| Author | Message | ||
| Jason Whitaker (Jeepme)
Rating: |
We're in Cody, WY on our first real bus trip and really loving it. It sure beats hotels! Coming into Cody on hwy20 we hit a few moderate hills that required downshifting. I've read the article by RJ (thanks RJ!). I've got to where I can downshift pretty well but some hills seem to run on forever with the bus slooowwwly loosing speed. It's a 4104 w/4sp. Should I keep chugging along at 55 mph in 4th or should I go ahead and drop to third and run up the rest of the hill on the governor? Just watch the temp? What is the best thing to do if I do start overheating coming up a hill? Drop to 2nd and put it on the governor? Pull over and let it cool off? Thanks for the help, we wouldn't be out here having fun if it wasn't for everyone on the board. Jason Whitaker 4104 | ||
| Bob Wies (Ncbob)
Rating: N/A |
One of things which JR didn't mention in his article is how to shift a clutch with a brake (Twin Disk). If half clutching will get the job done without grinding...you're home. If your clutch brake is working and you aren't sure what I'm talking about go to the MC5 (Yahoo) board and read the article by Roger (Beadknot). He's an old Over the Road truck driver and opened up a whole new world to me. I'm having to pull my engine and replace the clutch because the previous owner only thought he knew how to drive the bus. My guess is that you have the 6-71...8V71 developes it's max torque @ 1600 RPM. Don't know about the 6. I don't like to run up against the governor. Detroit's like to operate between 1400 and 2000 RPM's. Otherwise your blowing fuel out the stack. Enjoy your trip...safetly. NCbob | ||
| Richard Bowyer (Drivingmisslazy)
Rating: N/A |
Everything I have ever seen or heard is that you do not want to lug the 6-71 or any other DD for that matter. Shift down and keep her revved up. It will probably run cooler that way than what you are doing now. And just keep shifting down. Once you determine what speed you need to be in a lower gear, shift down about 5 mph above that speed. In other words if you need to be in 2nd at 35, then shift about 40. And enjoy! That is what it is all about. Beautiful country thru there isn't it? Richard | ||
| Geoff (Geoff)
Rating: N/A |
You are not going to hurt the engine running it up against the governor climbing a grade, keep those rpms up and don't keep it in a gear that is losing speed to the point of lugging it. You really need a tach to see what is going on. The worst thing you can do is lug it up a hill with oversize injectors-- you'll overheat and seize a clyinder or two real quick. | ||
| Bob Wies (Ncbob)
Rating: N/A |
Hi Geoff, Not trying to be argumentative, but I can't understand why you would recommend running a Detroit well above the highest point of maximum torque (against the governor) when a higher gear, within the recommended operating range of the engine, would give the same or better performance and fuel economy? Mayhaps is it that Rev's is better? I've always followed the recommendations of the Engineers who designed the engine(s) I had underneath me be they Detroit, Cummins or Cat. Please, show me where I might be mistaken and I'll give you a 'tip o' the hat', and might even throw in a bottle of "Jack"! NCbob | ||
| Bob Wies (Ncbob)
Rating: N/A |
Richard, my best information recommends that with the Spicer 4 Speed 'crash boxes' the shift points are: 15, 35, 55, and 'go for the ticket! That comes straight out of both the MCI and Greyhound Operator's manuals. We're not out there running a 'gymcana' or whatever they used to call sports car races...we're out there attempting to see the country and transport all of us from one point to the other...safetly. So, if I'm in the slow lane, and below 45 MPH and have the ICC flashers on, let the world pass me! I'm not a hazard...and hopefully..never will be. Sometimes the turtle wins the race...he lives! NCbob | ||
| Dallas Farnworth (Dal300)
Rating: N/A |
Bob, Your information doesn't take into consideration differential gear ratio, tire size, trans gear ratio, etc. Detroits, and I've driven and worked on them for years, like to be opened up. Yes the torque curve drops rapidly as you raise the rpm, but 2 cycle detroits drop off so quickly when you let off the accelerator that it's really easy to miss a gear when down shifting. As an example, my 350 Cummins BCIV would drop from 1800 to 1600 in the same amount of time that my 8V71 drops from 2150 to 1600. Another issue is cooling, the faster the engine turns, the more coolant circulates through the system, the more heat is exchanged at the radiator. The engine fan also turns more quickly moving more air, allowing more cooling. All of the engine manufacturers describe 'Lugging' as any time the engine cannot be accelerated under load. By the way, my GMC will do 30 in first, 51 in second and 65 in third. Fourth gear got me stopped at 96mph on a two lane road in NC. The main point is that you want to keep the engine as cool as possible while making the best forward movement. I also am in no hurry. I spent my life putting on 1000 miles over night. I refuse to do it now. If I drive 200 miles in one day, fine, if I do 50 in a day, that's fine too. But I'll always take care of my engine and drive train. And the one way to do that is to keep it cool. Dallas | ||
| Geoff (Geoff)
Rating: N/A |
Bob, we are talking 2-stroke Detroit Diesel in a highway vehicle climbing a hill. You drive a 2-stroke according to the horsepower curve, not the torque curve. The highest horsepower for an N engine is at the top of the band, 2100 rpms full load, (set at 2300 no load). The highest torque tops out at 1200 rpms, which is nothing more than fast idle. You can run the engine at 2100 all day without hurting anything but your pocketbook. Now if you want to talk about driving on flat ground, you can run higher gears and get your rpms down to 1600-1700 for fuel savings, but the original post was about downshifting to make it up a hill, and whether it is better to lug it or keep it rev'ed up. I also don't understand your response to Richard-- he used to own a 4104 and knows what he talking about. --Geoff | ||
| Richard Bowyer (Drivingmisslazy)
Rating: N/A |
Bob, like Geoff, I do not understand the point you were trying to make. I put many thousand miles on my 4104 thru a lot of desert in southern California pulling a 24 ft trailer in very hot weather thru places like Indio and further east. I never ever tried to win a race. I did try and always keep the rev's up above 1750-1850 to try and keep her cool and keep from lugging her. I always made it a point to start the down shift at least 5mph above the governor stop for the next lower gear. That way I was always at the top of the rpm curve. It worked for me. At least I never had any engine problem or overheating problem. Richard (Message edited by drivingmisslazy on June 03, 2006) | ||
| John MC9 Rating: N/A |
Jason - "some hills seem to run on forever with the bus slooowwwly loosing speed. It's a 4104 w/4sp. Should I keep chugging along at 55 mph in 4th or should I go ahead and drop to third and run up the rest of the hill on the governor? Just watch the temp?" Uphill, it's better to run at 55 in third, than run 55 in fourth. During the Federally imposed 55mph limit during the 70s, the hound drivers were running in third up and down the pike. It was easier to stay to the limit doing that, and a hell of a lot less wear and tear on the engine. Don't wait until the engine starts to bog down, shift to the next lower gear early and let the engine breath. | ||
| Greg Peterson Rating: N/A |
My trip to Cody is what got me motivated to change out the 190hp engine in my RV as you may have read from my other post. The engine had a turbo but overheated twice on that steep climb. I had to pull over and let it cool down. I was in 1st gear and the pyrometer was still pushing 1300F. I stopped at a Wal-Mart in Cody and talked to a bus nut there that had a MCI 8. My wife was in Walmart and I was hoping to hear that he climbed them mountains with that big 8v-71 without any problems. I would tell the wife ya see that what we should build. Unfortunately, he told me that the auto shut down had tripped several times even in 1st gear. He had the rear doors tied open in order to help cool the engine. He said he wished he could add a turbo to the engine. This convinced me to only buy a bus with a turbo charger and a large engine. Sure is beautiful country at high altitude but you need some big horses with adequate cooling. | ||
| Tom Caffrey (Pvcces)
Rating: N/A |
Bob, I would like to clear up something about peak torque in these engines. The standard 8V71 bus configuration used C60 injectors with standard valve timing. It's peak torque is about 1200 RPM. The higher horsepower configuration uses N65 injectors with advanced valve timing. It's peak torque is about 1600 RPM. I was just reviewing a 304 horsepower version that shows peak torque of 1400 RPM. I also note that the torque is nearly flat from 1200 RPM clear up to 1800 RPM. While I haven't seen where anyone has said it yet, I've had the distinct impression that reduced air going throught the engine can contribute to engine overheating. High elevation really can bring this condition on, with the warning coming in the form of abnormal amounts of black smoke on full throttle. Our engine (standard configuration) seems to start running short of air around 4000 feet; below that, no problem. This is where the turbo setups shine. They can keep enough air going through the engine even when they are well above our best operating elevations. For what it's worth. Tom Caffrey PD4106-2576 Suncatcher Ketchikan, Alaska (Message edited by pvcces on June 03, 2006) | ||
| Marc Bourget Rating: N/A |
Cooling systems at altitude. General comments It takes mass of air through the radiator to cool the engine. Thinner air reduces the mass of air flowing through the radiator. Also, thinner air, to the extent that it richens the fuel air mixture (or a too heavy foot with larger injectors) increases the heat load the cooling system has to get rid of. Turbos make up, fuel-air mixture-wise for the thinner air. | ||
| FAST FRED Rating: N/A |
If the engine hasant been hot rodded with defuler injectors, the choice is made by the engine itself. IF you can stay above the shift point 15,30 &45 in each gear , the fuel milage will be better and the wear lower if you do not shift down. If the coach can not "pull" the gear and you are loosing rpm , a downshift IS called for. The engines were designed not to have any problem like overheating at full throttle at any reasonable RPM.There is no reason to scream them just to get up a hill. The usual shift brings most rpm down to about 1100 rpm (stock gov setting) so running at the torque peak of about 1200 is fine FOREVER. You're only overloading if you CANT hold the rpm, FAST FRED | ||
| Bob Wies (Ncbob)
Rating: N/A |
Once again I have to bow to the more experienced OTR drivers...if you'll unpile off me long enough for me to catch my breath. <g> In my case I'm running N65's with advanced timing which a floor dyno says I'm developing 314 HP and Max torque is at 1600 turns. Perhaps it's my sensitivities to hearing a Detroit revved to the point where it seems like the pistons are trying to change holes and/or the lack of a few more holes in my gearbox that formulates my opinions. But that's all they are...opinions. Please don't mistake them as authoritarian. Even the Instructors at the DD school in Detroit opined that Detroits were "Inefficient, hot running Mother -------". But I do so dearly love the sound they make! NCbob | ||
| Marc Bourget Rating: N/A |
Can't see how you can over rev a properly goverened engine. "Seems like the pistons are trying to change holes . . ." is a lack of adaptation to the "new" sound of a DD 2-Stroke. I'd be the first to agree that they don't sound anything like a car engine and are horribly loud in comparison, so the "seems" is reasonable. For the most part, it's low rpm and heavy right foots that causes harm to the (otherwise properly maintained) 2 stroke. | ||
| Greg Peterson Rating: N/A |
I believe you should have a gauge monitoring the exhaust temperature. It is especially important when at high altitude. When I was going to Cody my Cummins was not lugging. The rpm did not drop as FF described. The first indication was that the exhaust gas went to 1300F. I had to pull over and let it cool down. I really had to watch the temp and be careful with my foot. Without this gauge I could have easily melted the engine down. My new 300hp engine has a pressure sensor so that the computer has an input to altitude. It measures the turbo pressure and water temperature also. It is in total control of the injectors so I would think it will back the fuel off before anything gets to hot. I put the exhaust gauge on the new engine and believe it is good to watch anyway. | ||
| Prather
Rating: N/A |
Just my experience, but my 8V71N seems to have the most power around 1600 RPM, when I go up a hill if RPM drops to 1400, I shift down, also the engine may hold at 1600 but if the engine temp starts to increase, I shift down. At higher elevations the best indicator for me is the engine temp. sometimes in the mountains your are climbing and don't know it. I also have a altimeter so I know if I am going up or down. Bottom line engine temp controls when to shift. I have a nine speed with 3.73 rear gear and overdrive 9th gear, 24.5 tires. | ||
| Jason Whitaker (Jeepme)
Rating: N/A |
Thanks for all the advice! I'm leaving out of Cody this morning headed for Yellowstone and I expect more hills ahead. Thanks! Jason | ||
| Richard Bowyer (Drivingmisslazy)
Rating: N/A |
Greg, what was the water temperature when you were seeing 1300 degrees on the exhaust? What is the maximum exhaust temperature you should run? Is 1300 degrees too hot? I never had a guage on the exhaust so I am a complete dummy on what it should be. Richard | ||
| herman Rating: N/A |
Greg wrote: " The first indication was that the exhaust gas went to 1300F. I had to pull over and let it cool down. " Greg, I'm also scratching my head. It's completely normal for the EGT to rise and fall quickly - it's sampling the engine load much more directly than any other temperature gauge possibly could, which is a big part of its value. Now, as for what constitues an acceptable reading, that depends a) on the engine (the materials that its exhaust valves and turbine wheel are made of), and b) on the location within the exhaust stream of the sensor, most particularly before or after the turbo. Turbine inlet temperatures can easily run a few hundred degrees above turbine outlet temperatures. My diesel pickup truck has its sensor ahead of the turbo, and it will very happily run 12-1300 degrees all the live long day, if you can find a hill / headwind that long and steep... While high EGT is certainly cause to back out of the fuel, it isn't cause at all to pull over. That decision should be made on basis of water and oil temperatures. And in my view that - or any - kind of overheating indicates a problem with the cooling system: it should be set up to cool an engine at full load, regardless of the RPM and road speed at which that load is occurring. Now, there is a minimum full load RPM for every engine, more related to the frequency of the torque pulses induced by the combustion events and the way they resonate with all the various masses in the motor (and the remainder of the drivetrain), and there are tachometers that enable you to honor that. Drive by the gauges, folks; its pretty damn simple. If your powertrain doesn't have a computer watching its back, an EGT gauge is invaluable in keeping you from flying blind, and even if you do have engine management it's a great backup. | ||
| Greg Peterson Rating: N/A |
I never had the exhaust gas go over 1100 F before I got to those mountains. The sensor is before the turbo. The water temperature started to climb into the red and then the hot light came on. My indication was the exhaust gas temperature first but the water soon followed. The old engine could melt stuff at 1350 to 1400F exhaust gas temperature. I talked to a mechanic at Cummins and he said I should check my exhaust manifold for cracks. Fortunately it was ok I must have pulled it over and cooled it down soon enough to prevent damage. The new engine has hardened valve seats and double anodized pistons along with 4 valves per cylinder. It probably can take 1250 to 1300F. I have heard of guys with Dodge Pickups hacking the computer and melting even these engines down. | ||
| Geoff (Geoff)
Rating: N/A |
I have a pyro on the exhaust side of the turbo on my 350HP 6V92TA and I have never had it go over 650 degrees, this seems to be the norm from what other people say. It gets up to 600 on a hill, by then the Allison downshifts and the pyro temperature goes down. The engine water temperature goes up after the pyrometer goes up, but the pyrometer works much faster and lets you know the water temperature is going to go up, especially pulling a trailer up a hill. That's when I hit the radiator water sprayer. --Geoff '82 RTS AZ | ||
| Brian Brown (Blue_velvet)
Rating: N/A |
I know I'm late to this thread, but with std. injectors and timing, the exhaust will give one a real good indication on whether it's getting too much fuel or not. Black smoke is unburnt fuel, and just heats up the exhaust, the oil, and the water. To keep the engine at its coolest on a climb, I always try to monitor the exhaust (not blowing or puffing black), on partial throttle, keeping the revs. between 1700-1900rpm (you don't need a tach with a std. tranny, just do the math with Daris' calc. and watch the speedo). RJ gave me this advice, and like most things he says, it's golden. My '06/4-speed with std. injectors could climb all day up any grade following with these caveats. Sometimes I was in 3rd, but many times in 2nd, depending on grade and altitude. Once I even had to bring her into first! (10% grade at 9,000 ft.) But she never got above 190 and seldom blew black smoke, so I knew the EGT must have been fine. My new bus with the auto and N65/std. timing blows black and gets hot on climbs no matter what I do. It's getting put "back to normal" as soon as I can sell the '06. The normally-aspirated motors do present a challenge to adequately cool themselves on steep climbs at altitude, due to the reasons Marc mentioned. Letting the engine determine its rate-of-climb helps immensely. Not gosel, just my expeiences... Brian Brown 4106-1175 4108-216 Longmont, CO USA | ||
| John Jewett (Jayjay)
Rating: N/A |
I installed a 7 blade, 5 inch pitch transit bus fan in my 4905, and have never had overheating problems in the mountains. The bus has a stock 8V71 with a Spicer 4 speed. Something to consider. ...JJ | ||
| rodger in wa
Rating: N/A |
It seems I'm a high RPM devotee. 1976 35' Prevost Champion with a DD8V71N and Allison MT 644 (4 spd). Since the Allison seldom shifts at the right time - usually to soon accelerating and too late on the upgrades, I usually force shift. Our typical traveling gross wt., is 31K LB to 32K plus a 3K LB car in tow for a combined GW of 34-35K. GVWR is 35K. My speedo, checked against the GPS, appears to read about 3-4% high. I cruise the flats at 1800 RPM/60 mph indicated speed (getting 8 mpg). I run the big hills at a target RPM of 2000 with one eye on the side mirror checking for black smoke. Seldom, if ever, do I detect black smoke below 5,000'. At the first puff of smoke, I ease off the accelerator until it disappears. When I cannot maintain an RPM above 1900, I back off a little, let it slow to my shift down speed and drop to the next lower gear that will bring the RPM back to 2,000. 3rd @ about 50 mph, 2nd @ about 35 mph, I've never had to drop below 2nd. This has worked fine for me, crossing passes up to 9,000' in warm weather. My only temp gauge is coolant. I have never seen the coolant temp gauge go above 180 deg. Normal is 170 deg. Traveling in speed zones below 45 mph, I keep it in 3rd. | ||
| Ron Walker (Prevost82)
Rating: N/A |
I have a pyro on the exhaust side of the turbo on my 475HP 8V92TA and it get up around 900 deg on a long hard pull. Ron |
