Gary_in_NJ wrote on 12/18/19 at 08:16:37:


That’s a pretty good boost for not a lot of upgrades. Do I really need a high compression piston to run the DR cam?

Dennisgb, check out DragBikeMike's thread on porting the head, here is a ton of good information in that thread. And there are gains to be had from improving the airbox and filter. On the exhaust side of things, a better flowing muffler and larger inside diameter pipe will make a world of difference too.

Go for the easy and cheap mods first, the ones that give the most bang for the buck. If you decide that you want more power, like a bigger bore, you have already taken care of those things that would need to be done to get the most out of that anyway.

Dennisgb wrote on 12/18/19 at 11:29:19:


There are a few aftermarket companies like MAC that make larger pipes. Or you can hit an exhaust shop and have them make you a new pipe if they have a tubing bender. Another option is buy pre-bent stainless tubing from Summit Racing or JEGS and cut and weld a new pipe. For a stock or mild engine, 1.5" ID is a good size. If you are getting more serious go for 1.625" ID pipe.  1.625" ID will be 1.75" OD which will fit your Dyna perfectly so you may want to go with that size anyway.

The stock pipe is a lie, that big 2 inch pipe has a tiny pipe inside, only about 1.3" ID. Way to small for a 650 to be able to breathe.

Fast 650 wrote on 12/18/19 at 11:42:18:


I know there is may be a loss of torque going to a larger diameter pipe. Has some one like drag bike mike done a comparison from the 1.5 vs 1.625 pipes?

That big MAC header doesn't seem to be hurting the low end with the 340b cam either.   The 340b does have a lot of overlap and the 1.79" header is working fine.  Wish I had a nice 1.62" ID header to do a comparison.  I think the 1.62" would probably work a bit better.  I could use the poor man's dyno to test it.

The larger filter had a huge effect.  It was very difficult to get the thing to run correctly.  I spent a very long time grooming the S&S carb with the stock replacement K&N flat-panel filter, and it wasn’t easy.  So, I figured that with a bit of effort I should be able to dial it in.  Lot’s of luck with that.  This tuning job was a real handful.  

I could tell right away that the thing made a whole bunch more power, but it ran terrible.  Waaaaay too lean at first.  I started with a #60 main jet and ended up with a #68.  The pilot jet and the two air-bleeds were all over the place.  Get it cleaned up down low and it wouldn’t run up top.  Get the top squared away and you could hardly ride the thing on surface streets.  The accelerator pump was also a headache.

I eventually got the thing pretty close, but a lot rougher around the edges than it was before the filter mod.  The WOT performance is absolutely killer.  It runs from 4K to 7K in 2.65 seconds, and it’s scary how quick it blows past 8K .  Glad those RD springs are in there.  Loads & loads of mid-range torque.  Starts great.  Idles good.  Part throttle operation is acceptable but it ain’t gonna win any awards in the “seamlessly smooth” category.

I was truly amazed at how much the filter affected performance.  It begs the question; how well would the stock DR cam have run with this filter?  I plan to do a separate post on the filter setup.  Seems to me that it has the potential to make significantly more horsepower, regardless of the carburetor.  I also suspect that a more forgiving mixer like the stock Mikuni or a VM would not present such difficult tuning issues.

So, the 340b cam with RD valve springs seems to be hackin it.  Oh!  BTW, it’s loud.