Tim,
For non-oxigenated fuels, the stoichiometric air/fuel ratio is 14.7:1.
Maximum power, on the other hand, occurs in the range 12-13:1 and maximum
mileage in the range 16-18:1. With oxygenated fuels, the ratios are
probably a point or two lower. But, I believe the point of the power valve
is to transition between "economy" and "power" air-to-fuel ratios
corresponding to the difference between "normal" and "spirited" driving.
While you can use the idle vacuum reading as a guide for selecting the
right power valve, the best way is to drive around with a vacuum gauge and
note the vacuum readings under "normal", steady throttle; say, driving
steadily down the freeway at 65 mph. (OK, so that isn't normal for a Tiger,
but you know what I mean.)
Here is what they have to say at the Holley Web site:
"A stock engine, or one that is only mildly built for street use, will have
high
manifold vacuum at idle speeds. To determine the correct power valve the
vehicle should be driven at various steady speeds and vacuum readings
taken. The power valve selected should have an opening point about 2" Hg
below the lowest steady speed engine vacuum observed."
I think you can take this procedure a little further and note how much
variation of the throttle and vacuum readings is within the "normal' bounds
and when you would like to have a more positive response. There is
obviously a compromise involved here between responsiveness and economy. If
you want to be sure what Holley calls the "power enrichment system" doesn't
come on until you really mean business, then pick a power valve with a
lower numerical number. If, on the other hand, you want your engine to
respond as quickly as possible to your every nudge of the throttle, then
pick a power valve that opens just a point or two below where it runs under
most steady driving conditions. Or, if you really don't care about gas
mileage at all, just block off the power valve and run at maximum power
ratio all the time. That's the way a lot of racers run their carbs, which
makes a lot of sense on the track.
One thing I'm not sure about and will dig into it and find out is whether
the "power enrichment system" controls both the primary and secondary
metering systems, or just the primary. It doesn't really make sense to me
that the power valve would control the secondary air/fuel ratio since any
time you are using the secondaries you are most assuredly interested in
power not economy. I have also read that the power valve accounts for an
increase of about five jet sizes; thus, if you are running 65's, when the
power valve opens they then flow like 70's. Mathematically, it would make
more sense for it to be a ratio, not just an additive factor, but "adding
five jet sizes" is a lot easier to remember. In any case, the point is to
go from economy mixture to maximum power mixture.
When I ran my engine on the dyno with the S.D. Shelby Mustang club my motor
was running too rich. Of course, all bets are off because of the spark coil
problem, but I have tended to put jet sizes around 70-75 in the secondaries
because think it is safer to run too rich than too lean. Now, if the power
valve adds about five jet sizes, and if the power valve only controls the
primaries, then it would seem logical to put jets about five sizes larger
in the secondaries.
I did some reading at the site Ron Frazer posted, which is a damned nice
one. Their 11.99 project dyno results had them with 73's in the primaries
for 15:1 and 80's in the secondaries for 14:1. These are both a little lean
for optimum power according to most sources. The difference between 15 &
14:1 almost accounts for the change in jet size (78 versus 80 in the
secondaries). This would suggest that the power valve operates on both
circuits. However, they may have remove the power valve from their carb,
since I didn't see anything mentioned about changing it. Also, the large
jet sizes suggests they were running a rather large cfm carb for the 302
motor, but ratioing the sizes should give a reasonable approximation for
other carb/engine combinations.
BTW, I increased my primary jets to 65's and left the power valve at 9.5. I
like this setup the best of anything I've tried so far. Now, I'll see how
the gas mileage goes.
Thanks to you and the others that have come up with a lot of great info on
this subject.
Take care,
Bob
At 09:47 PM 8/13/00 -0500, Ronak, TP (Timothy) wrote:
>Bob,
>It would seem to me that there are 2 things at work here. Power Valves and
>the Vacuum Secondary Diaphragm Spring.
>The power valve (unless I am very rusty) controls the flow of fuel to the
>main fuel circuit only as an on or off switch. While an engine that dips
>below the value of the power valve at Idle will run "fat" and usually floods
>out and dies, my understanding was that that was the extent of the
>relationship of power valves in tuning. The correct valve is determined by
>letting the car idle at its slowest idle setting 650-850 (SB 4.5-8.5in as
>memory serves but I did a lot of high stall automatic cars) depending on
>cam. Then drive the car starting in gear and watch to ensure that you never
>go below that lowest value as a reading. Once you determine the vacuum at
>idle choose a power valve 1 full range higher. Keep in mind that the term
>"power" is a misnomer as it simply directs fuel to the main fuel circuit and
>it acts like a switch either on or off with it preferably off at idle.
>If you wish the car to "come on" earlier you need to install a softer
>secondary diaphragm spring in the vacuum secondaries. This will result in
>the secondaries coming in a little earlier. Another option is to play with
>the accelerator pump cams to tweak the squirters to better suit the demands
>of the engine under transition.
>I hope that helps.
>Best regards,
>Tim
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