I saw in a very old dirtwheels buyers guide that said the lt500 was rated at 51 hp at the crank by suzuki.. Dont remember if it was 87, 88 etc. could also have been marketing BS too.

That has to be crank hp.

If i remember correctly, the same mag said the banshee was 48 hp at the crank figure that means around 40ish for the shee and 42-44ish for the zilla at the wheels??  Those # dont sound too crazy to me.
I have a stock 87 zilla that i want to dyno just for hahas,  but i havent been able to locate an atv dyno anywhere remotley close to me.

If i could find a atv dyno within 2 hrs of westminster md, i would!!

Only reason I ask is cause I have a 89 540HRD motor that dyno's 59hp and I have a stock 87 motor on the shelf that I was thinking of geeting done up and wondered how much more HP it would produce

AAEN pipe 39mm kehein carb by Duncan , Vforce reed cage and K&N filter with open air box. 58.90 hp and 45 torque

58hp is low for a 540, does it have porting lol???

Could be that the dyno is just stingy.  My raptor 660-makes 50.2 hp on one dyno and 58.4 on another.  Thats a big variance, but be more concerned with how it runs than the number it puts out.    We all want our quads to put out the big numbers we see from Dales bike on Jerrys dyno.   Perhaps jerrys dyno is just one of those that tests high.  Run dales bike on a stingy dyno and it doent look as good.  But it is just as good.  Nothing changed but the number.  And dales bike probably has every last once of power extracted from it too.