This is the kind of carbon...
This is the kind of carbon coloration you want in the combustion chamber. This engine wasn't run long, yet there are no areas where the chamber is shiny. That shows the flame travel is even throughout the chamber and therefore the air/fuel mixture is evenly spread around the combustion chamber.
There is no substitute for...
There is no substitute for flow bench testing. The OEM carmakers spend millions of dollars modeling port flow on computers, but it all still comes down to flow bench and engine testing.
See caption below.
The advantage we have today...
The advantage we have today with port and chamber design is that a combination that works can be exactly duplicated using computer numerically controlled (CNC) equipment. Everyone making CNC heads is still learning about this technology, so the ability to make great heads is only going to get better and the price will continue to come down.
See caption below.
See caption below.
A little over a decade ago,...
A little over a decade ago, Chapman Racing Heads was doing some serious work like this. These are early rolled-over 23-degree Chevy cylinder heads that were used in NASCAR Winston Cup. We welded about 3/8 inch on the deck surface, welded up every hole, and remachined everything. Some heads received considerable grinding and were blasted on the outside so they didn't look molested. The advantage of the rolled-over head was it allowed the use of a small combustion chamber, flat-top or dished piston, and gave the intake port a straighter shot at the intake valve. We built 18-degree, 15-degree, and other versions that won on all different types of tracks, including Daytona.
One thing we've found from...
One thing we've found from doing CNC heads is that since the CNC machines index off the valve centerlines, we need to have the valves located within +/-0.010 inch of specifications. To achieve that, we do all the prep work in this large shop area.
The grinder is in hand. The race-engine cylinder heads are laying in front of you. You're ready to make some power, right? Well, not exactly. It's probably important I share this with you. Of all the lessons I've learned in racing, the hardest have been how to make more power with the ports and combustion chambers in an engine. My company, Chapman Racing Heads, might be creating some great heads today, but, and this is tough to say, for about the first two years I was working on heads, every race car that had some of my work in it went slower. It was horrible, but I learned a ton making those mistakes.
I continue to learn, but I'm seeing fewer people get into port design, which is part of the reason I wanted to do this story--I hope to generate some interest in this field. I believe the lack of interest is due to the availability and low cost of CNC-ported heads (which my company, along with Weld Tech and a few other companies, make and sell). People now think there's nothing else to be gained in this area, which isn't true. If you want to work in this area but don't know where to start, I hope this helps you learn some basics of flow.
Even today there are things I incorporate into the port or chamber designs of our heads that make power, but many of the designers and engineers I am partnered with don't know why they work. These are things I've learned through experience, but other than that, I can't give you a mathematical equation that shows why they make power. Because of that, there is still plenty to learn in the art of flow.
When I was young in this business, the cylinder heads I was working on could flow, which made power. The problem was that it wasn't usable power. I didn't have any experience--no flow bench, no dyno, no computer simulators, nothing. I just headed out to the shop with the grinder and had at it. This showed in the results.
My suggestion is that if you want to modify some ports, or even port match your heads and intake, you should first read this section on flow, then read anything you can find. Talk with talented engine builders about what they see work on the flow bench and dyno. Then, if you want to port something, practice on some heads that are damaged or otherwise not valuable to learn what makes them flow. This is a tough game to get up to speed on but there are gains to be made. I hope this will save you from the slow-down effect I experienced.
With all that said, it's time to start talking horsepower. Like many young head porters, when I first started my priorities were all wrong. I wanted maximum airflow through the intake and exhaust ports at maximum valve lift. The result would have been great if we were racing flow benches, but we weren't. What I have learned over the years falls under a term I call charge motion. This term defines the strength, direction, and condition of the air/fuel charge as it enters the cylinder through the intake valve. If you have good charge motion, the cylinder will continue to fill even after the piston hits bottom dead center (BDC) and the air/fuel charge will evenly disperse in the chamber. These working together make horsepower.
I've found you can have a head that will flow 20 cfm more than a similar head, but the engine equipped with the higher flowing head won't run worth a lick and the head with less flow runs strong, and that's the truth. This is because we're dealing with charge motion.
This all goes back to how you approach the system. Yes, we are testing airflow on flow benches. But those ports need to be sculpted so the engine gets as much of the cool mixture of air and fuel, in a ratio of about 12:1 or 14:1, flowing past the intake valve while it's open. Then, the exhaust ports need to get rid of the extremely hot mixture of gases and particulate. This might seem easy, but add valve overlap into the mix and you need to be concerned with overscavenging the initial intake charge out the exhaust. This can make your low-lift flow seem like it's not working in the engine, when in fact you're just pumping the air/fuel mixture right out of the exhaust. Do you see how port and chamber design are much more than just peak flow?
It's been shown that the more thorough the combustion process is across the face of the chamber, the less detonation will occur and the more of the force generated by the combustion of the air/fuel mixture will go to pushing the piston down the bore. This results in more power output. So how do you know if you have this type of combustion? You study the carbon left over in the combustion chamber after the engine has been run.
I've learned a lot from looking at the combustion chamber carbon buildup after a race engine has been pulled apart, and so can you. Many combustion chambers in a typical gasoline fueled, wedge chamber, overhead valve, cam-in-block engine will show a clean area between the intake valve and the combustion chamber wall. Everyone, including me, used to think this was the fuel washing the chamber clean in that area. This is not the case at all.
In fact, there is no fuel in that area to burn. It's all piled up on the exhaust side of the chamber. Through real-time data acquisition testing, engineers have found the combustion flame front travels unevenly across the combustion chamber. This retards the combustion process and reduces the effectiveness of the combustion process pushing the piston down the bore. I agree with the engineers, because when we started changing the port shape to get a more even carbon coloration across the chamber face, what I refer to as improving the swirl into the bore, the engines ran better with less ignition timing and made more horsepower overall.
Even I was surprised by how much of a difference it made. You wouldn't think you could do something inside the ports that would affect flame travel, but you do when you affect where the fuel goes.
If you want to port your heads but don't know where to start, you probably have been reading this story looking for the nuts and bolts of how I port heads for power. The problem is that there are few surefire things in this game. For instance, for years it's been thought the best intake port design was a venturi-type port with the inside radius at the valve low and the port turning rather sharply into the intake valve. The tide is turning, though. The new Chevy SB2 head being used in NASCAR Winston Cup has an intake port that resembles a 2x4 board pointed at a steep angle right into the top of the intake valve. Does it work? You bet! Do the ports look like everything else out there? No. It's another case of a different way to make power.
The nuts and bolts I can give you in this short story would begin with not considering the flow numbers as everything--they are one part of the puzzle. You should also consider how the air is flowing as it goes through the ports, the sound of the air flowing, and the methods you use to make changes. Conduct your testing in as scientific a manner as possible, so that your results are repeatable. Keep good notes on your changes.
I think port and chamber design is one of the last true artistic venues left in racing. There is testing going on from the Big Three manufacturers on down to the lowliest racer. Everybody is coming up with ideas that can work. You don't have to be a computer geek, Ph.D., engineer, or lifetime racer to be in this game. If you spend the time and effort, there are still gains to be made. I think that's pretty exciting, and I hope you do, too.
So, don't be afraid to pull that grinder out to make some changes. But instead of doing it blindly, try to use some of the hard-fought lessons of others to get good results out of the box. Then, you can skip some of the slowdowns most young head porters encounter and move right on to making power.