The purpose of any cylinder head is to fill the cylinder with fuel and air, and to empty the cylinder once combustion has taken place. Below, you'll see a comparison between our Heads and other popular brands. The numbers speak for themselves!
As you can see, our Heads outflow the competitors- even the ones with larger valves. There are two reasons for this:
Remember, no matter how well the port is designed, it had better flow unrestricted, there is not a head anywhere that can do this better than our "Hemi Performance Engineering" Head!
We see a great deal written about peak flow numbers. It's our opinion that peak flow numbers are almost meaningless. The cylinder head with the best overall flow curve wins every time, whether at the track or through the curves on a country road. The reason is simple... the cam spends more time getting to peak lift than at peak lift and the majority of the filling takes place early in the cycle. The mid lift and low lift flow is far more important than the flow at peak lift!
It's also important to understand that adding the total flow at various lifts and comparing them to other totals is also somewhat deceiving.
Here's an example:
| Lift | Hemi Perf. Eng. | Brand B |
| .100 | 42 | 38 |
| .200 | 87 | 75 |
| .300 | 125 | 110 |
| .400 | 149 | 132 |
| .500 | 157 | 144 |
| .600 | 160 | 150 |
| .650 | 161 | 150 |
| Total | 881 | 799 |
As you can see, "Hemi Performance Engineering" Heads outflow Brand B through all lift points, netting 54 CFM more than the competition. More importantly, the fact that the "Hemi Performance Engineering" Head is far superior between the .100 to .400 lift, where the majority of filling takes place, means that the "Hemi Performance Engineering" Head is better than the numbers indicate.
Given comparable cam lifts and compression ratios "Hemi Performance Engineering" Kits will deliver more torque and more horsepower over a much wider power band than any other brand on the market!
Click on the image to the right to view a flow chart comparing the performance of the "Hemi Performance Engineering" Heads to some of the other products on the market. |
 |
You may have seen other flow charts displaying higher peak numbers. However, some of the charts you see are misleading. Most use 28 inches of water, which is the automotive industry standard of measure. In the Motorcycle industry, the standard rate of measurement is 10 inches of water. We have provided the chart below so accurate flow comparisons can be made between the many cylinder heads now on the market.
EXAMPLE: Our Big Twin head flows 160 cfm at 10" with a ported intake manifold. To convert to 28": simply multiply 160 by 1.669 = 267 cfm. To convert to 22": 160 x 1.446 = 231 cfm. 28" to 10": 267 x .598 = 160cfm.
| Factor .037222 | ||
| 10-10 x 1 = | 10-23 x 1.483 = | 18-10 x .746 = |
| 10-11 x 1.0372= | 10-24 x 1.520 = | 19-10 x .749 = |
| 10-12 x 1.090 = | 10-25 x 1.580 = | 20-10 x .709 = |
| 10-13 x 1.116 = | 10-26 x 1.595 = | 21-10 x .709 = |
| 10-14 x 1.148 = | 10-27 x 1.632 = | 22-10 x .691 = |
| 10-15 x 1.220 = | 10-28 x 1.669 = | 23-10 x .674 = |
| 10-16 x 1.223 = | 11-10 x .964 = | 24-10 x .657 = |
| 10-17 x 1.260 = | 12-10 x .917 = | 25-10 x .633 = |
| 10-18 x 1.340 = | 13-10 x .896 = | 26-10 x .626 = |
| 10-19 x 1.334 = | 14-10 x .871 = | 27-10 x .612 = |
| 10-20 x 1.410 = | 15-10 x .819 = | 28-10 x .598 = |
| 10-21 x 1.409 = | 16-10 x .817 = | |
| 10-22 x 1.446 = | 17-10 x .793 = | |
It has been widely discussed as to why a Harley can only make 1.5 horsepower per cubic inch, when other motorcycles and autos can nearly double that. Let's look at some possibilities. Our ntl. #2 pro stock has 160 cfm of flow per cylinder. A good 4-valve, 4 cylinder Suzuki has 130 cfm of flow per cylinder.
Comparison:
114 cu. in. Sportster vs. 77 cu. in. 4 valve, 4 cylinders Suzuki
H-D = 57 cu. in. per cylinder, or 934 cc's.
Suzuki = 19.25 cu. in. per cylinder or 315 cc's.
H-D = 2" intake. Sq in of area = 2.0 x 2.0 x .7854 = 3.14 sq.in.
160 cfm/3.14 = 51 cu. ft. per sq in.
Suzuki (2) 1.181" intakes 1.181 x .7854 = 1.09 x 2.0 = 2.19 sq.in.
130 cfm/2.19 = 59 cu. ft. per sq in.
At this point the H-D doesn't look too bad!
Let's try another approach. Let's compare circumferential area of the valves.
H-D: 2.0 x 3.1416 = 6.28
Hmmm!!! This actually makes the Harley look better. So where's the truth?
The math above, although accurate, has little to do with reality. First, the area of the valve is diminished by the diameter of the valve stem. Second, the distance around the valve is much more complicated than what can be discussed here. The real problem is as follows: H-D 160 cfm times 2 = 320 cfm feeding 144 cu in Suzuki 130 cfm times 4 = 520 cfm feeding 77 cu in
The Suzuki has a 32% smaller engine with 69% more flow. This is why the Harley cannot rpm, nor come close to equaling the horsepower of more modern power plants. At 57 cu. in. per cylinder, our race engine is almost the same size per cylinder as a 500 cu. in. pro stock car. (62 cu. in. per cylinder) The heads need to be much larger in order to accept larger valves. Because of the bottom dead center (bdc) piston to piston interference, it would be impossible to fit a 4.5 inch bore and still maintain the traditional look of the Harley Davidson. The rods could be made longer, but it is not practical. You could lay the cylinders over to 60 or 70 degrees, but again, why bother. We own Harleys because that's what they are - Harleys!
If you want to make 2 1/2 horsepower per cubic inch, consider buying another brand. But if you love Harleys the way we do, we can help you get the most out of what you love.
While we have tried to include as much information as possible, we realize that you may have additional questions. Please feel free to contact our Technical Service Department. We will always strive to be as thorough and informative as possible. Technical Support
Phone: (708) 656-HEMI (4364)
Fax: (708) 652-0940
Email: info@hemiengineering.com
tech overview / dyno results / hemi heads / |