Small valve early head with late model short duration cams

[bretts]

I recently had the Large valve head and long duration cam on the bike with a wiseco 1200 kit. I had only done 1200km in this arrangement and due to other issues it never ran perfect so I can’t say how well this set up went.
I have the head of again at the moment and thinking about using the small Valve head and late model short duration cams.
Dan Hodges I believe has stated this would give a lot of toque down low but I’m thinking you lose a few hp up top.
Since I don’t tend to ride up around 100mph I’m thinking this could be a fun setup.
I’m a layman and wanted to bounce around what I’m thinking. I believe the wiseco kit puts compression ratio up around 10.5:1 but with the short duration cams from what I’ve been reading that’s going to make even more compression than the long duration cams.
Another option after talking to a local performance head guy was he suggested using the long duration inlet and short duration exhaust that’s available from the stock variations.
What do you think and any ideas on dialing for even more power down low?

[MPittma100]

Cams

There should be a noticeable performance enhancement from increased compression ratio alone. Hard to speculate if cam swapping would yield more performance. Limitations will be air intake and exhaust options. You would have to spend some time on the dyno to see what worked or not. Enhancing cylinder head intake and exhaust ports may help. But are still limited to available cam options and valve sizes.

Some have increased compression by leaving cylinder to block gasket off. Only concern with 10.5:1 and higher ratios is available fuel with enough octane to accommodate higher compression.

Higher compression, More CC displacement, more lift and duration, and larger valves is a good recipe for more performance

Mike

[bretts]

Thanks Mike,
Premium fuel here equivalent to USA 93 octane is very common. I find it harder to get the equivalent to USA 87 which I believe the standard Xs11 engine runs best on.
I think I’ll experiment between USA 91 and 93 to see what works best at that compression. That’s 95 and 98 here in Australia.
I mostly go by Dan Hodges advice here who I believe has done a lot of these combinations on the dyno.
From what I’m gathering cc’s And compression are good ways to increase power but longer duration and more lift is not always better especially when it’s low down torque and not actual horsepower you are chasing.
But that is what I’m trying to understand.

[Bonz]

Longer duration and more lift is going to lower dynamic compression ratio, which hurts power down low. At higher RPM more lift/duration flows better so the lower dynamic compression ratio is made up for and surpassed to some degree typically.

Higher static compression ratio given by higher compression pistons or removing the base gasket, etc will make up for the power loss down low and then add even more up top with higher lift/duration cams.

Static compression ratio is what we talk about when we say an engine has a 9.0: 1 or a 11.0:1 compression ratio.

Dynamic compression ratio is the compression ratio that exists as the engine is running based on the effect of valve's opening and closing with the specific duration and lift of any given cam.

Running too high of compression pistons and stock lift/duration cams can put a hurt on an engine and burn things up at higher RPM pretty quickly due to detonation as dynamic compression goes sky-high.

Have to find that balance between increase in static compression with moderate pistons and using that to boost lost low end power due to the the higher lift and duration cams that give the top end.

[BA80]

As long as you keep the timing components that go with the cams so the timing curve remains correct that part should be fine.

The only other worry would be the valves hitting the top of the piston. The longer duration cam will keep the valve open longer as the piston comes up and the later engines have a taller crown on the piston and may cause valve/piston interference.

Some testing/measuring would need to be don in that area.

[Bonz]

Search for Dan Hodges posts, he did so much comparison work between the different heads, pistons, cams and timing mechanisms.

Also 3phase did some stuff too but not sure how much was posted up.

[Dan Hodges]

XS Modifiactions

Hello Friends
I have read a bunch of questions about heads, pistons/compression, cams, carburetors and so forth on the XS and from 2008 thru 2010 I ran the table on all this stuff and posted it and so you can find this stuff if you search my post because I'm not going thru all this stuff again.

Lastly, whatever you do don't go over 160 lbs. of pumping compression because this is all 93 octane gas will support on an air cooled motor on the street. Basically, the Wisco 1196 pistons with the early cams and late heads are a good combination and will fetch about 9.5 to one static compression without cutting the head or about 150-160 lbs of pumping compression once the rings have seated.

Good Luck.