Hey Mike, what causes.....

[Paniolo]

The low end dip in some dyno TQ curves? I've been studying the library of dyno charts, and see some with a low end dip and a bell shaped curve. Is it the cams, heads, tuning, build or all of the above? Would the dip be acceptable in a light FX but not a heavy FL, along the lines of a flat broad TQ curve for a bagger and a peaked bell curve for an FXR?

Thanks

Mark

[Mike]

You are exactly correct! The shape of the torque curve is a function of cam overlap, duration, compression ratio and exhaust back pressure. All of these affect how the air moves through the motor at any RPM. Of these, the exhaust has the greatest influence. As back pressure is added the dip gets better but at the cost of both peak hp and tq. Increasing compression helps make up for some losses that come with late intake valve closing. This is why it is really important to properly match comp ratio's and cams. It really is about how much of a low end dip are we willing to accept to get the higher end hp/tq we expect. Lighter bikes can accelerate through this dip easier and faster, so it tends to be less noticable and more acceptable. For heavier bikes, it is more of a problem. It simply takes longer to accelerate through the dip and so it is more noticable because you spend more time experiancing it. Touring riders tend to want to leave their bikes in high gear and resist downshifting, this can put them right in the torque dip at cruising speed or when climbing a hill and the bike may not have enough power to pull itself out of the dip. If we did not have dyno's we would likely install more restrictive exhaust systems because seat of the pants feel would be better (we would not know the hp losses without the graphs). For those that are willing to downshift to take advantage of the greater torque after the dip would realize much more acceleration. So then it becomes about riding style and what it is that we want the bike to do for us.
Regards, Mike