[Supras] mystery knock

[Rockey Fox]

I've had only one episode where the knock was alarming right after
startup and it diminished rapidly after oil pressure built up. That was
a week or so ago and hasn't recurred. I've been running Pennzoil full
synthetic since the second oil change (1st at 100 miles, second at 3000
miles) after initial break in. I'm guessing I've put about 25k miles on
since the rebuild and the synthetic oil and filter is changed
religiously at 5000 mile intervals. It takes 6 quarts to fill due to
the oil filter relocation kit and the larger Ford oil filter.

I'll keep up with it and let the list know of any further developments, hopefully it's as Jeff predicted that something, probably a piston, is slightly smaller than it should be when cold. That being the case I should be able to continue driving without major failure for quite a few more miles.

Thanks for sharing your thoughts,
Rockey 91T

----- Original Message ----
From: "berniek at technicaldevelop.com" <berniek at technicaldevelop.com>
To: supras at supras.com; Rockey Fox <supr91tt at yahoo.com>
Sent: Monday, April 16, 2007 9:19:50 PM
Subject: Re: mystery knock

Dear Rocky and group:

    The knock problem may be a variant on a phenomenon I noticed before my
'90T blew the head gasket at 156K miles (have not yet finished rebuilding a
JDM core after 18 months).

    I had noticed piston slap for quite a number of miles before warmup,
which as in your case was when the engine was cold.  One surprising element
was that I had used synthetic oil since the car had about 38K miles on the
odometer, so wear should not have been a factor, especially considering the
oil jets used in turbo engines.  .

    I never went in to boost until full warmup, but I can tell you that with
light throttle loading when cold, the sound changed in vowel character, from
"oooh" to "aaah" as throttle opening was changed.  It almost sounded like
subdued diesel clatter except for the vowel character change.

    Another surprising characteristic that works against noise production is
the stock pistons:  They are autothermic (the have cast-in steel struts
under the wrist pins to control expansion for noise control when cold).  Yet
another surprising factor is that stock pistons have wrist pin offset to
load the major thrust side of the skirt, which JE and maybe other forged
pistons do not.  The offset was for noise control.  In American cars of the
60's and such, it was usually 1/16" toward the major thrust side.

    Yet, the sound (it could not really be described as a noise) disappeared
as the engine warmed up.  You might want to listen carefully under light
load conditions as the engine warms up.  I'm curious to know what you find.

        BernieK








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