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Old 08-19-2011, 06:40 AM   #1
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High RPMs?

I have been reading about gear ratios and engine RPMs and highway speeds on this forum. I have a question.

First, I will say that I have a CAT 1160 (V8 diesel precursor to 320 motor. The transmission is a 4 speed auto. I presume it is an Allison, though I do not know. A placard on the dash says the ratio is 6.8.

My question is: why can I go 67 MPH with pedal to the metal on the highway? My water temp never gets over 195. I do not have a tach. I sure would like to know how to hook one up.

It seems that the motor must be turning some high RPMs to go that fast.

Oh, one more thing, the bus does not go very fast up hills.

I am thinking that either the rear end has been changed or the motor is weak and may be dying.

I get 8 miles per gallon at 67 MPH. The tires are 11R 22.5

Please give me your theories. If possible, perhaps you can tell me how to hook up a tachometer.

Thanks
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Old 08-19-2011, 09:01 AM   #2
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Re: High RPMs?

Quite a few of the aftermarket tachs actually run off the alternator signal.
The factory rpm limit is set to 2750. I'm pretty sure the gears did change, because the Allison is not overdrive, and the engine would have to spin to about 5500 with 6.80 gears to get to 65.
If the 1160 is not turbo, then you're dealing with a max of 210hp, and that's just not going up any hill very well. Drop in a turbo 3208 rated at 250-275hp, then it'll do a whole lot better. Adding fuel(fuel rate) to it will help it even more.
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Old 08-20-2011, 01:19 AM   #3
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Re: High RPMs?

Nev, Thanks for the info. The motor is not a turbo. I would not even know how to add one on there. I have two friends with Hawkins Motor Coaches, and they both have turbo 3208 motors. They also have 4 speed Allisons. They can both go 80MPH.

I never want to go that fast. Today I went 60 miles and kept it at 60 MPH. It is amazing how the motor sounds quieter and like it is turning way less RPMs than when I go 67 MPH.

It's funny that you said it would have to turn 5500 RPM with 6.8 gears to go 65. I did think it was kind of weird that I could go that fast without blowing up the motor.

My bus used to be the Johnson City (Tenn.) emergency management vehicle. Those guys did a lot of work to it, including the exhaust stacks. I have dual exhausts.

Now I have to do the calculation that I read about on this site about jacking up the bus and rotating the tire and counting the driveshaft rotation.

No wonder my bus can't climb hills very easily. I am excited to know what the real gear ratio is now.

Thanks again for your info.

Oh! Is it possible to put a turbo on my engine? How much $$$?

I'd really like to have a DT466. I drove a rental truck with a DT466 last month. It looks too long to fit in my doghouse. As you may know, my engine is in the front of the bus.

I will take pics of the transmission. I sure would like to know what model it is.

There is SO MUCH info on this site, and lots of helpful people such as yourself.
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Old 08-20-2011, 10:28 AM   #4
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Re: High RPMs?

We're here to help.


Help spend your money!

Rest assured, the 1160/3208 cant rev that high, it'll grenade between 4500 and 5000rpm. I just did a gearing calculator, and it says it's 5.17, based on 42.4" tire, 67mph, and 2750rpm.
To turbo your engine, you would have to do a major overhaul, get the right pistons, oil squirters, then all the turbo & parts from a 3208. Much less hassle to just buy a running 3208T. Although the DT466 is my favorite, I would advise keeping with that engine series(1160/320. They run a lot smoother and quieter than a 466.

If you go under to take pix of the trans, the ID plate on the side(just above the pan) will tell you which model it is. Hopefully it'll be an MT-something.
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Old 08-20-2011, 09:07 PM   #5
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Re: High RPMs?

Nev, thanks for the info. I will go under the bus (with a flashlight) tomorrow and look for an ID tag on the tranny.

Are you thinking that I may now have 5.17 gears?
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Old 08-20-2011, 09:57 PM   #6
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Re: High RPMs?

Our Blue Bird All American has a plate in the front of the bus (above the windshield near the door) that says what the rear end ratio is.
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Old 08-20-2011, 10:07 PM   #7
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Re: High RPMs?

I'm just saying it may be 5.17, approximately. That's just a rough calculation.
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Old 08-20-2011, 11:15 PM   #8
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Re: High RPMs?

Never seen a 5.17 ratio...probably either a 5.13 or a 5.29 gear. Best ratio I have found for highway running with most diesels is a 4.10--1900RPM at 60MPH. I would keep it under 60...long runs at the max RPM are not good for the engine.
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Old 08-20-2011, 11:37 PM   #9
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Re: High RPMs?

I was only saying it was the calculation, not the actual gears. I'd be more inclined to say 5.13s or 5.29s too. With at best of 210hp & no turbo, it would have a real hard time with 4.10 gears. If it had more power, 4.10s would be great.
Yes, going 60 will also help fuel economy too.
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Old 08-21-2011, 11:11 AM   #10
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Re: High RPMs?

I just looked at a picture of the plate on our bus. It says it has a 5.29 rear end ratio and we have a turbo diesel engine. you should have a plate that looks like the one pictured on your bus.



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