[Bullet Whisperer]

Here is a short video I decided to make while I had the opportunity, it might be of interest to some who run the separate gearbox 350 and 500 Bullets.
 B.W.

https://youtu.be/GsxnrzWtTj0

[AzCal Retred]

Nicely done - the 350's 70mm bore must be more tolerant of advance?

[Bullet Whisperer]

Nicely done - the 350's 70mm bore must be more tolerant of advance?

Thanks!
 It would seem so. That is the first time a figure of a full half inch BTDC for full advance timing for a 350 caught my eye and it was not long ago when I backed off the timing of a Redditch 350, thinking it was over advanced in that region, but it lost it's 'edge' and I advanced it again and the performance returned. It is easy to assume 8 mm [8.8 mm through the plug hole] BTDC is a blanket figure these days, but that is purely for the 500 cc machines.
 B.W.

[ace.cafe]

Nicely done - the 350's 70mm bore must be more tolerant of advance?

Shorter flame path.

[Bullet Whisperer]

Shorter flame path.

Although the 350's have a shorter flame path, this should make the mixture in the combustion chamber light and burn more quickly than for a 500 with a larger bore, suggesting the differences in timing advance might work better the other way around, but I am sure RE knew what they were doing. I haven't gone into the reasoning too deeply, but size for size, multi cylinder engines can run higher compression ratios than singles for similar reasons, connected with combustion chamber and piston crown areas. This is also why a twin plugged set up can run slightly later ignition timing to a single plug set up.
 Throwing another thing into the mix, I also am a big Triumph Tiger Cub fan and know the high compression 'Sports Cubs' ran more ignition advance than their lower compression counterparts, which again, might seem to contradict logical thinking.
 B.W.

[Paul W]

Over about fifty years I’ve experimented with timing on a variety of engines that I’ve owned. My preferred technique is to gradually advance the “static” setting from that normally recommended until the engine just pinks, then to back it off again until it doesn’t.

This has made me realise that manufacturers are often quite conservative in their published figures.

For example, one car manufacturer recommended TDC on their 850cc four cylinder engines. I had mine remanufactured and then modified and tuned it, fitted twin carbs and a performance cam etc. Due to an error at the engine shop, it had a very  high compression ratio (13 to 1). It ran best with 17 degrees BTDC, using super unleaded fuel. I  backed it off to 14 for peace of mind.

I have a 993cc, three cylinder engined Suzuki Swift (Geo Metro to our friends in USA), which I use to commute to and from work (45 miles each way). The “book” static setting is 5 degrees BTDC. I tweaked mine to 15 BTDC and it runs much better.

Annoyingly, my iron barrel, 350 Bullet Electra has no facility to alter the ignition advance, or even to check what it is.

[ace.cafe]

Typically we react to pinging as our detonation indicator, and pinging is our indicator of too much compression or too much advance.

Detonation is an end-gas phenomenon, meaning that pockets of mixture spontaneously "explode" violently well after the spark plug fires because these end gases are superheated by the normal combustion activity and light off later. It happens generally in a hemi type chamber because the large combustion chamber has places too far from the spark to burn quickly and completely  before the end gases detonate from heat.

A  more compact combustion chamber has less risk from end gas detonation, so it can often withstand more compression and/or timing before pinging becomes a problem. It can burn the mixture in a controlled way before end gas can become as much of a problem.

It is because of this that I mentioned the shorter flame path in the more compact chamber size of the 350 vs the larger 500. The 500 has a large enough bore and chamber size to be more challenged in the detonation matter, so that it can't take as much advance as the 350 because of that.

Here is a link to an informative article on the topic.
http://www.contactmagazine.com/Issue54/EngineBasics.html

[Paul W]

My job used to be teaching engine theory to military student pilots, as well as teaching them how to fly. We were flying aircraft with Lycoming flat four engines which have quite large cylinders by today’s motor standards; well over a litre each.

They were fitted with constant speed propellers. The one thing we had to emphasise to the students was how to avoid detonation because the engines run at a maximum of 2750 rpm but cruised at only 2400 or even as low as 2100 for maximum economy and it was relatively easy to cause detonation by opening the throttle too far before increasing the rpm again using the propellor control, even though the engines were normally aspirated.

The propellor noise and that from the airflow masked the “knocking” sound that would normally alert a bike rider or car driver.