[Diy_efi] The Hunt effect

[John Gross]

>> 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").

>Mean Best Timing, as I understand it, is the ignition
>advance that gives the greatest power output for a
>given set of conditions.  These conditions would
>include throttle position.  I could simply change the
>size of the intake tract on the motor and achieve
>exactly the same effect and impact as having a
>partly-closed throttle.  Your definition would mean
>that if I restricted intake flow by a throttle plate,
>I could not achieve MBT, but if I did the same thing
>with a change in the intake tract itself to create an
>equal restriction, I could.  That doesn't make a whole
>lot of sense to me.

Ok, this is a misunderstanding between ourselves on definition.  To me, MBT
means "Maximum Brake Torque" timing for an engine, which is the ignition
advance for maximum torque output at a given RPM under WOT conditions, thus
the reference in the name to "Brake Torque".
 

>>> A "true hemi" will have a hemispherical combustion
>>> chamber (i.e., flat-topped piston).  That is what
>>> the "hemi" refers to.

>> 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.

>Then it is meaningless.  Also, there is on combustion
>chamber in the head, unless it is separate from the
>bore of the piston by a smallish port.  It's not a
>"combustion chamber roof", it's a "combustion chamber"
>-- a 3-dimensional object defined by all its external
>limits.  That includes the piston crown.

>How can something that is not mostly sealed off from
>other things be referred to as a "chamber"?

>If someone specified "hemi head", that would stll not
>be terribly accurate, as a hehisphere is by default a
>closed, contained volume.  However, the intent of the
>person describing a head that way would be reasonably
>clear.  The current Mopar engines, AFAIK, have neither
>a hemispherical recess in the cylinder head, nor a
>hemispherical combustion chamber.  I am led to believe
>it is a marketing gimmick.

There is most definitely a combustion chamber in a cylinder, where the last
"face" of the 3D volume is an imaginary face coplanar with the deck of the
head.  While it is in and of itself not a physically closed volume, it is
ALWAYS called the "combustion chamber" because that makes a whole hell of a
lot more sense than saying "that part of the engine's combustion chamber
that is the undulating pocket in the deck of the head"....

>> 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).

>This was in a post I replied to.  I never said
>anything about Toyota motors, and since almost all the
>engines I deal with have had pent-roof chambers since
>the mid-80s, I'm pretty well-versed on what they are
>and look like.

Oops, sorry for not knowing what you've been working on for the last 20
years...