Input Needed: 273cid to 318/360 Swap

Hey there,
My OEM 273cid LA motor runs well enough but it's on my list of things to do to build a mild "street performance" replacement. Due to the rarity/cost of 273 performance parts and the smaller displacement of said engine I'm considering either building a 318 or 360 instead and saving/storing the 273 as "original parts".

Since the physical size of the 318/360 is the same I'd rather have the 360 for the added CID. I can think of a couple concerns right off the bat which I'll list below, anyone else know of anything I need to pay attention to?

concerns:
1) My 273 has an A745 3-speed manual behind it, I think the slant6 and 273 used a 10" clutch assembly, the 318 used a 10.5" (which I could use with the 360 I guess). The diameters I listed may not be spot on since I didn't refer to my docs but I do know there are two different clutch sizes for the different engines.

My basic build will be:
~4bbl 600'sh CFM carb and intake
~shorty/hugger headers (wrapped or coated)
~pretty heavy cam, more so for the sound and "Car show appeal" than anything else
~high flow water pump (and probably a new 4 core radiator)
~possible upgrade to electronic ignition but that's not a must have.

I'm not going to race it, just want to have a little better performance, sound and looks.

slowflapper wrote:...Since the physical size of the 318/360 is the same I'd rather have the 360 for the added CID.

All 4 small blocks (273, 318, 340, 360) share external size. The 340/360 has a different driver's side mount boss for the bracket. Fairly easy to modify or space out, but be aware. Also, the 360 started in 1971. As such, it has the different front cover and longer water pump. It and other engines after 1970 have different bolt spacing on the damper. You have to accomodate this with the pulleys. Another difference is that all 360s and 72-up 340s have cast crankshafts. There is no issue with this for street use, but it does mean that the engine is externally balanced. It has extra weight on one side of the damper and the flywheel or converter.
Be aware that the 360 also has a different oil pan due to larger main bearings and caps. It is also the size used on all magnum engines.
None of this is a major roadblock, but you need to match your components and plan ahead.

slowflapper wrote:...
concerns:
1) My 273 has an A745 3-speed manual behind it, I think the slant6 and 273 used a 10" clutch assembly, the 318 used a 10.5" (which I could use with the 360 I guess). The diameters I listed may not be spot on since I didn't refer to my docs but I do know there are two different clutch sizes for the different engines.

273/318 are internal balanced. This means the damper and flywheel have no offset weight. Others are externally balanced as mentioned before.
You have 2 choices with a manual trans behind the new engine.
1. Use whatever bellhousing, flywheel, clutch and transmission you currently have, with upgrades to handle increased power and addressing balance issues if using a late 340 or 360.
2. Use different parts and make them fit. This greatly increases the research, time and effort.

slowflapper wrote:...
My basic build will be:
~4bbl 600'sh CFM carb and intake
~shorty/hugger headers (wrapped or coated)
~pretty heavy cam, more so for the sound and "Car show appeal" than anything else
~high flow water pump (and probably a new 4 core radiator)
~possible upgrade to electronic ignition but that's not a must have.
I'm not going to race it, just want to have a little better performance, sound and looks.

HEADERS If you wrap the headers, you should get the insides coated. they tend to rot away in a few years if just wrapped due to the increase in retained heat.
CAM I would not suggest anything bigger than the 292/509 Hyd cam. I had one in a 340 Demon 4 speed and it was too big. You can tune the exhaust for your taste and still be able to enjoy driving it.
COOLING increasing power means increased heat. Good to plan for this. Do research on this however. There is a lot of junk online trying to hand you magic beads for the same or more than good quality parts.
EI is probably one of the best upgrades. If you have a running van now, I'd suggest that you do the upgrade now so you know it's working properly before doing the engine. One less thing to worry about.

Well put Rod .also if you are running a hi- volume water pump also use a hi- volume stat

Rod covered pretty much all the important bases here.

Pretty much any part that you can buy to fit the 318 will bolt right on to a 273 (except the very early 273s that had their little nuances, which I noticed you probably have). The block is identicle, the crank is the same between the 2, the only difference is bore. You can put 318 heads on a 273 if you are willing to clearance the top of the cylinders a bit for the larger valves. Cams will fit in any of them the same. If you put the 318 heads on then you can put any intake you want on it that is available for the 318/360 engine. There are many different routes you can go depending on what you want to spend and what you want for the final results.

If you want to put Magnum heads on it that is also a possibility with off the shelf parts. My Wrangler has an LA 318 block with everything else Magnum from a '96 Ram. The the block wasnt so tired I would run it for a lot longer but 5 PSI of oil pressure at idle with a high flow pump indicates slight wear problems in the bottom end.

Basically dont discount the 273 unless you have another engine to build and want to keep the 273 for its original factor.

Another way to go if availability is local is a good used magnum. These are 1992-up 5.2L (318) or 5.9L (360). They will swap right in with the same differences noted earlier. For ignition, any small block EI will work in place if the fixed computerized dist.
For fuel, the intake bolts are at a different angle. You can have a machine shop redrill and spot face any pre-magnum intake (check for someone who knows what they are doing) or buy a new magnum carb intake.
So why go through those issues for a maggy?
1. Roller hyd cam. With a carb, this opens up some pretty tasty cams. Pre-magnums are a big pain to retrofit with a roller, and are pretty much impossible to do a hyd. roller cam.
2. Better heads with a cheaper valvetrain. They flow more stock and can be ported for even more. The valvetrain is similar to a SBC, so cheaper upgrades are possible, especially a mild to middling street build.
3. Ability to convert to port EFI with stock parts.
4. Depending on local deals, these can be more plentiful and cheaper than an older, more worn pre-magnum. You may be able to find one with less than 80K miles that you can bolt right in, rather than rebuild.
5. Aftermarket heads for both are available. Guess which flow better and have better options for compression and quench?

Another thing on the 273 the 68-69 heads are the same casting as 68-74 318 with the same size valves. My 273 has the late 273 head casting with the mechanical cam. I am putting a LD4B small port Edelbrock manifold and 600cfm Holley next fall/late summer. Do some searching on the 273 at mopart forums and you will see that they can be pretty peppy engines. With more modern parts it can make more power than they did in the 60s.
Swapping in a set of the 4 barrel 10:1 pistons and a cam similar to the factory 4 barrel with some head work and good dual-plane aluminum manifold should net around 300hp from what I have heard. Now the 67 closed camber 273 heads would be ideal with this setup.

Good stuff guys, I'm leaning more towards an early 318 now just for simplicities sake. I see them pretty often on CL for cheap, of course they need a rebuild but I plan on that anyway.

The general plan is get a motor and spare trans and get them ready. When the van goes offline for body work I can install the new drive train. I'd like to keep the original 273 stock and just save it for future use. I wouldn't want to start doing performance mods unless I rebuilt it and if I'm going to rebuild something I'll get the extra CID out of a 318

All of my mods are "bolt on", I probably wont have any fancy head work etc done to it.

A warmed over 318 is a great motor for sure. They are readily available as you are probably already aware. You can do whatever you want to them for not a ton of dough. If you are leaning toward keeping your 273 stock for originality sake (nothing wrong with that at all) then your plan sounds like the right way to do it. Like you said, no use rebuilding a motor that you dont want to run when that money can be put into something more along the lines of what you want.
If you were close to me I have a 318 block you can have.
Be sure to keep us updated on what you do and how you do it. You know we can never get enough A100 talk around here!

Sure thing Freak, it will take me a while to get and build the motor so I'll definitely be documenting it.

For what the machine shops are charging for the work that is needed to rebuild most engines the best bet is a long block that is already done and comes with a warranty. I have talked to these guys and they will do additional work if you want them to.
http://www.hiperformer.com/engines/dodge-a100-rebuilt_engines-mop_318lb.html

I did a early 340 with J heads and I'm loving it.