Originally Posted by 100th Monkey

How would you suggest I get more low end torque out of my bike? The guys at my local Harley shop recommended the SE 585 and you are telling me I can't buy a cam that will give me more torque. Very confusing and I know nothing of the mechanics of how torque is achieved. I just know when it feels right. My bike is much faster than it was as an 883, but I feel it is not using it's HP efficiently at low rpm's. Any suggestions?

Originally Posted by aswracing

OK so now look at the cam specs:



One of the first things I always look at is the intake close timing. The later the intake closes, the less of the compression stroke is used for compression, and therefore the less compression you get from a given compression ratio. Knowing your CR is going to be 10.5:1, you can run the numbers ... I'll use my own compression calculator ...

585's: 45 degree intake close point at 10.5:1 comes out to 202.6psi
536's: 50 degree intake close point at 10.5:1 comes out to 195.8psi
575's: 52 degree intake close point at 10.5:1 comes out to 192.9psi

This just comes down to how hard you want to push it, and how good of gas you can get. None of these numbers is really pushing things hard though, for an EFI bike. They're all manageable.

Later intake close points work best at higher rpm's, too, so keep that in mind. Your stock "W" cams for example have a 25 degree intake close point as you can see on the top line, the earliest of any XL cams on the market. It's really well optimized for bottom end and mid range.

Another number I pay a lot of attention to is the overlap, which is the sum of the intake open and exhaust close points. More overlap gives the exhaust more influence over the intake flow, which gives it a lot of control of the powerband. A motor with a lot of overlap will make a bunch of torque (and thus power) at any rpm where the exhaust pulls hard. It'll also run like crap at any rpm where the pipe is pushing back. But it's a huge, huge opportunity to make power. It can make a motor act like it has much bigger valves than it actually has, it's great for undervalved motors, and these motors are pretty badly undervalved.

585's: 11+21 = 33 degrees of overlap - this is fairly mild to middle of the road
536's: 26+30 = 56 degrees of overlap - that's a good hot street grind
575's: 28+29 = 57 degrees of overlap - that's a good hot street grind

Also I look at intake duration as it relates to exhaust duration. Cams that have longer exhaust lobes are called "dual pattern". This is done largely to help overcome pumping losses associated with restrictive exhausts.

The 585's are 236 intake and 261 exhaust - a whopping 25 degrees dual patterned. That's a lot. In fact, I'm not aware of any other cams on the market that come close to that.
The 536's are 256 intake and 266 exhaust - 10 degrees dual patterned, which is more or less a typical number on a hot street grind
The 575's are 260 intake and 266 exhaust - 6 degrees dual patterned, again not unusual on a hot street grind

Lift is another key number. Ideally you look at the lift as it relates to the head flow data. All ports have a nose-over point, where opening the valve more doesn't cause more flow to occur, because the valve is far enough out of the way that it's no longer relevant. The bigger the valve diameter, the higher the lift at which this occurs. A person can actually look at the flow data and run some numbers and come up with some reasonable relative values from different cams in terms of the airflow capability with a given set of heads. I don't have the flow data on the SE CNC heads in front of me however. But from memory and just speaking in broad generalities, I think they can take good advantage of the higher lift numbers you're looking at here. The 536's aren't going to get everything out of them that they have for you.

Bottom line is that the 585's are fairly mild, not a lot of overlap, and made for a more restrictive exhaust. Note that they're an emissions legal grind, too. That doesn't surprise me, looking at their specs.

The 536's and 575's on the other hand are really very typical hot street types of grinds, with the 575's having slightly hotter timing and some additional lift that the SE CNC heads can make use of. Neither is a really super hot cam, but they'll work well for what you're doing. The 575's will likely work a little better.