Dual spark plugs or squish?

[Truckedup]

I am not convinced it will add power compared to squish for these engines....that's why I started the thread...
My bike hasn't been raced for several years...The engine needs some attention, it would be no big deal to add the extra plugs and find out for sure....

[naukkis79]

It shows to me like squish in this engine is directed towards combustion chamber roof. Squish should be directed to piston top instead, target is to make charge motion to where combustion takes too long time - gases stuck to piston and traveling downwards are main priority so targeting squish to that part of charge is important - charge in near of plug is burning in time even without added motion.

There's some strange designs, like late BMW M20 engines with 885 head where they use piston with squish dome which also includes reverse dome and deposits always show that combustion in that reverse dome is piss poor. Those can be cured with proper dome or some head milling and flat top pistons and engines will tolerate much greater compression ratios.

With that 90 degree valve angle changing squish angle towards piston dome could be challenging.

[David Redszus]

It shows to me like squish in this engine is directed towards combustion chamber roof. Squish should be directed to piston top instead, target is to make charge motion to where combustion takes too long time - gases stuck to piston and traveling downwards are main priority so targeting squish to that part of charge is important - charge in near of plug is burning in time even without added motion.

There's some strange designs, like late BMW M20 engines with 885 head where they use piston with squish dome which also includes reverse dome and deposits always show that combustion in that reverse dome is piss poor. Those can be cured with proper dome or some head milling and flat top pistons and engines will tolerate much greater compression ratios.

With that 90 degree valve angle changing squish angle towards piston dome could be challenging.

Prior to TDC, squish motion is inward toward the spark plug. After TDC, the squish motion is reversed and moves outward toward the perimeter.

[falcongeorge]

From memory, the Pro Stock dual plug allowed more Engine setback.

Thanks
Randy

Yes, thats right, well thats how racers ended up using it anyway. That wasn't the original intent of the D-5, but because NHRA measured engine setback from the centerline of the spindle to the first plug, when the racers found out there wasn't any power gain with the twin plugs, they wound up using a single plug in the "front" hole, and pushing the engine back an extra 1/4".

[naukkis79]

Prior to TDC, squish motion is inward toward the spark plug. After TDC, the squish motion is reversed and moves outward toward the perimeter.

Naturally if squish area is in edge and spark plug in center of cylinder. But squish motion can be aimed to different location, aiming towards spark plug will lead charge in near piston to be almost stationary and burning is compromised, directing squish to piston surface will result much more uniform charge motion in cylinder - and better and faster complete burn.

I mean squish directed like this:


Designs where squish isn't aimed to piston surface but to combustion chamber walls like that BMW M20 with 885 head always have thick layer of soot at piston clearly indicating that burn at near piston is compromised.

[falcongeorge]

This begs the question what are the intake and exhaust valve angles for this Triumph?

Both are 45 degrees from vertical...Dorset's engine makes a few more HP at a higher rpm than mine, but we are crowding 1.7 HP per cubic inch from a 1930's design production motorcycle engine.A well tuned stock engine makes 1 HP per cubic inch...Mine still has a kick starter,one kick gets it going and settles into a reliable 1000 rpm idle. My engine cams have 255 degrees duration at .050 lift, .407 lift at the valve.HP peaks at 7050 rpm, torque peak at 5100 rpm and at 7000 rpm it still has 90 percent of peak torque as run on a Superflow Dyno..
The engines look like this.....2.8 inch bore, 3.25 stroke with 1.6 in intake valve and 1.45 exhaust..



Me riding the dyno a few years ago

https://www.youtube.com/watch?v=-y94Yje2lzc

Man, that video is some serious brit bike p*rn. Good stuff!

[David Redszus]

Prior to TDC, squish motion is inward toward the spark plug. After TDC, the squish motion is reversed and moves outward toward the perimeter.

Naturally if squish area is in edge and spark plug in center of cylinder. But squish motion can be aimed to different location, aiming towards spark plug will lead charge in near piston to be almost stationary and burning is compromised, directing squish to piston surface will result much more uniform charge motion in cylinder - and better and faster complete burn.

I mean squish directed like this:


Designs where squish isn't aimed to piston surface but to combustion chamber walls like that BMW M20 with 885 head always have thick layer of soot at piston clearly indicating that burn at near piston is compromised.

If we assume an ignition angle of 28 deg BTC and an ignition delay of 11 degs, then peak squish velocity will be at a maximum at 10 deg BTC. At that crank angle only about 3% of the mixture has burned. The flame kernel has not moved very far from the spark plug electrodes. A secondary squish velocity peak will occur at aprox 10 deg ADC, which will have a greater effect on the flame propagation.

Squish vector is often directed at the piston crown in order to lift liquid fuel from the piston into the air in order to make it combustible. If it does not, carbon deposits can form in stagnant zones.

BMW generally gets things right. If only we knew what their actual intentions were.

[PackardV8]

BMW generally gets things right. If only we knew what their actual intentions were.

For true, David.

I once asked a Mopar engineer, when they knew how to do better, why they put out such crap BBC head designs in the early-to-mid '70s? He told me, "The B-engine was obsolescent. The directive was to spend as little as possible to meet that year's emissions regs."