225 swap to 318?

Hello guys, I'm new here, my name is Eduardo, from the north of Mexico. I have a Dodge Dart 1973 with a slant 6 225 engine. I filled my tank and did a 3hour long road trip this morning and surprisingly, I ran out of gas. I thought the gas was leaking somewhere but it wasn't. Then I checked its fuel economy and found out that it gives 16mpg, which is too low on my perspective for a slant 6.

Anyway, I did some more research and it seems that the 318 has better fuel economy than the 225 with 22mpg. My question is, is the swap as easy as taking away the 225 and putting the 318 engine? or my stock transmission, alternator, etc. are not compatible with the 318?

Thanks!

Welcome.
Would need to change the tranny, starter, radiator.
Change or modify the wiring harness.
Change the K-frame to a V-8 one or use conversion motor mounts.
I would change the torsion bars to bigger ones for better handling with the heaver V-8.
Probably some minor things that I'm missing.

Possibly the driveshaft.

I "think" the transmission might be the same....A904 I believe. In general, the 225 six cylinder should get better mileage than a 318 eight cylinder. Maybe just rebuild the 225 if mileage is your main concern?

/6 and SB V8 take different trannys. Different bell housing.

Too many chefs here spoiling the soup but no need to change the starter.

Too many chefs?

Having swapped a small block in place of a slant six in a 72 Dart I can tell you this.

It's not a simple swap if you don't know what you are doing and don't have all the parts on hand. I did the swap in one day, about 9 hours, but I had everything on hand.

You will need a complete engine and transmission, your slant six transmission will not bolt up to the 318.

You will need the proper three piece kick down linkage from a small block. The slant six linkage will not work, one piece kick down linkage will hit the firewall and not work.

You will need conversion motor mounts or a new K frame, when I did this swap there were no conversion mounts and I had to replace the K frame with a V8 one. Go conversion mount if you can.

If you have power steering you will need the V8 mounting brackets to put your pump on the V8. If you V8 donor has power steering you can probably use it's pump instead.

If you have AC you will need the V8 mounting brackets to put your AC compressor on the V8 and probably the V8 AC hoses too. My donor was an AC car and it's hoses fit my Dart.

Your alternator will work, if you use the V8 mounting brackets.

Wiring, you can use the slant six wiring with minimal mods. Basically, the wires to the oil sending unit, distributor and coil will be longer than needed, you can coil them up. The wires to the alternator and coolant temp sensor work as is.

Radiators are different, you might be able to use the slant six radiator with some creative upper hose routing, especially if you do not have AC.

Exhaust will not work as it, but you should be able to get/make a Y pipe to connect the small block to the slant six exhaust.

Assuming you have a 904 style transmission behind the slant six right now and your v8 donor also uses a 904 style transmission you can keep your current driveshaft.

I believe the accelerator cable will work so no need to swap that one out.

Not sure if the slant six fan shroud will work with a V8, I can't remember since I did my swap in 1994 it's been a few decades now.

I had a complete donor for my swap, 79 Cordoba, so a lot of the stuff you will need I had with the donor. Some of it I had to buy.

Before I did an engine swap I would check the health of the 225. Cheaper to put a good tune up on it and even buy a carburetor or rebuild it before swapping a motor; a big job. Your 225 should at least get 19 mpg in a reasonable state of tune. Do a compression test, check the condition of the distributor, cap, rotor, plugs, carburetor.

^this^ The '73 model year was reportedly the worst as far as discombobulated tune up specs with Ma & everyone else trying to stay ahead of the gov't emission and economy regs of the time. initial was TDC on a 198 or a 225 with an automatic & 2&1/2 BTDC if a 225 cube & a stick!. bump the timing up to 15. get the FBO $22 plate to limit the total to 32. play with the jets (hopefully it is a Holley). toss the heavy dist spring & replace it with a mr gasket #925B or MP light spring in its place. Maybe advance the cam 4 deg. If so tune the dist last. If you are under the pinging point at any time you are OK.

I would not bet on a Mexican market Dart having anything close to US spec's in 73. I know for a fact that points were still used in the Mexican market GM's until at least the 80's.



Assuming it is a Mexican market Dart and not a US spec one.

Good point!

I just sold a 72 Mexican Dart with a slant six that was originally a 3-speed on the column but changed to a 904 and all the parts were the same for a US Dart. It had points distributor too.

Yeah, but 73 is when a bunch of stuff changed.

Electronic ignition, bigger bumpers, smog stuff

As said before, something is wrong. Try fixing it first. I'd bet if you're disappointed with Slant mileage, 318 will cause you to frown too.

I just did a 340 install in a '73 Slant Duster. A good tune-up on your Slant will be much easier and cheaper.

You be hard pressed to get a steady 22mpg out of a 318. A well tuned slanty will do that no problem.

Really? My friend drives his 70 340 Cuda all over the country and gets 20 mpg all day long.
I got 23 mpg in a 73 D100 318 no problem. My 79 B200 318 w/ 833OD got a steady 26 mpg.
Over 20 mpg for a 318 A-body should not be hard to do.
Like stated before, maybe see what all is wrong with the /6.

Good running 318 in a A or B body with 2. series gear will just be loafing on highway, getting good mpg. I had 2 different 318 cars that got 24 mpg in summer traveling.

OK, assuming the Dart has a 16 gal tank and it can pull all the gas out of it, and it got 16 mpg, that means he went 256 miles. Divide that by the 3 hour drive, and it averages 85.33 mph. I believe 16 mpg isn't bad mileage for a slant 6 at 85 mph for 3 hours. I doubt anything in the 1973 era would do much better.

One also needs to consider the road conditions of that 3 hour drive. I would be concerned about how much gas would remain after a 260 mile trip in anything I own, especially at 85 mph. Gene