Now that we have an entire rotating assembly to pump air, and a full valvetrain to control when that air is pumped, it’s time to actually get the air in to (and out of) the engine – which is the job of the heads. One of the most important and expensive parts of any build, heads link nearly everything else together. They determine your compression ratio, what valves and cam will work best, what intake you want, what fuel and timing you’ll ultimately run, and even play a major part in determining how much your engine will weigh! Without further ado, let’s do some research into these air-flowing wonders…
Airflow Research – Speed vs. Volume
The best short block and valve train in the world won’t do anything if your engine can’t flow the air it needs. Engines really are air pumps, and they need to breathe as well as possible for optimal performance. There are a ton of heads available for the Big Block Chevy: rectangle or oval port, steel or aluminum, aftermarket or OEM. To make matters slightly worse, there’s no “right” answer on which head is the best for your engine – like everything else, it really depends on the goals of your engine. Very generally speaking, there are two important things that heads need to do from an air flow perspective. First, they need to get as much air into the engine as possible, as the more air you can get into the engine, the more fuel you can burn and the more power you can produce. But it’s also important to get the air into the engine quickly – that is, you want the air speed of the incoming flow to always be as high as possible, to promote complete filling of the cylinder, adequate mixing of the fuel charge, and all sorts of other things.
This becomes a bit of an issue because anyone who’s played with a garden hose knows that flow and speed are usually inversely correlated for a given pressure. In other words, with no hand on the end of the hose, it flows a lot of water…slowly. If you cover the nozzle with your thumb, speed skyrockets, but flow plummets. Since our engine is naturally aspirated and only has ambient air pressure trying to fill the cylinders, picking the right cylinder head is extremely important. Larger heads, like factory rectangular ports, flow a ton of air – but since we only plan to hit 5000 RPM and have a relatively conservative cam, those huge ports will be “lazy” in our application, and actually hurt performance at our intended lower redline. Small heads, like factory “peanut ports” found on heavy trucks and RVs, flow air very quickly at low RPM – generating a ton of bottom end torque – but severely constrict airflow at higher RPM, reducing horsepower potential. For me, a middle of the road solution – something that would flow a decent amount of air, decently quickly, at a higher but not extreme RPM, was the solution.
Enter AirFlow Research – one of the leading cylinder head companies on the market today. They have very “small” port heads that still flow a ton of air – meaning that I get both the high speed and high flow needed. I ended up going with their smallest offering because it’s all I needed for my build, and they were significantly more affordable than the next size up.
Compression and Chamber Design
In addition to getting air into the engine, heads help determine two more very important things – the compression ratio of your engine, and how well your fuel mixture burns when it’s ignited. These two qualities are intrinsically linked to the combustion chamber design, which has seen a lot of evolution since the Big Block was first introduced.
There is a huge amount of computational simulation that goes into modern combustion chamber designs, especially with companies like AirFlow Research. Generally speaking, the more turbulent a mixture right before ignition, the better it will burn. Similarly, the more efficient the combustion chamber, the more power an engine can extract from it’s combustion process. So beyond flow numbers, picking cylinder heads comes down to how well designed the combustion chamber is. This is one of the few areas where there isn’t as much of a tradeoff. The better your combustion chamber, the more performance you’re going to get.
The three things that influence the combustion chamber efficiency are the compression ratio, the quench area, and the shape of the chamber itself. The compression ratio is the most talked about and most important, so we’ll start with it. Gasoline engines follow the Otto Cycle, detailed below. Because of the physics of the Otto cycle, the higher your compression ratio, the more energy you can extract from a given volume of gas. Engine design therefore tries to maximize the compression ratio as much as possible before anything else, because ultimately it is the #1 factor for determining how efficient your engine is. Unfortunately, there are a lot of factors that limit how high compression can be. The octane of your fuel, the design of your combustion chamber, the ignition timing you run, and various other factors all limit the compression you can run in a given engine – even what material your cylinder heads are made of. Higher compression makes it harder to avoid pre-ignition or detonation, which is when the fuel / air charge explodes instead of burning in a controlled manner – which is really bad for your engine, and can destroy it within seconds! Modern cars with near perfect combustion chamber design, running premium fuel, and with full computer control of everything can eek out 12:1 or even 14:1 compression – but because I’m on mid grade pump gas, with an old engine design, and no real time monitoring of combustion processes, I’m limited to around 9.5:1.
Besides compression, the quench and combustion chamber shape help promote a more complete and efficient burn. “Quench” is basically a large flat area that nearly kisses the piston at top dead center, shown on the left of the below photo. As the piston rises rapidly, the fuel / air charge is squished out of the quench area at a very high speed. This promotes a high degree of turbulence, which fully mixes the fuel air charge, and ensures a more efficient combustion process. It also allows the combustion chamber itself (shown on the right below) to be quite small, which promotes a quicker and more complete burn of the fuel air charge.
Ultimately, there are a lot of factors that influence how effective a combustion chamber is, that go way beyond the scope of this blog. For me, going with a very well known, proven company like AirFlow Research was the best way to ensure that not only would my heads flow lots of air, but they would provide efficient and powerful combustion processes as well. It’s important to match your short block to your heads to get your compression ratio right – in particular, the “volume” of your pistons, combined with the volume of your combustion chambers, are critical to ensure you get the right compression ratio. Similarly, your head gasket thickness and deck height dictate your overall quench distance.
It’s worth noting that, once all of the above is taken into account, aluminum heads are better than steel in every way except one. They’re notably lighter, run cooler (which allows a higher compression ratio), and generally have more modern designs from the aftermarket. The only downside is cost – they can easily run a few thousand dollars for a good set, compared to a few hundred for a used set of OEM steel heads! In my case, it was worth the extra cost to go with a phenomenal pair of AFR 265 cylinder heads, given how important they are to the overall build!