cam choice

[4speed]

I have 88 sporty 883 /1200 conversion with stock cams,looking to put in new cams any suggestions,heads are stock
thanks 

[aswracing]

I have 88 sporty 883 /1200 conversion with stock cams,looking to put in new cams any suggestions,heads are stock
thanks

Well, here's my view FWIW ...

883 heads as you probably know have really small valves - 1.580" intake and 1.350" exhaust. As a result, they nose-over, on the flow bench, before they even get to .400 lift. By nose-over, I mean they reach the point where opening the valve more doesn't cause more flow to occur. Look-up "L/D ratio" sometime to learn more about this phenomena, but in a nutshell, on any port, you reach a lift point where the valve is no longer the thing that's constraining the flow, and generally speaking, the bigger the valve, the greater this lift point.

So since the stock cams are already quite a bit taller than they need to be, there's no power to be gained by increasing lift, that's my point.

Next issue is overlap. 883 heads have really shallow chambers, and the valve stems are longer so that the valves will reach it. This puts the valves much closer to the pistons to begin with, and as a result, it doesn't take a hell of a lot of overlap to make the valves hit the pistons. Be really, really careful with TDC lift specs as you choose cams for stock 883 heads, because you can get into trouble in a hurry.

So scratch overlap as a method for getting more power. Oh, you can increase it some, don't get me wrong, but not a hell of a lot.

What does that leave you? Exhaust open point and intake close point. The intake close timing is the dominant spec, it's what's really going to affect the rpm range of the cams. But moving the intake close point around tends to just move the power around, not actually give you a nice gain over a wide range.

Bottom line, stock 883 heads kind of tie your hands with respect to getting nice gains from a cam swap. You can gain some, don't get me wrong, but it's not going to deliver a particularly good bang for the buck.

Prepared 883 heads are a whole different story. Valves can be enlarged to gain flow at all lifts and sunk in their seats to gain both valve-to-valve and valve-to-piston clearance, allowing you to runs some overlap. A prepared 883 head gives you all kinds of opportunity to gain from a cam swap.

There are still trade-offs to be made all over the place, for example how much valve size vs. how much overlap since those things get each other in trouble. Piston design is a factor too, since these trade-offs affect chamber size and the depth requirements for the pockets. Exactly how you make all those decisions and balance all the conflicting parameters has everything to do with the final result. Of course, that's true with any head/cam/piston combo, the three need to be a carefully matched set for the best results.

I don't mean to burst your bubble, hell, try it, pick a cam grind that fits and slap it in. Like I said, there's some available there. Just not a hell of a lot in my experience, for the reasons outlined. Other combos respond well to cam changes because you're not fighting the same constraints that the 883 heads place on you.

[andyxlh]

Wow that last reply is really excellent!
I have andrews V2 grind cams in my '89 4 speed, with a 1200 conversion, 883 heads and new 883 valves, with the heads ported. KB pistons 30 thou overbore. Runs really well IMHO, and there was some gains with the cams, although as stated the heads are probably a limiting factor. I changed the cams as the stock ones were ruined when a lifter collapsed. V2 give a little more duration and lift from memory.
Big job to change the cams in the sportster engine though as the shimming and setup takes some time.

[Panzer]

Nice, ASW, very nice explanation, I learned something too. 

[sfmichael]