A mate who works for a Plant Hire company lent me a stationary Honda side valve cylinder head. I'm intrigued as to what Honda hoped to achieve with the design over and above BSA's efforts..

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Looks like the've gone for a swirl effect. Inducing a swirl is an attempt to get better combustion. I wouldn't try it on an M20 unless you also wanted a "stationary" engine.

You can see from the 'shadow' of the cylinder that this head has a very high degree of 'squish' area over the piston...

Typically a squish clearance of .040" (1mm) or less will force combustible gas out of that area and concentrate it around the valves..

This has the effect of reducing the size of the combustion chamber, thus reducing the distance the flame front has to travel and eliminating some of the irregular shape of the combustion chamber where gas can be trapped..

The purpose is to reduce the tendency towards pre ignition and to raise the effective compression ratio by compressing the gas into a smaller space...

Honda also appear to have moved the spark plug to what, these days, is considered a more favourable position...

The shaping of the combustion chamber roof is also interesting, though not entirely clear from the photos..

The key feature that requires careful attention at the design stage is the area over the piston that does not form part of the squish and the roof of the combustion chamber above it and the inlet valve.

This area forms the 'port' that allows the explosion of the fuel/air mixture to drive the piston down the bore so care must be taken not to restrict gas flow at this point...

I have previously studied the Honda patents that apply to this particular head design (though it is the nature of such patents they are both a little vague and misleading) and it has merit over the M20 design...but you would expect that after all this time...

On the patent drawings I found the shape of the roof of the combustion chamber to be the most interesting feature along with the position of the spark plug which is carefully positioned within that space, or shape...

All the principles, though not the exact details, were laid down by Harry Ricardo in the 1920s and you can see this in the squish pattern present in the M20 head which is the same in principle as the Honda example...Ian

email (option): ian@wright52.plus.com

Looking at the chamber shape, Honda appear to have tried to give their head a 'heart'...but they'll never manage to give an engine 'soul'....

Thanks for your thoughts Ian. I have also been told that the position of the spark plug ground electrode may be important. So some people try plugs in the head off the barrel to find one where the spark is open to the combustion chamber and not masked by the ground electrode.

Hi all,

I found some general info regarding sidevalves barrel heads.
It talks about "Single Cylinder Kohler Engine Cylinder Heads"
But I think the facts may also be valid for our sidevalve engines..

Interesting to see the development in shape, and the explanation,
Such as
"The newest style head manufactured by Kohler is much like the previous one, but has the spark plug located directly over the exhaust valve. There are two reasons why Kohler did this:
• It prevents cold-starting incoming raw fuel from fouling the spark plug.
• It helps to produce more engine power and torque by retaining the majority of the combustion heat in one area."

http://gardentractorpullingtips.com/cylhead.htm

Noam.

email (option): noam10@gmail.com

That's an interesting article Noam....Ian

email (option): ian@wright52.plus.com

Sidevalve technology is strange and seems to go against everything you know about tuning an engine, great stuff

Does this mean you're going to redesign your sprinter head now Ian..?

email (option): horror@blueyonder.co.uk

I certainly am..I've already pretty much finalised the shape and layout of the combustion chamber...

Currently I'm completing mods to the cylinder...When that is done the whole engine, apart from the head will be ready to assemble..

I'm treating the head as a separate project and will probably get 3 or 4 cast without the chamber so that I can make further versions if I need to...

Regarding tuning sidevalves you are absolutely correct...You have to more or less forget most things you may have learnt from OHV designs...

Key points are gas flow inside the head and the conflict between gas flow and compression ratio..but there are many other differences between the two types...

Also, there is a very large body of information to tap into when tuning OHV engines...This is not the case with SV engines where much less development work has been done and much of that was done a long time ago...Ian

email (option): ian@wright52.plus.com

Maybe we should change our bikes to OHV ! hahaha

No, serious guys; once the right shape of comb chamber is figured out, maybe new heads can be made like that, as they are hard to find already
I wouldnt mind a bit extra power without changing the looks of the bike

It looks simple Mike, you fill the combustion chamber with weld, then drill a sparkplug hole over the exhaust valve = extra 10bhp

email (option): horror@blueyonder.co.uk

What could possibly go wrong?... ...Ian

email (option): ian@wright52.plus.com

I may be wrong, but it seems like the last (improved) Kohler head is pretty much like an original m20 !
The BSA factory wasn't that much off