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“The power is in the cylinder heads.” Brad Morgan, engine builder for Reher Morrison Racing Engines. There is little dispute that cylinder head airflow equals power. However, it takes much more than just making a port bigger to make real power.
Porting is one of those areas that is shrouded in mystery. The Black Magic of the performance world. Practice by the few who seem to make magic from mediocrity. Legendary cylinder head porter and engine builder, Joe Mondello, once said, “You need to think like air to be successful. Joe didn’t own a Flow Bench before 1997 and yet engines with his cylinder heads set more records than any single individual to date.
The science of cylinder head flow is based on physics and fluid dynamics. A scientific paper done for NASA, Vena Contracta, changed our understanding of cylinder head design, says Daren Morgan, head of cylinder head development for Reher Morrison Racing Engines. It addressed how to get air around a bend while maintaining air speed. Cylinder head design is really about taking the amount of air you need and managing the velocity it is traveling. Flow numbers are important but velocity is critical to obtaining the full potential of the engine. "If the velocity, size, shape and venturi design are correct in the port, the port will flow a great deal of air. Unfortunately, a port that is too big, misshapen and has a poor venturi will flow just as much if not more air."
You can calculate the amount of air your engine needs based on some assumptions. A well tuned Harley Davidson Engine should make about 1.25 HP per Cubic Inch at the crankshaft. (15% less at the rear wheel) They seldom do by the way and the stock engine; forget about it (0.80 HP/CI). A really well tuned performance engine can make 1.5 HP per Cubic Inch and the truly high performance V-Twin engine in race trim, 2.0 Hp/CI. This discussion is assuming naturally aspirated engines. So if we consider the HP we want to make, we can determine the air required. As an example let's look at the development of the M3 103 kit.
A 103 cubic inch V-Twin style motor with a 4.375 inch stroke and a 3.875 inch bore will require 322 to 345 CFM of air at 28 inches of depression depending on the RPM range. (193 to 206 CFM at 10 inches) That is a lot of air. None of the Motor Company heads will get you there. S&S, T-Man, Elderbrock, Mackie, Head Quarters, Patrick Racing, Branch, Hot Shot, and Delkron won't get you there either. Zipper and STD may be able to get the number but they are not doing it on stock castings. S&S offers a B2 head, but it will not fit a stock bolt pattern. To my knowledge, at the time of this writing, no one is making more than 310 CFM at 28 inches on stock castings that are a street able advertised head. Our heads changed that. Our M3 head will flow 327 CFM at 28 inches AND velocity in the port is appropriate to allow the head to function in a wide range of applications. How? "Knowledge is Horsepower............You can't fake what you don't know." Kyle Mathews
So if you have bothered to read this far, here is the math.
To calculate the amount of air needed for a given motor combination you need the following information. The desired horsepower and torque. The expected volumetric efficiency (VE). The expected RPM
For our motor we are assuming a VE of 100%, a HP of 1.65 Hp/CI, and a maximum RPM of 7000 RPM's. (For those of you a little more advanced, BSEP of 210, BFSC of 0.45, Maximum piston speed of 5000 fpm).
Horsepower= Torque X RPM/5252
Torque: A normally aspirated gasoline engine can produce 1.4 to 1.65 FTlbs/CI.
RPM is limited by piston speed with is directed affected by stroke length. To calculate the piston speed= Stroke in Inches X RPM's/ 6 You want to keep the piston speed below 5000 fpm.
Theoretical Air Flow at 10 inches = CI X RPM's/3468 To convert to 28 inches multiply by 1.67.
Horsepower/cylinder form CFM= .43 X CFM at 10 inches (HP is at the crank)
So Theoretical HP is around 170 Hp from a 103 CI at the crank. That is about 144 HP at the Rear Wheel. Theoretical Toque is 134 ftlbs. Maximum RPM is 6700 RPM's
STILL THINK YOUR 124 CI MOTOR IS A BAD ASS?
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