I am in the process of repowering my boat with a '76 318. The engine I am pulling, has a 4bbl Edelbrock carb and manifold that I am going to install in my car ('68 318), and put the 2bbl manifold and carb on the boat engine. While shopping for intake gaskets I noticed that some adds indicated that they fit the 4bbl heads. My question is: is there a difference between the 2bbl heads and the 4bbl heads? Should I plan on swapping heads too?
There are two sizes of intake ports, that are quite different...the LA 318 from 67-77 used only a 2bbl carb and had the small ports. The 340 and 360 used the large ports. The 4bbl 318 came along in 78, and I'm pretty sure it used 360 heads with the larger ports. They kept making the 2bbl version for a while, too. Look at what you have....look at pics of the gaskets on rockauto.com....see what you can make work.
I thought maybe there was a difference in the intake ports when I found a separate part number for the 4bbl gaskets. Hopefully the heads are still decent, as I really can't afford to have them rebuilt right now. The engine currently in the boat is seized up from sinking on land. (P.o. neglected to pull the drain plugs when he had it blocked.) I would prefer the bigger valves in the car anyway...
A friend of mine decided when rebuilding his 318 to upgrade to 360 heads using information he got from one of those "hot-rodding" magazines. Unfortunately after re-assembly the engine ran like crap because he failed to read the entire article which mentioned that when swapping heads you also need to swap pistons to increase static copression. Unless you like completely tearing down a freshly rebuilt engine to fix an easily avoided mistake you really need more information than we can give you on this website.
Small block Mopars had 2 different intake port sizes, the large port, and the small port. A cast 4 bbl could have had either port size, but there are better odds the intake has the bigger ports. The only 318 cast 4bbl intake would have been the new intakes, and they likely have an egr port, and are for a spread bore carb (Thermoquad or Quadrajet or their aftermarket replacements). The good news here is that the different port sizes will work with each other. The difference in the port sizes are the thickness of the wall between the two intake passages and the height of the top of the ports (big port heads have almost a 1/4" higher port then the small port ports). If either the heads or the intake (or both) have the big ports, you need the big port gaskets. If both the intake and the heads have the small ports, you need the small port gaskets. The difference between the small ports, and heads with bigger or smaller valves show up in the higher rpm range (4,000 rpm and up), an rpm range most street cars seldom see. I've run a radical high lift, long duration cam in a motor with small port and small valve heads and a large port intake. It made more power then my tires could handle on the street. When we put that motor in the dirt track car, we needed the bigger valves and the bigger head ports. If you really feel the need, you can always use a grinder and match the port size on the small port heads to the big port gaskets (Chrysler at one time offered porting templates), but I sure wouldn't want to add all the grinding debris to the inside of a motor I intended to use. That might be an option if you ever pull the 318 heads though. The combustion chambers on the 340/360 heads have a larger CC then the combustion chambers on a 318 head, regardless of the difference in the valve size (which changed with the year of the heads, and not all heads had bigger valves), installing 340/360 heads on a 318 will lower the already low compression ratio. Gene
Your motors should have the large port low compression heads. The 67 and earlier had small port closed chamber heads. The later intake should port match the earlier heads, LA motor specific.
If your not against switching to a different intake manifold, your best bet is to go with the later magnum small block heads. - EM
Wow! So much that I didn't know about 318 L.A. small blocks. Thanks guys, this is really good information. (Always been a small block Chevy guy...)
I forgot about the 273s, but Mopar didn't put a 4bbl on a LA 318 until the early 80s that I know of. Desertrat#1, a 68 318 from the factory will have the small port heads. The boat motor, who knows what it has? The old 273 iron 4bbl intakes would have the small ports. Any 340/360 iron intake will have the big ports. I suspect the 80s 318 iron intake was an original 340/360 with the big ports, but I've never had one in my hands. Gene
I know it's best to match intake and head port sizes. For my 273 build I found an edelbrock ld4b intake and built it a little past the original commando specs. My research back then was that the large runner intake on small runner heads would cause a lot of turbulence unless they were port matched. I built my combo for 5500 rpm and it gets to 6k if I don't shift quick enough.
To answer your question again, Squirrel is correct; all 318 heads had the same ports and valves. These heads have always been the cork that keeps the 318 from making any real power. 360 heads can be utilized as someone else pointed out. You do not need to change pistons, however. Just take a set of common 360 J heads, have them rebuilt and mill the deck on the heads .050" to get the compression up. Mill the intake side .007 for every .010" you take off the deck. This will be worth probably 50 hp and is well worth the effort. If it were my engine...i'd skip all of that and cruise over to Aerohead Racing and buy a set of their reconditioned LA-X heads. These heads were developed by Indy/RHS decades ago and feature big valves and big ports. They are also closed chamber (unlike stock 318 heads) so they bump compression without milling. Awesome bang for the buck. $995 a pair, built and ready to go. http://www.aeroheadracing.com/chrysler.html
And yes... I have a set of original RHS heads. They're still mounted to an old 318 that's on a stand in my garage. I ran that motor for years in an old sleeper Dart I had. It slayed many foes...including a number of corvettes that refused to accept that it was just a 318 that bested them. I keep it around for when the right car shows up.
The "273 Commando" is a slightly different animal than the garden variety 273 2bbl. I had a 273 Commando 4bbl in an old Fastback Formula S Barracuda I had. Higher compression pistons (like 10:1) and smallish 530 Carter AVS carb. It was a great little motor but it suffered from the same problem that ALL stock 318's suffered (including the 4bbl models); small ports and valves in the heads.
The Jalopy Journal
