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Bigger is Not Always Better
Choosing the correct cylinder head design
for an engine has become a nightmarish task for the average racer. Now
more than ever there are a blinding array of cylinder heads to choose
from. Take the conventional big block Chevy cylinder head for instance.
It’s been around for over 40 years and the list of available choices
grows by leaps and bounds every year. It seems that every six months or
so a “new” trick head comes on the market and everyone flocks
to see what all the hoopla is about.
A bad choice costs thousands of dollars in wasted time. As a matter of fact, a bad choice costs you more than double what the correct one would, because now you have to sell the heads you purchased and buy the right ones the second time around! Since this type of decision has the potential of becoming a very expensive mistake, it’s no wonder it’s one fraught with anxiety. The bottom line is, with all these heads on the market, how is the average racer going to make the right choice? It’s a complex issue with no simple answer. How complex, you ask? Well, considering how many different engine sizes there are, not to mention the multitude of parts combinations, RPM ranges, types of fuel, as well as the intended uses the engines are meant for. All these variables and many more go into choosing the correct cylinder heads for an engine. a
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I wish I could lay out some quick and easy mathematical equations or
some simple guide lines to help, but there simply aren't any. Anyone who
states otherwise is analogous to an Old West snake oil salesman. This
issue is complex and this is why people have so much trouble with it.
I have seen in the past where individuals have tried to concoct irrational
ratios or simple equations for advertising purposes, only to confuse and
perplex people even more.
So it can be said that the most important rules regarding the induction
system design are: Velocity, Velocity, Shape, Velocity and last but not
least, Air Flow. Most people think that my job as a head porter and induction system designer
is to maximize air flow. Actually, flow is the last thing I look at. Surprising,
isn’t it?
Well, acceleration, lower E.T.s and a boat load of mile-per-hour for starters. When the velocity profile of the induction system is tuned properly to the engine combination and the RPM range in which it works, it will accelerate faster off the throttle stop, have faster and more consistent reaction times and just plain go faster from point A to point B. Many of my professional counterparts and I often discuss the fact that flow numbers have blinded the general public. Actually, The four words my professional counterparts and I hate to hear are: ” What do they flow”. When an individual calls me seeking my advice about his future cylinder head purchase, the words, “what do they flow” usually come up within the first minute or so of the conversation and tend to dominate that conversation for its entirety. a
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Even after I have explained that the velocity profile of the induction system as a whole is far more beneficial to producing power than high flow numbers, the conversation will undoubtedly turn on a dime and slap me in the face with those famous words once again. "What do they flow?" I shudder every time I here these words because I know the individual on the other end of the line has been misinformed by the mass media. He or she will blindly except air flow as the primary design feature to look for in a cylinder head. I also know that if this individual picks a set of heads solely on flow criteria, he or she will more than likely end up with an underperforming engine. Many who choose this route end up with an underperforming engine and never know it! Put it this way. If you have only experienced what you have, you don’t miss what you could have had. a
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When people compare port CCs they are really talking about average velocity in the port. Take a look at all the conventional BBC heads on the market today and the range of port sizes that are offered. The range of port sizes extends from 290cc to 420cc. When choosing a head for a racing engine many people understand the fact that you wouldn’t put a 420cc port on a 500cid engine just like you wouldn’t put a 290cc port on a 632cid engine because the corresponding velocities would be too low and too high respectively. Due to raw experimentation over the years, many engine builders never had to know how to quantify the average velocities of the heads they where using. They just used the old trial and error development routine. They tried a 320cc head on their 572-inch engine and then tried a 360cc head. The 360cc head obviously made more power but they often didn’t realize it was because the average velocity profile fit the power band of the engine. They then tried a 420cc head and found that it made less power. This was because the average velocity was too slow. Trial and error will get you there eventually, but man-oh-man is it expensive! Professional engine builders still have to try new cylinder heads to evaluate them, but they have already mathematically quantified whether or not it’s a good match to the engine combination before they try it. When people compare the volumes of various cylinder head intake ports they are, in a round-about way, comparing average air speed.
This method of comparing port volumes all started when everyone used the same factory cast iron heads decades ago. The ports in these heads were too small even after porting, so the bigger you made them the more power you made. With the advent of aftermarket heads this method, albeit improper, stuck and people still use it to this day. You must be careful to only compare like cylinder heads! The one common denominator is that the ports are the same length. You can’t compare the port volumes of the new 18-degree conventional heads with a 24-degree conventional head just like you can’t compare the volumes of a SBC to a BBC port because their ports are different lengths! a
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Once a particular engine has been perfected everyone is privy to what size head should go with that engine. Take a 572-ince piece for instance. The 355cc to 360cc heads work perfectly on engines of this size operating from 5500-7500 rpm. With a smaller port, like 345cc, the average velocity will be too high, moving the power band down and decreasing top end power while at the same time increasing the bottom end torque slightly. With a larger port such as a 420cc, the average velocity will be too slow, decreasing both top end power and bottom end torque. The old saying about big ports making top end power just doesn’t hold true. It’s a total fallacy and nothing could be further from the truth. You can take from this a very important lesson. You’re always better off with a port that’s a little too small than you are with a port that’s too big! Many professionals in the field of cylinder head development have built spreadsheets or written programs to aid in design work because the equations are complex and time consuming. Many of these software programs can help the novice understand the basic principles of the induction system. These software programs are not meant merely for the professional. I use a combination of over-the-counter software such as PipeMax from Maxracesoftware, Engine Analyzer Pro and a multitude of spreadsheets I have written over the years. Programs like PipeMax are ideal for educating yourself on the pitfalls of choosing cylinder heads based on airflow alone. Its simple user interface is easy to use and it can aid in picking a cylinder head for your particular combination.
Mathematical engine modeling software such as Engine Analyzer is a highly complex program but even the novice can learn a lot with it. Programs like these aid anyone seeking to learn more about induction system and exhaust system air speed issues as well as show you the likely result of choosing heads that are too large for your particular engine combination. Another way to help your decision-making would be to gather the opinions of professionals in the field. Call and get their input. In my personal opinion I would have to define a “professional” as someone whose good reputation spans two or three decades and his or her trustworthiness is well known in the industry. A professional is not your friend’s second cousin’s ex-roommate, who purchased a flow bench and started porting heads on the weekends. The bottom line here is that there are many well-designed and powerful heads on the market, but your type of engine and its intended use would ultimately determine the correct cylinder head, not how much they flow.
Got a comment? Drop us a line at comppluseditor@aol.com.
a d v e r t i s e m e n t
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