Listers,
Tim Ronak has been having a problem posting to the List. He has now
realized that forwarding his messages through the company computer doesn't
work because the list server rejects any mail that doesn't have the exact
same return address as when you registered. However, until he rectifies
this problem, he asked me to forward this posting he sent directly to me.
(I'm not nearly so fussy about the mail I accept. ;-)
>From: "Ronak, TP (Timothy)" <Timothy.P.Ronak@akzo-nobel.com>
>To: "'Bob Palmer'" <rpalmer@ucsd.edu>
>Subject: FW: Pumped Up Compression Test
>Date: Tue, 24 Aug 1999 12:35:28 -0500
>X-Mailer: Internet Mail Service (5.5.2448.0)
> >
> >
> > Bob and Listers,
> > I have been on the road a lot lately but I saw the older posting and just
> > had to put in my 2 cents on the high cold and low hot cylinder pressure.
> > My assumptions:
> > In both cases you had ALL of the plugs out. The engine rotated at the same
> > speed. The Carb was propped in Wide Open Throttle (WOP). The choke was
> > held open and the aircleaner removed. I would propose that the Hot reading
> > is the correct reading.
> > I have had a lot of experience with this as I use the compression as an
> > indicator of the health of the engine, kind of like taking the temperature
> > of a person. Given identical test situations the Hot engine should have a
> > higher value than the cold due to the better seal provided by a larger
> > piston and tighter end gaps on the rings. This assumes that the Valve
> > seats are in good condition but normally if they seal they seal hot or
> > cold, unless there is a crack in the valve face or seat that can open
> > under temperature. I read one posting that oil could have leaked down the
> > guides and could be raising the compression on first crank. If this is the
> > case there would be a big puff every time you start the car and you would
> > probably see this Bob as I know that you are highly mechanically inclined.
> > I gave it some thought and the only thing I can think of is the timing of
> > the valves opening and closing. If you have a hydraulic cam it may be that
> > the lifters are leaking down at rest and are not holding the vales open
> > long enough while you crank them during your test because the cold oil is
> > not able to fill the lifter fully and as a result allows the valve to
> > close quicker (the duration of opening is shorter) and raises the
> > compression by permitting the piston to compress a larger volume of air by
> > closing when the piston is further down in the cylinder bore. When the
> > engine and the oil are hot and the lifter is fully pumped up (the oil
> > viscosity is lower and it is easily pumped into the lifter and fills it)
> > it has the ability to hold the valve open for the full amount of time
> > while fighting against the pressure of the valve spring.
> > If you have a Mechanical or Solid Roller tappet cam the lash will be
> > different (Larger) when cold and this too will close the valve marginally
> > sooner although I would never expect that type of pressure swing as I
> > never saw that on any of my race or street cars.
> > I hope that helps answer your question.
> >
> > As an aside: back years ago I remember reading that the optimal cylinder
> > pressure to ensure full combustion of a naturally aspirated gasoline
> > charged (reasonable quality fuel) engine was 170-175lbs. The theory was
> > that pressures below this were wasteful as some of the charge would go
> > unburned and would exit the engine without all of the energy being
> > extracted. Any pressure over 175lbs and you were not maximising the output
> > of the engine as you were expending too much energy trying to compress the
> > charge and the result was an engine that generated a LOT of heat. I took
> > this to heart and built our racing engines around this theory and while we
> > don't run Nascar we produced 620HP out of 355 cu in and it was 14:1 but
> > had roughly 175-180 lbs. of compression. We controlled cylinder pressure
> > through valve timing and duration. For you Bob, if you wanted to increase
> > the compression Hot then if the lifters are fully pumped up you may want
> > to close the intake sooner by advancing the cam 2-4 degrees. It would
> > raise cylinder pressure but given the dynamics of cams you would also
> > close the exhaust sooner and on a Ford that is bad with the inherently
> > poor exhaust flow compared to a Chevy head (unless you are running after
> > market heads). It is my understanding that Comp Cams and Crane manufacture
> > dual pattern cams that are designed for just this type of optimisation
> > theory.
> >
> > Bye for now from long winded Tim!
> > Regards all,
> > Tim Ronak
> > B382000680
> >
>PS: I may be moving to Oceanside in San Diego county as I was formally
>offered a job on Saturday based out of Irvine with a home office.
There goes the neighborhood!! ;-)
Robert L. Palmer
UCSD, Dept. of AMES
619-822-1037 (o)
760-599-9927 (h)
rpalmer@ucsd.edu
rpalmer@cts.com
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