Ask Away! with Jeff Smith: Rectangle vs. Cathedral Port Heads – Which is Right for Your LS?

Everything I read says that the big rectangle ports for LS engines flow better than the cathedral ports. But then I know some pretty quick street cars running cathedral port heads. So for a 6.0L LS street engine, what would you recommend? My engine will be backed by a T-56 six-speed and 3.42:1 gears in 2002 a fourth gen Camaro.

C.J.

Jeff Smith: The simple answer is that the flow bench will always report that the rectangle port heads will flow more air than even CNC-ported cathedral ports. However, this simple flow test does not really give all the answers. You’ve probably heard that an engine is a combination of many independent systems that all contribute to the overall power curve, so no one component like the cylinder heads will be the cure for all ills. But having said that, let’s dive into this issue head first.

A flowbench delivers a number delivered in cfm based on laminar or smooth flowing air into a cylinder. But that’s not what happens in the real world of an intake port. One thing that is not discussed with regard to port flow is the velocity of the inlet air. Ideally, we want both high flow and high velocity. But the much larger rectangle port heads sacrifice port velocity to gain high cfm of airflow. So with larger intake ports, the flow increases but the velocity is down. If we were to measure the velocity and multiply it times the flow, we’d have something called port energy. While this can lead to a far more in-depth discussion, suffice to say that large ports tend to enhance the top-end power but tend to suffer with regard to low- and mid-range torque. Smaller ports like the cathedrals tend to improve the low-end and mid-range torque at the sacrifice of top-end power. It comes down to where the port energy is the highest. So if you wanted to build a high-rpm race engine where the goal is peak power, the rectangle ports do a great job. But a street engine spends a majority of its time at part throttle, spending a majority of its time in the mid-range rpm area – which would suggest using a smaller, cathedral port.

There are lots of other systems that have an effect on power. For example, the intake manifold has a tremendous effect on the entire power curve. This is pretty important when it comes time to build a street engine. I built a 404-cubic-inch LS street engine a few years back and was disappointed with the results. This engine had 10:1 compression, but was fitted with a 231/237 degrees at 0.050 and 0.617/0.624-inch lift camshaft. I used a set of CNC-ported L92 rectangle port heads and a carbureted single plane intake manifold. The engine made 561 horsepower at 6,400 rpm and peak torque came in at 510 ft.-lbs. at a rather high 5,200 rpm. At first, I blamed the big-port heads but lately I have moved my attention to the intake manifold. I think a better way to go now would be some kind of a manifold with longer runners which will increase the mid-range torque without necessarily killing the top-end power.

One manifold that might help do that is a budget-based intake that Holley debuted at the 2015 SEMA show called the Sniper EFI manifold. It is basically a production-line, fabricated, sheet aluminum front-breathing EFI manifold that looks like it might do the job. Holley has part numbers for both LS cathedral and rec-port heads and should have parts on the shelf by April or May of 2016. It would be fun to try this manifold – which will run around $530 (part # SNE-822031). Of course, you will need injectors, fuel rails, pressure regulator, a throttle body, and an EFI system to run it, but that might be a way to save my particular combination. Another idea would be to use the FAST LSXR composite manifold for the rec port heads to increase the runner length and build a bunch of mid-range torque. This manifold (part # FST-146102B) will also need injectors, fuel rails, and a 102mm Big Mouth throttle body to complete the installation.

Of course, rather than try to crutch these big, 280cc port volume heads, a better idea might be to replace the big-port heads with a set of smaller, CNC-ported cathedral port heads like the 225cc intake port versions from TFS. This head will flow almost 340 cfm at 0.600-inch lift but will also improve port velocity compared to the rectangle port heads, which will build torque in the midrange. Then add a really good intake like the FAST LSXr intake (146302B) with good runner length and this combo with a cam of around 226 degrees at 0.050 would make gobs of torque that would be extremely fun to drive. Assuming you could hook up all this torque – it would also be very quick on the drag strip. The rec-port 404-cubic-inch LS ran 11.70s in my ’66 Chevelle but with this high-torque, longer-runner intake package and cathedral port heads – I think this could easily run 11.20’s at probably 122 to 124 mile-per-hour. That’s a half-second improvement on a package that probably wouldn’t increase the peak horsepower but would drastically improve the mid-range torque. It’s really that simple.

This is not to say that we couldn’t fine-tune the original engine combo with a long-runner LSXr intake for the rec-port heads and run almost as quickly. It could easily be done and would probably be less expensive since we already have the heads. Perhaps the point of this answer is that if you already have a set of cathedral port heads, don’t give up on them as having the potential to make good power. Tuning with the intake and exhaust can do quite a bit if you concentrate on mid-range rather than just peak power.