What's the point of a soft RPM limit? According to my Hydra manual, it cuts fuel. I haven't done a lot of it, but I've seen knock on my knocklite right where the soft limit comes in, which makes sense to me because I'd think that trimming fuel at 18psi would cause some detonation.

So, what's the point of this setting? Do I have anything to lose by setting it to the same RPM as the hard limit?

 

Soft limits are usually a spark retard, pulling straight fuel is kinda harsh.

The point is to slow the addition of rpms as you near the redline, makes actually hitting redline a lot less harsh (on you and the car).

 

I have my soft limit set to retard spark to 10* at 7000 rpm, with the hard cut pulling fuel at 7200. At 100 kPa under WOT (not turboed yet) I'm usually running 28* advance, so you definitely notice when you hit the limit and it retards to 10*, but it's not so hard as to upset the balance of the car too badly if I happen to be accelerating out of a corner when it happens. Also because the car is not yet turboed, that 200 rpm difference between soft & hard cut gives me enough time to react and get the shift done.

In practice I don't make a habit of hitting either soft or hard cut. The stock tach reads a little high and I usually shift when that reads 7000 or just a hair above, which is only about 6800 rpm in Megasquirt's reality. I'm only doing HPDEs so far, I just don't need to wring the last 100-200 rpm out of it while hitting the limiter before every shift.

 

I don't use the soft-cut limiter. Retarding timing like that up top just spikes the EGTs. If I rev to 7400, I want the car to make as much power as possible at 7350rpm.

 

I think soft cut might be beneficial if you want to set up flat-foot-shifting.
Though most new ecu's have that as a seperate feature.