[Diy_efi] The Hunt effect

[John Gross]

>That wasn't what I said.  Smaller volume means a
>quicker rise in end-gas presure, and thus end-gas
>temperature.  This can be easily seen in two engines
>that are scaled differently but are in perfect scale
>to each other, fueled by the same air/fuel mixure. 
>The one with the larger volume will make less power
>for a given displacement unless you advance the
>timing, and that is a combination of changes in flame
>front speed and the physical distance being covered.

You're ignoring some of the other factors when trying to scale two engines.
Friction losses, pumping losses, and inertial losses do not scale linearly
either.  I'm not saying that a physically larger engine doesn't require more
timing advance to reach MBT, but I'm just pointing out that there are other
factors associated with trying to provide scaled power comparisons.  In a
different email, you said something about MBT timing and part throttle.
Technically speaking, there is no such thing as MBT timing at part throttle,
as MBT timing is the timing at which maximum torque is produced at WOT (thus
the usage of "Brake" in "Maximum Brake Torque").  But it is most definitely
true that the most "efficient" ignition timing for a given RPM is a function
of throttle position. 

>A "true hemi" will have a hemispherical combustion
>chamber (i.e., flat-topped piston).  That is what the
>"hemi" refers to.  A piston that is domed does not
>make a "true hemi", but is necessary if you want to
>increase CR without increasing displacement and still
>keep the rounded head contour.

I disagree with this, as the "hemi" refers to the shape of the combustion
chamber in the *head*, not the shape of the entire combustion chamber.
Additionally, unless Toyota has come out with something I haven't seen yet,
they definitely do not make a true "hemi".  If you are referring to the
4-valve headed motors, those are not considered "hemi" chambers, but rather
pent-roof chambers, as they are truly shaped as a 5-sided pyramid (including
the bottom of the pyramid).