Jim and Listers,
I too am interested in building the 347 motor. A couple of considerations
that you need to noodle are Wear, Heat and Rod angle. In talking with a "Old
Ford Racer" guy named Jim. (He is the guy locally building the killer
"Alger" 302 powered) He indicated that the 347 suffers from severe piston
skirt wear and friction (read HEAT) due to the extremely short connecting
rod in comparison to the larger stroke (Rod / Stroke Ratio).
Example:
In a small Block Chevy going from the stock 5.65" rod in a 400 to a 6" rod
improved the Rod / Stroke ratio from 1.5 to 1.6 ...the equivalent of a stock
350 cu. In. motor. (Key point is that stock 400 Chevy's were notorious for
overheating and piston wear, the low rod/stroke ratio was a contributor).
The result was about a 15% INCREASE in Torque across the RPM range due to
the effect of increasing the Piston Dwell at TDC. This also did not result
in any extra wear on the engine components and the engine was very
insensitive to timing and jetting.
Compare to the stock 260 - 289 Rod/stroke ratio of 1.79 which is excellent
for wear, RPM and loading of the engine and contributes to the high amount
of low RPM torque. The 302 has a rod/stroke ratio of 1.69 and the 351 W has
a ratio of 1.702 which maintains the rod angle similar to the 302. The 347
however if I am not mistaken uses the 351 stroke of 3.5" with the 302 rod
length of 5.090" yielding a rod / stroke ratio of 1.45 which is lower than
the 400 in. Chevy.
Also, Jim indicated the motors are very touchy when it comes to ignition
timing due to the amount of Dwell the piston sits at TDC. This decreased
Dwell may in fact result in incomplete combustion and may not be as
efficient an engine as a 302 with a longer Dwell at TDC (better Rod/stroke
ratio). This means that you may not be able to "fill" the cylinder with fuel
as completely as the 302 motor with all other variables constant. While I am
still interested in putting the extra cubes into my Tiger I am concerned
about the extra friction that side loading the pistons will create and the
loss of piston Dwell. The longer stroke may also tend to lower the optimal
(read peak) RPM of the motor. The extra friction (heat) may be the
proverbial straw that breaks the taxed Tiger cooling system. I am curious if
other guys out there running the 347 have had cooling, wear or tuning
issues. I am still very interested in building the 347 but may be leaning to
a .060 over 302 with some compression and excellent heads for more reliable
HP with less heat at higher RPM.
On the other hand every Race motor I have worked with the underlying rule
that I have abided by is:
"There is no REPLACEMENT for DISPLACEMENT!!"
Any one else have comments??
Way to go Jim we are back talking performance stuff !!
Regards,
Tim Ronak
B382000680
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