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Topic: U 270 top end speed and RPM's (Read 859 times) previous topic - next topic

U 270 top end speed and RPM's

Have any of you U 270 owners ever checked out your WOT performance in regards to top speed and RPM's? I have always been told by the diesel experts that if your engine performs the way it is supposed to in relationship to it's set up with gearing and transmissions under load performances that all other aspects should pretty much be on the money. Those of you that have checked yours out, would you mind posting your results....thx
Rick Barna
St. Petersburg FL.
1998 U270 36'
Build # 5232
1972 CJ 5 Jeep

Re: U 270 top end speed and RPM's

Reply #1 –
See my threads on this topic, including an excel spreadsheet where I calculated rpm/speed for different gear ratios. The excel spreadsheet numbers are correct as verified by my swap to 4.33 gears. 5.13 was way too low of ratio. I topped out at 92mph on the flat...wouldn't climb any faster. I cruise 70-75. if it's rolling hills, I put it in 5th and still cruise 70-75.  6th gear is 60mph 1600, 66mph 1700, 70 1800.

Rear end gear change from 5.13 to 4.33 on 8.3 Cummins u270
and
Rear vibration 65-70mph+
and
Rear end gear change from 5.13 to 4.33 on 8.3 Cummins u270
1998 U270 34'

Re: U 270 top end speed and RPM's

Reply #2 –
I can tell you that one trip to Iowa, I drove too many hours and miles but got there before my Father in law passed away.  I know by the GPS that the max speed I hit on one stretch of road was 96 mph. Slowed way down when I realized how fasti was going but that wasn't the end of an 1100 mile day.
I had a 34 foot so I wonder if that had a different rear end then the 40 foot coaches. I think my 320 is different but I would have to look it up.
2025 Wanderbox Outpost 32 on F600 Expedition Motorhome
2015 Born Free Royal Splendor on Ford 550 nonslide version  for sale
Former Coaches  covering. 360,000 miles
1999 34 U270
2000 36 U320
2001 42' double slide U320
2018 Jeep Rubicon

Re: U 270 top end speed and RPM's

Reply #3 –
John S:  I think the 270 , 295 and 320 may all have different ratios as associated with length of coach.  My 40 ft has never gotten over 7.8  miles per gallon loaded.  I hear many others with tags doing a lot better.  I may be geared down due to length and weight.
Dan & Shirley Stansel
2002 U295 4020 AGDS Build#6054
Towing Buick Enclave & M &  G Braking
Emerald Bay, Lake Palestine, Texas
MC# 16650

Re: U 270 top end speed and RPM's

Reply #4 –
I'm not going to admit my gps logged top speed with my 270 running solo....but the wheels on the bus have turned at a pretty good clip on a eastern Nevada well maintained road with a 25mph tailwind.
Doug W.
96 36' U270 CSGI #4946
04 Toyota Tacoma 4x4
PNW

Re: U 270 top end speed and RPM's

Reply #5 –
According to my calculations, 83mph with 5.13 gear will be max governed speed of 2400rpm.
1998 U270 34'

Re: U 270 top end speed and RPM's

Reply #6 –
Read the tag attached to one of the bolts on the rear end housing for the rear end ratio then use your 4th or 6th gear ratio (depending on transmission), and tire size into a plug-in internet calculator. Will give you your revolutions per mile as well as speed for the desired RPM.

Transmission Ratio RPM Calculator

Pierce
Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)

Re: U 270 top end speed and RPM's

Reply #7 –
So Krush are you saying that 2400 rpm should be max rpm under a load?
Rick Barna
St. Petersburg FL.
1998 U270 36'
Build # 5232
1972 CJ 5 Jeep

Re: U 270 top end speed and RPM's

Reply #8 –
Forrest's Cummins 8.3 325 engine does 1600 RPM at 60 MPH.  That's the only speed I care to drive any more.  It's not as tiring as keeping it at 70 or 75 MPH and stressing over the possiblilities.  If I am in a hurry, I just leave earlier.  But I am never in a hurry any more.

Clearly the Foretravel is many times more capable than my current abilities.  So I don't push my luck when it comes to safety of self and others.

As to the axle differential ratios, I believe the engineers got it well covered in their design in order to keep the coolant temperatures under control for EGT - output HP/RPM and cooling system capabilities.
Peter & Beth Martin
No Forrest? What have you done?
MC# 15890 until Dec 2016; FMCA #F329677
Cincinnati, OH

Re: U 270 top end speed and RPM's

Reply #9 –
So maybe a little off topic, but if I am going down a hill the engine shouldn't be allowed to rev over 2400. Is that correct. Going to hit a few mountain passes on the way north to Reno and up to the Siskiyou pass. Been running at about 55 / 57 and much slower down the other side. Probably a dum question.
Robert and Susan
 1995 36' 280 WTBI 8.3 3060r
 1200 watts on the roof, 720 Ah of lithium's
 Build # 4637. Motorcade # 17599
        FMCA  # 451505
        18  Wrangler JLUR

Re: U 270 top end speed and RPM's

Reply #10 –
Please consider that commercial truck tires aren't designed for sustained speeds in excess of 70 mph. This has been getting a lot of attention in the past few years with 80 mph speed limits in some states now.
Scott Cook
1991 U300 36' 6V92TA
Old Town Penobscot 16
1984 Honda VF1100C (V65 Magna)

Re: U 270 top end speed and RPM's

Reply #11 –
Quote from: krush –
According to my calculations, 83mph with 5.13 gear will be max governed speed of 2400rpm.
Last trip out, no complaints averaged 9 MPGs at 27k pounds with high-end Toyo's @ 90 psi all around.
Some say highway diesels should run at 80 to 90% of its governed rpm at highway speed for maximum efficiency.
Doug W.
96 36' U270 CSGI #4946
04 Toyota Tacoma 4x4
PNW

Re: U 270 top end speed and RPM's

Reply #12 –
Quote from: barndog –
So Krush are you saying that 2400 rpm should be max rpm under a load?

2400 RPM is what cummins says is the goverened RPM. It's what the governor will stop giving fuel to the engine at. Push pedal to floor in neutral and it will go to 2400rpm and stop.

Engine makes max horsepower is 2200rpm. Look in threads I linked for dyno plot. For climbing hills, max HP is needed. For cruising on the flats, not so much. I can cruise 75 all day long at 1800rpm. Rolling hills make me drop it down to 5th and leave it. Steep hills, well 4th gives 59mph at 2200RPM.
1998 U270 34'

Re: U 270 top end speed and RPM's

Reply #13 –
Quote from: RRadio –
Please consider that commercial truck tires aren't designed for sustained speeds in excess of 70 mph. This has been getting a lot of attention in the past few years with 80 mph speed limits in some states now.

My Michelin chart says 75?  And the truckers on interstate 10 across Arizona would run me over at 70. 
"Riding and rejoicing"
Bob
1997 U320 40' Mid entry, build 5132,  wtbi ce27, 4th owner
2007 Solara convertible
2 prodeco tech outlaw ss electric bikes

1095 watts solar
08 Ls 460 and a sc430
2000 Ford F-250 superduty 4x4

Re: U 270 top end speed and RPM's

Reply #14 –
Quote from: Bob & Sue –
...if I am going down a hill the engine shouldn't be allowed to rev over 2400... Is that correct?
Bob & Sue,

Not exactly...  When going down a hill, with your foot off the throttle, the engine is not "under load".  As Krush explained above, for the mechanical 8.3, 2400 is the governed (under load) max RPM.  What you are asking for is the allowable "high idle" speed for your engine.  You may find this discussion thread enlightening:

Retarder use, RPMs, etc. (split from Rocky Mtn. High-way)

If you haven't already discovered it, there is a excellent resource available online which contains much valuable information about your specific engine, including critical RPM numbers.  It is called Cummins QuickServe Online.  Follow the link below, register your coach, and enjoy learning all the details about your 8.3!

Cummins QuickServe Online
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: U 270 top end speed and RPM's

Reply #15 –
I drive 60 in the 80 zones all across the country and I'm still alive. Let 'em hit me if they wanna... I ain't skeered... unless I'm on my motorcycle that is...  :o
Scott Cook
1991 U300 36' 6V92TA
Old Town Penobscot 16
1984 Honda VF1100C (V65 Magna)
 

Re: U 270 top end speed and RPM's

Reply #16 –
Quickserv didn't have a dyno plot for my engine and cummins wrote me some stupid email saying "we can't give you that, blah blah blah I am useless". But a friend got me one (he sells lots of cummins engines).

Of course, then when I was all done with the project, I found the dyno plot in the back of my owner's manual......sigh.
1998 U270 34'