Squish band?

I suggest that when you take four-stroke motors to extremes, you end up with the same problems which occur with two strokes. If detonation is going to occur due to lean mixture or too much advance, it is more likely to occur in the squish band area . I have been looking at the programmable Dynatek ignition system used on some Harleys. There is one wire which is usually left disconnected. It is intended to be connected to a vacuum switch so that the advance curve is less aggressive when the motor is running at full throttle with less vacuum. It could easily be attached to a throttle position sensor to stop detonation under similar circumstances.

As Dances has pointed out, a stock Commando has no squish effect, and still seems to run pretty well. I think Smokey Yunick is the one who did testing that showed once you get up to .050" or so clearance, you don't get a squish effect, and you start getting full combustion of the end gases. In setting up Commando engines for racing, the only issue with squish is making sure you don't get too little, and start hammering the head with the piston. The benefit of having a squish effect is to add turbulence to the fuel-air mixture, and the Commando head design does a pretty good job at that just from the swirl effect of the offset intake port design. At high rpm in a Norton motor, you have so much turbulence already that there's no real benefit from the squish effect. It might help a little at low rpm in terms of less emissions, but I'm not even sure it does that. You might recall that the short stroke 750 engines had a full hemi chamber and no squish band, and made more horsepower than the standard 750.

I think the concern about detonation is kind of a red herring. As long as you have the timing advance set properly, and have jetted to just slightly rich of optimum power, you should have no problems with detonation on a Commando race bike. Modern race fuels are available in very high octane ratings. It's really difficult to get the compression ratio in a Commando 750 or 850 up high enough that it will detonate on decent race gas. Detonation is much more likely to happen in a street bike with a borderline CR and crappy fuel. I've never had a problem with detonation in Commando race engines, either road racing or landspeed racing. If you're getting detonation, the solution is to get your tuning right to start with.

I can't say much about the issue with Triumph engines. The have a much less efficient combustion chamber design than Commandos, and might have much different behavior.

Ken

Ken

There was an article in Which Bike magazine in, I think, 1977 / 78 about an 850 belonging to NVT tester Bob Manns. He picked a head & had the machine shop leave out the machining op. that produces the 0.120" counterbore, leaving a full hemisphere. He found that it not only ran better but did more miles to the gallon also. This was just one of the changes he tried to get into production but sadly his ideas fell on deaf (daft) ears.
Unfortunately I can't locate the magazine at the moment but the article was called the Commando that never was. Anyone remember this?

Of the triumph piston I have seen an certainly the higher C/R ones they have a domed crown which is bad for the progression of the flame front through the combustion chamber. Norton ones that i have seen as a rule are the flat type so surely that design difference has more effect than squish band differences.

If you operate any race motor near it's limit of comp.ratio , lean mixture and ignition advance, you are in the realm where you can get detonation. This generally applies to top end motors designed to give high horsepower at high revs, so probably does not apply to many Commando motors when an improvement in mid-range power is more important. In the extreme, what applies with two-strokes motors also applies to four-strokes. I've never road-raced a bike which had the motor on 14 to 1 comp. However I know people who have done this stuff and what happens is sometimes not what you might expect. 12 to 1 comp. with methanol fuel can give problems if you run it lean enough, and you have to lean off to get best power even with methanol. Common practice when using ultra-high comp. is to retard the ignition beyond the timing used for petrol. So whether any real gains are made by going that way are a bit doubtful.

Are the bathtub chambers an exaggerated squish (quench) or is there some property they provide? Hobot would likely discuss Singh grooves here as well. I'm a novice in these matters but enjoy learning.
Lance

Can we be a bit clearer on terminology. Unless I'm mistaken Detonation should refer to spontaneous detonation and is thus not related to the moment of spark and thus not related to ignition timing, advanced or otherwise.

Clanger said:

Can we be a bit clearer on terminology. Unless I'm mistaken Detonation should refer to spontaneous detonation and is thus not related to the moment of spark and thus not related to ignition timing, advanced or otherwise.

Click to expand...

Detonation [pinging] is the explosive combustion of the end gasses after the spark has occurred and the mixture is partially burned and can easily be caused by advanced spark timing or poor fuel grade [octane too low for the CR].

Detonation can lead to spontaneous combustion.

Spontaneous combustion or combustion before the spark is referred to as pre-ignition. This can be caused by a glowing spark plug electrode or the edge of a gasket or valve. The overheated plug, gasket or valve can be a result of detonation, or mixture or timing problems or simply too hot a plug for the application.

Ah thanks , looks like I was back to front with pre-ignition and detonation terms. Previous comments all make sense now!

Matchless said:

There was an article in Which Bike magazine in, I think, 1977 / 78 about an 850 belonging to NVT tester Bob Manns. He picked a head & had the machine shop leave out the machining op. that produces the 0.120" counterbore, leaving a full hemisphere. He found that it not only ran better but did more miles to the gallon also. This was just one of the changes he tried to get into production but sadly his ideas fell on deaf (daft) ears.
Unfortunately I can't locate the magazine at the moment but the article was called the Commando that never was. Anyone remember this?

Click to expand...

This is an interesting slant on this, with distinct possibilities.
(Haven't seen the article in question.)

Someone might be able to take a not fully machined FullAuto head and try it ?
Expect howls of derision at this suggestion ??

That has already been done on a recent short stroke build.

And the winner is? Was it a good idea for this short stroke engine?

Rohan said:

Matchless said:

There was an article in Which Bike magazine in, I think, 1977 / 78 about an 850 belonging to NVT tester Bob Manns. He picked a head & had the machine shop leave out the machining op. that produces the 0.120" counterbore, leaving a full hemisphere. He found that it not only ran better but did more miles to the gallon also. This was just one of the changes he tried to get into production but sadly his ideas fell on deaf (daft) ears.
Unfortunately I can't locate the magazine at the moment but the article was called the Commando that never was. Anyone remember this?

Click to expand...

This is an interesting slant on this, with distinct possibilities.
(Haven't seen the article in question.)

Someone might be able to take a not fully machined FullAuto head and try it ?
Expect howls of derision at this suggestion ??

Click to expand...

Not a silly suggestion. I can have the machining for the step made shallower or leave it out altogether leaving a hemi head. Might do some research and try one or two variations in the next batch.

NKN said:

And the winner is? Was it a good idea for this short stroke engine?

Click to expand...

See Herb Becker's build of Doug McRae's 750 Short Stroke. There's a thread somewhere here on the forum which presents data and discussion. A FullAuto head (with incomplete machining) was used by Herb.

comnoz summed it up nicely above.

Having worked with explosives throughout my career, I appreciate the considerable distinctions between Deflagration (where our IC engines really need to be) and Detonation. As an example, black powder deflagrates whereas nitro glycerin, C4, PETN and other high explosives detonate. PETN has a velocity of detonation of around 22,000 fps whereas black powder is probably closer to something like 1,000 fps to 3,000 fps. Rather than "quote the wheel" here, I have copied a link on the subject which I feel provides an introductory description.

https://en.wikipedia.org/wiki/Deflagration_to_detonation_transition

Apparently, both detonation and deflagration are categorized as combustion according to the above link. I always assumed the stoichiometry and chemical end products were the same. I suspect the article discusses overpressures resulting from an air/fuel mixture starting at or near atmospheric pressure. After reading the article, think about the opportunities in designing and building an IC engine that operates on detonation alone. Near zero ignition advance required so near all combustion pressure as after TDC! Pistons and combustion chamber components fabricated from unobtanium.

At the risk of getting too far off topic: Anyone on the forum who has direct experience with Nitro Methane interested in chiming in on how Nitro Methane behaves in an IC engine. Is the combustion subsonic or supersonic?

As I understand it nitromethane improves the torque characteristic of the motor, so the combustion rate is probably still subsonic unless the comp. ratio is too high. What the nitro does is provide more oxidising agent, so you use more fuel - thus more power. I have also done a lot of work with explosives - when detonation occurs the combustion rate occurs at the speed of sound. In one of my jobs, I was involved in closed vessel testing of gun propellant. Octane rating of motor fuels is similar to the term 'brisance' which is the combustion rate of propellant divided by the pressure rise time - an acceleration.