Listers:
Too stupid to keep my mouth shut, so I will chime in on this
thread...
=:-o
>> 1) There were 2 reasons why electronic ignition was developed? Both
>> answers pertain to the consumer, or auto owner.
> It is cheaper to manufacture, install and maintain. Car manufacturers
will
> bend over backwards to save 1 cent on an item. An added benefit is that
> distributor-shaft wobble is no longer a main issue.
There are some other advantages; The improved accuracy due to lack
of points to wear - this means emissions will be more stable. Hotter
spark - you can pump some real energy through the coil with capacitive
discharge techniques, which means you can fire a spark plug no matter
how fouled it is. Again, an advantage for emissions. No moving
parts in the ignition system - much more reliable, in theory.
>> 2) Upon introduction: Hydraulic Camshafts were touted to be a technology
>> breakthrough. Why? Was it really a ruse? Today many people believe that
>> hydraulic cams have a grind profile that is far superior to anything
>> that could be achieved in a mechanical cam. Is this True or False?
> Hydraulic cams were introduced because they did away with the valve
re-shim
> at 500 miles on every new car. I don9t really know the answer to this,
but
> I would guess False.
The only draw back to hydraulic is that the lifters want to pump
up at high RPMs, which means that the valves never fully close. Thus,
hydraulic cams generally are not suitable for much beyond 6,500RPM.
The plusses of hydraulic are numerous; They run zero lash so
they are quiet, and the maintenance advantages are obvious.
What is less obvious is that hydraulics offer the potential for
more 'area under the curve' for a given duration, and hence better
power and efficiency. This is because a solid lifter requires a very
gentle 'take up' ramp, to take up slack in the valve train resulting
from the non-zero valve lash needed for valve train expansion.
As a result, at low lifts the hydraulic will have a higher valve
acceleration than a mechanical, ALL OTHER THINGS BEING EQUAL.
The differences are small in this area, but measurable. So
for a street driven car, hydraulic is clearly superior in every
regard. You would use a solid lifter cam only in a car that will
spend time above 6,500 RPM, in which case solid is your only real
choice.
>> 3) What does an air pump, (smog pump), do? Explain the entire
>> cycle...peripheral equipment!
> Sorry, I ripped off all smog gear on my car. The air pump is a notorious
> consumer of hp!
The air pump supplies oxygen to the very hot, but oxygen
deficient exhaust gas stream. This completes the oxidation process
and reduces unburned hydrocarbons and CO. There is no flame
present (if there was you would have a lot of popping sounds)
but the gases are hot enough that the reactions will proceed in
the presence of oxygen.
Incidentally, you must have a diverter valve for carbureted
cars with air pumps to shut off the air flow during deceleration.
If you do not, or the valve malfunctions, you get backfires during
deceleration as the unburned gas in the exhaust ignites (a flame
appears). Ask me how I know.....
>> 4) Why will roller rockers of an equal ratio open the valves further?
> I thought we covered this in a thread last week? And I'm not convinced
they
> do. (Nomex suit on!).
In theory, no. In practice the rocker ratio is varying slightly
throughout the valve lift cycle. Because of this, a roller tipped rocker
might differ slightly from a plain rocker as the valve is lifting. So
I *SUPPOSE* you could end up with *SLIGHTLY* more (or less) lift with
a roller rocker. Offsetting this is the greatly improved matching and
accuracy of a roller design, which should *MORE* than make up for
any (tiny) lift losses incurred. Add to this reduced frictional losses,
and the roller rocker is clearly superior. The only fly in the ointment
is that they cost substantially more.
>> 5) What performance gains will you achieve by installing a long
>> duration...(extended period of valve opening)..., high lift, camshaft in
>> an otherwise stock engine?
> The gains will come at high-rpm, because more gas-air mixture can enter
the
> combustion chamber than with the stock cam. Air has mass, and it takes a
> finite duration to move it. The hi-po cam allows the valve to open wider
> and for a longer period of time, allowing this mass to slide on in.
As they say, "It depends". All other things being equal, this is
true.
But if the cam is *TOO* big, you will end up with less power everywhere in
the RPM range, because the rest of the motor cannot keep up with the big
camshaft's demands at high RPM.
Also, if the lobe centers are closer together, a longer duration
cam might give more power *LOWER* in the RPM range than the stock cam.
Lift and duration are not the only cam parameters that affect amount of
power and it's location in the RPM range. Two other very important
factors are lobe center spacing (in degrees) and valve acceleration
(in inches/degree**2).
The above is why it is simply impossible to compare cam timing
and draw conclusions about which is "best" except in a very broad sense.
The cam manufacturers do not give enough information about their
profiles to make precise comparisons. You cannot even compare
the rated durations because there is no standard method for measuring
them. The only values you can compare are net lobe lift and duration
at a known lobe lift (most often 0.050"), and many cam makers will
not even give you that much information!
<snip>
>> 7) Suppose you have two 152ci TR engines side-by-side; same, make,
>> model. One engine has a compression ratio of 12:1 with a static
>> compression of 90. The other engine has a compression ratio of 7.5:1
>> with a static compression of 200. Which engine would you want in your
>> car? Why?
> Anyone on this list knows my love for power-bands and zero torque at low
> revs. The 12:1 motor of course!
Impossible to say. Assuming that both motors are properly
set up for their respective camshafts and compression, it would
depend on what you intend to do with the car. The 12:1 engine
would be all but undriveable on the street, the 7.5:1 would be a
sure loser at the track. You would also have a tough time finding
high octane fuel for the 12:1 motor.
>> 8) Is compression ratio related to static compression? Yes or No, and
>> why?
> Ooops. Better go back and read my books.
Compression ratio is usually taken to mean "Theoretical"
compression. That is the ratio of the stroke volume to the combustion
chamber volume, irrespective of valve timing. This can also be referred
to as static compression ratio.
Dynamic compression means the MEASURED compression, with the
engine crankshaft in motion. This value is heavily affected by camshaft
timing and engine RPM. Dynamic compression will always be at it's
peak when the engine is generating peak torque (not peak horsepower).
Dynamic compression is directly proportional to the volumetric
efficiency of the engine, and for a NA street engine, will virtually
never be 100%. (As I recall, 80-90% is a typical value). Racing
motors can actually exceed 100% under some circumstances, but only
in a narrow RPM range.
<snip>
>> 11) How are engine firing orders determined by the engineers? Is it
>> rhyme or reason?
> I always figured it was to minimize harmonics in the crankshaft, but I'll
> defer to the engineers. But this is a good question: why do BMW & Toy
> straight 6s have different firing orders? Length of crankshaft?
Right on. The position of the crankshaft throws will be chosen
to minimize shaking, rotational couple, and rocking couple as the
crankshaft spins. The firing order will then be optimized to further
minimize these effects, and to evenly spread the power strokes around
the rotation to further minimize shaking. The crank damper is used
to minimize torsional forces on the crank, and improve engine longevity.
All of this gets much more complicated if you are running balance
shafts, which rotate at 2x in the opposite direction of the crank. =:-)
>> 12) If the piston to be fired is approaching compression...(15 degrees
>> BTDC)... and you have the timing set to fire at 10 degrees Before
>> TDC...why doesn't the piston travel back down...moving in the direction
>> from which it just came...?
> Ummm, inertia from all the rotating mass?
Inertia is correct. Also bear in mind that combustion does not
occur instantaneously. In fact the flame front only moves across the
combustion chamber at about 100mph or so.
The fact that the ignition begins before TDC means that there is
some negative work delivered to the piston (the ignition event adds
additional downward force to the piston as it nears TDC). This is more
than made up for by the positive work on the power stroke, but it does
represent lost efficiency and power. This is why a compact combustion
chamber with lots of swirl and tumble is so desirable; it allows the
combustion to complete as quickly as possible, reducing the amount of
advance needed, and hence reduces negative work. Better mileage and
more power.
This is also why Kastner recommends decking the TR6 block; It
decreases the clearance between the piston top and the combustion
chamber's squish zone, increasing tumble. It means more power and
efficiency.
Ciao,
Vance
------------------------------
1974 Mimosa Yellow Triumph TR6
Cogito Ergo Zoom
(I think, therefore I go fast)
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