MKIIT: Cam and Turbo Specs

From: uunet!cal-tech.com!cal (Cal Smith)
Date: Wed, 7 Aug 1996 17:25:13 -0600
Subject: MKIIT: Cam and Turbo Specs

        Richard Soluk was kind enough to send me OEM cams and a CT-26 Turbo
from his MR2T. Measurement of the parts yielded the following results:

3S-GTE Camshaft Specifications:

Duration @ 0.010" lift: 260.0 degrees
Duration @ 0.020" lift: 211.0 degrees
Duration @ 0.050" lift: 160.5 degrees

Max Lift: 8.52mm (0.335")

Error: + - 0.02mm or + - 1 degree--about as accurate as we're ever going to get.

        Both pairs of lobes for each cylinder's intake and exhaust
valve-pairs have the same lift, duration, lobe profile, and lobe center.The
intake and exhaust cams have the same lift, duration, and lobe profile. In
fact, the only difference between the two is that the intake cam has a
notch cut into the undriven end to turn the distributor. FWIW, two intake
cams could probably be used on the same engine with no ill consequences as
the intake cam characteristics are identical to the exhaust cam and the
distributor-drive slot should not interfere with any oil seals, etc.
        The cams were used and obviously broken in (Polished shiny and
smooth at certain points of each cam lobe). The acceleration face of the
camshaft showed a measurable, albeit small, amount of wear, right before
peak lift. The data should still be valid and should not be more than -
0.02mm or 1/2 degree vs. a new OEM cam at any of the above lift points.
        One very interesting thing is that this engine has a power-peak of
6000rpm with very conservative-duration cams. This is not as huge of a
shock as it first seems, as small-displacement engines naturally shift the
power-peak up (all things being equal) when compared to big V-8's, but it
is still a big deal. To get a 6000rpm peak in a 350 Chevy would require at
least 310 degrees of duration (measured at 0.020" lift) and the engine
would idle horribly and have no low-end power.
        HKS has cams for the 3S-GTE, advertised as having 256-, 264-, and
272-degree duration (presumably at 0.020" lift) and the 264 is regarded by
many as being the longest-duration unit of the three that is
practical/streetable. The lift in all the three is more than stock, but is
nothing to write home about (9.0mm, 9.2mm, and 9.4mm, respectively). Where
you get more power in these cams is by holding the valves open longer, not
higher. The power peak undoubtedly moves up, but HKS has no quantified data
as to how high or how the overall power curve changes. The cams are about
$600/each and take a couple of months to materialize from Japan, so don't
expect any under the tree at Christmas. We'll probably never see hard data
on the profiles of those cams, either.
        Richard Soluk is the only MKIIT person I know with aftermarket cams
and maybe he or someone else can help shed some light (as in real numbers,
not..."the dude at AEM said you'd pick up about 1000rpm and 75HP with their
secret grind") on this.
        For those who are thinking about changing timing belts, also
consider advancing the cam timing a bit. The factory made the cams/drive
cogs so that you could change the timing + - 2.5 degrees and + - 5 degrees
by simply shifting the location of a dowel pin. A 2.5 or 5 degree advance
should fatten up the midrange torque without hurting the top end much at
all. The shape of the torque curve will change, but basically, your E.T.'s
will drop a little and the car will be a little more fun to drive.
        If you are thinking about doing a shim job on the valves, remember
that any clearance over the minimum specified cold-clearance will waste
what cam duration you do have. I would not lose too much sleep over it, but
if your clearance is at the wide end of the spec, 0.010", rather than the
tight end, 0.006", your effective duration at 0.020" lift will be cut from
211 degrees to 201 degrees. Do NOT reduce the clearance to less than 0.006"
cold under any circumstances--there are better ways to gain duration.


CT-26 Turbo Stuff:

        I measured the exhaust housing of the CT-26 and found that the A/R
ratio of the turbo is between 0.48 and 0.50 (OK, OK--it came out to be
0.49! ;-) ). This number could be helpful if you get an upgraded
aftermarket turbo and want to know where to start with turbine housing
selection. You might not be perfect on the first pass, but at least you
know what the stock unit was and can judge from there.


Sorry I've been idle for a while--the wife claimed other things in life are
more important than the MR-2...Yeah, RIGHT!

Cal Smith

P.S.-Use this information freely, but please give me credit as the source
if the information is to be passed on, published, or posted on the
Internet.