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steven_george88
Active Member
30 Posts |
 Posted - 07/17/2006 : 10:04:34 AM
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Hi All,
I have some questions about my bus, how it's geared, and how it performs with respect to fuel economy and speed.
It's a 1989 BlueBird short bus (19 passenger, and no handicapped lift, so it's very short, weighs under 13,000 lbs). It's got a DT-360 (turbocharged, non-intercooled, 170hp) with an AT545 transmission, 4.78 rear end, and 285/75R24.5 wheels/tires that I installed.(Diameter = 41.3") I usually get 8MPG with it, maybe as high as 8.25 if I run at 55mph or so.
Now according to my calculations, without any TC slipping, engine speed vs road speed should be:
Engine
Spd MPH
1,100 28
1,200 31
1,300 33
1,400 36
1,500 39
1,600 41
1,700 44
1,800 46
1,900 49
2,000 51
2,100 54
2,200 57
2,300 59
2,400 62
2,500 64
2,600 67
2,700 69
2,800 72
When I drive the thing, it seems like I'm loosing a lot through the torque converter, but I don't know if it is necessarily abnormal because this is the only bus I've ever driven. It will typically shift into 4th around 45mph, then the engine seems to fairly quickly climb to governed speed (2800-2850 rpm), however the bus very slowly gains speed up to about 60-65 mph, with top speed sometimes up to 65mph. What has me most curious is that usually even if I'm driving 60mph on level ground, the engine is pretty much still riding on the governer; whereas based strictly on gearing it should be running around 2350 or so. Would it be normal for the TC on an AT545 to eat up 400rpm? Now if I come to any decent hill, road speed will easily drop to 50 or so, with almost no drop in engine speed until it gets down to nearly 50mph or below.
Does this sound pretty typical for a bus like mine? And would 8mpg be about right for my engine and gearing? Seems like I've heard people with a full size bus getting nearly 12mpg, so it seems like I could do that well being that my bus is a bit smaller and lighter. And it just doesn't seem right that the engine would have to work so hard and run so fast just to go 60mph, but I just don't know.
Is it possible that the Torque Converter is slipping too much, causing poor fuel ecomomy and causing the engine to run faster where it is also less efficient?
If I were to pull the AT545 and replace it with a 5 speed manual, do you think I would notice much of a fuel economy/performance benefit?
The engine seems to build good boost pressure (about 19psi, which seems like it should be about right) so I think the engine is probably doing alright as far as power, but I don't know. Should I expect more boost? Should I look somewhere else for lost fuel economy (leaky injectors, etc???)
Finally, does anyone have any idea how high I could turn up the power before needing an intercooler, or before hurting anything else? Is 190-200 hp possibly without doing anything but having the fuel pump changed (It's got a Bosch pump).
Thanks for any input you might have! |
Edited by - steven_george88 on 07/17/2006 10:05:44 AM
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ModMech
Top Member
USA
948 Posts |
Posted - 07/17/2006 : 3:32:35 PM
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When the TC goes, you WILL know it - it will either stall the engine, or sound like someone tossed marbles into it... :(
I have never seen enough MPG advanatge to a MT to warrent the annual clutch jobs they require, and certainly not the expense of a change-over!
There are a couple things to ckeck:
1) Make sure that the throttle arm is FIRMLY against it's stop on the injection pump when the foot-feed is floored. 80% of the time this is the problem.
2) Let us know what each upshift is in RPM. Each *should* occur at about 2600 RPM.
3) 8 MPG is 2.5 MPG less than we got on the open road with our DT360s with the same RAR and 11R22.5 rubber (502 Rev/mile).
4) Is your speedo correct? I'd bet not. In fact, I'm almost sure it is off by at least 10%. Check with a GPS.
5) You can increase the HP 10 or so w/o an intercooler, but I would not do it. You will LOSE mileage. |
If you want customer service, you NEED an International! |
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Brad Barker
Administrator
USA
874 Posts |
Posted - 07/25/2006 : 2:25:31 PM
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Unlike the MT 645, the AT 545 converter is a non lock up converter. There are basically three converters. Depending on your application. Like Modmech says, check to make sure you are getting full throttle first and that the engine is performing at its peak.
The converter reaches its peak torque at stall speed. When the turbine speed approaches the speed of the oil pump, oil flow to the stator begins striking the backs of the stator vanes. This rotates the stator in the same direction as the turbine and pump. At this point, torque multiplication stops and the converter becomes a fluid coupling. If your engine speed is not what it should be or for some reason, like wear in the transmission, the converter cannot reach its peak speed it will continue to slip and never reach the fluid coupling stage. Thus mileage will drop.
I question the boost pressure you stated? It seems a little low to me. I would expect around 25 but I may be wrong. |
Brad A. Barker |
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steven_george88
Active Member
30 Posts |
Posted - 07/26/2006 : 1:44:46 PM
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I will check the throttle arm. Upshifts definitely happen in the vicinity of 2600 RPM, I'll check this the next time I drive it.
I don't have a GPS to check the speed, as near as we could figure, it looks like we are actually traveling 64mph at an indicated 60mph, so the speedo is reading low.
My usage of the bus has diminished a lot within the last year, and I no longer want to throw much money at it, but I would like to gain some mpg's if possible.
The reason I posted was because I'm not sure that I'm really reaching the fluid coupling stage, or else reaching it very, very slowly, and I don't know if that is normal. I haven't driven any other busses like this, so I don't know. It just seems like I've heard others with the same engine, tranny, and axle ratio getting 10mpg with a full size bus. Since my bus is only a 29 passenger unit (it weighs just under 13,000 lbs) It seems that I should also get 10mpg or better, but I almost never achieve over 8 mpg. And I didn't expect there to be so much lag between increasing engine speed and road speed.
I can't really think of other no-cost ways to diagnose my loss in mileage. I just mentioned boost pressure because (as long as the turbo is functioning correctly) boost is directly correlated to exhaust energy, which is directly related to power produced. If 19psi seems low, maybe I'm not making the power I should. Since my mileage also seems low, maybe fuel injector rebuilds would be in order, does this sound reasonable? Although it still doesn't explain the percieved "fluid loss" between engine speed an road speed.
It's just tough because I really don't know what this machine is supposed to feel like.
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IBTMech
Top Member
USA
973 Posts |
Posted - 07/26/2006 : 5:29:11 PM
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| Your description of a DT360/AT545 sounds accurate. We only had one unit years ago. Without the lockup converter you'll experience poor high speed mileage. |
If it doesn't fit, FORCE it.
If it breaks, well, it needed replacing anyway.
Pullin' wrenches for 45 years. |
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AT545/torque converter questions
