The debate has gone on since probably the 1980s- can you run a 318 BBD on a slant six? Many people have taken both sides of the debate. Some say they have run a 318 carb with no problems, others say only a true 225 BBD will work properly on a slant. Today, I found a strong reason why you should really try and use a slant six specific BBD.
Before I get to it, I will admit that I am in the camp that says you should try and use the slant six specific carb. First, there is the choke issue. The 318 BBD has a slightly different choke linkage that will not work on the slant six without addition effort. The workaround for that problem can be found HERE.
Second, the big nipple for the PCV hose is oriented differently. This likely won't make that much of a difference, but if you have lots of stuff in the engine bay or if your hoses are short, you might need to run a new hose.
But the difference I found today between the slant six BBD and the 318 BBD is fairly large, and is really a confirmation of something I have been saying for years based off of factory literature.
Over ten years ago, I found on eBay a 1976 Chrysler factory reference book introducing the "new for 1977" slant six BBD carburetor and highlighting the differences between the slant carb and the 318 carb. That factory reference book is included at the back of the BBD carb manual available for free download HERE.
In addition to other differences, on page 1 the 1976 reference book states,
Quote:
Although the throttle body bore for the 1977 Carter Super Six and 318 engines are identical (1-7/16"), there is a difference in the main body venturi size. Since a Super Six has less cubic inch displacement than the 318, the main venturi needed for the Super Six is smaller (1-1/16") than the venturi for the 318 (1-3/16"). This is one of the primary reasons for non-interchangeability of the two-barrel Carter for the Super Six and the 318 engines.
Emphasis added.I have been referring to this difference for many years as why one should always try and find a slant six specific carb, but today I obtained photographic proof of this large difference.
I happened to be rebuilding a 318 BBD this afternoon with a Super Six BBD also loose on my workbench. Both these carbs were assembled from loose parts I obtained as part of a bulk purchase of various BBD parts. As I reassembled the 318 BBD, I noticed that the venturi cluster had a "6" cast into the top of it. I remembered the internal differences referenced in the 1976 manual, and checked what I knew was a slant six BBD. Sure enough, the slant six BBD had an 8 cast into the venturi cluster. This got me curious so I decide to take the Super Six BBD apart and compare the venturi clusters.
This is what I found ("6" cluster on top in both photos)
As you can see, the hole in the end of the fuel tube inside the venturi is visibly larger on the "8" venturi cluster. Not visible due to the blurriness of the picture is the fact that the hole in the top of the brass tube that pokes up out of the venturi cluster is also smaller on the "6" cluster vs. the "8" cluster. This makes sense since the smaller air flow of the 225 would required a smaller hole to amplify the effective venturi effect and draw the fuel out of the float bowl.
I determined that I must have unknowingly assembled the carbs with the venturi clusters in the wrong one so I swapped the venturi clusters and reassembled both carbs.
I then tried both carbs out on the 225 in my 76 D100. The 318 carb ran OK but was a touch rough at idle and a bit sluggish when revved. The 225 BBD (with the "6" venturi cluster) idled smoother, would idle down to a much lower RPM, and was much more responsive when revved. I didn't do a test drive, but the 225 BBD with the "6" venturi cluster performs better, at least in the driveway.
I believe the 6 and 8 cast into the top of the venturi pack signifies if the venturis are for a 225 or a 318, and that the right venturi pack should be used with the specific engine it is intended for. If anyone has contrary info to this, I would appreciate being set straight.
Casting numbers ,useless. there a 2 and 8
