Bob,
That sounds like a lot of work. I've been using a leak-down style tester that is
only a little more cumbersome to use. The major effort to use it is to ensure
that
the cylinder under test is at TDC-compression. I detected a very bad leak in two
exhaust valves before I noticed any performance degradation. The gauge uses
compressed air. (I use 80 psi since that is what airplanes use and I am familiar
with the numbers.) The cylinder is pressurized through a small orifice. There
are
two gauges on the unit, one reads applied pressure and the other reads the
pressure
in the cylinder. The difference is lost due to flow through the orifice caused
by
the cylinder leaking. If the cylinder is perfect, the pressure is the same on
both
gauges. On aircraft anything above 70 is considered very good, 60-70 is OK, and
less than 60 is suspect. My two bad cylinders read 5, yet it still ran OK,
although
it wouldn't for long. Another advantage is that you can listen in the intake,
exhaust or engine breather to determine what is leaking.
Craig
Bob Palmer wrote:
> Craig,
>
> As usual, some very thoughtful ideas from Tim, but a bit off the mark in my
> case. First, my lifters are solid and I was taking the compression with
> only that particular plug pulled. Yesterday and this morning I did some
> more checking and it appears the biggest factor is the cranking speed. This
> time I shut the engine down and pulled all the plugs a quick as I could (
> <10 min), and, with the primary butterfly full open, checked cylinders 1-8
> in order. Of course cranking speed was noticeably higher with all the plugs
> pulled and consequently compression was too, in the range 185-195 psig.
> Mind you, this test was "hot", whereas previous hot results were around 140
> psig. This morning I repeated the test, this time with the engine cold. In
> the meantime, overnight, I had charged the battery. This time the pressures
> were all real close to 200 psig. Without any way to measure the exact
> cranking rpm, I can't be sure whether charging the battery accounted for
> the increase, but I tend to think not. By the time I had gotten to #7 and
> #8 the first time around, the cranking rpm had dropped noticeably and
> compression had dropped about 20 psig, but after charging the battery only
> about ten minutes, they were back up to 190 psig each.
>
> It would be interesting to know what the compression versus cranking rpm
> is. I would expect the ring sealing to be very important here; i.e., the
> better the seal, the slower you need to crank to get the maximum reading.
> And I am also assuming there is a maximum reading, within reason of course,
> versus rpm. Whether my readings of 200 psig is maximum or not, I'm not
> sure, but there wasn't a whole lot of difference (20 psig) between when the
> cranking was noticeably slower and with the battery freshly charged, so I
> would think I was pretty close to the maximum I can get, even if the motor
> was turned over even faster. I did put oil into the one cylinder that read
> the lowest and then it read 200 psig too. It would be interesting to repeat
> the experiment with the engine hot and squirt oil in the holes and see if
> this brings all the cylinders up to 200 psig. This would indicate that the
> difference between hot and cold is due to the better sealing of the thicker
> cold oil in the cylinders, or has been suggested, that oil leaks down
> through the valve guides into the cylinders after the engine sits awhile.
>
> Another suggestion from Tim I am taking is to index my plugs. I have an
> indexing tool that I have now marked for my engine for all cylinders. All
> of them lie in a range of less than 180 degrees. Four of the plugs I have
> match up real well, the other four don't fit any of the cylinders, lying in
> the opposite 180 degree range. Guess it's time to head back to MM&J for
> four more plugs.
>
> Well, back to wrenching and other chores,
>
> Bob
>
> At 01:25 PM 8/27/99 -0700, Craig Wright wrote:
> >Bob,
> >
> >That's the best explanation that I've heard. Are the lifters hydraulic?
> >
> >I'm sending out invitations for the Car Coral at the Coronado Historics this
> >weekend. Who would be the best person to receive the info for dissemination
> >amongst the Tiger owners in SoCal? The race is October 23 & 24.
> >
> >Craig
> >
> >Bob Palmer wrote:
> >
> > > 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
>
> 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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