This article first appeared in the May, 2016 edition of SportsCar Magazine. SCCA members can read the current and past editions of SportCar digitally here after logging into their account; To become an SCCA member and get SportsCar mailed to your home address monthly in addition to the digital editions, click here.

Part 2: Cylinder heads are key to finding power, though attention to detail is essential

Last month, we laid the groundwork for building a solid competition engine (otherwise known as building the bottom end of a motor). This month, we take a look at finding power up top. Cylinder heads are the gateway to the engine – air and fuel flow into the combustion chambers, and after the magic happens there, spent exhaust gasses hurry out the exhaust. And, it turns out, the more efficiently you can make this process happen, the more power the engine will make. Who knew?

Much like the engine block, the cylinder head you start with needs to be in great condition. If you’re going to utilize anything other than a brand-new cylinder head, the first step is to disassemble and inspect. “It’s a visual inspection first,” says John Edwards of Costa Mesa R&D Automotive Machine Shop. “There are about three different options to check it for leaks. First is a pressure test, which is what we routinely do on aluminum heads – you may have a crack that goes into an area where a die penetrant can’t be seen. For cast-iron heads you can use a magnetic particle inspection, or you can use Zyglow to check for cracks.”

Surprisingly, it takes a lot to write-off a cylinder head. Even cracked units can often be put back into service. “A lot of the heads we can rebuild,” Edwards explains. “Porsche heads, I have to weld those up because they get external cracks to the combustion chamber. But corrosion is more of a problem than cracks.”

If the head is physically sound, you can turn your focus to the valve job and valve seat. “Some shops are still grinding valve seats with stones,” says Edwards. “We don’t do that; we use a carbide profile cutter that will give us all of the angles at the same time, and they all come out exactly the same size. If you have valve seat recession, you will have to cut a pocket in the head and install a replacement seat.”

Jesse Prather, of Jesse Prather Motorsports, echoes those thoughts on a proper valve job. “Angles are good, but the way I do it is more of a radius cut with a special cutter when the rules allow. The valve job is far more important on the intake side than it is on the exhaust side, because that is where the engine is trying to pull the mixture of air and fuel into it.”

When it comes to the valve job, it’s not all about flow – a proper seal is arguably even more important. “You want to make sure your valves seat well so you don’t have any horsepower loss – that is one of the things you are checking during a leak-down test – it needs to be even all the way around,” says Prather. “When you lap the valves, you are making sure everything sits properly in the seat. Valve-to-valve guide clearance is also very important – if there is too much space, the valve can walk at high rpm and you won’t get a good seal.”

Part of making sure your power is not getting lost due to improper valve seating is matching the valve springs to the specific need. “There is going to be a valve spring recommendation by your cam grinder on how much pressure the cam lobes can take,” says Prather. “On push rod motors, you have to run a lot more seat pressure because the valve train is so much heavier. At high rpm with a heavy valve train, it will bounce if you don’t have enough seat pressure. It’s double what we could run on an overhead cam motor. The other part of picking valve springs is over the nose pressure, which means how much pressure you have at full lift. Certain cams can only take so much over the nose pressure.”

If class rules allow it, another way to improve airflow – and increase power – is porting. Opening up the intake and exhaust ports can help improve efficiency. Some classes limit this type of porting to one inch in, but others allow porting much deeper into the motor. “To support what you are attempting to do, a flow bench becomes a must-have item,” says Prather. “If you don’t have a flow bench, you don’t know if you are improving or hurting the flow of the cylinder head by porting it. If you are going to do some basic porting, the key is in the short [turn] radius of the valve – removing the sharp edge underneath the seat. Your exhaust needs to support it properly, but the flow is in the intake side.”

Selecting the right camshaft for your application is a combination of experience, math, science, and maybe a little bit of luck. “Picking camshafts is tough,” says Prather. “Cylinder head efficiency really makes a difference on how much or how little camshaft you need. A lot of times a cam grinder can give you a suggestion depending on if the engine is overhead cam, four-valve, three-valve, two-valve, and bore and stroke – all of this stuff comes into play when picking cams.

“Most of us have a limitation on valve lift in the classes that we run,” Prather continues. “There is a little bit of black magic to it. The F Production motors don’t get very much lift at all, so when you are lift limited, you start working on duration and how quickly the lifter opens. I can’t tell you how much time I have spent working on that kind of stuff to get to where we are on those [F Production] motors in particular. But every motor likes something different.”

All of this may sound daunting, but the reality is that if you race one of the more popular models cars, building a complete race engine should go smoothly. If, however, you’re building something unique, be prepared to experiment, and make sure you give yourself ample time to truly develop the engine package. But either way, the journey begins with finding a builder who’s intimately familiar with your engine as well as the rules you’re building to, and has a proven track record to back up that reputation. Once that’s complete, the only thing left to do is race and win!

 

Words by Jason Isley
Image by StockiPhoto.com/NecipYanmaz