From MBaxterm Wed Feb 16 17:42:24 2000
From: Michael Baxter 

Jamie.L.Phillips writes:

>> To keep things simple, I will just go over information on the 360 engine
you mentioned (2bbl) from '71 to '72.

In 1971 the 360 (2bbl) had its intake valves opening at 18 1/2 degrees.
In 1972 the 360 (2bbl) had its intake valves opening at 14 3/4 degrees.
This indicates that they were indeed adjusting valve timing during this
period.

In 1971 the 360 (2bbl) had an "advertised" compression ratio of 8.5:1.
In 1972 the 360 (2bbl) had an "advertised" compression ratio of 8.5:1.
No change, no effect.

In 1971 the 360 (2bbl) had an "advertised" horse power of 245@4400.
In 1972 the 360 (2bbl) had an "advertised" horse power of 175@4000.
A significant drop in horse power.
<<

In 1971-'72 the SAE standardized the way engine power was measured.
AMC adopted the standard for the '72 model year. The rule of thumb to
convert from Gross HP to SAE NET HP is to take 30% off of the Gross
numbers.

Your reference is right about the 8.5 to 1 for '71 and '72. At least
that's what AMC reported. It may be the 304 which had 8.4 to 1 that I
was remembering. AMC increased the size of the combustion chambers in the
big valve dog-leg heads from 51cc to 58cc (I measured 57 cc on a '76 big
valve head) during the '71 model year. That's when I believe the
compression ratio dropped but, AMC seems to have continued to report 8.5 to
1 until '73. Maybe the engineers changed the pistons and maybe the
marketing dept. took advantage of a lack of communication? I don't know and
I've never researched the Parts Book for piston part number changes.

AMC engineers were working on the 360s performance just after its
introduction in '70. My educated guess as to why AMC increased the
combustion chamber volume in '71 was to unshroud the valves somewhat and
improve flow. I've never had a '71 small chamber head to play with on the
flow bench, but I'll bet it doesn't flow as well as a '72-up head. I
believe the extra flow offset the small compression ratio loss. Raising the
compression ratio from 8.0 to 9.0 would only net a 2% increase in power
anyway. 

AMC also bumped the exhaust valve from 1.625" to 1.68" in '71 and they
dumped the log type exhaust manifolds for the much better flowing ones we
have today.

And it looks like I missed the cam change by 1 year. I have the following
for '72, '76, '79 and '91. But please post the full '71 cam specs. because
I'd love to see what they were doing back then. I'm rounding the event
timing to the nearest degree BTW. AMC was a little anal with their valve
event timing specs. IMO. You can only get so accurate with a cam driven by
a timing chain and 1/4 degree increments is asking a lot from an inexact
drive.

IVO 15 BTDC
IVC 69 ABDC
EVO 57 BBDC
EVC 27 ATDC

Overlap 41.5
Duration 263.5 intake, 263.5 exhaust (seat to seat, AMC doesn't specify)
Lobe Separation Angle 111 degrees. (calculated by me)

Intake & Exhaust lobe lift is .266"

It's a really good grind IMO. I'd like to see a little more lift and it
could be considered too radical for moving 4,500 lb. plus vehicles. But for
an OEM grind, they don't come much better.

Robert Barry  writes:

>> What were the closing events? This could be a retarded timing gear, if
not an actual change in the camshaft (a book with part #'s would be good to
solve this).
<<

Of course, If AMC just changed the keyway as James pointed out the
possibility and left the grind the same, it would still affect performance.
Valve event timing is in crankshaft degrees and if you advance or retard it
relative the to the crankshaft, the performance will change. However 4
degrees retarded is only moderate and we could easily achieve '71 cam
specs. with an aftermarket gear with 4 degree either way and straight-up
keyholes if this turns-out to be what the AMC engineers did. I've theorized
on the list before (with John mainly) that what someone really ought to do
was advance the stock cam at least 8 degrees. I believe that'll pick-up the
bottom-end, flatten the torque curve and increase mileage. At the expense
of the top-end though.


 Michael Baxter,
 From Reno, NV USA on 16-Feb-2000