An internal combustion engine is like a big air pump, and the more air you can get it to pump, the more torque and horsepower it will make. Things like bigger camshafts, aftermarket intakes and exhausts can help increase airflow to a certain point, but ultimately, to get the most out of your high-tech air pump, you've got to make the cylinder head more efficient.
The goal of modding any cylinder head is to increase its volumetric efficiency (VE). The VE is a measurement of how efficiently an engine can pump air relative to how much it displaces. For example, if an engine has a displacement of 1000cc and moves 700cc of air with every stroke, it has a VE of 70 percent. If it moved 1000cc with each stroke, the VE would be 100 percent.
1. Cleaning up any imperfections...
1.Cleaning up any imperfections where the valve meets the head, as shown here, will help the intake charge flow into the head with the least amount of resistance possible.
Most modern multi-valve engines have a peak volumetric efficiency of around 80 percent; which leaves plenty of room for improvement. All things equal, bringing the VE up by 10 percent should yield an additional 10 percent increase in power and torque-in theory at least. There are different ways to increase the VE of a 4-stroke cylinder head, and the key to getting the highest VE is to make sure all the methods and parts used are compatible.
Aftermarket cams can increase airflow by keeping the valves open for a longer period of time (more duration), or by just pushing the valve farther into the combustion chamber (more lift). Be warned, if you just install bigger cams without making any changes to the head, you're not getting all the power the cams can offer. Because OEM heads often have many manufacturing imperfections in the ports and combustion chamber that inhibit flow, often times opening the valves up more or longer won't increase airflow much over stock.
Even without changing the camshafts, mild headwork can increase an engine's VE substantially, and gains of around 10 percent or more are common if the job was done well. If that's not enough and you decide to toss some cams on too, consider this: Camshafts and heads are designed to work as a single unit. In order to get the best results, have the same shop that does the headwork grind you a set of custom camshafts to match the head.
In this article, we'll show you what you need to know if you deicide to take your air pump to the next level, as well as showing you a few tricks you can do at home to get the most out of your head.
2. This strange-looking machine...
2.This strange-looking machine is used to force an abrasive putty-like substance through things like intake manifolds and cylinder head ports to remove casting imperfections and increase airflow.
Valve JobsBelieve it or not, all the cylinder head gurus we spoke with agreed that 50 percent of the gains of a full port and polish could be had by simply performing a quality valve job. This is good news for budget racers, considering the cost of a quality port and polish.
The reason behind this phenomenon is a simple one. All of the air that flows into the engine must flow directly past the valve, making the aerodynamic profile of the valve very important. If the valve has sharp angles and casting imperfections, it will create turbulence and decrease the amount of air that can move past it. Making the valve as aerodynamic as possible can pay in dividends.
Multi-angle valve jobs are the most common way to make a valve more aerodynamic and make the head more efficient. A multi-angle valve job puts extra cuts on the valve near where it mates with the cylinder head, streamlining it and helping to move air past its surfaces with the minimum resistance and turbulence possible. OEM valve jobs are usually one or two-angle valve jobs. This is done to keep costs low, and even if they want more airflow, OEMs will often just switch to a larger valve instead of using multi-angle valve jobs because it's cheaper than performing the extra machining of a multi-angle job. However, for the average enthusiast, the opposite typically holds true because oversized valves can be very costly.
3. Here's a head that's received...
3.Here's a head that's received some mild work. As you can see, the ports are smooth and the combustion chamber has no imperfections. This is how it should look.
The more angles a valve job employs, the better it will generally flow. The most common multi-angle valve jobs are 3-angle, 5-angle and radius valve jobs. A radius valve job is basically a 5-angle job where the transitions between the 5 angles are smoothed out. The downside is that the more cuts, the higher the cost. For this reason, 3-angle jobs are the most common and offer a good balance between cost and performance. Don't underestimate the importance of a quality valve job.
Porting And PolishingWe've all heard the term "ported and polished," and it's likely we've all got idea what it means. To the average enthusiast, this means enlarging, and possibly reshaping the intake and exhaust ports and polishing the combustion chamber. Sounds simple enough, right?
4. Here's a head that's received...
4.Here's a head that's received some mild work. As you can see, the ports are smooth and the combustion chamber has no imperfections. This is how it should look.
Wrong. Cylinder head porting is an art. While we can explain some of the common methods and techniques, because of the advanced physics involved, porting is something best left to the professionals. In fact, without expensive and complex equipment, there is no way to even measure any changes.
Contrary to popular belief, a good port job doesn't simply include making all the ports as big as possible, as this will often result in a soggy engine that has lousy throttle response. Things like air velocity must be taken into consideration, which sometimes requires making the ports smaller than stock to increase the velocity. That's the thing about porting-sometimes the results are counter-intuitive; which means unless you are willing to destroy a few heads through trial and error, make sure your head shop knows what they are doing. The only real way to test if the changes that were made helped is with the use of a flow bench or a dyno... and do you have either of those?
What the car will be used for will generally dictate how much porting will actually be done. Proper porting takes into consideration things like engine size, RPM range, horsepower, etc and must be done to match the other mods. For a street car, this usually means only cleaning up casting imperfections, a mild un-shrouding of the valves and smoothing everything out. For a full-race motor, a port job might include enlarging the intake and exhaust ports as well as reshaping and polishing the combustion chamber. It all depends on the goals. Any reputable head builder should be able to tell you what is best for your application.