Which motor for trouble free cruising including highway miles

-
W

Willy Moose

Member
Joined
Jan 28, 2025
Messages
9
Reaction score
11
Location
Milwaukee
My mind is made up. Done with the tip toe grampa’s ride. I’m getting the 1990 124” Dakota frame to put my 1950 Desoto Custom onto. My question is what engine has the most reliable reputation 360 LA or the 360 Magnum. This will be bottom up brand new frame start. Money is not an obstacle (to a point, I’m not rich) and I just retired. Plenty of time and I do all my own wrenching (again, to a point). I was commercial HVAC repair tech, big RTU’s, Boilers, and Process equipment. I’ve been wrenching on that stuff for 47 years, so i should have a good leg up. I’ll be doing simple 4 barrel and which ever automatic transmission (slap stick) makes sense. My problem is lack of knowledge on engine and transmission quality for my type of driving. I’m also thinking 3:55 rearend, and with that in mind is 8.25 or 9.25 better. Any input is greatly appreciated. Thx. My driving will be mostly cruising all the time daily driver and 300 miles Hwy twice a month up north. Looking for reliability.
 
Last edited:
Welcome to the site!!!
Both 360's are reliable. I would get a roller cam, which makes the magnum an easier find (LA 360's with roller cam were just a few years). Both the 8.25 and 9.25 will hold up just find for a mild small block. Enjoy the project and post pictures!!
 
If you end up with a 5.9 magnum, put a 318 cam in it. the 5.2 cam has more lift/duration than the 5.9 Buddy has a 5.2 in a 49 Plymouth and we got almost 24 mpg at 73 mph coming back from Moparty last sept. He has an a518 and 3.23 gears in an 8 3/4.
 
i'd press the easy button and go magnum with an eddy air gap and leave it as is up front with the magnum front cover and accessory drive. a reasonable cam, hughes spring/retainer kit, headers, AVS 2 650 and ship it.

if you get the trans with the 90 dak frame, that should be some variant of an overdrive A500/518 or RH42/46 which would be a wonderful match for a cruiser looking to gobble up highway miles.

a stock 8.25 will be plenty strong and 3.55's are plentiful for those. you can also upgrade to disc brakes easily.
 
Magnum parts are more plentiful and cheaper in most cases, if I had to start over that's the way I would go.
 
I built a roller LA out of my '85 th ave. cleaned up/reringed the shortblock, added magnum heads I redrilled and did a little porting to, IIRC I used hughes 1110 springs, 2.2L retainers, and had bullet cams regrind the LA roller cam with HR259/314 on the intake and HR262/312 on the exhaust Bullet Cams Master List. headers, air gap and 600CFM eddie, and it was a nice, strong motor.

was a really nice motor, and in my 5th avenue with a 42RH & 3.55's would keep up with a 5.7 hemi 300C, and would put down 20-22mpg on the highway.

if I were starting with a motor from scratch though, I think I'd get a '09-up "eagle" 5.7....the one in my '11 charger is awesome.
 
That era of Dakota's has great drivetrains. The 5.2 in my Dak's are real neck-snappers and still run great on the freeway. If it's over 150k I'd probably put a new timing chain in it (and replace the gasket and plate on the bottom of the intake manifold), and rock on. The overdrive makes cruising a snap and those trucks handle great.

Edit, if you want the EFI and are just building a cruiser, the stock 5.2 is probably just fine. It's not hard to convert the EFI to run the 5.2 or 5.9 if your chassis is V6, but I like mine closer to stock if there's a chance I might be far from home and need to remember where this or that part came from.
 
Willy Moose said:
My mind is made up. Done with the tip toe grampa’s ride. I’m getting the 1990 124” Dakota frame to put my 1950 Desoto Custom onto. My question is what engine has the most reliable reputation 360 LA or the 360 Magnum. This will be bottom up brand new frame start. Money is not an obstacle (to a point, I’m not rich) and I just retired. Plenty of time and I do all my own wrenching (again, to a point). I was commercial HVAC repair tech, big RTU’s, Boilers, and Process equipment. I’ve been wrenching on that stuff for 47 years, so i should have a good leg up. I’ll be doing simple 4 barrel and which ever automatic transmission (slap stick) makes sense. My problem is lack of knowledge on engine and transmission quality for my type of driving. I’m also thinking 3:55 rearend, and with that in mind is 8.25 or 9.25 better. Any input is greatly appreciated. Thx. My driving will be mostly cruising all the time daily driver and 300 miles Hwy twice a month up north. Looking for reliability.
Click to expand...

I was at the same crossroads you are with my Dart.
Went with a mild built 5.9 and 42RH overdrive and 3.55 sure grip 8.25 rear end with the Grand Cherokee disc brakes.
No EFI, Holley 770 vacuum secondary.
Oregon Cams roller regrind with Hughes springs, retainers and locks.
Most every part mentioned above.

25 mpg hiway at 80mph running about 2,300 rpms.
(Vac advance tuning is key for mpg)
Who cares what city mpg is. :D
Did mine about 8-10 years ago and it has been a flawless daily driver since.

IMG_5384.jpeg




A500OD.jpeg


ShifterArt.jpeg
 
Last edited:
Thanks trail beast. This is exactly what i needed. Did you do wiring harness as i will have to do. And what exact disc conversion did you use.
 

"Which motor for trouble free cruising including highway miles"​

If you're doing the overdrive, (which I too highly recommend), which lets you use a deeper rear gear,
and you're looking for hiway fuel economy,
If it was me then IMO,
I would go with a 5.2M, over an LA 318 for it's higher compression ratio and roller cam, and I might get a higher than stock stall, still with a lock-up. But as to reliability between these two, IDK I think it would be a Toss-up. But if you are planning an engine overhaul, then the LA318 might be back on the table. I say might because, the Magnum still has the roller cam, which has got to count for something.

> As for the LA360 vs the 5.9M, it's a wee bit harder to get fuel-economy out of a 360/5.9M, unless you gear it down (in numbers) one gear size. and if you do that, then your cubes will still get you a lil more take off, require less stall, and have better passing power, in what looks to me like might be a lil heavier than an A-body..

BTW-1
If you're in hill-country, your passing gear needs to be given some thought; if you have to go down to Second Gear with a screaming engine, I find that highly embarrassing.

BTW-2
I really like TrailBeasts combo.
Not too many years ago I ran a similar combo, (68-Barracuda) but with a Manual trans, and geared 80=1960. That combo made a lil better fuel economy, (than TBs) with a tuned for economy 600 Holley, and dash-mounted, adjustable timing module.
and I agree with TB, cruise-timing is key.
 
85-up M bodies (5th ave) if you can still find one have roller cam 318's. the cam can be reground, lifters reused, and can still run a mechanical fuel pump with an LA accessory drive if you are sticking with a carb
 
Willy Moose said:
My mind is made up. Done with the tip toe grampa’s ride. I’m getting the 1990 124” Dakota frame to put my 1950 Desoto Custom onto. My question is what engine has the most reliable reputation 360 LA or the 360 Magnum. This will be bottom up brand new frame start. Money is not an obstacle (to a point, I’m not rich) and I just retired. Plenty of time and I do all my own wrenching (again, to a point). I was commercial HVAC repair tech, big RTU’s, Boilers, and Process equipment. I’ve been wrenching on that stuff for 47 years, so i should have a good leg up. I’ll be doing simple 4 barrel and which ever automatic transmission (slap stick) makes sense. My problem is lack of knowledge on engine and transmission quality for my type of driving. I’m also thinking 3:55 rearend, and with that in mind is 8.25 or 9.25 better. Any input is greatly appreciated. Thx. My driving will be mostly cruising all the time daily driver and 300 miles Hwy twice a month up north. Looking for reliability.
Click to expand...

Reliability is t the issue. IF! You can reuse the fe injection on the 5.9, id to that route as the factory did an excellent job with it and the engines bores will look great and last a long time before the cross hatch hairs are gone.

The biggest key to longevity for an engine is to keep the fluids and filters changed on a regular basis. The other item is low cruise rpm. Keep it cool, fresh liquids and filters at low cruise rpm’s and that set up will go and go and go miles and miles and miles.

IMO, the best thing you can couple to that engine with 3.55’s is an overdrive transmission. You’ll get a deeper first gear for that excellent 10.0 ratio leaving the line (or close to it.)
And a OD gear for a good cruising gear.

I think, (so forgetful right now) 3.09 first gear multiplied by the rear ratio @ 3.55, 3.55 X 3.09 =10.97 and the OD at 3.55 X .69 =2.45 for a stour Hwy. cruise gear.

The Chrysler based OD is 904 based and you’ll need the early version that is Hyd operated. Not electronic.
 
Most dependable cruiser? Luke rusty said /6 or else a 318!!!
 
Had a slant six before, it lacked guts in its original shape. I’m going to go with a little more power this time.
 
Thanks AJ. I wont have to worry about mountains or big hill country, I’m in Wisconsin where people call hills mountains, haha. What rear end gears would you use for minimal flat rolling hills. Up here everybody has been doing 85 or 90 for a few years now, however I’m still thinking daily driver cruiser. Yeah, looking for the best of both worlds, go figure.
 

Willy Moose said:
Thanks AJ. I wont have to worry about mountains or big hill country, I’m in Wisconsin where people call hills mountains, haha. What rear end gears would you use for minimal flat rolling hills. Up here everybody has been doing 85 or 90 for a few years now, however I’m still thinking daily driver cruiser. Yeah, looking for the best of both worlds, go figure.
Click to expand...

By wiring, if you mean the engine my car is basically a 360 style setup, save for the ignition.
I have all the LA style V belt system and run an 8 pin HEI module with a factory E core style coil and recurved factory style electronic distributor.

I did do all the automatic shifting wiring for the 42RH trans.
In the pic of my shifter you will see the two switches.
One marked 4th and one marked 5th.
Those are for overdrive and converter lockup.
In the pic “Auto” position all shifts are automatic 1-lockup.
With both switches off it shifts 1-3 like a 904, but with lower first and second of the 42RH.
Also I never have had to downshift on hills unless I just wanted to.
If I really wanted to, I could just turn off lockup and OD, but I have never “had to” to pass even on hills.
If it’s going to be a hard pull I will turn off lockup to take the load off the clutch in the converter.

My differential is a 73-74 8.25 with a mid 90’s carrier, ring and pinion and sure grip.
With different side bearings it all bolted right in.
The rear disc brakes from a late 90’s Grand Cherokee also bolted right up to the 73-74 rear axle housings.
Longer wheel studs are required due to adding the discs.
 
Last edited:
TrailBeast said:
I was at the same crossroads you are with my Dart.
Went with a mild built 5.9 and 42RH overdrive and 3.55 sure grip 8.25 rear end with the Grand Cherokee disc brakes.
No EFI, Holley 770 vacuum secondary.
Oregon Cams roller regrind with Hughes springs, retainers and locks.
Most every part mentioned above.

25 mpg hiway at 80mph running about 2,300 rpms.
(Vac advance tuning is key for mpg)
Who cares what city mpg is. :D
Did mine about 8-10 years ago and it has been a flawless daily driver since.

View attachment 1716359349



View attachment 1716359351

View attachment 1716359352
Click to expand...
What's up with the switches for 4th and 5th?
 
So on a 300 mile road trip it can be set it and forget it?
 
Thanks TB all of this information is helping a lot. My brain got side tracked on the wiring thing. Mine is a 1950 with 6 volt. I’ll have to hand wire the car 12 volt with fuse panels, but NP I find that kinda stuff relaxing. Did a lot of custom wiring during my career. Thanks for 73-74 rear info and the disc brake stuff. Who did the new longer studs for you? How did you accomplish this work?
 
Willy Moose said:
Thanks TB all of this information is helping a lot. My brain got side tracked on the wiring thing. Mine is a 1950 with 6 volt. I’ll have to hand wire the car 12 volt with fuse panels, but NP I find that kinda stuff relaxing. Did a lot of custom wiring during my career. Thanks for 73-74 rear info and the disc brake stuff. Who did the new longer studs for you? How did you accomplish this work?
Click to expand...

Well, after one failed attempt with Autozone I was able to get the right studs a friend let me use his press to put the new studs in.
Just used a hammer to knock out the originals.

disc.jpeg


IMG_0819.jpeg
 
rumblefish360 said:
Reliability is t the issue. IF! You can reuse the fe injection on the 5.9, id to that route as the factory did an excellent job with it and the engines bores will look great and last a long time before the cross hatch hairs are gone.

The biggest key to longevity for an engine is to keep the fluids and filters changed on a regular basis. The other item is low cruise rpm. Keep it cool, fresh liquids and filters at low cruise rpm’s and that set up will go and go and go miles and miles and miles.

IMO, the best thing you can couple to that engine with 3.55’s is an overdrive transmission. You’ll get a deeper first gear for that excellent 10.0 ratio leaving the line (or close to it.)
And a OD gear for a good cruising gear.

I think, (so forgetful right now) 3.09 first gear multiplied by the rear ratio @ 3.55, 3.55 X 3.09 =10.97 and the OD at 3.55 X .69 =2.45 for a stour Hwy. cruise gear.

The Chrysler based OD is 904 based and you’ll need the early version that is Hyd operated. Not electronic.
Click to expand...

A833OD manual overdrive is 3.09/1.67/1/.73. 46RH is non electronic and 727 based, with 2.45/1.45/1/.69 ratios, 42RH is 904/998 based with 2.74/1.54/1/.69 ratios...

I had a 42 RH and 3.55's in my 5th ave with both a flat cammed LA360 with a comp XE262, and a roller cammed, magnum headed 318 in the 208@.050 range and both setups worked great
 
You must log in or register to reply here.
-
Back
Top Bottom