The Black Art of Head Porting
In the past, cylinder heads were ported by hand and tested on a flow bench. This was the norm for decades. With the production factory automotive heads of the past, this approach worked fine due to the very un-efficient ports of the day. Basically as long as you made gains in flow on a flow bench you usually made more power. But it was soon discovered that even though you kept enlarging the port and the flow kept increasing, the power didn’t keep going up. In many cases the midrange torque would start to suffer.
It was soon realized that average port velocities played a major roll in power production. Ports started to get smaller and more efficient. They were smaller but flowed more air. They were straightened out and valve sizes grew. To make these ports work the cam designs changed as well. You no longer needed massive cams with big duration numbers that didn’t like to idle. The midrange torque increased and high hp engines became more street-able. This trend continues today.
The modern shallow chamber 4 valve engines of today typically found in Japanese motorcycles has taken this a step further. The velocities in these stock production ports are very fast compared to port velocities just 15-20 years ago. As a result these engines are making nearly and in some cases more than twice the HP of the early 4 valve motorcycle engines.
But like every mass produced engine that has to meet the EPA standards of the day, there are compromises. These compromises are engineered in to the product for many reasons some of which are durability, cost of manufacturing, emissions requirements, drivability, etc. The OEM’s have to warranty these engines and they are designed to last many tens of thousands of miles.
These compromises in many cases cost HP and can be modified and or eliminated. We can do this on a race engine because we don’t have the wide operating and usage conditions that the OEM’s have to take into consideration. We can make the engine a more focused engine performance wise.
So now that we are free to “focus the engine” for a particular use, there are several things we can do to make major power improvements. These include raising the compression ratio and using bigger cams etc. But the cylinder head is where all the power is really made. Optimizing the ports for a focused application is a very complex engineering feat.
Yes you can keep making the ports bigger and flow more and more air but there is a balance, a proper ratio of velocity to flow that must be maintained. Raw CFM numbers are just one of over a dozen important numbers we look at. This is where the “black art” comes in.
The valve seat profile, valve size, seat choke, bowl diffusion, minimum cross sectional area, the MCSA location, port taper, length, localized velocities, velocity bias etc. all come into play. A mere .010” of increase in the ports MCSA can affect velocity by 10 fps or more. Take out your feeler gauge and see how much material that really is. Yes this is what it really comes down to with these ultra high efficiency ports, thousandths of an inch. Much, much different than your fathers old Chevy fuelie heads. Controlling the geometry of the port has become so critical to squeezing the last few % of hp from these engines that it has to be done in a computer. You won’t see any hand ported heads anymore in any form of top professional racing for a reason. Does your head porter know all of these critical dimensions of there port?
One of those reasons is also repeatability. With CNC porting the repeatability is incredible. Each port is exactly what the doctor ordered.
We offer our porting in stages, and we do this because the ports geometry is matched to the engine size and usage. For example, if you have a stock displacement Busa engine at 1299cc and its at 10,000 rpm it will draw a given volume of air and have a given average port velocity. If you have a 1441 cc engine at 10,000 rpm it will draw more air but you still want to maintain the same velocity in the port. A 1696 cc busa engine will draw even more air but you STILL want the same velocity in the port. When a ports localized velocity on a running engine spikes and exceeds the speed of sound, “a specific velocity based on the temperature of the air in the port” it forms a shock wave perpendicular to the direction of flow and acts like a restrictor plate. This is commonly called “sonic choke”. This typically happens on a port that is improperly sized at its MCSA for the size and RPM of the engine. This severely limits the hp output past the RPM that the shock wave starts to form at. Ideally every engine combination needs its own specific MCSA for that combination. But that’s impractical because of the number of engine and cam combinations available. So we offer 3 carefully sized ports that overlap to suit any engine and cam combination. This is why the first question we always ask is “what size is your engine” and “what are you going to be doing with it” before we can determine what port you need.
Does your Busa, zx12r or zx14 engine make .164 -.165 hp per cc?
Here are some very typical real world examples of our customers results on there OWN Dynojet dynos with mild compression ratios under 14 to1. With high compression engines over 15 to 1 you may expect slightly more.
Stage 2 Busa head on a 1397cc engine, 231 hp.
Stage 2 Busa head on a 1441cc engine, 238 hp.
Stage 2 Busa head on a 1507cc engine, 248 hp.
Stage 2 Busa head on a 1580cc engine, 258 hp.
Stage 3 Busa head on a 1650cc engine, 271 hp.
Stage 3 Busa head on a 1696cc engine, 278 hp.
Stage 2 ZX14 head on a 1417cc engine, 232 hp.
Stage 3 ZX14 head on a 1534cc engine, 254 hp.
Stage 3 ZX14 head on a 1570cc engine, 260 hp.
Stage 2 ZX12 head on a 1290cc engine, 213 hp.
Stage 3 ZX12 head on a 1427cc engine, 234 hp.