Are there any upgrades for the 225 slant six engine that would increase the torque from idle to 3000 rpm for towing?

I have a 1964 W200 (4x4, 3/4 ton) Dodge Power Wagon that I got without an engine. This truck originally had a slant 6 with an NP-435 transmission and a 4.10 Spicer 60 rear end. I'm wondering if this truck with a rebuilt slant 6 could be used for towing small trailers (from 2,700 to 5,500 lbs.) at speeds up to 65 mph?

That is a heavy truck and you want to tow a fair bit of weight. You can do it with a 225, but you will need to spend some money and time.
Some questions before we go further:
1. Are you building a fresh motor from top to bottom?
2. What is your budget for just the engine?
3. What trans and clutch do you plan to use?
4. Do you have a good machine shop nearby?

We’ll want more info after you answer these questions, but this is a start. The easiest path will be a v8 swap but that will take you out of this forum and the wacky world of slants…

Lou

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Home of Slant6-powered fun machines since 1988

The upgrades I would recommend for this application are as follows:

Simple route:

(1) two barrel carb with the intake divided into two halves, each half fed by one carb barrel
(2) upgraded camshaft to stock 1972-1981 mechanical lifter cam
(3) free flowing exhaust - 2 1.2 inch from manifold to muffler (walker quiet flow) and 2 1/4 from muffler to bumper
(4) mill head enough to achieve 8:1 dynamic compression ratio
(5) custom tune the distributor mechanical and vacuum spark advance curves
(6) reduce parasitic loss: clutch fan, synthetic fluids, run correct tires pressure in skinny tires


Difficult/expensive route:

(1) same carb upgrade
(2) contact Oregon Cam grinders and get a custom ground cam (search the board for the RV10 or RV15 cam profile)
(3) Dual exhaust using a Dutra front manifold and a modified stock rear manifold. Connect the pipes using a quality y-pipe to promote the scavenging effect. 2 inch pipes from the manifold to the Y, 2 1/2 inch from the Y to the muffler (walker quiet flow) 2 1/4 inch from the muffler to the bumper
(4) rebuild the head and clean up and bad spots in the ports
(5) install a supercharger (eaton M90 or torquestorm)
(i) adapt your carb to work in a draw or blow through system, whichever you prefer, or go all out and install a form of fuel injection
(ii) tune the timing curves or install some form of computer controlled ignition
(6) reduce parasitic loss as in the simple recipe above

The hardest part will be achieving 65 MPH. Assuming a 28.5 inch diameter tire (this is roughly a 235/75/15 tire), with 4.1 rear gears and in 3rd/direct drive, the engine will have to turn roughly 3200 RPM at 65 MPH. If you have an overdrive the engine will turn around 2200 RPM, but that would be putting quite a load on a 225 hauling that truck and trailer around.

I own a 1976 D100 that was built with a 225, a 727 three speed auto trans, and it has 3.9 rear gears. I can cruise all day empty at 70-85 MPH but the engine is spinning 3300-3500 RPM to do it. Not good for economy or longevity. I have been gathering parts to convert it to a more modern A500 four speed auto trans with overdrive to get my highway RPMS down. That said, this truck has a tough time going up moderate grades with a load in the bed and a trailer. It will do it, but it won't do it very fast. Granted, the engine is tired and I haven't spent the time to really fine tune it, but it is still a little six cylinder being asked to motivate a heavy load.

Honestly, if I were in your situation I would be thinking about a switch to a more modern and powertrain. Installing a Magnum 318 or 360 would be fairly easy and give you plenty of power and much better economy than you would get with a slant six. But if you are set on installing a slant six then I would probably do the "simple" recipe I laid out above.

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Casually looking for a Clifford hyperpak intake for cheap.

Since the 4.10 Spicer 60 rear end gear was set up for a slant 6, is it feasible to reduce that ratio to something more suitable for a 318 or 360? That way the NP 435 that came with the slant 6 could still be used with the V8.

If what you are after is low end torque, then leave the 4.10 gears and install an overdrive manual transmission. A magnum 318 or 360 can mate to a NP 435 with the right bellhousing, but it is different than a slant six bellhousing. Further, changing front and rear gears gets expensive. Finding a manual overdrive transmission would be cheaper than swapping gearsets.

You might want to keep the slant six and then NP 435 trans and find a separate divorced overdrive unit to bring the highway RPMs down. Brownie box, Laycock overdrive, etc... Many have been made over the years.

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Casually looking for a Clifford hyperpak intake for cheap.

Agree. Carefully-chosen carburetor is crucial.

'71-'80. Disagree. Not enough difference in the metal to make a difference in how the engine runs sufficient to justify the work and expense. If anything, the '64 cam is a bit better for low-low-end torque, because of its lesser overlap. If we're taking the engine apart anyway, and changing cams at all, go directly to the Dutra RV-10RDP, Oregon Cam grind № 2106R. That one is a worthy upgrade.
Agree on principle, disagree on particulars. No benefit to 2½" headpipe, and it'll create seal hassles at the exhaust manifold outlet flange. Also, 2¼" all the way to the end of the tailpipe means just pointless noise for no benefit. I would instead use the recipe here.
Agree if we've decided to have the engine apart for some other reason. Like the cam swap, this one's not worth the hassle on its own.
Yup! This should be much higher on the list. And a thoughtful ignition upgrade. And speaking of, before we get into upgrades and rebuilds and stuff, a thorough tune-up (parts and technique suggestions in this post) and carb go-through (Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download) should be first on the list.
Meh. They're not terrible or anything, but I wouldn't put much priority on this; see here.
Sure, carefully-chosen ones.
Disagree. We're towing a trailer with a heavy truck. For safety's sake, we don't want to skimp on tire size.
…in a brick being pushed through the wind.
Agree. The Slant-6 is a lovely engine, but it is not the one and only and always-best.

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Too many people who were born on third base actually believe they've hit a triple.

315-75-16 tires will make your 4:10 rear end more like a 3:10 gears.. as they are 34.5" tall... The shortcoming is that they also reduce the leverage the brakes have... so towing loaded with tall tires might be a bad idea...

Reed's idea of using a V8 is wise... I didn't, and don't plan to, but if you want to go 65... that is the way to go.. the early trucks were designed for local use... and so getting them to go quickly at a distance is a big undertaking... basically everything needs to be considered and upgraded.. brakes, transmission, engine..

I have a 54 M37 that I installed a slant 6 into.. it weights 5600 empty, and from the factory it has 5:83 gears.. the engine is stock, and someday I will put a cam in it, and do head work... but I also only drive it locally...

A slant six could tow that - at slower highway speeds. Maybe take back roads. This is where the Ford 300 six does well, that extra 75 inches of oomph is needed. In the '40s and '50s Dodge used "Job-Rated" transmission and axle options to tow, at maybe 45 mph with a 230 L-head on 1 tons, then step up to a 236 on 1 -1/2 tons, then 251 on to as large as 413 six on road tractors towing 40 foot semi trailers. But, most roads were then like back roads. Is it is something you do every six months vs weekly?