Anyone can hog out a port...
Anyone can hog out a port and get it to flow copious amounts of air. The real story is in the velocity of that air as it rushes into the port. Groh uses an air-speed probe to test air velocity. For example, these TFS heads flowed 290 cfm at 0.500-inch lift with an air speed of 340 ft/sec. According to Groh, the air was moving fast through the port, but he cautioned that speed is only part of the story. Air swirls and tumbles around in the port. He also said too much air speed would create a dead spot on the short turn, which is a no-no for performance. Porting heads can be considered an art form, and there is no particular way to do things. Every head porter does something different to arrive at the same result.
We picked a set that was CNC-ported in-house at TFS and features 2.08-inch intake valves and 1.60-inch exhaust valves. They came complete and ready to bolt on, but we chose to pop off the valvesprings and use a different set. Groh selected stiffer springs from Comp Cams to match the camshaft he designed for the engine. One thing we forgot to mention to TFS was that we were running larger pushrods. One phone call and TFS swapped out their standard 51/416-inch guide plates to the larger 31/48-inch stuff.
Groh carefully ran the TFS Street Heat heads on his flow bench to gather data. The flow numbers tell only half the story, as Groh also evaluated air speed (or velocity) to get an idea on how well the volume of air moves through the head. The flow numbers are an essential part in the camshaft design. According to Groh, the lower lift numbers are decent and this cylinder head shines in the peak numbers area. "It's a good cylinder head with good peak numbers. For a street car, I would work the chamber to get better low-lift numbers, but then the high-lift numbers might suffer. Like everything in racing, preparation is full of compromises," Groh says. The out-of-the-box heads were fine to go on our project engine. Of course, there is more to be had with these cylinder heads, but we were content with the flow capabilities of the set we had in our hands.
In addition to getting the cfm results, Groh inspected the port velocity using a special probe. The probe provided the airflow data in feet per second. "This gives me an idea of the speed of the air," he says. "The air moved through this head at 340 ft/sec at 0.500-inch valve lift and moved 290 cfm. It is a very fast cylinder head and will fill the cylinder quickly. There is no industry standard for the air speed and its relationship to flow. Knowing what works and doesn't work is based on experience."
The cam is key in this application because of its giant cubic inches and supercharged nature. That is not to say an off-the-shelf grind wouldn't work, but the custom-designed cam will help get the most out of our engine. It is a custom hydraulic roller piece designed by Groh, ordered from Pro Power, and cut by Comp Cams.
Maximum valve lift comes in at 0.570 inch on the intake side and 0.575 inch for the exhaust. The duration at 0.050 inch checks in at 242 degrees for the intake and 248 degrees on the exhaust side. Groh put the lobe separation at 115 degrees. "That is a fast lobe, so it will idle nicely and use the air from the cylinder head and supercharger," Groh says. Some people may question the smooth idle, but Groh assured us the idle and street worthiness will be just fine with the lobe he picked out.
The Comp Cams valvesprings were specially selected for this engine combination. They are required due to the fast opening and closing of the lobes. The valves will not float with these springs in place, even as the engine soars to 6,500 rpm or so. If we had used titanium valves and retainers and triple springs, we could have turned the engine to 7,200 rpm without floating the valves. Unfortunately, more spring pressure leads to more maintenance and increased valvetrain heat-which will wreak havoc on valvetrain survival rates.
The exhaust ports were flowed...
The exhaust ports were flowed using a pipe to simulate the effect of having a header bolted on.
We used ARP fasteners throughout...
We used ARP fasteners throughout this engine. Here is ARP's head stud screwed into the Man O' War block.
The Comp Cams valvesprings...
The Comp Cams valvesprings were specially selected for this engine combination. They are required due to the fast opening and closing of the lobes. The valves will not float with these springs in place, even as the engine soars to 6,500 rpm or so. If we had used titanium valves and retainers and triple springs, we could have turned the engine to 7,200 rpm without floating the valves. Unfortunately, more spring pressure leads to more maintenance and increased valvetrain heat-which will wreak havoc on valvetrain survival rates.