Jackson....
I seriously doubt you will have any sort
of a valve clearance problem at all, even
with flat-tops, BUT....I don't know the
combustion chamber capacity of a Longman GT17 head.
The factory never went beyond a min.
cc capacity of 16.4cc's. I have no idea
what Longman does in this respect with
any of their heads, inclusive of the one
you've selected. So I am reluctant to
say for sure with 1.5 rratio with their head.
In any event, you will need to ascertain
this information so as to accurately
reach your target compression ratio
with your low compression application.
So....while you've got the supplier on the
phone, you might as well ask about
"clearancing" of piston/valves with 1.5's,
11cc dished pistons and CR's up to
say, 10.0-10-5 to 1.
But, before you do, be aware the VP6
and VP7 cam profiles were optimized
for the stock 1.25 rocker ratio, NOT
1.5's. That doesn't mean you can't use
1.5's of course. But staying with the
more "profile compatible" 1.25's will
do at least four things for you.
1. greater mixture velocity for the lower
range (below 4-4.5K) = "torque" or
"response"....all other things being equal.
2. you'll knock .065 off your valve lift...
for sure, no piston clearance problems,
unless the Longman head is virtually
a pancake!
3. "1.5" lift will result in better mass flow
at higher rpms but, higher lifts are harder
on the cam and valve train in general...
so you'll have less parts wear and tear
with the cam's intended 1.25 rocker
ratio.
4. more quiet operation of the valve train,
since there will be no "cushion" of
lead to "soften" the impact of the valves
closing. On today's unleaded fuels, 1.5
rockers are VERY noisey, relatively speaking.
I have 1.5 bushed roller-tips and
am running .018 on a Cooper
"S" cam (normally .015) with an allum.
head and the first time I opened it up
I thought for sure I had bearing failure.
Went immediately back to the shop.
"Valve clearances can't be right, Tom"!!
"Oh...they be right, Parrot Breath"....
and they were!! Time for Dynamat!!
Now...as to target compression ratio
on today's 93PON. Basically you are
talking 96RON, conservatively speaking.
So you're OK with 30-32 degrees of total
advance, more or less, even at 6.500 revs
as long as you don't hold it there up to
9.5-10 to 1 with colder plugs than
stock, assuming a cooling system in
good order and a decent med. bore
LCB and 1.75" exhuast system to get
the mass flow heat out efficiently, without
compromising low end torque.
9.5-1 with 1.25's and 10.1 with 1.5's.
With 1.25 rocker ratio, but
without the upgraded exhaust, I'd
stop at 9.0 to 1. With an allum. head
I'd go to 10.0, personally, and "balance"
this with "cooler plugs" if need be. Last
thing I would want to do is have to "balance" with a "softening" of the
ign.
advance curve, though I might "adjust"
the distributor's total advance curve by
"shortening" 2 degrees (4 crankshaft degrees) without changing the
intial setting. Cylinder filling at low end with
the VP7 profile will be less than with
stock so a little more advance at the
low end is ok, despite the increase in
static compression ratio. At the top
end cylinder filling with be relatively
greater than stock and we'll be getting
good heat, even with a better exhaust
and colder plug, considering the increased
compression ratio. So a little less total
advance than stock MIGHT be called
for.....maybe not!
Years ago I used 11.0-1 with a VP3
cam with the iron (19cc) head, 1.5's
and "flat-tops" (no valve clearance problems, Jackson) and everything
else like carb and exh system to match
but I was running 2 gallons of CAM2
110 with 5 of 96 (93PON) for 100RON
and factory racing dizzy "capped" at
30 degrees. Excellent...went like a bandit!
Today we use straight 93PON in the
summer and 89PON in the winter.
I go with low and mid-range
torque over high rpm horsepower and
cam accordingly. Everything happens
under 6k and I haven't even installed
the 45DCOE, yet. The engine was tailored
for a Judson, but the application is not
compatible so back to the previously used
45DCOE (suitably calibrated) for the present 8.3-1 motor. I don't worry
about
octane or total ign. advance anymore! :)
Speaking of carbs, Jackson. What are
you gonna be using? If stock 1.25X2
set-up...H6 or M needles and red springs
should be pretty much in the ballpark, at
least to begin with. An A/F ratio set-up
should point the way from there....what
size intake valve is the GT17 using?
Cap'n. Bob (Ret'd)
'60 Frog
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