Thanks for the tip MarkT, I will try sending him a PM

I bought it based on an article in the June 1975 issue of Cycle World. The main take away for me was, "a 10% improvement in cooling" But it also gave a head to head comparison of HP increase over the stock head. Improvements in HP and torque started as low as 2500 rpm and continued all the way to 8500. Max on each was:   HP at 7500 = 40.43  (a 2.45hp increase over stock)    TQ at 6500 = 30.70  
Asco has suggested the purchase of supplemental insurance for future right leg injuries. A recent inheritance has me considering a Bionic leg transplant. Or I may pay the neighbor kid to start it for me on a per kick deal (he seems to do OK on that big Harley).   
 

Per kick… I’d have it flooded… make out like a bandit! Kicking… and kicking and… (:
Sneezles61

Buy one of those battery powered paddock starters and strap it onto the rack! 

MACE6772 thanks for the head to head comparison information 

Just as an experienced opinion I can tell you it REALLY woke up a '73 MX360 I was racing some years back.... one of those "no doubt" seat of the pants feelings.....it had made the engine stronger from low revs up. I did a same day comparison between a stock cyl head and the Webco.... never took it back off lol.

I never had any data on a Webco MX400 head. Here is what I do have:
Machine ________________________Part Number_________Combustion Chamber Volume in CC_____Weight of head in pounds
75 MX400, 76 IT400, 76 YZ400___510-11111-01-00______42.0_____________________________________4.1
75/76 DT400____________________500-11111-00-00_______54.7_____________________________________5.0
75/76 DT400____________________Webco #1310__________49.7_____________________________________6.7

More discussion here: yamaha-enduros.com/index.php/forums/74-7...amber-volume?start=0

I also have some scans from a Webco catalog of Yamaha heads with some pictures and select dynomometer info somewhere.

Wrench66 I had one brand new 1973, 360 MX I still have the paperwork for it today. I just got to find it where I traded in my CT2. I had a lot of memories on it. It was very fast for a 18 yr old I was working at Butler’s Yamaha Triumph at the time 

Some more Yamaha/Webco head info from an old catalog:

imgur.com/G1uhX0Y

imgur.com/erdruLw

imgur.com/To91qIu

I am an imgur rookie, so let me know if I should insert them differently.

Now selling gun collection to support Webco head collection. This is going to be worse than my phonics addiction.   Your going to have to be older than 60 to get that one.

.

Thanks to everyone for all this info. Glad I made my purchase.