When it comes to valve sizes, common sense tells us that the bigger the valve, the bigger the volume of air that can be moved through the port. However, not only are there many more considerations than just valve size when it comes to multi-valve cylinder heads, the simple premise of “bigger equals more” has started to prove less than true in some cases.
Zeke Urrutia, Ferrea Racing Components’ Director of Marketing recently appeared on a webinar with Luke Wilson, owner of 4 Piston Racing, put on by Epartrade.com to discuss the matter of valve sizing — specifically in multi-valve cylinder heads — in modern times. “We focused a lot into the multi-valve technology early on, because we saw that coming,” says Urrutia.
“The market has basically become two-valve technology against four-valve technology. A lot of people have embraced the four-valve technology, because of both the performance gains as well as the gains in reliability. Ford has done an exceptional job with their valve technology, while GM is just starting to embrace it, but Ford has really captured that multi-valve V8 segment. When you look at the flow characteristics of a cylinder head and how well the dynamics work, the four-valve is just tremendous. I think four-valve technology would enhance any system it’s applied to, even hybrids.”
When More Isn’t Necessarily Better
While on the surface, that might strengthen the “more is better” idea, it’s a lot more complicated than that. Because more valves take up more room in the cylinder head. To give the valves the real estate they need in the cylinder head, compromises need to be made. “When we are looking at a four-valve head, we look at the limitations we are faced with, which are the water jackets and the valve angle,” Wilson explains.
“We’re typically seeing much different valve angles that are seen in a race-developed cylinder head. For packaging’s sake, the OEM might have gone with a 25-degree valve angle. We’re dealing with a small-bore engine, and you can have the valves running so close to each other they are almost running into each other already, which makes increasing the valve size extremely challenging.”
Besides the sizing considerations in the same plane of existence, the dynamic location and clearances of a larger valve come into play, when you are trying to modify something that is packed into a box within an inch of its life. “Sometimes, we have to weigh the 10 or 12 cfm we’ll pick up from an extra millimeter of valve size, against losing our ideal camshaft centerline and having to alter the valve timing to physically fit the larger valve in the system,” Wilson says.
One of the key points that both Urrutia and Wilson kept touching on is that the valve size, whatever that might be, needs to match the rest of the system. Having a larger valve, just for the sake of having a larger valve, doesn’t always produce positive results. “We need to size the seat, the throat, and the port to be able to physically fit in the head. In a less developed, more stock cylinder head application, usually, there is more to be found in the port, using a stock-sized valve than just putting bigger valves in it,” says Wilson.
Is Less More?
When you start to look at the system as a whole as opposed to individual components, you start to realize that bigger indeed isn’t always better. To illustrate the point that the valve itself is only a single part of a larger ecosystem, Urrutia brings up Pro Stock cylinder heads. “Even though it’s not a multi-valve arrangement, Pro Stock has seen cylinder heads transition to a smaller exhaust valve and a bigger intake valve. Traditionally, they tried to get every inch out of the ports and valves on those engines. Now we’ve gone from say a 1.900-inch exhaust valve to 1.820 inches, 1.830 inches, and 1.840 inches, and teams are getting more power,” Urrutia explains.
“That same premise stands to be true on the four-valve technology. For example, we used to run large valves on the 2JZ engine; usually, 2mm oversize on both the intake and exhaust valves. In today’s world, we’re going back to standard size and we’re finding the same or more power with smaller valves.”
Wilson agrees that sometimes there is such a thing as too large. “On a naturally-aspirated combination, we could easily have a valve that is so large that it’s almost hitting the side of the bore. Then we need to figure out if that valve shrouded so much that it’s actually hurting us,” says Wilson. “If it’s a turbo application, the valve timing becomes more important than valve size at that point. In that case, we have to let the turbocharger do its job and prioritize valve timing over the last few CFM of an oversized valve.”
So as time and technology march on, it’s clear that besides multi-valve arrangements inherently providing more efficiency than more traditional two-valve arrangements, that a better-developed port, throat, and seat can be worth more power than larger valves alone, whether in a two-valve or multi-valve arrangement.