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Thread: Increasing Towing Capacity
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chuyler1
Hauling
Posts: 140
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posted September 04, 2012 01:49 PM |
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Increasing Towing Capacity
I'd like to have a discussion about towing with the REPU. It seems like 1,500lbs is what the truck was rated for back in '74. My question is what would it take to increase that to 3,000-3,500lbs? That's roughly the weight of a small single axle travel trailer or a lightweight auto transport with a lightweight race car.
If you've done some towing, please elaborate on your experience with specific details. Distance traveled, estimated weight, types of terrain, temperature, vehicle configuration, etc. I understand no one is going to be climbing the rockies with insane weight behind this truck (or descending the other side) but my focus would be on typical interstate grades with speeds between 40-60 mph.
Lets try to break down the discussion into several different areas that make up a decent tow vehicle.
Horsepower/Torque:
The REPU is never going to pull as hard as a full size pickup but how much power would be necessary to maintain 55-60 mph on the highway? Would a street port and high compression rotors be enough? How about forced induction? What mods would you install to increase torque?
Suspension/Chassis:
Air adjustable shocks in the rear maybe? Would a sway bar upgrade be required? Wider rims and appropriate tires are a given. A class 3 hitch (aka not a bumper hitch) would have to be installed.
Cooling:
Would the factory radiator keep the engine cool on a hot summer day? Is the oil cooler large enough? Would you install a transmission/rear diff cooler typically used for race cars?
Gearing:
My truck is a '75 with the 4.33 rear end. I imagine the '74 4.625 rear end is the best option, but would the 4.33 suffice? When it comes to transmissions, what's the best option available? 5th gear on the 5 speed wouldn't get much use so are the '74-'75 4-speeds OK?
Brakes:
Any upgrade to the brakes would help, but what would you recommend? Drilled/slotted rotors? Or a full big brake kit?
Trailer:
Trailer brakes with a controller would be necessary as well as a leveling hitch and anti-sway kit.
Other?
What else do you think would be necessary? Tow mirrors, etc.
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Mazdamark
Hauling
Posts: 139
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posted September 05, 2012 10:30 AM |
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Chuyler,
I somewhat regularly tow a tandem axle trailer with my REPU. I'll sometimes tow it with my RX7 on the trailer. WIth tools and stuff it comes out to a trailer weight of ~4500. Do a search on my username I've done some threads in the past on this topic.
My truck is about the worst possible combination of driverains for towing (I have a stock 85RX7 12a, 79 RX7 5 speed, and a 3.90 rear) and I've towed this combination on 100-150 mile trips several times. I live in Texas, so there are only rolling hills to contend with, but it is damn hot here. Now, I also have a larger vehilcle to tow with if it's very hot, hilly, or far away (it's a V10 Ford Excursion) but the REPU will do the job IF you set it up right, (tires, brakes, shocks, etc.); you balance the load on the trailer correctly, and you drive intelligently. (No tailgating, conserve momentum when you can, downshift as needed, etc.)
Horsepower/Torque: Porting is a definite no. You'll kill what low end torque you have, and trust me, you'll need all of it. High compression rotors would be nice for the low end bump. I have a good condition GSL-SE 13B I plan to swap in when my 12a finally goes. I've never driven a REPU with a stock 13B, but I think the 13Bs used in the S3-S5 RX7s would be a big upgrade from what my truck has now. The key to towing with a rotary is RPMs. I cruise in 4th most of the time. I'll hit 5th on shallow grades, keep it in 4th or 3rd on steep grades for the engine braking. Uphill, keep your revs up and you'll be fine. I tow 60-65 all day, 45 or so on the steeper hills. I would not try to tow an enclosed trailer or RV, as the drag would kill you, but towing a car does pretty well.
Suspension/Chassis: I run stock suspension, Tokico shocks on the front and those rear shocks with the coil-over helper springs, and my truck does fine. Air shocks would be nice, but are not necessary, especially at 3-3500.
I run the stock tire size, (185R14) with Hankook RA08 tires. These tires are 6 ply, Class D comercial tire. The reinforceded sidewalls are a nice plus to control any sway in the setup. I air them up to 50 or 60 pounds when I have a load on, but drop them down to the high 30s when unloaded. These tires are fantastic on the road btw, but if you're going to tow off road, the 185s are too narrow.
Cooling: If it gets really hot, or you have some serious hills, I would upgrade the rad. Stock under Rad oil cooler is fine. I do run synthetic fluids in my trans and diff, but no external coolers. Those would be nice if you tow long distances or have some serious
heat where you are.
Gearing: This is key. Your 13b with 4.30s beats my 12a with 3.90s all day. 4.65s would be nice, but you can always downshift if you need it. The 4.65s would be great in stop and go traffic though. They'd really save your clutch. I looked into upgrading my trans to a latter B series at one time. From the guys that swap rotaries into their B series trucks, I think you can use a rotary bell housing on the Mazda truck box. I believe the mazda truck gearboxes are based on the Ribcase TII boxes, fwiw. One nice thing about the B series, is some of the gearboxes have a very short 1st gear. This "granny" ratio would be nice getting your rig rolling.
Brakes: My truck runs stock brakes, Porterfield R4S pads and I happen to have the Re-Speed braided brake lines (though good condition stock lines are all that is needed). The Porterfields are grippy and I've nnever had an issue with fade. I've thought about upgrading to a larger brake kit, but a stock system in good working condition with a semi-race pad (to prevent fade) has been adequate with a trailer equipped with trailer brakes. Stay away from slotted/drilled rotors. They'll crack if you get any serious heat in them. Better to run a pad that can just take the heat.
Trailer: I have found no need for a load leveling hitch or sway kit at the weights I tow. You definitly will need a heavier hitch than the stock bumper. Trailer brakes are a necessity, i'd say once you get over 3K in weight. My tandem wheel trailer has brakes on both axles and I run a Prodigy P1 brake controller.
Best advice I can give you is set up your truck with the things needed to tow safely, and try some small trips with lower weight to see how you and your truck do. Then add some weight and distance and try again. Don't go out and tow it cross country with a huge load right off the bat.
____________
94 RX7
89 RX7 GTUs
85 GSLSE
88 323 GTX
1975 REPU
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chuyler1
Hauling
Posts: 140
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posted September 05, 2012 10:54 AM |
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Awesome post Mazdamark. Exactly what I was looking for.
Ok, so port job is out. I guess I gotta decide if its worth pulling apart the engine just to install high compression rotors. I'm thinking I should just try a few tows first and see how it goes, maybe by renting a uhaul or something. Any thoughts on a bolt-on supercharger?
I'd like to tow a canned ham or a 17' airstream eventually. I'm not planning any cross country excursions. I live in Southern NH and the plan would be to do some camping around New England, 200-300 miles tops. The weather hear can get hot, but nothing like in Texas. The grades on the highways aren't bad, and if they do get steep there's always a climbing lane for the slow pokes. I just don't want to end up like this guy...
http://www.youtube.com/watch?v=lAfZ1N56qjY
Just curious, what kind of RPMs are you running at highway speeds? My truck hovers around 4,000-4,200 at 65-70mph.
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chuyler1
Hauling
Posts: 140
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posted July 22, 2013 08:40 PM |
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Here's an update. I scour the internet for as much info as I could find. I'm still not sure what the actual towing capacity is for the REPU but I'm guessing it is between 1,800-2,000 lbs based on ratings of similar vehicles. I've found reports of owners hauling race car trailers and even a Datsun owner towing horses with his similar vintage truck.
Anyway, I installed Courier front springs which lifted the front suspension quite a bit. I removed the blocks in the rear and added 80 psi to the air shocks to match height. I may actually cut a little bit off the springs to lower the suspension back down but I haven't decided yet. I got a brake controller and class 3 hitch installed.
Then I purchased a 16 foot Scamp. Dry weight is reportedly 1,750 lbs but that is without options. I haven't weighed it yet but fully loaded I'm guessing it is between 2,200 and 2,500 lbs.
I've done a few trips around the white mountains of New Hampshire and here's my report. Trailer weight distribution is very important. The trailer wags like crazy if the tongue doesn't have enough weight on it. Ideal tongue weight seems to be between 250-275. Below 225 and it becomes unstable above 55 mph. I do not have a Weight Distribution or Sway control hitch but I plan on installing one if I make longer trips.
Acceleration is what you might expect. Below 3,000 rpm is very sluggish but once you get the revs up it pulls fine up to 65 mph in 4th gear. I don't have a 5th gear but I doubt I would use it anyway. Between 60-70 mph the rpms are between 3,500-4,000 rpm which is the sweet spot for torque. On steep inclines (5% or more) it will eventually run out of power in 4th gear and with your foot on the floor speed will creep down. Once the rpms drop below 3,000 you have to shift. It will do 45-50 mph up a 5% grade in 3rd gear.
Braking is a little twitchy and I haven't got the brake controller dialed in just right. I think a sway control hitch will help with heavy braking.
Cooling is doing well. I don't have an oil temp gauge but I've been closely watching the water temp gauge on the dash. All of my trips have been in 90+ degree heat. This summer has been brutal. I did the 5% climb in that heat and saw the meter bump from 1/4 to almost 1/2 before I shifted from 4th to 3rd. It hovered around 1/3 until I reached the top. If the climb were any longer I probably would have stopped or slowed down even more.
Visibility is fine with the tow mirrors I have in the photo. I'm not even sure the dealer installed mirrors would pop out enough to see around this trailer...and this one is pretty narrow compared to aluminum sided campers.
Ride quality is harsh. The Courier springs lift the control arms up so far there is only 1/4" of clearance between the top bump stops. This causes some whacky behavior on potholes but allows for near flat cornering when I'm not towing. As I mentioned before I'm thinking I should cut 1/4 or 1/2 a coil off to get the ride height down a little.
Fuel economy is about what I expected. I've been getting anywhere from 16 to 17.5 mpg without any load. Best mileage is around town when I usually drive 40 mph. With the trailer in tow on the highway I am getting 12 to 13.5 mpg. It's not great, but when you consider the gas mileage the average full size pickup gets it is actually quite good.
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Brad
Rotorhead
Posts: 1672
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posted July 23, 2013 03:01 PM |
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Nice work. I towed a racecar with a 99 Isuzu Amigo short wheelbase SUV. 1500lb trailer + 2250lb car plus 400lbs of stuff. Upgrading from T to H speed rating tires REALLY helped, stiffer sidewalls and less tire sway. I'd find some V rated if you can.
Weight Distributing Hitch was godsend, overall more stability and less rear dip when hitting bumps. It absorbed the trailer bounce and kept the tow vehicle straight and steady.
I think you should put more air in those rear shocks, like 125lbs to handle the load. If u haven't installed the missing leafs or aftermarket comparable then I'd do so.
I adjusted the trailer brake so the trailer's brakes pulled just a tad more than the tow vehicle. You can hear the trailer brakes lock when coming to a stop. THEN back it off a hair so you don't flat spot trailer tires.
Great picture and thanks for sharing. Glad to see Gary's Orange Creamsicle is being put away wet.
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chuyler1
Hauling
Posts: 140
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posted July 23, 2013 03:40 PM |
 Edited By: chuyler1 on 23 Jul 2013 15:42 |
I've got BF Goodrich A/T tires mounted on 14x6 rims: LT195/75R14 99/96R D. They are pretty stiff even at lower PSI. Any idea what I should be running for air pressure with and without load on a tire like this? I think I have them at 35 but it feels stiff.
For the shocks, I bumped them up to 100 psi at first but that put the tongue too high. I will try it again once I have a WD hitch that will allow me to configure the ball height since the high tongue contributed to the sway.
With the brake controller, I have it set with moderate levels. I know I could use more braking power but when I let the trailer do more braking it tries to redirect the truck out of its lane. I'm guessing the wd hitch will straight this out and allow the trailer to brake harder without upsetting my direction.
BTW, if you see Gary at one of your meets let him know the truck is performing great!
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Brad
Rotorhead
Posts: 1672
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posted July 25, 2013 10:05 PM |
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I used to run mine near the max psi (44psi) when fully loaded. It made a huge difference. The more weight, the higher the psi. I've never useda LT tire but it says it'll take 65psi. That sounds real high. If the truck doesnt handle as well or squirms under braking add some more air. I'd want a wider tire but even with flares u can only go so wide.
Yeah keeping the tongue level with the ground is key if 80 keeps it level then go there. I didn't think 20 more psi lifted the rear of the truck too much, like another 1/8".
U don't want the trailer brakes to move the tow vehicle that's bad. Sounds like you're on the right track with that.
The WD hitch also keeps the trailer straighter but it's job is to really keep it level and reduce bounce and dip. The optional anti sway kit would keep it straighter. I don't think with that u can dial in more trailer brake. Can you put more weight in the REPU?
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chuyler1
Hauling
Posts: 140
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posted July 26, 2013 12:39 PM |
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Interesting tid bit of info I have. I saw a similar vintage Ford Courier for sale on eBay a while back. It still had the original tire pressure sticker inside the glove box. It read:
Air Pressure In Tire
Driving Speed More than 60 mph:
28 psi front, 49 psi rear
Driving Speed Less than 60 mph:
26 psi front, 46 psi rear
Tire Size:
6.00-14-6 PRLT
Higher rear tire pressures are common among light trucks as it assumes you will be hauling a full load. As I think I mentioned before I have been running 35 psi all around. I may try 30 psi when I'm not hauling to see if it soften things up a bit but when I load up I'll probably increase to 40-45 psi in the rear only.
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Xp882
Redlining
Posts: 294
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posted July 26, 2013 02:31 PM |
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Yeah has anyone heard from Gary?
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mazdamark
Hauling
Posts: 139
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posted July 31, 2013 12:16 PM |
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Nice setup, chuyler1. I really like the look of the truck lifted and with those BFG All Terrains. LT Tires are where it's at for these trucks if you are hauling loads.
____________
94 RX7
89 RX7 GTUs
85 GSLSE
88 323 GTX
1975 REPU
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chuyler1
Hauling
Posts: 140
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posted October 18, 2013 02:19 PM |
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Another quick update after the summer of traveling...
Without the camper, I'm running 80psi in the shocks and 32/32 psi in the tires. Whenever I leave higher psi in the tires without a load the rears lock up under heavy braking.
With the camper, I'm running 110psi in the shocks and 32/42 psi in the tires. I generally put as much gear in the front of the camper to keep the tongue heavy and that's the reason for the higher shock psi. This gives me a firm stable ride with no sway.
I still haven't purchased a anti-sway/weight distribution hitch. I may get one for next season but I didn't feel it was necessary for the short trips we made this season.
Gas mileage is almost always between 16-17 mpg without the trailer. The worst I saw with the trailer was 11.7 and that was under a full load through the hills of North West Connecticut. The rest of the trips were around 13 mpg.
Regrets? I wish I had A/C and the exhaust is pretty loud. If I changed those two things I'd venture further from home in the truck.
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