Speed Equipment: Taking a look at the Honda Four's Power Potential

Here is another vintage article from Richard Bean. I got this one from the SOHC4.net site and I do not know where the information originally derives from.

Mr. Beans words.

If the big Harley twins can be likened to the Chrysler engine which has dominated automobile drag racing for a decade, then the Honda 750 engine must represent the Chevy V8 of the motorcycle world. With its ability to rev clear out of sight and stay together, it cannot be overlooked as a potential winner in any form of competition. Its appeal as a street engine is as large as its hold over certain classes in both drag and road racing.

The purpose of this article is not to follow the step-by-step construction of any one engine, but to report on the state of the art and inform the engine builder as to what is available in the way of hop-up parts and techniques.

The basic engine is by itself a fine powerplant, capable of giving a good account of itself on the street. It develops considerable power and, for an engine that needs rpm to produce that power, is fairly tractable on the street even in heavy traffic. In its stock form the Honda 750 is able to turn the quarter mile in the high 12's which definitely puts it in the superbike category.

Several riders have written us complaining of the fact that the Honda is a pipey type of bike which comes on quickly at the upper limits of its rpm range, but we haven't found this to be the case. To be sure, the Honda engine doesn't develop much torque at low rpm's, like the Sportster or 74, but it is mostly a matter of getting used to holding the revs up. Another thing is that the engine exhibits very little flywheel effect and revs fall off quickly in between shifts. Again, this is a matter of getting used to the engine and carrying the shift point a little past the point of maximum torque.

For the rider who wants to get additional power out of the engine, two things are immediately apparent. One is that the stock carburetion isn't worth a damn for real high performance operation, the other is that the engine can really benefit from better exhaust.

The 28mm Keihin carb used on the stock Honda 750 is a good, reliable carburetor for a stock engine, but if internal changes are made to the engine to increase the power, the restricted airflow through the carb just about cancel out any gain. In the December issue of HOT BIKE, we ran a story about porting the head on the Honda 750. In the story we pointed out how flow bench testing showed that the stock Keihin restricted the airflow through the intake port to the point where installing a better cam or modifying the port would be almost useless. Honda Four owners are lucky, however, because there are a number of different carburetion setups which can solve the problem.

Probably the best from the standpoint of cost and ease of installation is the Mikuni. In addition to being a better flowing carburetor, the Mikuni offers a substantial number of jets, airbleed correctors, and other tuning parts to get maximum performance. The testing we did on the Honda head used four 32mm units and we feel they are about the right size for racing use, with the smaller 30mm size for street operation. Another good setup which we have seen on several bikes is the constant velocity carbs from the late Honda 450. These can often be found in wrecking yards and will do a good job. These are also a 32mm unit and need little modification to fit.

For the rider interested in maximum performance without worrying too much about the cost, Russ Collins at R.C. Competition Engineering has started production of a manifold which adapts a pair of DCOE -40 Webers to the 750. Russ also has a single Weber manifold for the Honda which is a log type in the works and we should be hearing more about it before too long. Jerry Magnuson, who designed one of the best manifolds on the market for the Weber to Sportster conversion is rumored to be working on a manifold for the Honda, so there is no scarcity of carburetion for the 750. For the all out racing effort, Yoshimura Competition in Waterford, California, sells matched sets of the 31mm GP carbs used on the Honda road racers, and Fuel Injection Engineering is putting the Hilborn injector designed for the Honda LSR bike (see page 52 of this issue), on the market for the drag racer.

Another place where the Honda has a lot going for it is in the area of exhaust systems. A number of manufacturers jumped on the bandwagon early in the game, and there are several good systems available over the counter. Because the Honda 750 is a four cylinder engine, a lot of hard earned design work done by the automotive exhaust system builders can be applied to the Honda in the form of collectors. Basically, the collector uses the pulse from an adjacent cylinder to lower the pressure in side the exhaust, creating a slight scavenging effect. By arranging the exhaust pipes inside the collector, additional effectiveness is gained at certain rpm levels.

The two companies which have the most experience with the collector header on the Honda 750 are Action 4's in Santa Ana, California and R.C. Engineering in Torrance. Both of these companies are active in competition at the dragstrip (against each other), and make a variety of custom exhausts for the 750. Action 4's newest collector header for the Four is a 180-degree tuned system which is just becoming available. Another recent entry into the Honda header market is Jardine Headers, well known for automotive competition exhausts.

In the non-collector field these same companies market make a variety of special exhaust systems for the Honda Four, drag pipes with and without megaphones, and street systems. Yoshimura Competition also offers racing headers with megs for road racing. These pipes tuck in under the engine for maximum cornering ability.

Next, in order of importance, is better internal breathing for the engine. If you add better carburetion and exhaust, probably the first thing that occurs to most builders is to install a better cam. This can be a losing battle in the unmodified Honda engine, because of the fact that the stock head suffers from restricted breathing. Most high performance cams rely on increased lift to get more air/fuel mixture into the cylinders, and this is something that the Honda engine can't make good use of until it has been modified. Our testing on the Honda head pointed out the fact that airflow through the intake reaches a peak at about the .300 lift point, and further increases in lift do not help. If you intend to install a hotter cam in the otherwise stock engine, get a cam which uses somewhat more duration and lifts to only about .350-inch. This is easier on the valve train, as forces increase dramatically with increased lift, and reliability will be increased.

Currently the best cam for competition seems to be the Kenny Harman F grind. This is an all out racing cam and several alterations to the engine, including cutting deeper valve pockets in the pistons are necessary with this cam. Action 4's offers this cam as part of their line and their custom racing pistons are clearanced to handle the extra lift.

If the head is ported and polished, lifts of .400 and above can be used. Valve shape on the Four is excellent and no modifications are necessary. In addition to opening and reshaping the intake port, the engine will gain considerable power by grinding away much of the shrouding around the spark plug tip in the combustion chamber. When finished, it gives the Honda a hemi-like look and really adds to the engine's horsepower. One word of advice that we should offer is don't do the job yourself unless you have quite a bit of experience in porting. This is one job that's best left to the shops that are equipped to handle it.

Increasing the displacement of the Honda is a sure route to more power. There are several kits on the market to enlarge the cylinders of the Four to as large as 915cc's! The popular sizes are 785cc, 811cc, and 836cc. These big bore kits are not too expensive and require only a minor amount of machining to install. Th two major suppliers of the big bore kits for the Honda 750 are Action 4's and Powroll, who supply all sizes and have a complete line of custom made and modified parts to equip the kits.

Several of the speed equipment suppliers are working on the problem of providing better rods for the 750 and we hear rumors that an entirely new forged rod may be available this year from a southern California designer. At the moment, Action 4's offers a carefully lightened and polished rod as part of their engine packages, and Russ Collins at R.C. Competition Engineering sells a heat treated and shot peened version.

Electrical systems for the Honda are limited to two magneto units, one by R.C. Competition, the other by Yoshimura. Later this year we expect the Joe Hunt Magneto for the 750 to become available. For most street applications, the stock ignition system works well, the magneto being necessary only for competition.

The Honda 750 represents a real challenge to the rest of the big engine field in motorcycle drag racing. It is tough, makes a lot of power for its size, and has the added advantage of having more speed equipment available for it than any other engine except the big Harleys. We've included a list of the major speed equipment suppliers for the Honda to assist you in getting the parts you need to turn your Honda Four into a real screamer.

Action 4's	2621 S. Main Street	Santa Ana, California 92707
Yoshimura Competition	P.O. Box 267	Waterford, California 95836
Powroll Performance	P.O. Box 926	Bend, Oregon 97701
R.C. Competition Engineering	2920 Sepulveda Blvd.	Torrance, California 90505
Branch Flowmetrics	1637 E. Burnett	Long Beach, California
Hunt Magnetos	2600 W. Vernon Avenue	Los Angeles, California
Mikuni K. American Corp.	7923 Gloria Avenue	Van Nuys, California 91406
K-H Cams	2163 S. Hathaway Street	Santa Ana, California 91705

Porting the Honda Head

Back in the December 1971 issue of Hot Bike Magazine, Richard Bean wrote a detailed article about the power potential hidden in the CB750 cylinder head. His article describes the research and experimentation performed by Jerry Branch of Branch Flowmetrics.

The Following is Mr. Beans words.



Next to a whopping increase in displacement, nothing adds horsepower to Honda's multi-cylinder 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 flow 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 pattern 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 Hemi-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 improve 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, multi-cylinder 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-percent 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-percent 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-percent 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!