XJ650, one thing I always wondered about

Mine runs very well, always has.

I've always been impressed with the rush of acceleration that comes on from around 7000 rpm to redline. It's astounding for a motor this small. Took me a while to find it, felt like I was hurting it, running it that hard. That was before I realized this engine lives for abuse.

What is it exactly that causes that surge? Is it just some quirk of the powerband, an artifact of the hp and tq curves? I always likened it to the rush you get when you open the secondaries on a four-pot carb on a V-8, or something akin to a VTEC Honda kicking in.

What produces it? Is there some induction trick? Is this YICS? Or no?

I feel like I probably missed out on something well known about these motors...I musta been sleeping in class that day

 

I think it's probably more a case of cam timing and porting, etc. The reason I say this is because the non-YICS motors (like the 650 Seca) have the same reputation.

It may also be YICS contributing as it begins to flow enough to fully affect the incoming charge.

I know the YICS 550s really take a leap above 6K; once I get my non-YICS bike running it will be interesting to compare just HOW radical the surge is.

 

My non-yics 650 does the same thing. I too was slow to get it into the upper end of the tach and the first time I did it about threw me off the seat.

 

Yep - what fitz said. Cam timing, porting, etc. The Seca 550s for sure had it. It was even remarked upon in the press back in the day.

Very much like a two-stroke motor coming "on the pipe". In that case, it's when the porting, intake tuning, exhaust expansion chamber tuning all come together to make a rapid, non-linear increase in the hp curve.

A lotta fun, for sure...

 

An engine is essentially an air pump. How much air a given engine pumps is dependent on RPM and the intake and exhaust characteristics (Bigfitz).

Looking at the rear wheel Torque/HP curves for the XJ650G (NON YICS):

There is a steep rise in torque to 4K rpm (28 lb*ft). It then drops to 26 lb*ft at about 5K. It climbs gradually back to 28 lb*ft at about 7K. It then spikes up to it's peak of about 32 lb*ft at 8K. In that range from 7-8K the Horsepower goes from 37HP to almost 49HP.

8K is the point where the motorcycle will achieve maximum acceleration.

I'd be willing to bet that the valves (timing, lift, duration), intake and exhaust are tuned for best breathing at 8K. Above 8K the system cannot maintain the air flow to keep up the torque. The peak at 4K is probably because it is a harmonic of 8K.