Ever since I test-drove my miata before I bought it, I found
out it has noticeable extra low-end torque than most other miatas
I was picking. I have even wondered if the previous owner had
swapped the engine with the detuned version that goes with the
automatic transmission. My intensive investigation on the engine,
fuel system, etc. later on ruled out that possibility and any
out-of-tune situation. Even after I put in the Trust exhaust and
customized the air-intake with removed resonance chambers,
the power-to-torque ratio was still lower than my RWD Toyota Corolla GTS. On the other
hand, the 180,000km-, 10-year-old-Corolla was equipped with
variable intake runners which are supposed to bring back some
low-end torque for its 7500-rpm high-revving engine.
While most aftermarket power mods usually attack the
characteristics of a stock engine or assume one with other
'expected' power mods, the HKS adjustable cam gears, on the other
hand, work on any engine mutation. In other words, cam gear is
not some mod that has been well-tuned for 'plug-and-play', but a
mod that requires the user to 'test-and-tune'. My point is that
cam gear is not a mod for the general public unless you are
encountering a situation like mine or want to alter an engine's
characteristics quite different than what most other people
prefer.
Installation notes
Installing the cam gears takes
less labor than replacing the timing belt. No need to drain
coolant or remove the crank pulley. However, the valve cover, the
upper belt cover and the middle belt cover are needed to be
removed to facilitate the installation. Before installing the cam
gears, loosen the six bolts and make sure the center plate can
turn freely relative to the gear. There is not much clearance
between the cylindrical wall of the center plate and the gear
itself. If it requires some force to turn the center plate, it
will be quite difficult to adjust a precise timing you desire
when the engine is hot. One important note, before tightening the
bolt on the cam gears, check that the key-holes on the cam gears
are big enough to fit the key from the camshafts. The key-holes
on my cam gears needed to be enlarged by 1 drill-bit size.
Adjust the cam gears to 0 degree positions before
re-installing the timing belt although it's tempting to make
initial adjustments before they are in place. The need of
zero-deg initial adjustment is because Mazda suggests adjusting
the timing belt tension by loosening the tensioner bolt when
crankshaft is at 1-5/6 turns after cylinder No. 1 passed TDC.
This is a special position that makes the camshafts apply certain
tension on the tensioner bolt. If the cam gears are not set at 0,
the 1-5/6 turn position will be meaningless and the tensioner
bolt will not apply correct deflection on the timing belt.
Now it comes the optional part. Since it
requires removing the valve cover to remove the upper belt cover
for cam timing adjustment, you may want to leave the upper belt
cover off before having a satisfactory setting. Alternatively,
you may also have the front part of the valve cover cut to nicely
expose the cam gears. For me, reversibility has a high priority
in my design criteria on car mods. Until I can't resist to expose
the beautiful cam gears any more, I just leave the valve cover
for now and have the upper belt cover cut in halves. The upper
belt cover has four mounting bolts. I picked a line to cut so
that the lower half stays in the car with two mounting bolts
while the upper half uses the other two bolts to fix it to the
car and does not require removing the valve cover. Although I
cannot uncut the belt cover, I can at least put the two halves
back in place. It's still kind of 'reversible'.
Adjustment
In order to see the result of different intake/exhaust
advance/retard settings more clearly, a dyno is almost a must.
But for me, I wanted to alter the engine's torque-to-power ratio
just to make me feel more comfortable with. After trying various
settings, I provide here a little guideline how different
settings affect the engine's characteristics.
Advance in general helps low-end, retard in general helps
high-end. Overlap also helps high-end but may do more harm to
low-end than can be re-gained by the improved high-end.
Basically, I would say that the factory cam timing has not much
room for adjustment if you want a smooth overall response.
When I first adjusted the cam gears, I started by retarding
the exhaust cam by about 6 deg and advancing the intake cam by
the same amount in order to increase the overlap. (Remember to
re-adjust the ignition timing if you changed the intake cam
angle, or exhaust cam for 1.8l engine). The first thing I knew
after starting the car was that the engine was very rough even at
fast-idle speed. I did not want to warm up the car that way and
decided to try a more conservative setting first. Then, I
mistakenly advanced both cams instead of retarding them. No
wonder why I felt like the engine's high-end was even worse than
before. So the next trial was of course with both cams retarded
(to about 6 deg each). Fast-idle quality was slightly worse than
stock. High-end power seemed a bit improved but it might be an
illusion caused by the deteriorated low-end torque. Later, I
tried adjusting the cam gears one at a time and also re-tried the
increased overlap setting by retarding exhaust a little and
advancing intake a little. After a number of adjusting/testing
cycles, I settled with 0 deg intake and about 6 deg retard
exhaust. This may not be the best setting to suit my needs, but
at least I am a little bit more comfortable with the new
power-to-torque ratio. I will try out different settings later
with the help of my home-made dyno simulating device.
March 98 modification, January 99 review
It's time I decided to nicely expose the
adjustable cam gears to ease tuning. The valve cover was
mutilated and hand-polished. See Mutilated
and hand-polished valve cover for more details.
Since the December 97 review, I have had a great number of
experiments with different cam timings. The basic idea stays the
same where retards on both cams means shifting power band towards
higher rpm, as is the similar result of more overlap except that
the engine is more sensitive to degrees of overlap than retarding
angles.
Terence and
I found out his engine with HKS 256-degree camshafts are more
sensitive to cam timings than my engine with stock camshafts.
While 3 degrees retard on the HKSes could mean shifting the power
band too high on the rpm range (power didn't drop at fuel-cut
7200rpm), 3 degrees advance on the HKSes could mean bringing the
peak power rpm from 6800 (0/0 deg on the HKSes) back to 6600. On
the other hand, I could dial in my cam gears to 8/10 deg retard
(8-deg retard on the intake cam and 10-deg on the exhaust cam)
with peak power rpm shifted up not more than 300.
Looking
back at my December 97 review, I don't know how I got the 0/6 deg
setting. I tried it lately and it seemed to produce too much
overlap to make the engine run rough. Anyways, I'm now settled
with some different timings. For street use, I like to run -1/1
retard (1-deg advance on the intake and 1-deg retard on the
exhaust) or 0/2 retard. Sometimes when I want a bit more
aggressive, I would try anywhere from 2/3 to 4/6 retard.
Sometimes 6/8 retard seems to give the kind of feel of Terence's
HKS cams, in smaller magnitude though. But other times, the
engine feels even worse overall than something like 4/6 retard.
Installing the cam gears takes
less labor than replacing the timing belt. No need to drain
coolant or remove the crank pulley. However, the valve cover, the
upper belt cover and the middle belt cover are needed to be
removed to facilitate the installation. Before installing the cam
gears, loosen the six bolts and make sure the center plate can
turn freely relative to the gear. There is not much clearance
between the cylindrical wall of the center plate and the gear
itself. If it requires some force to turn the center plate, it
will be quite difficult to adjust a precise timing you desire
when the engine is hot. One important note, before tightening the
bolt on the cam gears, check that the key-holes on the cam gears
are big enough to fit the key from the camshafts. The key-holes
on my cam gears needed to be enlarged by 1 drill-bit size. 
Akimoto cone filter
with custom-made adaptor