The rear cylinder has a richer charge of fuel/air because of how the engine fires one piston then the other, then pauses, then does it all again. And the rear cylinder is the one firing first, getting the full initial charge of fuel/air while the front piston is following closely behind and takes "less of a breath" of fuel/air, not getting quite as much fuel as the rear one did (has to do with pressures/velocities created at the intake with this assymetrical firing, and also therefore manifold design contributes as well). A richer mixture (more fuel per unit air) will run cooler. It's also why when you jet the carb, you should jet against the front plug first since it will be the one to run leaner (hotter). Then the rear plug is checked after setting things up against the front plug. (Oh, and a lot of people don't know that plug reads should be done after the engine is warmed up, and then swapping in fresh plugs to read with...) And it's also why in some applications, slightly hotter plugs are run on the rear cylinder (not needed normally, but mentioned to make the point).

The attachment is an animated .gif file that can help visualize what's happening when looking at the left side of the engine (it's kinda crude, but if you can imagine how the cylinders fill and fire, you can start to see how the fuel/air can be slightly different from back to front...)


[IMG]local://upfiles/2613/A44A8BA8D2B446E3BE8F46D921A1EE48.gif[/IMG]

 

I would take the temp at the base of the spark plug (like we do in RC Racing) and at the base of the exhaust pipe just inside the flange.

 

Yep - if you shoot at the base of each plug (or at the same points at each exhaust flange) with a laser-aimed IR temp gun, you'd get more accurately comparable readings I think...

 

Once I knew a gal named Jugs and for good reason. She did have a pretty hot temper, but near as I could tell both hers were the same. Oh, wait.... you're talking about motorcycles.

 

Everyone knows that the sister's jugs are always hotter! Or was it her best friend's jugs that were hotter? I think I need to go check...

 

Wow this turned out to be a very informative thread. Thats why I like to read them. Especially when someone gives an explanation for the answer instead of just giving the answer with no reason why it's that way.

 

AZ: Thanks for your very complete response. I understand you say the rear cylinder fires "first" in the cycle, therefore gets more fuel/air charge than the front cylinder because of the shorter amount of time the front cylinder has to "draw" in the fuel/air charge. You say the rear cylinder is running "richer" than the front because of this therefore it is going to run the coolest. If there is a difference in the amount of cylinder fuel charge drawn in between the cylinders, couldn't H-D engineers take that into account and grind the cam lobes for the intake valves slightly different giving the front intake a slightly longer duration than the rear? I am a Construction Engineer by education and not a Mechanical Engineer, but I do appreciate all the many variables that take place between even just two internal combustion engines of the same design and manufacturer. I can see the possibility that the dynamic pressure flow of the fuel/air mixture thru the intake manifold could be slightly lowered when the rear intake valve shuts and the front intake valve has only just opened. I also wonder what percentage of cooling effect is contributed by air/fuel mixture and by outside airflow.

I am not trying to be a pain in the a** on this, but I have heard from many H-D mechanics that the rear cylinder tends to run hotter than the front strictly because it is subject to less cooling air flow and because the exhaust pipe points to the rear and gets much less cooling air than the front exhaust pipe. I vaguely remember some experimentation by H-D on their flat-track 750cc and 1000cc engines by switching cylinders to have both exhaust pipes facing the rear and then again with both facing front. Do you know of any results of those experiments?

On my '05 XL 1200 R, I have examined the plugs after a true shut down from 70/75 mph with a magnifying glass and could tell that the front plug was running the coolest. I have lots of experience in racing single cylinder two-strokes and single cylinder four-strokes and can read a plug.

I used to own an airplane with a six cylinder horizontally opposed air-cooled engine with a six position Exhaust Gas Temperature gage. I could see that the rear two cylinders always ran just slightly warmer than any of the other four. By flying the airplane in a "Yaw", I would watch the rear cylinder on the "sheltered" side warm up slightly over the other one that was receiving more airflow.

I wonder if H-D or anyone else for that matter have published any definitive studies on cylinder head temps during normal use. Wouldn't it be fun to have unlimited funds for a engine test stand/Dynamometer?

Again, thanks for being so willing to educate us. .................... BC

 

Quite Frankly I lost myself watching that gif. file. . It was kinda like watching a bondfire, don't know what it is, but you can't take your eyes off it. But looking at it, if I see it right, the red arrow near the crank center line is when the plug fires? And if that is true, the plug also fires at the end of the ehaust stroke?

 

Yup, you got it right AZFlyingDiver! The sister's jugs are always the hottest!

Thumper26

 

Being a real old time redneck "shade tree" mechanic, I'll simply add this observation. With all the addatives and with the removal of lead from todays fuels, Plug reading is not the "science" it used to be, with one exception, EXTREME cases of rich or lean. If the engine/carb settings are remotely close, Plug reading is virtually useless, in my opinion. Also if you look at the cam specs on a V-twin engine of any displacement, you will see the lift and duration takes into account for the "unique" situation.