Re: [AMC-List] Continuing Saga of Doc the Jeep, Part II.5

[adh@xxxxxxxxxxxxxxxxx (Sandwich Maker)]

" From: "Jim Blair" <carnuck@xxxxxxxxxxx>
" 
" 
" Here's a little something to put in your pipe: Altering the cam advanced 
" also mods the power band (as in where it starts/stops in the RPM band) I 
" used the 3 key set in my '66 Fairlane's 289 and couldn't get any top end 
" RPM. It had brutal power to 1700 RPM then fell flat... It also pinged like a 
" banshee! Retarded the cam and voilà! no more probs!

that rings bells...  advancing the cam moves the power band lower,
retarding it moves it up - a common racing tuner's trick.  but as a
side effect it would also affect cylinder pressure, and that's where
you could get your pinging!  it works with the inertia of the intake
and exhaust streams; at high rpm when the gases are really moving,
leaving the valves open a little longer helps cylinder filling and
scavenging, but at low rpm the gases can blow backwards before the
valves close if the cam is retarded.

[side note: if the engine runs best with a lot of cam retard or
advance, it's a sign you need a different cam]

the '87-'90 jeep 4.0 efi has a knock sensor and egr.  the '91-5 efi
has neither, but uses the same cam - retarded something like 8
degrees iirc, to reduce pinging and give it a sort of 'poor man's egr'.
btw this cam is pretty radical compared to older amc six cams.  the
'76-80 2bbl and '81-'90 cams are in the upper 250s, 257 and 259 deg
iirc.  my '66 cam is 244 and i have a book that lists that as stock
replacement for all '64-'79 except the late '70s 2bbl.  when i went to
build a hot 232 up in the early '80s, the most radical hydraulic cam i
could find was 278 deg.

the '87-'98 4.0 cam is 280.  in '99 they reduced exhaust duration to
the 260s.
________________________________________________________________________
Andrew Hay                                  the genius nature
internet rambler                            is to see what all have seen
adh@xxxxxxxxxxxxxxxxx                       and think what none thought

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