Some time ago I bought some 620 cams and sent them out to Ian Drysdale to have them modified to suit the earlier 3 bearing heads so I could try them in other things - namely my 400. I've also made a rig to hold a spare 400 head I have that allows me to read cam lift as the cam is rotated. It's all very analogue (ie, me sitting there looking at a degree wheel and dial gauges) and gives me valve lift every 4 degrees of crank rotation. You'll see more of that in the future. This way I could compare the 400/600/750 cam (labelled R) and the 620 cam (2S), as below.
Spec wise, the 2S is like the R, but with the inlet closing 15 or so degrees earlier and with more lift on both inlet and exhaust. In profile, the exhaust is a bit bigger and has an earlier peak while the inlet is as expected. The reason I wanted to try the 620 cam is that the 400 has a long flat power curve at the top end - it's pretty flat from 9 to 11. As such, I was wondering if it was over cammed, and figured the 620 cam should be better and a good illustrator of why. What I was hoping to see on the dyno with the 620 cam was peak power of the same or more, the peak moved down the rpm range and the power dropping away after the peak.
Spec comparison as below:
I fitted the 620 cams at the inlet centreline spec of 112 degrees, and with the drilled pullies I could advance them to 107 to see what happened. I was also considering retarding them on the dyno as well, but didn't bother in the end. As usual, the results sucked all the motivation right out of me.
Red is R cam at 108 degrees, blue is 2S cam at 112 degrees and green is 2S cam at 107 degrees. The midrange improvement is between 5,700 and 7,100 rpm, and the top end loss starts at 8,500 rpm. Given we're only dealing with 38 hp at the peak, we're not talking great differences here.
As an aside, the runs below are all done with an original 2-2 header and the Megacycle mufflers. I pulled the 2-1 a few weeks ago and jammed the original horizontal header onto the modified vertical header. A dyno run showed the same power as where we started all that time ago, and with much the same richness as all the std airbox lid (both snorkels fitted) runs for the 2-1.
Mixture wise, it didn't do much at all. Still too rich (std 2001 my M400 carbs and jetting). Top graph is horizontal, second vertical from memory.
To lean it out a bit I thought I'd try to usual trick of pulling a snorkle, which had the usual effect, as below.
Still generally too rich, but better.
As the graphs show, what I wanted to see and what I did see were two different things. I have become accustomed to dyno disappointment with this bike.
But my mechanical sympathy has me cringing as I rev it over 9,000 rpm looking to use the last of those 38 hp, it's not something I like doing when there's no performance gain to be had. When I first made the 2-1 and fitted the Acewell dash I was running it up to 8,000 rpm or so and it felt great. Dyno shows that the 2-1 with Danmoto muffler turns to crap around 7,000 rpm, but under that, and especially through the midrange, it's as good or better than the 2-2. With the Megacycle muffler it holds the power a bit longer, and definitely felt better on the road between 8 and 9,000 rpm when I held it open just to see how it felt. But I wouldn't normally go that high riding it around, so I'm a bit caught in the dyno number loop here as it would be. Or, more to the point (let's be honest here), I didn't expect the things I did to work so badly nor require so many excuses.
Now with the 620 cams it's better again in the midrange, and I think I'll go back to the 2-1 and one snorkel and either tolerate the Danmoto noise or get another muffler made. I have a design in mind, whether it comes to fruition or not is another matter.
As a comparison to where it was all std, it's not too bad. Mufflers, cams and one snorkel out of the airbox lid in red, all std in blue. As a graph, it looks great until you read the numbers on the LH side and think "oh". Its got another 4 - 5 hp for most of the rev range, which, when you've only got 37 all up, is at least 10%, and more as you go down in rpm. Woohoo! Doesn't make it much faster though. It's slow. Kinda fun, but still way slow.