Scott asks...."So what does it all mean? How
do I put this information to practical use?"
(or words to this effect).
it all depends on how you are tuning YOUR
application.....is YOUR emphasis on low and
mid-range torque (port velocity) or high end torque and HP (mass flow
Smaller valves and less radical porting favor port velocity over mass flow,
generally speaking, bearing in mind that
"torque gets you there, HP keeps you there".
Fr'instance, years ago, when limited to the
Spridget 4 speed and 3.9 diff, I needed a
car which would be comfortable cruising at
70 with power to spare for passing, etc.
for limited access highway driving as I
was commuting to the office once or twice a
week for 4 or 2 day trips.
So I tuned for 3.5-4.5 and up, obviously favoring the higher rpm range, at the
expense of bottom end, low mid-range. IOW more toward "mass flow" at the
expense of some low end port velocity.
I went with an APT big valve head, VP3
cam, 1.5 roller, big bore header, 2" exhaust.
So the car was nothing under 3k for "in town" driving (low port velocity in the
lower rpm range) but once it got to 3.2K
and came up on the cam, it could go to 7k
Thanx to the availability of CAM2 for a 100RON "mix" with unleaded hi-test, I
able to bring up the bottom end a bit by
going to 11.0-1 CR which bought me not
quite stock performance under 4k for town/country puttering around. Without
this "squeeze" to improve low-end "trapping efficiency", I'd be slower below 3k
for sure. A stock 1275 Sprite would probably be more "quick" around town below
3k, even so.
Anyway, an 8 port head or cross-flow
would have been very nice for this state
of tune, considering it will probably give
you in the region of another 20HP or so which is a substantial per-centage gain.
'Course, it's a lot of money so I went with
the modified APT cast-iron product instead
as I wasn't looking for that "last available
HP" to that extent ($$$$).
Today I'm retired, work part-time locally
and hardly see a limited access highway.
99.9% of my driving is town/country use and I've gone 5 speed as these have
become available. Bearing in mind the first
3 gears are of a lower ratio than a Spridget ribcase, I can go with a 3.7 or 3.5
diff for good low end without hi-rpm...I don't need a hairy cam for the high
range with a 5th (overdrive) gear, either.
So NOW I want excellent low and
low mid-range torque. I want something
"quick" below 4.5K and the hell with the
high rpm ranges...don't need it. I've got
a 5th gear and a 3.7 for occasional
limited access highway use.
I need a car thats "quick" and
fuel efficient around town. I need more"port velocity" rather than "mass flow"
capability while spending "reasonable" bucks.
And gas is getting more expensive. I don't
want to hafta use CAM2 racing fuel for a
100RON mix if I don't need it to get what
I want in terms of "quick" around town. So
I need to limit compression ratios to some
Boy....does the aluminum head with stock
valve sizes ever fit the bill for THIS application or what!!!!? You bet it does
perfectly. Flows about 30% better than the
stock head, handles heat better so I can go
about a full point higher in compression ratio than I could with the cast iron
on same octane fuel. For a "reasonable" price for all this at about $1k. And
it's lighter weight with all the foregoing advantages is a nice "freebie"!!
I'll team it with a Cooper S cam and following Vizard's advice, advance same
4 degrees for Metro inlet timing (even
better low, low-midrange torque). In keeping with the "port velocity" over "mass
flow" objective, we'll use a smaller, medium
bore header this time around, along with
a smaller diameter 1.75" exhaust. I don't
need the 1.5 ratio rockers, but I already
have this assembly and it's worth a pound
or two of torque at "high velocities".
Answer your question to some extent?? :)