Getting my vintage mini truck ready for towing

[chuyler1]

Hi everyone, I already posted a welcome thread but I figured I would share here as well. I'm an avid Mazda enthusiast...an odd brand to be loyal to but it fits me. I've owned 4 total in my life and have two right now.

I purchased this 1975 Mazda REPU over a year ago. The previous owner did the restoration but never actually drove it more than 500 miles. His style was...well not mine...but I saw potential in the truck. I've spent the last year sourcing parts to get it back to a state where it can be a truck again. I haven't towed with it yet, but I plan to tow a 16' Scamp as soon as it gets delivered.

This is what I started with... Keep in mind these photos make the truck look larger than life. It is small, measuring only 5' fender to fender. I think a Toyota Prius is wider. It does however have a 6' bed and a 1,400lb payload.



Not bad, for the car show circuit. I purchased it sight-unseen after getting an appraisal. Within the first months I had to...

* Replace clutch master/slave cylinder
* New ignition coils
* Install speedometer cable
* Repair dash light wiring
* Replace turn signal relay
* Install a radio
* Complete the exhaust
* Replace the horn
* Install and repair windshield washer
* Repair some gas lines that were dry rotted
* Install evaporater emmissions lines
* Repair choke

So yeah, great job on the body work, not so great on the mechanicals...that's OK though. I was looking for a project. The motor is an original 4 port 13B 1.3L rotary. It will rev to 7,000 rpm and the only way you'd know it is from the buzzer that goes off in the dash to warn you to shift. This particular motor had 0 miles on it when installed by the previous owner. It was one of a few lost in a dealer warehouse for decades, part of a manufacturer swap program that Mazda offered in the 70's when apex seals started to wear out prematurely. The emmissions equipment is removed and a header and high flow exhaust is installed...but other than that, what you see is original.




More recently, I've been doing tow-specific preparation.

* I sourced replacement springs for the front and installed them
* Removed the lowering blocks in the rearl
* Installed air shocks 25-250psi in the rear
* Had a hitch installed
* Wired up a 7-pin harness
* Replaced the low profile wheel/tire combo with something more appropriate. The speedometer was way off before and now I've gained back a few highway cruising rpms.





Some of the things Mazda was kind enough to provide from the factory...

* Front disc brakes, an uncommon feature for a truck this size in 1975 (I've got slotted rotors which should help with heat dissipation)
* Front mounted oil cooler (rotaries use the oil for cooling so it is necessary)
* 4.33 final drive ratio (not so great for gas mileage but perfect for towing)
* 4 speed transmission (probably wouldn't use 5th gear while towing anyway)
* 4 barrel dual stage carb (pretty big for a 1.3L motor)



Final details that I'm missing and hope to resolve before my Scamp arrives in June...
* Brake controller, Not sure what to get yet and could use some opinions.
* Spare tire winch, I think I have a line on this...I don't want to go too far without a spare tire!
* Tow mirrors, I'm trying to find a set of original dealer installed West Coast Jr mirrors. I've got a line on a set that may need re-plating. I've seen some available from online parts stores but I think they're too big.




Anyway, that's where I'm at. Given some of the tow vehicles I've seen on this site, I think the truck is prepared for the job. I don't know the original towing capacity and haven't had much luck finding a copy of the owners manual but the Scamp shouldn't be much more than 2,500lbs and if the truck was rated for 1,400 lbs in the bed, it shouldn't have trouble with a 200lb tongue weight. I tested standing on the hitch and with the shocks at just 50psi i get virtually no sag.

[Bobbie Mayer]

Piston engine goes chunka chunka chunka
Rotary engine goes Purrrrrrrr.

(Remember the Mazda commercial?)

[chuyler1]

Yep...

Mazda RX-3 Commercial - YouTube

[chuyler1]

But these days, most people think of the rotary a little differently. If you've never heard one in race trim it is quite an experience. We go to Limerock Park every year for Grand Am racing. They used to bring the 20B cars like the one below but the race program moved to Diesel and we won't get to hear the swarm of bees in the foothills of Conneticut anymore

Mazda RX-8 3-rotor 20b race car BTCS - YouTube

[Matt in SV]

That's a nice truck!

The Piston engine goes sproing sproing sproing but the Mazda goes Mmmmmmmmmmmmmm.

Mazda RX-3 Commercial - YouTube

We've had three. A 1987 Miata, 2005 Millenia-S and 2007 MPV. No rotaries, but I bought one to take apart.

Regards,

Matt

[Cam A]

A very nice little truck. I'm not sure about towing, though; were not rotaries notably shy on torque?

[Thomas G.]

Cool truck, but I'm not sure that I'd use it for my regular tow.

[chuyler1]

Based on the 1974 Road & Track article, the motor had...
110 hp @ 7,000 RPM
117 ft-lb @ 3,500 RPM

Rotaries are not knowing for having copious amounts of Torque and the RX-7 and RX-8 sports cars were tuned with high rpm in mind and despite having upwards of 250hp never had more than about 160 ft-lbs to go with it.

Anyway, those numbers above seem low by today's standards, but keep in mind the truck's curb weight is only 2,800 lbs. Also compare that to an early 70's Ford straight 6 that came standard in many full size pickups weighing more than this truck...

105 hp @ 4,400RPM
156 ft-lb @ 2,400RPM

Ok, so it would be nice to have a little extra torque but I'll see how things go. The truck has quite a bit of get-up-and-go right now with the free flowing exhaust. It revs at about 4,000 rpm at 70 mph on the highway and should be right at max torque at 60-65 mph. I can upgrade the carberator if I feel I need a little more power.

Another thing to consider is the torque curve. It has a low value but the curve is very different from a piston engine. The more you rev the rotary, the more power you get.

Rotary Engine power curve:


Typical Piston Engine Power Curve:


I'm not saying we should all be running rotary engines...but if it weren't for the gas mileage all the major manufacturers would have released a version in the 70's. Chevy actually built a prototype of the Corvette with a mid-mounted rotary engine. Can you imagine that? If you are gasping, you'll understand why GM tried to hide this from the public, but one of the prototypes still exists.

[floyd]

Stunning! I'm betting that there is no better example in the world.(Zoom-Zoom)
Am I right in assuming that you may be able to source some parts from the Courier should the need arise?
I once installed a turbo on the 70CID engine in an RX7,
REDLINE IN OVERDRIVE!

[chuyler1]

You are correct. Many of the parts from the Ford Courier swap over. Door panels, door seals, windows, suspension bits, etc.

About 15,000 were built originally. I'd guess there are less than 1,500 left, possibly less than 500. This is probably among the top 20 since the bed, fenders, and motor were all NOS. There are a few really nice resto-mods that have replaced the entire drivetrain down to braking system. There are a few original examples with just fresh paint jobs. Mine fits somewhere in between. Body work and under carriage is immaculate, engine bay is mostly stock, inside has updated interior but all the stock parts are there.

I paid much less than a new Ford Ranger or Toyota Tacoma and I plan on driving it around 5,000 miles a year until I get bored with it. I don't believe in trailer queens and I told the previous owner it was going to get driven. I appreciate the hard work put in, and I have put in work of my own to keep it running...but its a truck and it needs to see the country side and do truck things like haul a trailer!

[stevebaz]

Cool truck I think it will do fine with the manual transmission you just wont be racing to get there especially if hills get in the way. I use to have a Datsun 520 pick-up and loved these old small mini trucks. The big issue with these is the lack of driver comfort. No air and worse no powersteering. Without power brakes I would consider putting larger brake drums on the trailer. With the trailer weighing as much as the truck you need to have control of the trailer or it could easily pitch you into the ditch. If it were me and the truck stays I would be looking for a smaller trailer.

[chuyler1]

No a/c could be an issue if we get stuck in heavy traffic on a hot day. While driving it isn't too bad. There are vents under the dash that when open allow air from the front grill to pass right in.

No power steering isn't a big issue. With 195 series tires the steering is light as long as you are moving.

Brakes are actually very good. Obviously no ABS though. The truck is about 3200 lbs with gas, driver, and a few items in the bed. The trailer will be max 2,500 lbs (probably closer to 2,200 if I don't fill it up with water before leaving).

We did consider the 13' scamp instead but I know we'd want to upgrade eventually. My daily driver is due to be replaced in a few years. If we start venturing further from our home and the truck doesn't quite cut it for those long trips, I'll consider something with a little more towing capacity for my DD so we can use it when we don't want to take the REPU.

[Brian B-P]

My guess is that there's no need to worry about the ability of the "little" engine... although the displacement of two chambers (like cylinders) is 1.3 L, there are three chambers around each rotor, so the total engine displacement is 3.9 L. It's not so small, and has no problem running fast enough to provide a bunch of torque.

We survived for decades without air conditioning, even in hot places. The car I drove daily didn't have power steering until 2002, and it didn't kill me.

Originally Posted by chuyler1

With 195 series tires...

That's presumably 195 mm section width. Just curious: what's the tire size? 195/70R14, perhaps? I have heard the stock size was 7.35-14 LT.

[chuyler1]

They are 195/75R14, 1,700lb load rating per tire and the speedo is within 1-2mph of being correct based on my GPS.

I could have gone wider but the next step for the tires was 215/75R15 and those wouldn't fit under the fenders without rubbing and would have been too tall. The tire size is a bit odd and there weren't many options to be honest, especially if I wanted white letters. Original tires would have been something like 185/80R14 so I've got a little extra rubber and plenty of load handling.

When comparing displacement, the rule of thumb is to double it. a 1L (10A) wankel is roughly equivalent to a 2.0L 4-banger. A 1.3L (13 wankel is similar in output and performance to a 2.5L 4-banger. That's typically how they are classed for racing...but they also restrict porting and rpms to make it fair for the piston engines. You can push these motors well over 300hp without using forced induction. They don't last long between rebuilds at that level, but you'll never get that much out of a 2.5L piston engine without slapping a turbo on it.

[Brian B-P]

Thanks for the tire info - looks like the truck is ready to go. That's presumably a load range D Light Truck tire (LT195/75R14). It's about the same size as is almost universally used with 3500 lb trailer axles (205/75R14), and thus with most 17' or so trailers - but not the Scamp 16', because Scamp thinks the same 13" tire is right for all sizes of trailer. If you changed the trailer's hubs to the available 6x5.5" pattern you could interchange wheels and tires with the trailer, as long as the wheel offset was close enough. Just more ideas for customization, not anything that's needed...

Originally Posted by chuyler1

When comparing displacement, the rule of thumb is to double it. a 1L (10A) wankel is roughly equivalent to a 2.0L 4-banger. A 1.3L ( 13B ) wankel is similar in output and performance to a 2.5L 4-banger. That's typically how they are classed for racing...but they also restrict porting and rpms to make it fair for the piston engines.

Yes, that's the result of the racing politics, disregarding physical reality... but I think we should spare the normal people in the forum from a rotary engine classification debate.

[jen b]

That truck is straight up adorable.

[chuyler1]

Originally Posted by Brian B-P

If you changed the trailer's hubs to the available 6x5.5" pattern you could interchange wheels and tires with the trailer, as long as the wheel offset was close enough. Just more ideas for customization, not anything that's needed...

Yeah, I would love for the trailer to be 6x5.5. The wheels I have are actually 14" trailer wheels. I couldn't find an original set so these were the closest. 14x6 0-offset.

[Brian B-P]

Originally Posted by chuyler1

Yeah, I would love for the trailer to be 6x5.5. The wheels I have are actually 14" trailer wheels... 14x6 0-offset.

The common Dexter Torflex #10 (and the same capacity 3500 lb leaf-spring beam axle) use the "545" hub group - named for the standard 5x4.5" bolt pattern - which includes 6x5.5" hub/drums as an alternative. Just swap the hub/drums and you're good to go!

[chuyler1]

Hi guys,

Since we last spoke, I've managed to get the brake controller installed and found a set of fender mirrors that won't mar the paint job. Oh, and the camper was delivered on Sunday!

I haven't loaded it up with gear yet but I did take it for a spin around the block. This included some 40 mph driving on curvy roads, a hill start, and a short 4% hill climb with a 55 mph speed limit.

Initial impressions:
I found myself driving about 35 mph in 3rd instead of typical 40-45 mph in 4th. 4th is definitely too tall for anything under 40 mph, even without a trailer attached, since the rpms drop below 2,000 and the rotary loses all sense of torque. The hill start was fine and the clutch wasn't a problem. Accelerating up the 4% grade from a stop wasn't a problem either and I gradually made it to 60 mph by the top without using full throttle. Braking from 60 to make a left turn was fine too but I didn't try an emergency stop. I still have to dial in the brake controller to get the right feel. While cruising at 35 I did experience a little porpoising over bumps. I may add more psi to the air shocks and/or rear tires to see if this firms things up, otherwise I'll get a WD hitch. Backing up into the driveway for the first time was a success. The mirrors weren't adjusted right so my wife helped a little (and that actually didn't really help since she doesn't understand how the trailer turns). Overall, I'm very impressed with what the truck can do. The trailer by no means goes unnoticed but with careful driving I should have no issues with short excursions. I'll have a more in depth review once I load it up with gear, but for now, here are a few photos...








Oh, and one other note, it looks like my hitch is about 1" too low. I think it should be fine, what do you think? All Uhaul had was a hitch that was 2" taller and I thought it was better to be low than high...if you think it's iffy, I will get something online.

[Brian B-P]

Looks good!

Originally Posted by chuyler1

... it looks like my hitch is about 1" too low. I think it should be fine, what do you think? All Uhaul had was a hitch that was 2" taller and I thought it was better to be low than high...

I agree - a bit lower than level is fine.