Ask Away with Jeff Smith: Choosing Cylinder Heads for Bottom-End Power on a Mopar 340

I have a ‘73 340 ‘Cuda. The motor is bored 0.030-over with headers, an Edelbrock dual plane intake and a matching cam, roller timing chain, and Holley double pumper carb. The motor I want should make strong bottom end power. Right now my heads are stock iron. I need to know the best heads that would give me the torque and horsepower that I need. I hope you can help me. This is a street car.
S.S.

In the tradition of full disclosure, I have to admit I’m not as well versed on small-block Mopar engines. But frankly all engines, regardless of their manufacturer, all respond to the same kind of improvements.

Many years ago, Bob Glidden decided to race a small-block Mopar in Pro Stock instead of his traditional Ford-powered cars. Somebody asked him if he thought it would be difficult to make power with this new engine. If I recall his comment, I think it was something like “It’s just a small-block with funny rocker arms.”

I think we can help you.

The Mopar/Chrysler LA 340 that got everybody’s attention in 1970 was a hot little small block with good heads and 10.5:1 compression. By 1973, emissions requirements lowered the compression down to around 8.5:1. I’m not sure about other changes to the heads but it appears the intake valve diameter in the hot 340 engines was 2.02 inches but shrunk to a 1.94-inch valve in 1972-73, accompanying the drop in compression.

So for a cylinder head, there are several out there including those from Edelbrock, EQ, ProMaxx, Mopar Performance, Trick Flow, and a couple others. To keep this answer short, we’ll focus on two popular heads: the Edelbrock and the Trick Flow castings.

The most interesting aspect of the Trick Flow heads is that this company has made the effort to build a head that can be used on both the older LA style engine that you have, but can also be used on the later Magnum 360 engines. This doesn’t apply in your particular case, but the advantage of using this head on a later Magnum is that this head can accommodate the earlier and far wider selection of LA-style intake manifolds.

To start with, let’s look at the Edelbrock castings. We’ll list the part numbers for all the pieces in a chart at the end of this answer. We’re not going to get into all the details necessary to perform this swap except to mention that with both heads its likely you will need new pushrods to accommodate changes to the valvetrain.

The big change with both heads we’ll look at is combustion chamber size. Our research indicates the 1973 340 engines used a large, 73cc chamber to reduce the compression ratio to 8.5:1. While this reduced NOx output for emissions, this loss of compression really hurts power.

The rule of thumb with compression is that, in the range between 8.0:1 and 12.0:1 compression, one full ratio is worth approximately 3 to 4 percent power. This may not seem like much, but with 400 hp, a 4 percent increase in power equates to 16 horsepower, so it’s certainly worth the effort.

While your engine is rated at 8.5:1, our experience is the true number is generally lower. So you may be looking at an engine with perhaps 8.3 or even 8.2:1. For the sake of this discussion, we’ll use 8.4:1.

We then plugged some numbers into our Summit Racing compression ratio calculator. The variables are bore, stroke, combustion chamber size in cubic centimeters (cc), head gasket thickness, position of piston below (or above) the deck surface, and whether the piston is flat with valve reliefs, dished, or domed.

We set up a combination using a 73cc chamber to create 8.4:1 compression and then plugged in an Edelbrock head with a 65cc chamber and larger valves. This 8cc smaller chamber pumps the compression from 8.4:1 to a much better 9.04:1, which helps a little. Our combination used variables where the piston is 0.035-inch below the deck as that seems to be what Chrysler did to lower compression. We also used a relatively thin 0.039-inch head gasket for our calculation.

If you can find a thinner composition gasket (like perhaps an MLS version), that would help a little as well since the piston-to-head clearance with our above combination is still very wide at 0.074-inch. Minimal piston-to-head clearance is something closer to 0.040-inch. Or another way to go would be to machine the deck surface of the heads to reduce the chamber volume. While this works, it is an added expense.

The other head is the Trick Flow version. This aluminum head also uses 2.02/1.57-inch intake and exhaust valves but uses a much tighter 60cc combustion chamber. This really helps the compression. From an 8.4:1 number, that 60cc chamber bumps the static compression up to 9.53:1. This is a full point more compression and that move alone will improve power. Plus you have the added airflow of the larger intake and exhaust ports and larger valves. All of that will help power. While we have no actual testing data to back this up, it’s entirely possible that these heads could bump the power by a solid 30 to 40 horsepower or more, along with added torque.

One quick note is that in addition to the heads, you will also need a set of specific head bolts to properly attach the heads to the block. Trick Flow changed a few of the head bolt lengths to accommodate port changes so this is a requirement. Take this into consideration as well.

You mentioned a “matching” Edelbrock cam that we’ll assume is the Performer cam. This cam uses 204/214 degrees of duration at 0.050 with 0.420-inch lift on both the intake and exhaust side. This should make for a great street package when combined with either of the previous heads.

You also mentioned you had an Edelbrock dual plane intake and a Holley carburetor with headers. Combining all these pieces should create a great little small block Mopar. My only experience with a small block 340 was an old Car Craft magazine project giveaway Dodge Dart 340 that we built back in the mid 1980s. Drag racer Bob Lambeck built the engine for us and that car ran really well, so with a good set of heads, your engine has all the pieces it needs to make decent street power. Good luck!

Ask Away! Mopar 340 Cylinder Heads