[Diy_efi] The Hunt effect

[Adam Wade]

--- John Gross <jogross3 at hotmail.com> wrote:

> 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.

Well, I wasn't trying to make a perfect engineering
comparison.  I'm well aware that I ignored those
things; I was making a point, and adding that level of
detail would distract greatly from the point I was
making.  While not absolutely precise, I believe I've
covered the major points with regard to burn rate
between the two differing cylinders mentioned in my
post.

> 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.

You seem to be meaning something quite different from
what I meant MBT to represent.  The "brake" in brake
torque refers to the method of measuring the torque (a
water or other sort of brake), however.
 

>> 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.

> 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.

| Honda GL500 Interstate (Slug)    Kawasaki Zephyr 550 (Daphne) |
| "It was like an emergency ward after a great catastrophe; it  |
|   didn't matter what race or class the victims belonged to.   |
|  They were all given the same miracle drug, which was coffee. |
|   The catastrophe in this case, of course, was that the sun   |
|     had come up again."                    -Kurt Vonnegut     |
| M/C Fuel Inj. Hndbk. @ Amazon.com -  http://tinyurl.com/6o3ze |




		
__________________________________ 
Yahoo! FareChase: Search multiple travel sites in one click.
http://farechase.yahoo.com