Even the best of things can stand some improvement ...

Porting the Honda head

By Richard Bean
Next to a whopping increase in displacement, nothing adds horsepower to Honda's multicylinder engine like porting. One of the best of the current crop of super bikes, the "Four" still has room for improvements in the ports and combustion chamber. 
To find out exactly how and where these improvements should be made, we went to Branch Flowmetrics in Long Beach to talk to owner Jerry Branch about the Honda head. Jerry had just started porting a head for a customer and we were able to gather our information while the final tests were being performed.To test the Honda head, Jerry had designed a jig which would support the head over the port on the flow bench and provide a means of opening the valves in precise .050 steps so that the airflow through the ports and combustion chamber could be measured at different valve lifts. The first test is a guideline test to determine the flow of the stock head, intake manifold, and carburetor. The bench uses a standard pressure measurement rated in inches of water with corrected temperature and barometric pressure to insure that the suction applied to the cylinder re- mains constant throughout the test range.

With the valve held in place by lightweight spring, this test jig allows valve to be set at any lift to check airflow though port and head. For accuracy flow bench testing is best.
Air is pulled into the carburetor intake and through the port at varying degrees of valve lift to check the capabilities of the port at any point in the cycle.
The first bit of information gained from the stock configuration test was that the Honda 750 head reaches maximum flow at a valve lift of only .250. At the .250 lift point, air flow has risen from zero to 54.8-CFM (cubic feet per minute). Going all the way to .450 lift only increased the flow to 58.7-CFM, indicating that the stock Honda Four engine requires a cam with a long duration, which will open the valve quickly and hold it open at moderate lift (about .300-inches) for an extended period of time to allow complete filling of the cylinder. High lift is not needed and cannot be used effectively until the port shape is improved to a point where the engine will flow much larger amounts of air at high lifts.

Using this radiused entry on the intake port gives a guide line reading to determine overall flow rate of the stock and modified ports. Finished port had 105.6 CFM frow rate!
Jerry also performed a test without the valve installed, but retained the stock 28mm Keihin carburetor. The airflow increased a very small amount to 60.3-CFM demonstrating that the design of the valve is very good just the way it is and presents no large restriction to airflow. The final test on the stock head consisted of removing both the valve and the carburetion and adapting a venturi to the intake port to sec how much air could be pulled through the port under ideal conditions. The test, called an "entry only" test, gave a figure of 77.3-CFM indicating that while the valve does not present much restriction to airflow, the stock Keihin carburetor does.

Jerry decided to adapt a larger carburetor to the stock head and chose a 32mm Mikuni. The Mikuni is a better flowing carb and is still small enough that four of these 32mm units will fit the engine and frame without much modification. The test showed- that the Mikuni installation was good for an increase of 5.9-CFM at the .250 lift point and would allow a further increase in flow to 65.7-CFM at .350-inch lift before the curve started to flatten out again. This is a substantial improvement for a bolt-on part.

A further test with the valve removed and the 32mm Mikuni gave a reading of 71.7-CFM, not too far down from the entry-only figure of 77.3-CFM. Jerry feels that the 32mm Mikuni would be perfect for racing use, or on large displacement street engines. For a stock cubic-inch Honda Four the 30mm size would work best.

The actual porting of any head is done in a trial and error manner using the experience of the man doing the work as the guideline for what must be done. Even with testing facilities like the flow bench, the porting business remains as much art as science. Jerry's first job was to establish the flow pat- tern of the port so that he could get a clear idea of the areas in which metal must be removed to increase the flow. Porting is not just enlarging the area of the port; such a move will sometimes lower the flow rate and destroy the usefulness of the head.

Most of the shrouding in the combustion chamber was ground away, giving the Honda head a Himi-like appearance. Valve sizes and seat width were left stock. Finished porting job gave a 25.6 increase in airflow.Illustrations at right show areas were metal was removed to alter flow around valve guide and implove velocity.
Jerry uses a tiny probe which can measure vacuum in different areas of the port. By inserting the probe into the port, he can find those areas which have less flow and can draw a graph of the flow deep inside the intake port. The first thing he found was that the majority of the air movement was confined to the floor of the intake port. The valve guide area was severely restricting the flow of air in the upper portion on the port and would have to be reshaped to allow an increase in CFM. Also the height of the guide projecting into the port should be cut down about 114-inch. Reshaping the upper portion of the in- take port caused an increase in the velocity of the flow which led us to the second discovery about the port shape. The curve of the port directly behind the guide is too gradual and forms turbulence, blocking part of the area around the valve head with a disturbed wall of air. Jerry ground a deeper curve, almost a pocket behind the guide and flow shot way up. Just the opposite condition exists on the floor of the port. There the curve is tight and causes the air to try and go around too much of an angle to reach the valve. The radius of the floor was reduced slightly and the sides of the port just ahead of the valve seat were opened up about 1/8-inch to form a mild venturi which would further accelerate the air/fuel mixture into the chamber. After these modifications had been made and the port polished smooth, Jerry removed most of the shrouding in the combustion chamber, making the inside of the head look surprisingly like a Chysler Hemi. The valve scat width was left stock.
Reinstalling the head on the flow bench, Jerry set up an open port (entry only) test to sec what improvements had been made. The results were astonishing! Flow in the open port was now 105.6-CFM, an open port increase of 37 percent!
With the valve and 32mm Mikuni carburetor in place, flow was up from 65.7-CFM at .350-inches lift to 73.3- CFM. The best part was that the flow continued to increase rapidly all the way up to .450 lift, drawing 81.5-CFM. In this configuration, the Honda Four engine can make use of a cam with really high lift, getting heavy charging of the cylinder, even at rpm's above 10,000. On the basis of dyno testing on other small displacement, multicylinder engines, Jerry believes that the required CFM for maximum use of the air/fuel mixture would be in the area of 75-CFM. This means that the modified Honda engine can be twisted to over 10,500 rpm without running out of air! The final figure for the modified port and 32mm Mikuni carburetor installation showed a 26.6-pereent increase in flow, a tremendous gain for the small amount of reworking required.
The final test was with the stock Keihin carburetor on the modified head. Flow was restricted slightly by the 28mm throat on the Keihin, but at .450 lift the engine was still capable of pulling 72.4-CFM over the original 58.7- CFM. Asked to estimate a horsepower increase on the ported engine, Jerry said a 15-pereent increase was in the ball- park!
On the exhaust side of the chamber, very little need be done other than polishing around the guide. This resulted in about a 10-pereent increase in the area of the port just behind the guide. In normal practice, the exhaust should be capable of flowing about 85-90 percent of the intake rate. Jerry recommends that using straight pipes with megaphones, a flow rate of 85 percent be maintained. Since the Honda exhaust port comes very close to this in its stock state, very slight reworking is all that's necessary.
There you have it. If the extra power is wanted, it's available with porting. Those of you running large displacement Honda Four engines at the races would stand to gain the most from porting and polishing the head The results speak for themselves!