RHS Designs for the Entire System, Not the Flow Bench

Advanced dyno testing with Fast manifold, valve body, fuel rails, and oxygen sensors allows RHS to see how the entire engine performs with a new head design.

Determining the best cylinder head for your needs requires more than simply comparing port flow numbers. There are many ways to measure a cylinder head: port volume, port flow, cross sectional area, air velocity, dimensions, chamber volume, valve size, etc. All of these things are important, some more than others. It’s important to understand what to look for and how to decide which of these aspects are most important to consider when selecting a cylinder head. While there are many ways to measure and compare heads, the only thing that’s really important is that the entire engine performs as desired. After all, it’s the way the engine operates and feels as well as the power it makes that will determine if the choice was correct. That’s why it’s important to understand how some of these things work together and why head designs can be so vastly different.

It would be really easy if you could just throw a head on a flow bench and declare the one that flows the most to be the best. Unfortunately flowing a head has become the easiest and most common way to compare heads. RHS® took the bold approach and designed their ports for the best performance on the engine and not necessarily on the flow bench.

Big-block Chevrolet cylinder head on flow bench, including a gauge that measures airflow at different lifts.

Port flow, which is an extremely important part of head design, has become the single most common factor used in the cylinder head arms race. The reasoning is simple: Although the factors that determine what cylinder head design works best with a particular engine package are numerous, and sometimes complex, most people only understand the concept that if a cylinder head restricts the air/fuel flow going into the combustion chambers, it will kill power. So the more air and fuel an intake port can flow, the better, right?

Not always, but that doesn’t keep someone from thinking that intake port flow numbers (measured in CFM, or cubic feet per minute) are the only critical factors when determining which head is best for an application. Unfortunately, that’s just not always true. In fact, it’s rarely true. It is easy to make a port flow more air – just make it bigger. But doing that creates an engine that, more often than not, makes no torque and displays lazy throttle response. Plus, because there’s no standard, unless you know exactly how the testing was performed, raw flow numbers are not of much value and can easily throw you off track. The list of tricks a smart operator can use to get artificially high numbers on the flow bench is nearly endless. So when it comes to flow numbers, the only ones that are useful come from when you are comparing two different heads on the same bench at the same time.

The engineers at Racing Head Service work to put together a complete package. Taking into account the entire system when they design a head and port, they ensure every cylinder head works for the entire engine package to maximize power, efficiency and throttle response throughout the RPM range. Port flow numbers here aren’t the goal, only a single indicator among many factors. After all, who cares how much CFM the intake flows as long as the engine hits your horsepower and performance targets?

“I have a customer who was buying heads from somebody, and he called up and wanted to try a set of ours,” says head porting specialist Mike Stensrud of Wegner Motorsports, who works with RHS® on many of their port designs. “I did up a set and sent them to him. He put them on his flow bench and then called me up and asked if he could return them. I said sure, but asked him to first tell me what was wrong. He said that on the flow bench they were 40 CFM off from what he was currently running and he didn’t even want to put the heads on the race engine he was building.

This is the RHS factory milling area where new cylinder heads are machined to spec.

“I asked him to do me a favor, go ahead and bolt them on the motor and see how they ran on the dyno. I told him if he still didn’t like them he could send them back and we’d not only refund his money but also buy him a new set of head gaskets and anything else that needed to be replaced from the test. So he did, and those heads that he didn’t like at first because they were 40 CFM down, wound up making 38 horsepower more than the better flowing heads.”

Stensrud says there are design cues he has learned in his many years of designing and porting high performance heads, and those features are regularly included in RHS® cylinder heads, that sometimes even result in lower flow numbers on a flow bench. But that’s OK because the results show up quite well when it comes to helping the completed engine make power. “The problem is a lot of people want to race the flow bench,” he says, “but nobody wins the real races with flow bench numbers. You have to consider how the cylinder head will affect the completed engine’s output. One trick that some people will use to help make their heads flow better is to blow out the chamber really big around the intake valve seat. And yes, that will flow well, but when you put those heads on the engine, it loses torque and you won’t have a happy motor. You also usually have to put a lot of timing in it to get it to run. We like to leave it tighter around the valve seat, which means we give up a little bit of flow on the bench but you really pick up the torque, which you can feel from the driver’s seat.”

Another area you can improve power but also hurt overall port flow is the throat diameter. The “throat” of a port is the area directly behind the seat. A smart head designer considers the throat an extension of the valve seat. By controlling how the throat tapers as it approaches the valve seat, it can make the changes in seat angle less noticeable to the incoming air/fuel charge. Generally speaking, if you are trying to target max power in lower RPM ranges, you need a smaller throat size. Stensrud says he aims for a throat size that’s 88 to 92 percent of the valve seat on a hot street motor, which helps the low-end torque. For all-out racing where the RPMs are going to be much higher, you may want the throat to be in the 92-98 percent range. A throat percentage this high will hurt flow bench numbers but will show up on the dyno in the upper RPM range.

The RPM range the engine will be operating within is also a major consideration that affects just how much flow is actually ideal. In many respects, the velocity of the airflow inside the ports is just as important as how much air the port can flow. The speed of the airflow in the port – often referred to simply as “velocity” – must be high enough to keep the air/fuel mixture in suspension, otherwise the fuel droplets will drop out of the airflow and won’t burn in the combustion chamber. And unburned fuel means lost power. Port velocity also helps improve signal at the carburetor, reduces the opportunities for power-killing reversion and increases cylinder fill at lower valve lifts. In plain English, this translates to better part-throttle response and more power throughout the RPM range. The trick is to size the port appropriately in order to get the necessary velocity and then shape it to maximize flow.

Assembling an aluminum small-block Ford head.

“The port size doesn’t directly relate to flow or velocity,” Stensrud explains. “Higher flow numbers usually mean a bigger port, which usually means less velocity, so you can actually end up going the wrong direction. A good example of this is the LS head I just worked on with RHS®. When we finished the design, the ports weren’t any bigger than the stock LS7 head, but yet it flows almost 40 CFM more air. It all has to do with intelligently changing the taper sizes and doing the things that help flow and horsepower without making the port any larger than it needs to be.”

This is why when it comes to choosing the correct head for your engine, you shouldn’t believe the first thing you read or hear. Consider the other important factors, including the port design, the volume and cross-sectional area, and all aspects of the head, not just the flow numbers. The smaller details are often more important than overall flow because, as with port volume and cross sectional area, they have a direct link to velocity and performance. Everything should be sized appropriately to the engine. And in order to do this – especially if you are building a brand new engine – it requires a little planning. If you aren’t already a cylinder head specialist, the best advice is to find someone who is. You can, for example, contact the tech help line for a cylinder head manufacturer. Or better yet, contact several different manufacturers and compare the answers they give you. The tech department at RHS® doesn’t mind spending some time with someone looking to find the right cylinder head for their application, and may even point you to a different brand if they don’t have what you need. Their philosophy is that creating a relationship with a customer is much more valuable than a single sale.

A micrometer measures spring height so the technician can get an accurate reading. Shims are used on the valve seats to get final spring height on this small-block Chevy head.

When calling the RHS® tech department for help, you will need to be prepared with some information of your own. In order to guide you in the right direction, they will most likely want to know what engine you are building, the displacement, how it will be used, the RPM range it will most often see, compression and even the fuel type (octane, gas or alcohol). Also, you can do a little research on your own. If you are a racer, take a quick survey at your track. What heads are the top dogs running? And we’re talking about the guys that are actually fast on the track, not the ones who simply talk a good game. That will tell you plenty right there.

Unfortunately, when it comes to determining which cylinder head is best for your application, there is no easy answer. And there certainly is a lot more to it than simply comparing flow numbers. Instead, ask around. Find a tech department that’s more concerned with how you want your completed engine to perform instead of selling the features on their heads. After all, horsepower and torque are the only numbers you should be interested in, not flow numbers and other mumbo-jumbo. Once you’ve found someone you can work with, the hardest part of selecting the best heads for your project is already complete.