Low gearing solutions?

[Mark R. Obtinario]

Before you start spending a lot of time and $$$ swapping stuff around you need to first check out if your engine has enough oomph to go faster.

Manifold vacuum at cruising speed needs to be in excess of 10".

If it isn't, faster gears is not going to make your bus go faster. I have driven several Ford gas V-8 powered trucks and/or school buses and unless they had the 534 most of them were hard pressed to go faster than 50 MPH.

Swapping from 20" tires to 22.5" tires will not create any difference since a 9.00X20 tire has the same nominal height as a 10X22.5 tire. Going from a 9.00X20 to a 10.00X20 (11X22.5) or 11.00X20 (12X22.5 or 11X24.5) would make a difference. But that is only going to work if you have enough room under the fenders for the taller tire.

Swapping in a 2-speed is also not going to achieve much of a difference because the high side is not going to be very different from what you have. In most cases, in a 2-speed the advantage is in the lower side for starting and climbing hills. Mated with a 5-speed transmission it usually meant that 5-low was lower than 4-hi so you really couldn't split each gear equally.

I doubt you will find very many axles out of a gas powered truck that have much smaller numerically ratios than what you have already. Most gas engines were governed in the 3400-3700 RPM range. Most medium duty diesels are governed in the 2600-2750 RPM range and most heavy duty diesels are governed in the 2100-2250 RPM range. With higher HP and torque at much slower engine speeds the rear gears are substantially smaller numerically in newer trucks.

Even in diesel powered rigs some of the gears are still in the slowpoke realm. I just sold a school bus that had the T444 V-8 diesel and it had a top speed of 47 MPH.

My recommendation to you is enjoy driving on secondary roads or the slow lane. The time, $$$, and aggravation of swapping isn't going to make much of a difference unless you plan on driving tens of thousands of miles every year.

[Sharkey]

Before you start spending a lot of time and $$$ swapping stuff around you need to first check out if your engine has enough oomph to go faster.

Manifold vacuum at cruising speed needs to be in excess of 10".

If it isn't, faster gears is not going to make your bus go faster.

I can vouch for this. In December 1980, I installed a over-and-under 3 speed Brownie transmission on my Housetruck. While the conversion made acceleration and hill climbing much more bearable, it did nothing for the top speed, the engine (GMC 305c.i. V-6 w/two-barrel carb and low-restriction exhaust) just doesn't have enough power to use 4th-overdrive, the truck just slowly loses speed from 55 MPH unless I have a tail wind or a slight downhill grade. In fact, the only time I felt that it actually had adequate power was when I used the Pusher on it.

As for taller tires, I swapped out from 7.00-18 rubber to 7.50-20, and this did make a big change in top speed, only because the 3,500 RPM red line of the GMC V-6 was keeping me at 40 MPH or under before installing the Brownie.

[teamgreen]

I guess I just won't bring it to Oakland.There is no secondary way in or out. Driving 45 around here on the freeway is DANGEROUS. I've had 2 six cylinder buses that went 45 and they were HUGE. My current bus is tiny in comparison and has a V8. I trust your judgement though. I would like to add that I personally don't mind going slow. Being screamed at for going slow gets old though.

[Jones'n4chrome]

My bus tops out at 50.
I took it to the final AMA National Motocross race at Pala raceway, about 20 miles into the 90 mile journey, the exhaust pipe broke at the inlet to the muffler.
With a Detroit 671 it was very loud. The people passing me were pissed. They flip me off just for going slow. But there were extra birds flying that day.

[Standaddy]

I vote for the faster gears. If you told which v-8 you have, I missed it, but a small bus with any decent v-8 that would have been factory-installed in a bus should be able to run 55-60 with the right gears. Large school-busses from the 50's and 60's that came with 6's and direct-4-speeds would usually run about 50 mph. You seem to be able and comfortable with axle-swapping, so I'd check the current axle ratio for sure, then locate and check a couple 2-speeds, you might find a nice surprise.
And I forgot who it was asking about putting a tach on a diesel. They are usually mechanical tachs, and run off the injection pump on mechanical engines, and on late model electronic engines, I'm not sure, but probably are electric tachs, ran from one of the crank or flywheel sensors.
Stan

[teamgreen]

Thanks for that vote of confidence. I was secretly going to continue to investigate this on my own. I will start with a vacuum check at speed as this has come up twice since I started looking around. I also need to get my tach working to really figure out where I stand. Since I have a 391 in a short B500 I would like to eventually break 50 mph without killing the engine. I hope my stubborness on this hasn't offended anyone.

[Jones'n4chrome]

You can't offend anyone here...I tried, but they all have thick skin

How much does your bus weigh?

[Mark R. Obtinario]

Weight is going to be important but at speed the determining factor is the brick shape of the bus. It takes torgue to get up to speed but it takes lots of HP to push a brick through the air.

Are you sure you have the 391?

Most of the full size gas powered Ford chassis buses I have seen had the 361 and the shorty buses had the 331.

I have had much more experience with IHC chassis buses. I know from experience that most of the SV392 and MV404 powered buses were able to cruise at 55 MPH but really slowed down on any hill. The SV304 and SV345 buses struggled to maintain 55 MPH and slowed down if you said hill.

It is your bus and you will do what you think is the best thing to do. But from what you have said, I would save the time and $$$ that swapping would take and spend it on upgrading the interior.

Good luck.

[Rudy]

It is I who asked about hooking up a tachometer in my bus. It is a 1973 Cat 1160, so I presume there are no electronics in the engine.

I don't know what the injection pump looks like or where I could attach a tachometer to it. On page 19 of my Blue Bird thread, there are pictures of what I thought was a governor. Mark Obtinario said it was the injector pump.

Anyone have an idea how a tach is hooked up to that?

I know it was going 65 to 70MPH because I was following it in my pickup truck on the 100 mile trip back from where I bought it.

I also have those dual exhaust pipes. Does that give the engine more power?

I have an Allison 4 speed automatic transmission. I have no idea if it has overdrive. I don't know how to tell the model number of the transmission.

I can take off the air cleaner and take a bunch more pictures if that will help.

Thanks to all who can answer these questions.

[teamgreen]

Mark, I definitely will take your advice into account as far as evaluating my bus's power situation before doing anything. I'm actually beginning to wonder if I may be wrong on the engine. The previous owners where clueless. They bought it from a tow yard to keep in their yard. I heard the 391's have a 4 barrel and mine has a 2 barrel so I most likely either have the 331 or 361. Any clues on distinguishing these two? Either way a vacuum check and tach will come first.
I'm not looking to break any speed records. Anything over 50 would be GREAT. I should add that it would be nice if I could occasionally pull my IH scout 80 behind me. That is why I was interested in a slightly higher 2 speed axle.
The interior requires very little $$$. It came converted so I am simply rearranging things and I am getting lucky with salvaged materials. Just got solid oak flooring. FREE

[teamgreen]

As far as time and $$$$ goes I appear to be looking at around $800 and a day worth of work with a buddy who enjoys this stuff. As a carpenter in California, I currently have TONS of time and ZERO money. None of this work is happening tommorrow. I'm just looking for ideas for my wish list. I plan on having this bus for a long time.

[Sharkey]

Rudy, a mechanical tachometer is connected to the engine using a cable and sheath that is identical to a speedometer cable. A fitting must be found on the engine that provides drive to the cable. This might come off your camshaft somewhere, or perhaps the injection pump. These drive locations would provide one-half cable speed, that is when the engine is running 2,000 RPM, the cable would be turning 1,000 RPM. The tach head compensates for this difference.

If your engine doesn't have a drive connector, there is not really an easy way to put a mechanical tach on your bus. In the event, you would need to get an electronic tachometer, a converter box and have your alternator modified to provide a pulse signal to drive the tach. This is not far-fetched, this method has been used on a lot of diesel passenger cars to drive a tachometer. In fact, I built a custom tach driver from an old ABS wheel sensor, a box full of scrap electronic parts and an old instrument cluster tach. This is installed in my diesel Rabbit. Using the alternator to run the tach is so-so accurate, close enough for normal driving.

Dual exhausts could provide your bus with a bit more power, but only if someone monkeyed with the injection pump to provide more fuel. Anything that can be done to make the engine breathe easier will allow an increase in fueling and a corresponding increase in power, but it's not simply a matter of bolting on parts. In your case, the exhaust stacks are best viewed as a set of fancy chrome tail fins...

[Jones'n4chrome]

It is I who asked about hooking up a tachometer in my bus. It is a 1973 Cat 1160, so I presume their are no electronics in the engine.

I don't know what the injection pump looks like or where I could attach a tachometer to it. On page 19 of my Blue Bird thread, there are pictures of what I thought was a governor. Mark Obtinario said it was the injector pump.

Anyone have an idea how a tach is hooked up to that?

I know it was going 65 to 70MPH because I was following it in my pickup truck on the 100 mile trip back from where I bought it.

I also have those dual exhaust pipes. Does that give the engine more power?

I have an Allison 4 speed automatic transmission. I have no idea if it has overdrive. I don't know how to tell the model number of the transmission.

I can take off the air cleaner and take a bunch more pictures if that will help.

Thanks to all who can answer these questions.

This photo is from page 17 in your thread.
The ID tag should be on the side of the transmission (right side in your bus) in this photo the exhaust pipe is blocking the view of the tag.



Your dual exhaust stacks may add more power, but more power will not increase top speed unless your bus was underpowered to begin with.
It will help get your bus up to max speed quicker, but you should not drive your bus above max RPM.

Your final drive gear ratio will determine your top speed at max RPM. Providing you have enough power drive it in the highest gear.

This one on your dash says "ratio-high 6.8"



But what does this one say?
The tags are on the back of the right axle tube, you can see them in this photo.

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[rlaggren]

Rudy,

A diesel tach, bolt on.

http://www.tinytach.com/tinytach/diesel.php

[Sharkey]

People I know who have used the "Tiny Tach" consider it to be pretty much a toy and not very useful on an engine that is varying in speed (i.e. driving a vehicle down the road). The resolution of the digital display is poor, it "flips around" a lot, giving a lot of jumpy numbers that are difficult to resolve into a useable RPM. It might have it's place in stationary engines and for tune-ups, etc.

A better solution in my opinion would be the Dakota Universal Flywheel Tachometer Interface and a standard analog four cylinder dial-type tachometer.

Digital instruments are the wrong solution for vehicle instruments. They change value constantly, and don't give a good impression of what is going on in the engine or on the road. Your brain uses a different process to compute the information on digital and analog displays. For example, you can glance at an analog clock and have a good idea what time it is without having to read the actual minutes and seconds. The face of an analog clock (or analog instument) can relay an instant impression of how-much-is-enough, how-much-has-passed, and how-much-is-left without bothering you with the actual value of the reading. Think analog speedometer. You know approximately where the needle needs to be without even having to focus your eyes on it.

I will give the Tiny Tach high marks for using a clever transducer setup to detect injection cycles right through thick injector lines. Now if they would just sell the transducer with an interface to an analog gauge or sell an analog gauge version, they'd have a winner.

Also see the Universal Diesel Alternator Tachometer Interface. This could be adapted to run on any alternator.

While we're on the subject, here's my version of a diesel tachometer sensor:



The cylinder between the two sprockets is the wheel rotation sensor (variable-reluctance magnetic sensor) from an Audi 5000. It's pointing at the injection pump drive sprocket, but I designed the mount so it could point at the top, camshaft drive sprocket instead.

The signal from this sensor goes to a simple frequency-to-voltage converter (one integrated circuit, half a dozen other components), which then drives the meter movement from a gasoline tachometer that's a bolt-in to my diesel's insrument panel. In this photo, I'm simulating the VR sensor using an audio generator for calibration:



Been using it for six years now, no problems. Total cost = $0 plus some of my time.