Fan pulley size and fan speed on MCI 5

[mcilee]

I have made a few changes with regards to pulley size etc on both the engine and the miterbox on our MCI 5. This has been a result of rebuilding the miterbox and upgrading the pulley a few years ago but unknowingly at the time with a different size pulley then buying the only one I could get for the engine which I know is smaller than the original but now don't know the original of either. Does anyone have the original size of both or have any suggestions?  What would be the optimun fan speed be for this 8V71 rig?

[Ed Hackenbruch]

My upper pulley is 9in. and the lower is either 10 3/4 or 11 in. I believe both to be original to my 68 5A.  I think i saw something in the book that said that 1700 rpm and higher is optimal fan speed for cooling.

[RickB]

MCI Lee,

I am not sure if the cooling system is the same on your 5 as it is on my 9 but with the 9's the preferred pulley is smaller which boosts the operating rpm's of the blower assembly by about 30%. I have an 8V71 and I completely rebuilt my cooiling system over the winter and I am so happy I went with the smaller pulley. It really is a remarkable difference. If you insist on putting a larger pulley back on , I still have my old one and you can have it although I think you may need a different length belt to run it. If your cooling system is basically the same as mine I would stick with the smaller pulley. If you ever decide to do a repower to a turbo model you will be wanting a smaller pulley as well.

Is the reason for your post that your bus is not cooling properly? If so, I would look at other possible explanations rather than the pulley.

Good luck.


Rick

[bevans6]

My crank pulley is 9.25" OD and my blower pulley is 8.25" OD.  1980 MC-5C 8V-71.  I find talking about the "smaller" or "larger" pulley hard to follow, without numbers the references mean nothing to me.  I'm presuming the sizes on my engine are stock, but I have no way to tell.

Brian

[RickB]

Brian,

That is good advice.  I will measure my old pulley and the new smaller pulley when I get home from work.

The parts places I shop at always refer to them as Large (OEM stock) or the smaller more desirable MC12 or 102A3  pulley that MCI went with exclusively when they started putting wet sleeve Turbo'd 6 and 8 cylinder 92 series heat sensitive engines in their coaches.

Rick

[bevans6]

Rick, thanks.  also, which pulley are you referring to, the crank pulley or the blower pulley?  I would expect the sizes matter less than the ratios, as long as the sizes are reasonable for the belts.

Brian

[RickB]

Brian,

I am referring to the blower gearbox pulley. I have never been led to believe that the crank pulley comes in different sizes. Whenever I have spoken to anyone about the different options of the blower gearbox pulley or mitrebox pulley they have always been referring to the pulley attached to the blower gearbox.

I am not saying that the crank pulley doesn't come in different sizes, I just have never heard of it.

Rick

[RickB]

Well here goes,

keep in mind these are all on my 9 which may not be the same as your 5.

The part # on my old part is 6F-6-2. Both pulley are the same size  (around 8 3/8") but the new style pulley has considerably deeper grooves.

The new pulley does not have a part #

hope it helps.

Rick

[buswarrior]

The only pulley I have heard any of the gurus talk of changing is the top one on the fans.

Smaller makes the fans spin faster, same as the gears on a bicycle.

I think it safe to assume there are no practical choices in changing the crankshaft pulley, or in the least, it would have been talked about as to how it wasn't feasible?

happy coaching!
buswarrior

[mcilee]

I measured the pulleys I have on now tonight and they are engine = 9.25 and top pulley = 8.25. A quick spreadsheet puts the blower speed at 2242 at 2000 RPM engine speed.  I am thinking that when I did the miterbox rebuild the replacement pulley was 3/4 inch larger than what was on it and that I started having some heating inconveniences. I then thought the engine pulley was worn and traction was a problem so I bought a new pulley from Luke and he said the size I had wasn't available. The only one was a 9.25 inch one. We got it and put it on. I think now that we had a 10.75 on the engine and a 7.5 ish on the top. Quick calc puts the blower speed @ 2865 RPM.  That would be a 22% drop in fan speed. Is that enough to affect engine heating to the point I am at now? OK, to answer more questions: We also this past spring had new radiators built by Motor Missions in Las Vegas.  I highly recommend them for quality work and quality products. I am confident the heat is not caused by the rads.
MCILee

[Len Silva]

http://users.cwnet.com/thall/fredhobe6.htm

[RickB]

Lee,

The stock crankshaft or bottom pulley O.D. is 9 and 3/8" on my 1981 MCI which came equipped with the 8V71 and an Alliosn 754. The dimensions may or may not be the same as your 5 but I'm sure someone else here (where's Dallas when you need him...) would know better than I on that. Fred Hobe knows alot about our cooling systems he may be a good person to contact.

With that said,

Do you still have the original crank pulley?? If not, you might want to get the bottom pulley back to OEM before you start trying to figure this stuff out. When you change both pulleys at the same time you are losing your ability to have a baseline that is anywhere near where the designers had it. If you were to severely overspin the blower gearbox and the squirrel fans you might have some fairly serious unintended consequences. If I were you, I would try and get your bottom pulley back to stock. Then, if you still have the 8 1/4 pulley I would look to see if the belt rides even with the outside edge of the pulley or if the grooves are deep enough to let the belt ride roughly a 1/4 inch below the edge (that would mean you have the "smaller" more desirable pulley on top) but either way you are close enough to the stock setup that it should only overheat at that point under heavy load on a very hot day.

With newly rebuilt radiators and the 8 1/4 pulley on top and the OEM pulley on the bottom if you are still overheating and if are using an infrared gun and your gauges to verify temps, I would be looking at thermostats, water pump, air tensioner for the belt itself, wrong belt size and I would IR the exhaust manifold on each cylinder to make sure I don't have anything major wrong like one cylinder running alot hotter than the others..

You haven't decsribed the symptoms well enough for most here to be of help.

Is it overheating all the time or just on long pulls. What kind of temps are we talking here? 200 plus degrees?

What is the ambient temp outside when it overheats?

Are you towing a vehicle?

Are you running an automatic transmission ? If so, have you IR'd the transmission and its side engine mounted cooler during an overheating episode? Allison transmission's are fairly notorious for overheating the motor when they have issues because they share the same cooling sustem.

The thermostats and water pump are a quick diagnosis with an IR gun. The temp of the coolant returning to the motor should be considerably cooler when it re-enters the motor.

Lastly, have you checked the seals around the upper door to your fan assembly?  That seal has to be as airtight as possible and all the seals around your radiators need to be airtight as well so you have virtually all the air that is entering the fan area running through your radiators.

There are also seals on both ends of the squirrel fan shrouds. Big rubber O-rings. Are they making a good seal with the shroud or are they brittle/ chewed up and allowing air to enter the fan around the seals?

I feel for you, there is nothing like white knuckling gauge watching to ruin a good bus ride...

Hope it helps.

Rick

[luvrbus]

I would do like Rick and check the bottom pulley there are tons of different size crank pulleys for the Detroit .
The standard being 9and 3/8 inches.
The reason people change the top is for price ever buy a 8.5 crank pulley from DD  

good luck

[bevans6]

The crank pulley is actually also a harmonic balancer, isn't it, he said, digging for his manual...

[luvrbus]

Bevans, no they are not the V71 does not use a harmonic balancer neither does the 6v92 and some 8v92 engines 


good luck