Rockys!

It all comes down to how much HP is being USED at the RPM running at. Again, refer to the HP curve in the first post. If you are running 1800rpm and going 60mph and this requires less HP than the engine is capable of making at 1800rpm (the line), then the EGT will lower.

If you overload the engine at 1600 and require more HP than it is designed to put up to maintain that speed (above the line) EGT will go up.

It's all about matching available HP at an RPM to what is required to maintain the speed desired.

The premise of my exercise here is that going 65 mph doesn't require all the HP available (approx 300hp) at 1900rpm.

My coach is a 1998 and has mechanical cummins (engine is 1997). I think 1998 engines are electronic--making 1999 coaches have electronic engines.

Re: 8.3 325hp mechanical torque and hp curve

Reply #30 – October 14, 2014, 12:29:15 pm

Quote

My coach is a 1998 and has mechanical cummins (engine is 1997). I think 1998 engines are electronic--making 1999 coaches have electronic engines.

Our 99' coach has a build date of July 98' and is an ISC (electronic 350HP), the engine was built in 98' as well of course, and was also as a 99' model year.
Don

Re: 8.3 325hp mechanical torque and hp curve

Reply #31 – October 15, 2014, 12:11:36 pm

So, original question: Where do I find a HP/Torque curve?

Re: 8.3 325hp mechanical torque and hp curve

Reply #32 – October 15, 2014, 01:00:27 pm

Krush,

I would choose one of these that is the closest to what you have. Don't know if yours is stock or modified but take a look: https://www.google.com/search?q=8.3+cummins+torque+curve+graph&espv=2&biw=1410&bih=779&tbm=isch&tbo=u&source=univ&sa=X&ei=sqc-VJS5LND4yQTRt4CgBA&ved=0CCoQsAQ

Pierce

Re: 8.3 325hp mechanical torque and hp curve

Reply #33 – October 15, 2014, 04:05:41 pm

While trying to figure how to hot rod your small engine, why not think of a larger Big Boy engine that gets the job done day in & day out ?
Could never figure why folks buy the little engines, guess due to being unaware and just looking at $$$$ only ? Looks like one needs to live with their toy as is.
Me, I love whopping butt on mountain up grades, everyone goes screaming past me on the down grades with brakes smoking. Another reason I laugh.

Re: 8.3 325hp mechanical torque and hp curve

Reply #34 – October 15, 2014, 04:15:56 pm

Take it to a Cummins dealer, have them put it on the dyno, you will have all the info you need for your engine, then you can be sure it is running up to snuf.

Re: 8.3 325hp mechanical torque and hp curve

Reply #35 – October 15, 2014, 05:13:06 pm

FYI, dyno's are hard on rear tires as they heat up too much. Also radiator may not get enough air flow to keep coolant temps low.

And while I am at it, steam cleaning pressure washing engines can be hard on gaskets, electrical parts, etc. I have seen Cummins require steam cleaning, even if not dirty, before working on engine. Also radiator fins can be bent with pressure washer.

Re: 8.3 325hp mechanical torque and hp curve

Reply #36 – October 15, 2014, 05:23:19 pm

Quote from: krush – October 13, 2014, 11:35:56 am

I don't have a power problem.

I think you do have a power problem. The curves you showed appear to be reasonable given the stated torque and horsepower for your engine. Your problem is that the engine needs to spin up to a high RPM to develop its rated horsepower.

Contrast that with the M11 in my U320 which weighs roughly the same as your coach, and develops roughly the same wind resistance at a given speed. My M11 develops about 370 hp at the 1500 RPM that it turns at around 70 MPH.

It's pretty clear why they chose the axle ratio they did for your coach. The small torque of your engine means that it has to spin at a high RPM to develop reasonable horsepower and that requires roughly the axle ratio they chose to perform adequately under typical conditions.

Re: 8.3 325hp mechanical torque and hp curve

Reply #37 – October 15, 2014, 05:28:11 pm

Quote from: Dave M – October 15, 2014, 04:05:41 pm

While trying to figure how to hot rod your small engine, why not think of a larger Big Boy engine that gets the job done day in & day out ?
Could never figure why folks buy the little engines, guess due to being unaware and just looking at $$$$ only ? Looks like one needs to live with their toy as is.

I completely agree! The M11 in our coach is perfect for the application. It develops 1350 lb-ft at 1200 RPM, and 370 hp at 1500 RPM at about 70 MPH, which is where we like to cruise. With 400 hp at 1800 RPM it performs very nicely both in the mountains and on the flats.

I'd be very happy to have your 500 hp, but I try not to be greedy!

Re: 8.3 325hp mechanical torque and hp curve

Reply #38 – October 15, 2014, 05:51:14 pm

Quote from: D.J. Osborn – October 15, 2014, 05:23:19 pm

My M11 develops about 370 hp at the 1500 RPM that it turns at around 70 MPH.

You could probably run lower RPM at 70mph on the flat. What is your boost at that speed/rpm? Or do you have a read out that gives you % of power.

Where did I state I was trying to go faster up hills? I stated I'm trying to go faster on the level. There is a known problem with the p3 carrier bearing going bad on these coaches. Coaches that are driven around 60mph don't seem to develop this problems. Coaches driven around 70mph do.

U295 Driveline vibration issues

A double overdrive (6th) isn't meant to be engaged at 50-55mph.

Downshifting for hills allows the engine to get to the proper RPM to produce HP needed.

I A) don't want to blow up my transmission by driving 70-75mph and B) hopefully get a tiny bit better fuel economy.

How about this...I realistically cannot drive my coach at 75mph. Driveshaft RPM is too fast (big deal) and engine RPM is unnecessarily fast (not a huge deal).

Another fact: in 1999, coaches went to 4.63 gearing (350hp) from 5.13 (325hp) in 1998. Hmmmmmmmm...the mother ship changed something!

Lastly, I have a 34' coach. I like it, I don't want a 36. I wish it was 30' foot or less. The 8.3 is a great engine and easy to maintain and gets great economy. I have all the power I need.

Re: 8.3 325hp mechanical torque and hp curve

Reply #39 – October 15, 2014, 06:42:25 pm

You guys are trying to out think the situation. Just stick the 4.63 (or lower) in and be happy. It will be the same final drive as our U300, be lighter and with 6 speeds with a higher numerical first gear, way quicker and faster in a quarter mile. Better (more) ratios for the grades so probably faster up those too. Foretravel got it right on a lot of things but also missed the boat on others. At the $300 plus shipping quoted earlier, that's almost free with not much labor for installation.

I have dropped ratios in many diesel cars and trucks and have been happy with the results every time. If ours were a 6 speed with OD 5th and 6th, I would drop the ratio to a 4.33 in a second.

Pierce

Re: 8.3 325hp mechanical torque and hp curve

Reply #40 – October 15, 2014, 07:44:32 pm

Pierce makes a good point: If you want the engine to turn at a lower RIM then change the differential to a numerically lower ratio. Simple!

Re: 8.3 325hp mechanical torque and hp curve

Reply #41 – October 15, 2014, 08:50:15 pm

Quote from: krush – October 15, 2014, 12:11:36 pm

So, original question: Where do I find a HP/Torque curve?

Gale Banks engineering might have it on their site.

They offer proven kits and help for 93-98 cummins "c' engines up to an including your 325hp.

Many customers with better mileage, more power, advice on gearing needed.

I would only buy a mechanical 8.3 knowing I could buy the banks kit for it.

You have the right coach and cooling and stopping systems.

Look at their website and call them IMO.

Re: 8.3 325hp mechanical torque and hp curve

Reply #42 – October 15, 2014, 08:54:33 pm

Presently using the stock gearing of 3.91, but thinking maybe need to check for other ratios like 4.11, 4.33,4.44, 4.56. Need to run the numbers and see what the present housing is capable of using. Always playing gear ratios, not looking for mpg, just get er done within reason. And yes having two over drive radios is good, Boys & toys.

Re: 8.3 325hp mechanical torque and hp curve

Reply #43 – October 15, 2014, 09:33:26 pm

I have an email sent to Cummins tech service...hopefully they return the specs I want.

I found this post via google on another forum. It's the internet, so I am always skeptical. But, the numbers sound right.

Quote

According to the specifications for my C8.3, the engine has the following performance characteristics:

1300 rpm: 820 ft-lbs and 203 hp (full throttle)
2200 rpm: 716 ft-lbs and 300 hp (full throttle)

peak torque rpm: 1300 rpm
peak hp rpm: 2200 rpm
allowable continuous full throttle rpm range: 1300 rpm to 2200 rpm
allowable 1 minute full throttle rpm range: idle to 1300 rpm
governed maximum rpm: 2200 rpm (at full throttle)
maximum operating rpm: 2400 rpm (when downshifting the transmission to slow down the vehicle)
idle rpm: 700 rpm
maximum continuous operating temperature: 212 °F
thermostat start to to open temperature: 178 °F
thermostat fully open temperature: 203 °F

According to my Cummins operation and maintenance manual the engine is most efficient when operated slightly above the peak torque rpm (1300 rpm in my case). Since it produces an incredible 203 hp at only 1300 rpm, as long as the cooling system can handle it, to get the best fuel economy it makes sense to allow the engine to lug down to close to 1300 rpm before downshifting to the next gear when climbing a long, steep hill.

https://www.rv.net/FORUM/index.cfm/fuseaction/thread/tid/23812743/srt/pd/pging/1/page/2.cfm

Re: 8.3 325hp mechanical torque and hp curve

Reply #44 – October 15, 2014, 10:13:59 pm

You have a 325

Re: 8.3 325hp mechanical torque and hp curve

Reply #45 – October 15, 2014, 10:32:19 pm

Quote from: Caflashbob – October 15, 2014, 10:13:59 pm

You have a 325

Yes, I know.

Re: 8.3 325hp mechanical torque and hp curve

Reply #46 – October 15, 2014, 11:04:16 pm

I would buy any coach knowing I am going to be happy with it as it is. I would never buy a motorhome wanting to later modify it for more power. If I want/need more power, I need to buy a different model with higher horsepower. All power increases can come with unknown issues, including the popular Banks.

We know several are VERY happy with their Banks systems and would not trade it for anything.

And we also know Banks user that found exhaust temps too high and removed the Banks rather than taking so much time watching EGT gauge, with the risk of damaging engine. Often more power can result in more heat.

Re: 8.3 325hp mechanical torque and hp curve

Reply #47 – October 16, 2014, 12:27:43 am

Aftermarket turbo kits always go hand in hand with adding fuel to the injection pump. Add too much and you get smoke and high EGTs. Unless the mechanical injection pump has an altitude compensator, the EGT will go UP with increased altitude. High altitude together with a restricted air cleaner can really raise the exhaust temperature.

As far as having to watch the EGT gauge, an engine with a turbo kit installed may lower the EGT if you take some of the extra fuel out of the pump. Some people get too greedy and boost too much or add too much fuel or both or drive their modified diesel with a restricted air cleaner. Hard to imagine a stock Foretravel with an Allison controlling the shift points having to worry about high exhaust temps. Another reason this old diesel saying was true, especially with manual transmissions, "Drive it like you stole it", meaning high RPM translates into lower EGT in turn translates into longer engine life. Very hard for many gasoline car drivers to be able to understand or do.

EGT probe goes before the turbine wheel, not after for correct reading.

Pierce

Re: 8.3 325hp mechanical torque and hp curve

Reply #48 – October 16, 2014, 12:41:23 am

I made excel sheet with speed vs engine RPM for 5.13, 4.63, and 4.33 gears---for each gear 1st through 6th. Lower on the sheet is RPM of driveshaft.

520 is what I used rev's per mile. This matches up to a michelin number and the number seem to match up to my Tach and Speedometer.

Doing a bit of analysis shows me that 4.33 is going to probably be the choice. 4.33 allows for a nice 72mph cruise of 1800rpm (which is a nice good blend of HP and efficiency). 65mph will give around 1600. This is for FLAT cruising.

If there is a problem, pop it down into 5th, and we basically are at the same conditions as the original 5.13 gearing....meaning 5th with 4.33 is about same overall ratio as 6th with 5.13.

And 4.33 in 4th gear gives 2050rpm at 55mph...perfect for the hill climbs.

Of course, this is all in theory

. But, since 1999+ coaches have 4.63 gears and nobody seems to complain, I think this should work out well.

I don't plan on doing banks kit or upping the fuel. My power is fine. Gearing doesn't change power.

The jump from 4th to 5th means that peak HP (2100 or so) is going to be ~55mph or ~75mph. But 65 in 5th gear should be fine for a fair grade.

Re: 8.3 325hp mechanical torque and hp curve

Reply #49 – October 16, 2014, 02:17:58 am

IF you ever sell your coach a 400hp ten mpg rig is worth more than the cost of the work. $10-$15k+. Trust me after selling a lot of high line motorhomes.

IF you were fortunate enough to have a modifiable coach and did not I would be able to buy it for less and add to its future value cheaply.

Especially for anyone who travels the Rockies which it seems you do not do.

With the increased power you could lower the numerical ratios even more. Better mpg.

Regeared many 3176 Marquis because of adding power.

Some future time the banks parts may be unavailable. Old already.

Safer having a coach that on a six percent grade does not have to go in and out of the truck lanes.

Or without a tow can be in the left lane up hills. Power sells rv's. Type "a" are not patient.

For sure adds resale and owners enjoyment.

I hated driving around the steep grades where the pretty was to stay on the flatter bottom of the canyon roads.

My two cents. You have an enviable coach that can legally and safely be rerated.

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