modern CBX engine

[cbxtacy]

I've been sitting here, studying and measuring rod's/piston's from the CBX and other bikes (CB1100F/VFR750) because I can dream, can't I. The reason for the CB1100F piston is because you can sleeve a CBX barrel with CB1100F sleeves. And the VFR is the same bore as the CB1100F. The first thing I will cover is rod angularity.

Rod angularity is basically the ratio of rod length divided by srtoke. Rod length is the length center to center. It's usually given as a ratio, and most engine builders recommend between 1.6 to 2.0. 1.6 to 1.8 gives good midrange but is hard on top end, with strong horizontal forces pushuing the piston sideways, accelerated piston wear, and so on. 1.8 to 2.0 gives better top end but the rod's are longer and therefore heavier. Longer rod's also means a taller barrel. I always figured the rod problems a CBX has could be partially attributed to low rod angularity. I was wrong. The CBX has 1.98 rod/stroke. If I had a crank and turned 4mm of the rod journal on the inside, it would increase the stroke by 4mm but the rod would only have to be 2mm shorter. On the other hand I believe new pistons could be made moving the piston pin 2mm closer to the dome with a 2mm cylinder spacer. That way you could use a rod 2mm longer then stock. That would bring the rod angularity down to only 1.85. Not as good for high rpm power as 1.98 but still acceptable. I would really like to do it without the spacer but that would create a weak point-the piston pin.

Rod construction. Piston construction. Overall weight. Moving the pin closer to the dome would essenctially make the piston shorter/lighter. That would help rpm's. Making the rod out of titanium would also trim much weight without sacrificing strength. Two of the considerations for how close you can get the pin to the dome is heat, and enough room for the rings. The CBX runs hot but with oil's not available when the X was developed and advances in metallurgy it would be easy.

So, things are now getting pretty expen$ive, aren't they? I'll save you some money-sleeve it with LASLEEVE CB1100F sleeves. 70mm bore. Design some pistons with a 70.5mm bore and you have a 1345cc CBX.

So what's going to happen when you rev the crap out of it? Valve float and lousy top end power. Shim under bucket (pain in the youknowhat), titanium retainers, bigger valves, regrind cam. Now you get the rpm's but no head-snapping top end. Blame it on the ports/carburation. Build a decent/adjustable ignition/fuel injection system and the rest of your problems are solved.

Now go win the lottery and remember me.

[Rick Pope]

Or, for the cost of sleeving the six holes, and buying a set of pistons, you can leave your CBX stock, and buy a faster, and reliable, motorcycle.

[cbxtacy]

like a new CB1100R or a new Norton ?

[Terry]

This brings to mind a question I've always had about the (mighty) six, and that is, why isn't it so mighty? I mean, why doesn't it make more HP than it does. After all, it is a six cylinder engine. I know its air cooled but its also a liter sized motor. So what gives? How did the inline fours pass it by in terms of power output?

[alimey4u2]

More cylinders give you greater valve area & lighter (faster spinning, easier to control) components. From the Italians, "one" way to make horsepower is work done per unit time. Make an engine rev like the clappers over that unit of time....
In a race motor you have little limitations for ergonomics but on a street bike rider comfort is a demand, hence the serpentine inlet tracts which negate the horsepower benefits of a 6 cylinder engine. You also now have a combustion chamber area which is larger & has a need for greater cooling to become efficient. Just a few of the reasons why a street motorcycle 6 cylinder engine really isn't more powerful than a 4 or 3 or indeed a twin.... As an engine, yes but in a street motorcycle, no...

[EMS]

The CBX has six cylinders, but it is not that big of an engine. It has 1047cc the engineers could use to make torque and horsepower with. The CB900F which was introduced at the same time as the CBX in Europe had 845cc and made 95 hp - 10 less than the CBX. So the CBX WAS mighty at the time. The first four cylinder that made more hp than the CBX was Honda's 1981 CB1100R with 115hp out of 1062cc and later 120hp.
Kawasaki's KZ1300 six cylinder had 1286cc and made 120hp with 3 double barrel carbs and 130 hp with fuel injection.
The compression ratio, a measure to produce mean effective combustion pressure (creating the downforce on the pistons) was also relatively low in the CBX with 9.3:1. The KZ1300 had a 9.9:1 ratio.
The new BMW 1300cc 4 cyl motor has a 13:1 compression ratio.

[Terry]

It just seems to me that with a little head massaging, a rise in compression and maybe a larger bore and some hotter cams, it should be able to make more power and still maintain some reliability.

[alimey4u2]

It just seems to me that with a little head massaging, a rise in compression and maybe a larger bore and some hotter cams, it should be able to make more power and still maintain some reliability.

Worked for me .....

[Mike Nixon]

as Irimajiri pointed it out long ago, but the main problem with the CBX is the valve angle, 63 deg included, due to the need for airflow. This produced a nearly hemi (read: unecessarily voluminous) combustion chamber, which hurt combustion, producing more heat for each lb of torque, and hp is a product of torque, so hp is limited also. Add to this basic problem the shear number of plain bearings... All kinds of efforts to enhance normally aspirated power meet with frustration, ultimately due to the valve angle. A look at a modern Asian engine will show something else. Note the cam holders. Attempts to put more lift into the equation flexes the stock cam holders. A solution would be to somehow tie them together, making them more like a modern bike's, i.e more rigid. If the caps don't flex and cause the cam to snap, the cam chain fails due to the stronger valve springs necessary. Can't hardly win. :) A good example of what might happen if the valve angle were tightened up is found in the early GSXr engine. Suzuki took an air-cooled design and gave it essentially liquid cooled valve angle, and compensated by bathing the combustion chambers in oil. This, if done to the CBX engine, would be the fastest way to increased normally aspirated power.

[cbxtacy]

One of the allure's of the CBX is the sound. I could take a recording of a CBX with a 6 into 6 exhaust. Set it up on a continuous loop. Get a BMW K1300RS (175+hp? should make me happy) and set the tape to play through an amplifier on it. Use a variable playback speed running off the tachometer. Volume control off the throttle setting. Now I bet that would sound bitchen and be a lot less expen$ive. When the neighbors complain, I'll toss the speakers and run the sound straight to headphones in my helmet

[silversurfer1050]

...parts of the parts of the first part meeting part of the story of the second part....

viewtopic.php?p=31555#31555

[Larry Zimmer]

Thanks for the good stuff Mike. It's all about combustion chamber design which is all about valve angle as required to get enough cooling. That's where Suzuki and BMW, and a few others have gone with their oil cooling of the piston dome, as Mike said. Certainly helps. BUT, there's nothing like water cooled to give better valve angles that give the necessary combustion chamber design for faster burning with its shorter ignition timing.

[EMS]

BUT, there's nothing like water cooled to give better valve angles that give the necessary combustion chamber design for faster burning with its shorter ignition timing.

So all the air-cooled Porsche engines that were dominating sports-car racing for decades were sort of second rate?
The only reason Porsche went to liquid cooled heads was emission requirements.

[bdento59]

Or, for the cost of sleeving the six holes, and buying a set of pistons, you can leave your CBX stock, and buy a faster, and reliable, motorcycle.

Jeez, what fun would that be...

[bdento59]

If I had a crank and turned 4mm of the rod journal on the inside, it would increase the stroke by 4mm but the rod would only have to be 2mm shorter.

George,

Turning down the journals will not increase the stroke of the engine, nor will it change the C2C length of the conrods. The stroke is determined by the distance between the centerline of the crank and the centerline of the journal. In the CBX engine with a 53.4mm stroke, that C2C distance is 26.7mm. After milling the journal down by 4mm, the C2C distance is still the same, thus the stroke has not changed. Unless you want to change the height of the engine, the only two ways you can increase the rod length is, 1) with custom pistons that have their pins up closer to the bottom of the crowns, or 2) by destroking the crank.