CBX Racing

[Warwick Biggs]

Sign of the times. Here we have a 68 year old wounded racer helping a 74 year old on his 300 Supersport after he was described by the announcer as "the cork in the bottle" holding up some kids during the race.

How rude. Rob actually spannered for 'Mike the Bike' Hailwood back in the middle of the last century as an Ozzie in Europe. That was b4 the 747 revolutionised travel allowing almost anybody to go almost anywhere and thus, facilitating pandemics.

[Warwick Biggs]

Here is Rob ...

[Warwick Biggs]

There are many advantages to basic tuning of a CBX which was sadly relatively undeveloped by Honda and it is interesting to compare it with a modern sports bike.

I have been tidying up all my race/track bikes and recently cut the catalytic converter off my 09' GSXR750 saving 4.2 kg in the process. Just basic re-tuning to compensate using Woolich racing software and re-flashing the bike's ecu produced a 10 hp increase across the range bumping peak rear wheel power from a stock 119hp to 129hp. See below:-

[Warwick Biggs]

It is a similar story with a stock '82 Prolink CBX. A simple port and polish, 2 mm larger valves and a mild grinding of the foot of the cam lobes increased power from a very healthy stock 89hp to 104hp.

[Warwick Biggs]

But that then shows up the restrictive nature of the CV carbs, not to mention that dreadful vacuum lag when you crack the throttle. So if you bore out the CV's to match the valves this is what happens:-

[Warwick Biggs]

So we have gone from a stock CBX peak power in the 80's to 115hp. A 30% increase just by letting it breathe. Still doesn't fix the throttle lag but the conversion to 31 mm CR Special carbs fixes that and with some careful jetting, gets us up around 120 rear wheel hp. That is not far off a modern superbike. With a more modern head design a lot more power could be released without even thinking about increasing compression or straining the basics.

It is such a pity Honda was not able to persist a bit longer with the CBX but the above shows what can be easily achieved by an owner with some simple tuning. I hasten to add that this is not a hotrod conversion that would require a stronger clutch or strengthening of other components. I know Roly has speculated on what might be achieved with a more modern head design but we are probably unlikely to ever know. Unless there is a CBX enthusiast with a practical interest in thermodynamics.

(I should have noted that the original 89hp was slightly up on a more typical 85hp because of a first overbore slightly increasing the cubic capacity)

[EMS]

I don't think it is that easy to dismiss Honda of not doing a job properly. After all, Honda made and still makes the BEST small engine, IMO.
We all know why the head is designed like it is and what causes the shortfalls of the intakes
There may have been a multitude of reasons why the CBX was not tuned further. After all, at the same time, Honda had a 4-cylnder inline with essentially the same displacement putting out 120hp....

[Warwick Biggs]

I will display my ignorance again EMS. Why was it designed that way? Because I don't know. You seem to be inferring that it was a conscious choice made by the designer and that Honda had the expertise to adopt a better breathing engine. Was it a packaging issue?

The point that I was making was that the engine is capable of so much more with basic tuning but it does have design limitations. For example a 2 valve Suzuki engine of the same era can be tuned to make more power than the 4 valve Honda engine.

To take that a step further it seems that the relatively short production run of the CBX of only 4 years did not allow for the sort of development that occurred with the 4 cylinder motors, for example. My understanding of the history was that the production costs and economic cycle conspired against Honda and therefore the 6 cylinder project was curtailed; that it was a bit of an economic disaster for the coy.

I would be interested in hearing more about this and in particular, whether anybody has taken the design any further.

[EMS]

O.K., this is not news for most.
Honda angled the outside cylinder intakes to the inside (which created the V-shaped carburetor bank and the uneven length of intake runners) to allow more space for the rider's knees. This has been tried by a couple of people to remedy by machining and re-shaping the intake stubs on the head and making a straight bank of carbs fit. More or less successful.
The Honda CBX inline 6 was not a stand-alone development. It is largely based on the DOHC 4 cylinders and was conceived just after design of the DOHC 750 and 900 had been started. The cam-in-head design is basically the same and the CB900F of 1979 and the CBX share the same bore size.
The CBX was an "afterthought" after Honda learned during the development of the DOHC 4 that their competition was working on a large in-line six and they had to up the ante because the CB900F would have been outsized from the start. The inline 4 was pushed back slightly and the inline 6 was rushed ahead. The fact is, Honda began working on a complete new concept in 1980 which was supposed to make the in-line obsolete: The V4 which came out in 1983 and replaced the inline 4 Superbike in 1984. And there was a "flat 4" in the big Tourer already. That's why the work on the inline 4 was not continued. Only when the V4 did not show the success Honda had hoped for the in-line 4 was brought back in 1987 with the "Hurricane" and continued from there on to today. While the V4 maintained its superior role on the track in various forms of RCs.

[Rick Pope]

And then there's cooling, or lack of it.

[Warwick Biggs]

Aha! I did not know that. A few of my more technically minded club mates have suggested "fixing" the intakes after I discovered splits in the bottom of the CR Special carb rubbers and they inspected the highly irregular rubbers. Running your finger inside the outer rubbers in particular, you can feel significant ridges or steps that would not assist even gas flow.

I have discussed with Roly the desirability of fitting the V shaped CR Specials that were apparently made in Japan but the only set I have seen in Oz were 35 mm and too big for my motor.

I would be very interested to read any further details on how the head was re-machined. My mates suggested making up an alloy inlet manifold and using straight rubbers to accommodate my straight line CR's. However V shaped 31 mm CR's would probably be a better start, if I could find them.

As for the cooling I can only increase oil capacity and radiator size to maintain 'the look' under our rules but even a water jacket may not be the final solution. A few years ago a fellow from Sydney turned up at the Island Classic with a liquid cooled Kawasaki 6 racer and I thought 'this should be fun, let's see which is quicker around Phillip Island'. But 2 years in a row he could not get it to stop seriously overheating and I think he eventually abandoned the project, which was a pity.

[Warwick Biggs]

This is Roly's idea to stop me knocking the velocity stacks off the outside carbies with my knees but I have to say it does not make hanging off very comfortable as the outside knee presses against the wire protector. The straight CR's are problematic in every way.

[Warwick Biggs]

Here:-

[Warwick Biggs]

Hmm, can't post pic for some reason. Not to worry. The CR specials are such a pain that they won't even pose for a pic!

[Warwick Biggs]

How time flies is a cliche` that resonates in the current environment. It is nearly a year since I had the lump out for testing and it was woeful; missing, backfiring and generally farnarcling about. Here is a pic from that session.

Since then we have rectified those problems and we could have it out again soon. In fact both my historic race bikes are fit for testing altho' because of my hand we have been somewhat restricted to having some fun with the GSXR750. Weather permitting we should be out again in a week or so but again on the Suzuki. Its' light 'n easy. Last time I could only manage a dozen or so laps. Hopefully, we will improve on that and there will be an officials course afterwards.