Info needed: Streetable CMP truck

[Hanno Spoelstra]

Further to my thread Chevrolet 235 c.i. conversion for CMP, I decided to split this topic off into a separate thread.

Why not keep the 216 c.i. engine, you may think? Well, I bought a 235 c.i. engine with approx. 140 hp. as a plan to build a C15 that can keep up with modern traffic. The primary objective is to have a CMP that is "streetable", rather than restore it to factory specs.

Here is my shortlist of requirements:

1. Chevrolet 235 c.i. engine with synchro gearbox to enable cruising speed of 80-85 km/hr to keep up with trucks - check
2. Road tread tyres - check
3. Balanced rims - set of four Canadian divided rims sourced, dismantled and blasted - now to send them off to a balancer
4. Upgraded brakes: fit special drum brake linings for improved friction; research options for vacuum power booster
5. 12V electrical system with provisions for camping lighting power and navigation aids; possibly intercom and/or 27Mc CB transceiver
6. ... further suggestions welcome!

[Hanno Spoelstra]

Mike O'Sullivan has set a benchmark for this with his C15A, I reckon. Mike drove it from Turkey to Normandy in 2009, which is where I met him.




Mike has since moved to the USA with his business and the C15A. In 2022 he did a Dallas Texas to Fairbanks Alaska round trip covering 10,000 miles and he does 55 on the way to a convoy and 35-40 miles an hour on the convoy.

His C15A is fitted with a 235 c.i. engine, different seats, CB radio and some more mods. I hope Mike can do a write up of the modifications he did.

[Grant Bowker]

For boosted brakes, have you considered fitting a CMP booster from a 3 ton truck to your 15cwt?
I see more modern (Michelin) tires under Mike's truck. I've never driven a truck with them but I've been told they make an extreme improvement to ride.

[Hanno Spoelstra]

Originally Posted by Grant Bowker

For boosted brakes, have you considered fitting a CMP booster from a 3 ton truck to your 15cwt?

Yes, but I understand spares for the CMP booster are hard to find so I am also looking at aftermarket power boosters like shown here: https://www.prinstruckshop.nl/c-5242...ster-complete/

[Grant Bowker]

Understood that original CMP boosters are about as common as NOS Chevrolet master cylinders...
For the style shown, you will need to check for available space in line with the brake pedal (easier to find this space on a C15 without the transfer case to get in the way...). Another option is a remotely mounted booster, mounted to the outside of the frame - sometimes fitted between the supports for the step (at least partly hidden by the battery box).

[Harry Moon]

1. The original transmission gives me no grief at all, you get used to it.
2. Taller tires! I run the 1050 X 16 .
3. Fenton exhaust manifolds, wakes up the motor and no more exhaust gasket leaks.
4. Check wheels for straightness with a dial indicator, use the best you have. Put a couple hundred miles on before balancing at a truck shop.
5. Pertronix ignition for reliability with matching coil.
6. Line the doghouse, the frame channels and the engine compartment with insulator/sound deadener. wouldn't hurt to do that to the underside of the floor to.
7. Spring seat cushions and don't use just a foam block. I use the original seatsas I need all the legroom I can get and a set of van seats that were in it when i got it pushed me to far forward.
8. Single wire alternator with 12 volts, led bulbs in the signal lights.
9. Brakes have never been a problem for me but if you want power install a remote hydro vac if you can't procure an original set off of a C60L

[Hanno Spoelstra]

Originally Posted by Grant Bowker

Understood that original CMP boosters are about as common as NOS Chevrolet master cylinders...
For the style shown, you will need to check for available space in line with the brake pedal (easier to find this space on a C15 without the transfer case to get in the way...). Another option is a remotely mounted booster, mounted to the outside of the frame - sometimes fitted between the supports for the step (at least partly hidden by the battery box).

The first modern brake booster I saw fitted on a CMP was on John Hotson’s C30. He modified his trucks to make them suitable for modern traffic. Look where he managed to fit it:

[Bob Carriere]

A top speed of 55 mph with standard C15a axle gear ratio.......

My 261 stock rebuilt gives me 44 mph with the 10:50 Chevron tyres and that is pushing near 2990 RPM........

Now my speedometer has not been calibrated using a GPS reading and I think my rpm meter is reasonably close to a testing separate rpm meter gauge.

Planning to calibrate the NOS GMC speedo with a telephone app this Spring.''I do have a mounted set of NOS 9:00 x16 Michelin that have not tried yet.. I believe from all account thats the radial are far smoother b ut not sure they would increase the top speed unless the rolling diameter is greater than the 10:50.......

How do I get more top speed without pushing the 261 much over 3000 rpm????

44 mph is a handful not sure I would feel comfortable at 55.......

Note that non of my tires have ever been balanced yet....... one truck tire shop would not tackle the 10:50 on my CMP rims ..... have yet to check with Benson truck shop....

Properly adjusted new brake linings on radius linings matched to turned drums seems quite adequate.

Comments/suggestions welcome before I paint RED racing stripes on my CMP.......

[Bob Phillips]

I think Bob C. has preempted my question about roadspeeds on stock 15cwt drivelines. I have had severaal roadworthy CMPs over the years and could never achieve speeds any where near 50mph without fear of engine destruction or vehicle instability resulting in catastrophe.
I have 20 inch wheels and tires on my current 15cwt chev it it wont do 50mph
without fear of imminent disaster.
So what experience do others have on this question?

[chris vickery]

60MPH in a 12 cab CMP. Not the most intelligent choice, but back in the day that’s what she could do.. Was it safe? Foolhardy? Probably

[Harry Moon]

in 2005 coming home on our first big freedom convoy I had on a set of 1100x20 on my C15A, we hit about 65 mph when we just had to pull out and pass an LSVW. Only time I ran the big tires on the C15A.
I regularly cruise at 50 mph (gps confirmed) in my C60L with 1100x20's with the 235 with Fentons.

[Grant Bowker]

Canada's Fighting Vehicles lists maximum speeds between 40 and 50 MPH. The maximums do vary within the same basic model (eg. C60) which I assume may be due to weight as to the best of my knowledge all C60 had the same power, gearing and tires.

[Harry Moon]

Originally Posted by Grant Bowker

Canada's Fighting Vehicles lists maximum speeds between 40 and 50 MPH. The maximums do vary within the same basic model (eg. C60) which I assume may be due to weight as to the best of my knowledge all C60 had the same power, gearing and tires.

I imagine with the 216 and 10.50x20 tires that is the upper limit.
Way more useable RPM with the 235 and a few MPH from the 1100 tires.

[Hanno Spoelstra]

Very usable info, will digest and get back on this subject later

[Lang]

There are lots of anecdotal figures on what people can and can not do with tires, gear ratios and engine speeds. There is nothing subjective about it - just straight maths.

The subjective bit is when one bloke thinks it OK to sit on 3,000rpm all day long while another is horrified if it goes over 2,500rpm on an over run down hill. More than worries about engines exploding is the fact revs-is-fuel.

There are lots of calculators on line. I find this one pretty quick and easy. Their 3rd gear number is just standard 1:1

https://spicerparts.com/calculators/...rpm-calculator


This tells the story, even with a modern efficient design quite clearly every extra rev is costing you money.

[Lang]

Here is a simple 60mph chart Easy to figure out revs at any speed eg for 40mph just do the 4/6 calculation, for 55 do 5.5/6 and 35 do 3.5/6 etc.

When measuring the tire height do it from the ground to centre of the axle and double. This is your true radius and diameter.

[Hanno Spoelstra]

Originally Posted by Lang

There are lots of anecdotal figures on what people can and can not do with tires, gear ratios and engine speeds. There is nothing subjective about it - just straight maths.

The subjective bit is when one bloke thinks it OK to sit on 3,000rpm all day long while another is horrified if it goes over 2,500rpm on an over run down hill. More than worries about engines exploding is the fact revs-is-fuel.

There are lots of calculators on line. I find this one pretty quick and easy. Their 3rd gear number is just standard 1:1

https://spicerparts.com/calculators/...rpm-calculator

This tells the story, even with a modern efficient design quite clearly every extra rev is costing you money.

Thanks Lang, helpful indeed to calculate rpms at a given speed.

For a C15, cruising at a speed of 80 km/h, with a 6.16 to 1 ratio rear differential and 844 mm diameter tyres, the engine will rev at 3096 rpm.

[Lang]

Even for a Chevrolet 3,000rpm cruise is getting right up there. I think a Ford or Dodge engine would be hurting pretty badly at that pace for long periods.

Maybe you could look at some sort of overdrive. Either a change to a modern 5 speed gearbox or a bolt-on unit. I think a higher diff ratio would probably be better as you Europeans do not seem to take your vehicles bush-bashing and the unnecessary crawler first gear is just along for the ride.

[Hanno Spoelstra]

Originally Posted by Lang

Even for a Chevrolet 3,000rpm cruise is getting right up there. I think a Ford or Dodge engine would be hurting pretty badly at that pace for long periods.

Maybe you could look at some sort of overdrive. Either a change to a modern 5 speed gearbox or a bolt-on unit. I think a higher diff ratio would probably be better as you Europeans do not seem to take your vehicles bush-bashing and the unnecessary crawler first gear is just along for the ride.

Yeah, a 5-speed conversion like you did seems the way to go. On the other hand, the engine I bought is fitted with a fully rebuilt SM420 4-speed gearbox already, so maybe an overdrive/ diff swap would be a better option? We'll see later.

There isn't a lot of bush left in Western Europe to bash through - this is about as rough as it gets
https://youtu.be/rAo71QQRqpE?si=-ua7mUj2BOwazXvp

[Lang]

Still fun to get them off sealed roads into their natural environment.

Here is another 60mph chart with a wider range of differential ratios.

[Bob Carriere]

....with the newer 235........???????

Consider yourself lucky you got a FULLY synchronized 4 speed...... no more grinding......

Not sure how accurate my NOS speedo is but top I can get at approx 3000rpm is 44 mph...... and that is with my 261...... at which point I have both hands on the steering wheel........

My 10:50 tires on 16 rims gives me 532 inches of rotation per mile....
.......tire is 38 inch tall.

On January first we will be taking it out of the catacomb for the traditional spin in the back field..... fist time since 1990 that we have absolutely NO snow on the ground.....lucky if the ground is frozen solid....

Cheers