Audio recording of XS doesn't reveal what I expected

[1980 Special Twin]

So I make industrial techno music using motorcycle samples. I've been meaning to post them and will soon. I use different bikes and have used the XS in the past, but I was excited to finally make some quality recordings of my 1980 xs650 with shorty mufflers. I don't believe it has been re-phased at all. I sync'd it with a homemade manometer, and although the tool didn't operate all that smoothly the bike seems to perform well.
My understanding of a paralell twin was that both cylinders fire at once. If that is the case, why do the left and right channels appear to fire differently? The left on top seems to fire a moment before the right. There is no fault in the recording gear or software.
Do I misunderstand the operation of the twin?

 

[Jim]

Unless it's rephased, the XS650 is a 360° parallel twin. That means it fires every 360°....
Left fires. 360° later the right fires. 360° later the left.....

 

[1980 Special Twin]

I felt like I understood the mechanism but now i'm confused. Are you able to explain it in relation to the image i posted? I think I need to understand it musically.

 

[gggGary]

Since these are 4 strokes (suck, squeeze, bang, blow) they fire only every other TDC event. Left fires on one TDC, right on the other.
360 twins have an even firing sequence the right cylinder fires exactly halfway between the left cylinder firings.

 

[1980 Special Twin]

I gave it another look and now I realize the piston has one stroke for ignition and then another for exhaust, right? So it's like "left fire, right fire, left exhaust, right exhaust?"

 

[1980 Special Twin]

Haha my brain figured it out right when you posted. Yeah now it makes sense!

 

[gggGary]

Your charts seem to be somewhat misaligned.

 

[5twins]

Although the pistons rise and fall together, they both don't fire at the same time. When one cylinder is at TDC on the compression stroke (and it fires), the other is at TDC on it's exhaust stroke. Another full (360°) rotation of the engine brings that 2nd cylinder to TDC on it's compression stroke (it fires) while the 1st cylinder is now on it's exhaust stroke. If you have a single coil with dual outputs (two plug wires) then yes, both plug wires are fired at the same time, but only one spark gets used, on the cylinder that is at TDC on it's compression stroke. The other spark gets wasted, does nothing, and is in fact called a "waste spark".

 

[Jim]

I felt like I understood the mechanism but now i'm confused. Are you able to explain it in relation to the image i posted? I think I need to understand it musically.

If your left and right cylinder traces are correctly synced, you've likely got a 270° or 277° rephased motor.
In a stock motor, the cylinders would fire 360 degrees apart from each other.

 

[jpdevol]

Reckon you're measuring sound waves @ exhaust pipes that occur after ignition, and after exhaust stroke, and the brief time it takes to exit pipe and make noise - same frequency though.

 

[gggGary]

or unequal length exhausts?

 

[jpdevol]

IDK - it's probably obvious to the rest of you that I know nothing of recording equip. nor its software, but I would think you need one mic per cyl. and a time overlay to determine phase

 

[Jim]

or unequal length exhausts?

Possible, but I don't think so. Speed of sound is roughly 1000fps (feet per second). Let's say one pipe is a foot longer.
One pulse emits 1 one-thousands of a second slower than the other. At idle, that wouldn't be much difference on the graph.

 

[Jim]

or unequal length exhausts?

One way to eliminate (or verify) that would be to rev the engine and see if the relationship changed on the graph. If the pulses get closer together (between cylinders), then it's prolly unequal length. If the relationship stays constant, it's phasing.... or rephasing in this case.

 

[SomervilleXS650]

The thing that jumps out at me is the start time of the clips (they're offset). Did you simultaneously record those and just crop the start times? A very simple explanation would be if you inadvertently dragged one of the clips forward or backwards and offset them. (As someone that's accidentally scooted a clip before, please don't take offense to that, haha).

 

[Jim]

I gave it another look and now I realize the piston has one stroke for ignition and then another for exhaust, right? So it's like "left fire, right fire, left exhaust, right exhaust?"

A 4 stroke engine has 4 events (strokes) for a complete cycle.
Starting at TDC, piston goes down drawing fuel/air mix into the cylinder. That's one event
At 180° the piston goes back up to TDC compressing the mix. Event number 2
At TDC (360° in) we get spark and the piston is forced down. This is the power stroke... Event number 3
At the bottom the exhaust opens and the piston goes back up (540° of rotation)... exhausting the burnt mix. Event number 4
These 4 "events" take 720° of crank rotation.... 2 full revolutions.

 

[hovel]

A stock XS650 engine at 7500rpm is said to be at full song. Through open pipes, that's music to me.

 

[1980 Special Twin]

I intentionally cropped the lower clip to make it clear where the pattern begins. I didn't change the timing at all.
Yes, i think what I see is what I should see. I didn't realize that "360° paralell twin" means they don't fire at the *exact* same time, and I didn't take into account all 4 strokes.
I have similar recordings of my single cylinder 2 strokes which are a joy to behold, but don't have the cool galloping chaos that this one has.
And I'm sorry to say that my Harley has the most useful sound, in that the bass is so deep and full. After all, that's the only reason anyone rides one, isn't it?
My pipes are the same length. It would definitely be fun to play with "tuning" in this context but I don't know if it would be musically productive. Most of my audio manipulation is electronic anyway.

 

[WideAWAKE]

Just post audio of your bike running.

It would take 2 seconds to tell you if it’s rephrased or not.

 

[1980 Special Twin]

I know it's not rephased! I just wasn't sure why I saw what I saw given my flawed understanding of the 360 paralell twin operation.