It is easy to see that a wrapped header is going to contain some of the heat within the pipe, that would have normally started to dissapate more at the pipe/engine connection with an unwrapped header.
This is obvious. Since the exhaust
gas stream leaves the engine at upwards of 1300 degrees (F), and the pipe does not exceed roughly 1000, even at an extended idle (1000 degrees is the approximate point at which a pipe would appear distinctly red in daylight), or often much less than that, that there must be some transfer of heat from the pipe to the surrounding air mass.
This containment of heat is what increases the exhaust temp, which in turn creates more exhaust gas velocity/volume.
This seems logical, but it fails to account for a couple of things. First, at the lower rpm that you raise later in your post, say 4000, there are 33 individual exhaust events per second, and even at that speed, each one only occupies the pipe for less than 80 milliseconds. The pipe gets hot because of the cumulative effect of a very small percentage of the total heat energy of each individual exhaust event being transferred to the pipe. If you have your thermal engineer friend calculate how much actual heat energy is lost from a combustion event to the metal of the pipe itself, you would find it to be less than 2%. Eliminating that loss of heat energy is not going to have much effect on the bulk volume of gas in any individual event, or on the cumulative gas stream as a whole.
...the exhaust is not the source of the heat as you stated, grayracer513. The exhaust is simply a means of guiding the gasses and heat away from the actual source, the engines combustion chamber.
The heat that is absorbed by the exhaust system comes from the exhaust gas. Of course it originated in the combustion chamber, but it doesn't go directly to the exhaust pipe, now, does it? Cute little bit of misdirection. The purpose of the exhaust system is to eliminate the spent gases from the combustion area, and secondarily to improve cylinder filling by correctly timed gas extraction. The heat would be better kept within the combustion chamber where it could become mechanical energy, but unfortunately some 33% of the total energy extracted from the fuel is lost this way.
by insulating the heat removal apparatus in the vicinity of the cooling unit (radiator), it only makes logical sense that the lesser heat going to the rads will improve their ability of overall function. And if the heat wrap can keep the exhaust heat from buring your leg as bad...it can also help to keep the radiators cooler than they would be without the heat barrier.
Have you ever burned your leg on an exhaust pipe that you did not actually touch?
Try this: With the engine off, and in still air, place a red hot 1 1/4" tube 3/8" of an inch from a radiator and see how long it takes to raise the temperature of 1/2 gallon of water 1 degree that way.
... The wrapping again increases the exhaust gas temps thereby helping to eleminate the scavenging that can occur with the cooler/lower volume/velocity exhaust gas in the unwrapped pipe at lower RPM's.
"Scavenging" only takes place within the rpm range the pipe was designed to operate in, based on its length, and the placement of the cross sectional increases (steps) in the system. It has only to do with shock and pressure wave speed, and is independent of exhaust velocity.
IF insulating the exhaust actually did cause a significant increase in exhaust gas volume by raising its temperature, the velocity would increase, but only as a result of an increase in the internal pressure within the pipe. That's just simple gas/fluid dynamics at the most basic level. You and others who suggest that increasing the exhaust temp can make it flow better are essentially arguing that increasing the resistance to flow within the pipe will improve the flow within the pipe.
The benefits to doing this on an off-road bike are the same as for a full bodied automobile; the reduction of radiant heat absorption. ISDE racing involves long sections of extremely tough, hard slow speed work, an under those circumstances, protecting the fuel system and othe adjacent components only makes sense. Navynuke pointed out a real-world example of exactly this kind of thing. But that isn't MX, which is a sport more dependent on power output, and wrapping headers provides no more benefit to an MX bike than tire balls
do (they increase the weight and rolling resistance of the tire too much for their occasional ability to run flat to be of any use).
As for the cross comparability of the Z28 experiment, it speaks directly to the core issue of whether insulation raises exhaust temperature. The attempts by you and others to discredit it on the basis of it being a different vehicle type are groundless. The controls were fine. The comparison was done on the same vehicle on the same day with the insulation on, then off, then back on.
And I am certain that Mr. Summers is a very accomplished rider deserving of a great deal of respect for what he has accomplished, but that is a completely impertinent point when it comes to his credibility as an engine builder, or his qualifications to recommend any performance modifications. In fact, some of the fastest riders I've ever known have al the mechanical good sense of a bag of hammers. Riding skill and technical prowess are not necessarily correlated in any way.