OK - I think its 65mm stroke and 88mm bore.  This based on a passing reference on the Heise Savage pages to the Suzuki Tempter 400cc single, saying it is similar to a Savage but the stroke had been reduced to 65mm.  65 x 88 would give around 395cc.  Odds are the LS400 uses the same engine as the Tempter.  

I wonder if the cylinder heads on these bikes are the same as the LS650 head. Of course, if the LS400 uses smaller valves and combustion chamber then it raises interesting possibilities - higher compression ratio  and smaller valves could work better than many people might think.  For example, Harley 883 heads works really well on a 1200 conversion.

Anyone got any better info?

Smaller ports can increase velocity of fuel/air mixture, improving fill rate.    With 140 CC.s less than a Harley 1340 Big Twin, a 1200 with 883 heads (smaller valves than a genuine 1200) can produce very similar torque and horsepower numbers. This is well established.
There is, of course, a trade-off.  You tend to get better low-down torque at the expense of top end.   If valves are much too small then there is a reduction in power.  

A VERY crude analogy is using a hose to water the garden.  If you want the water to flow faster and further you restrict the opening at the end of the hose.  It all depends whether you want a lot of water all in one place or slighly less water but fully distributed around the garden.  When a large cylinder is filling rapidly high velocity from a smaller valve may be more important than the slower fill from a big valve.

justin_o_guy wrote on 12/31/69 at 16:00:06:


Actually, that was a very good analogy of how it works out, and indeed the difference is Low RPM torque vs a high reving engine that will "pull all the way out the back door" (in drag racer terminology) due to it's free flowing design.

Some internal combustion engine science is counter intuitive. I was not really well informed that 2-stroke engines were capable of pulling at low RPMs until I got a Trials bike. The first being a 70s Suzuki RL-250. They make torque (and HP) all the way down to just above idle, by having very different carb/head flow/valving/compression ratio and exhaust tuning, plus spark timing than a higher HP Motocross type bike. But a 2-stroke can indeed pull like a little tractor. They don't all have to go "zing" to pull.

In the case of the LS400, being such an "under square" motor (stroke being much less than bore; and "square" being where bore and stroke are the same) it may need the restriction of smaller intake and exhaust to allow it to produce torque and prevent it from being an all top end and wound up motor. Not what you want on a cruiser style bike. As DrT suggested, this principal only works to the point that the 'restriction' starts to cause power loss instead of more complete combustion through higher intake velocity/swirl etc...