The Panhead & Flathead Site

Hi All

Here are some heat gun numbers off my 51 Pan. Air temp was 56 degrees and I ran the bike for 45 minutes. I let it idle while I shot the temps (all in degrees F).

Front Cylinder Head Rt Side 306
Rear Cylinder Head Rt Side 287
Gear Case Lid 185
Front Cylinder Head Lt Side @ Plug 330
Rear Cylinder Head Lt Side @ Plug 299
Lt Side Case 210
Oil in Tank 201

King

Good information - thanks for posting it.

I've been thinking of getting one of the IR temp guns. It would be handy to have a baseline so that you can take periodic measurements to see if anything is getting out of whack.

Kind of interesting that the rear head is cooler than the front. Conventional wisdom would lead one to think that the rear would be hotter, but there must be some other "dynamics" involved. I'd like to hear what the theory behind this phenomenon is.

King , here are temps. I got at idle after a 50 mile ride @ 80 degrees ;

front head-312
rear head -312
front cyl. -311
rear cyl. - 308
oil in tank 109 ?
oil tank out side -190
tranny case - 124
front drum - 98
rear drum - 109

The one for the oil inside the tank kinda thru me, thats why I put question mark next to it , you have to watch those infrareds, some reflective surfaces tend to produce funny readings even chrome or polished surfaces.I'm going to wait till it gets back in the 80's to do a running test to see how much they change , I could have done it yesterday but I was putting up a fence in my back yard to keep my critters in and the unwanted lizards with teeth out .
The phenomena could be the way these old machines are timed, but I'll leave that up to the experts here that may know better .
I think these readings will change under way , with a load on the motor.

Great info. Thanks King. The start of flame propagation at the spark plug might have something to do with the hotter temps there. I had always heard the the rear cylinder is colder, but forgot the reason why.

Kent

i had always thought the front ran hotter because the exhaust pipe is in front of it radiating heat. the rear has the pipe off to the side and to the rear.

make sense?

john

When I asked Vern the guy who makes the Velva touch lifter kit why his instruction sheet said to adjust the front pushrods 1/2 turn more than the rear he said that the front runs hotter so that it expands more. He said something about the front getting a leaner mixture somehow ,he kind of lost me on that....

I got the cheapo Harbor Freight gun for Christmas, but not the riding weather to test it out..

My tongue shows 94F, so I'm impressed already.

It will be most interesting to see if the exhaust port is measurably hotter than the rest of the head.

I have long been certain that the front "sees" more heat, but nobody likes my explanation.

.....Cotten

cotten
Its not ,like or dislike( we or at least I , think you word is Gospel in the HD department ) , its just that the way the motor is configured, I think when we start seeing motor @ load temps you will see some thing completely diff. from an unloaded condition (idle ). Its been shown by the readings we have so far that you are indeed correct about the front running hotter( at idle, we do not have data yet to see how things change at high way speeds ). The fact that we run these motors more running down the road then at idle and the configuration of this particular air cooled engine lends most to believe that the rear should run hotter.

Cotten, does it have something to do with the timing of the "waste" spark?

Kell!

Reality is probably that a lot of different factors add, and subtract, from one cylinder's heat or the other.

But, I was taught that front heads are hotter because of the same thing that makes hot water pipes freeze before cold water pipes.

If tuning is precise, one cylinder shouldn't burn an appreciable number of fuel BTUs more, or less, than the other.
And friction should be very, very close to the same.

So it is only the variance in cooling that a cylinder sees that makes a significant difference.

Internal oilsplash may make a very minor difference, but we are addressing an aircooled motor.

I believe that the actual average temperature of the heads are the same, but more energy fluxes through the front head, and you will read that at its surface.

I've been told I'm full of it many times over this.
But a great many dissections of motors have confirmed for me that the area of greatest heat 'signature' is the front exhaust guide and port, because that's where the most heat is dispersed.

...Cotten

Are we heading for a molecular description of thermal dynamics? Mike

Here is my theory, it may not be molecular, but it does involve a little Newtonian physics.

I always thought the front cylinder did just a little more work than the rear due to the basic design of the engine: single pin crank and 45 degree cylinder separation. The engine has to rotate 405 degrees (360 plus 45) after the rear cylinder fires before the front reaches TDC and fires which means that the flywheels are doing all the work at this point, running the front cylinder up through the compression stroke and they are slowing down or losing inertia at this point in the cycle. When the front cylinder fires it has to hammer a little harder to get that heavy flywheel back up to speed. The rear cylinder then fires after only 315 degrees of revolution, residual energy from the front cylinder's power stroke having done most of the work to compress the rear cylinder.

I tried a little experiment like Cotten did but I pointed the heat gun at my forehead instead of my tongue. The left lobe of my brain was running about 2 degrees hotter that the right lobe while I was trying to write this stuff, I have no explanation for the difference.

mike

That's usein' yer head! actually your theory makes the most sense to me yet, on the other hand I would think the rotation speed of the flywheels at any speed faster than an idle would be the same for both cylinders, especially with stock wheels. Maybe we should be looking at quantum physics? Mike

The problem with looking at quantum physics is that by looking you change the outcome of the experiment! We will never get an answer unless we can sneak up on the motor. Personally, I subscribe to the tandem bicycle theory. The guy in front is getting hot because he is doing all the work. He hasn't noticed that his buddy is sitting back with his feet on the crossbar.

I get the whole picture now ! I fried my brain when I was twenty something so no chance of hurting it now with any thought process.
You may find that all your theory's have some grounds for fact.