read my sig again. Fact Always displaces theory.
I can't dispute your findings, I wouldn't try. My problem is I've never run a 4 cycle. I have raced 2 cycles. In every case, with a 2 cycle, the higher the EGT, the faster the kart goes.
It really puzzles me why plate motors have been found not to follow this rule.
Every engine builder in the world knows that the higher the compression, the higher the exhaust temperature, the faster the engine. It puzzles me why the plate motors don't follow that rule. It puzzles me that your engine runs best at 800 to 900 on the EGT. It puzzles me that, when you get the EGT up to 1200 degrees, it makes less horsepower. It is universally accepted that the hotter the burn the more power that is produced. It's all really confusing, for me anyway.
From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory. (Al Nunley)
First, I am NO EXPERT on this stuff, so take whatever I am saying with a HUGE grain of salt, but I can use google, read and try to think logically...
In 2-cycle sprint racing, we generally gear our karts and tune the engines to pull high rpm's at the end of the straights. How do we normally achieve high rpm's? By running the engines "leaner" on the top end - I won't try to define "leaner", but let's just say you'll never achieve high rpm's when you're "rich". Basically, we tune the carb to gain extra horsepower at the top end of the rpm range, or to increase the usable rpm range. The thing is that once we hit that high rpm number, we are then backing off the throttle. If you plot the rpm vs. EGT, EGT climbs as rpm's climb (unless we are "too lean" at which point EGT will drop). Al, you have to admit that if we ran at WOT (or close to) all the time, we would definitely gear and tune the carb differently.
On dirt ovals, I don't think that karts are generally geared and tuned to turn huge rpm's. They tend to run in a narrower rpm range where the engine is making the most power, thereby sacrificing peak rpm's (where the engine makes less power). It's not entirely different from road racing in that sense.
So why might a kart on a dirt oval run better when "richer" or run at a lower than maximum EGT? Well, if you plot rpm's and EGT together, the EGT will continue to climb as rpm's climb. But on an engine and track conditions where you're not tuning for max rpm's, and rather to be in a given rpm range that is lower than max rpm, you'll never get to that maximum EGT number. If the carb was set up to run at a high EGT, you'd either be geared all wrong (above where the engine is making its real usable horsepower), or you'd be running that high EGT number for too long a period and things would be blowing up, or as Brian alluded to, the fuel curve is not linear, and thus peak power in the optimal rpm range will be made by running with a "richer" carb setting (needle, clip, tube, jets) rather than a "leaner" setting.
Read up on AFR and it becomes apparent that it's pretty much accepted that you want to be on the "rich" side of the stoichiometric ratio to achieve peak hp, which is where you get peak EGT.
Dave E.