News:

Due to a technical issue, some recently uploaded pictures have been lost. We are investigating why this happened but the issue has been resolved so that future uploads should be safe.  You can also Modify your post (MORE...) and re-upload the pictures in your post.

Main Menu

Compression pressure for 49 331 engine

Started by Sam Moore #21210, October 15, 2006, 07:06:25 PM

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.

Sam Moore #21210

Ive been checking a few things on my newly acquired 49 while awaiting the receipt of a shop manual that I ordered.  In the meantime, can anyone advise me of the compression pressure at cranking speed (throttle open, warm engine)?  I know that a 54 engine should be 150 psi.  The 54 has a 8.25:1 compression ratio and I have seen sources that say the 49 is 7.5:1.  I am curious about cranking speed on a 57 year-old 6-volt system (may be a little slower than when new) and how it might relate to compression results.  Anyway, my readings were about 100 psi plus or minus 4 psi for all cylinders.  The good part is that they are close.  I didnt try putting oil in the cylinders yet to see if it would increase.  

John Washburn

Sam,

New engine, at sea level, 1949 is 110 plus/minus 4 lbs. I would say that 100 lbs is damn good.

John Washburn
CLC #1067
Elizabeth, Colorado

Sam Moore

Thanks, John. Im beginning to feel pretty good about the car now. I ordered carb & fuel pump kits, oil and trans pan gaskets, and some other things from Bob Cooper yesterday. Once I get  the carb  & fuel pump done, it should run like its supposed to. Then I can put a toggle switch on the auxiliary electric pump that was already on it and use it only when needed.

Bruce Reynolds # 18992

Gday Sam,

Putting a couple of squirts of oil in each of the spark plug holes is done as the second stage of a compression test, to give the top ring a better seal in the bore and the pressures obtained allow one to deduce if Piston Rings or Valve Seats are the reason for a poor initial reading.

The Base Compression check is always done dry, and whilst the test is being done, it is always adviseable to take note of just how many revolutions on the Crankshaft, or pulses of the compression gauge, it takes to build each cylinder to its maximum reading.

Plus, always keep the results of every compression test you do on a given motor for future reference.   This way you can chart over the years as to how it is performing.   And, again, note any differences or imbalance.

Lastly, always buy the best Gauge you can, and keep it for ever, as doing the tests with different gauges could give differing readings.   No two gauges are the same.

Bruce,
The Tassie devil(le),
60 CDV

Sam Moore #21210

Hi Bruce,

Thanks for the response.  While taking the readings, I did note the reading on the first pulse and it was a consistent 90 psi for each cylinder, which I thought was good news.  I didnt count the pulses to max pressure, as it occured so quickly after the first one, it was hard to tell.  I guess thats good too.  I plan to take readings again, both dry and with oil, after a good tuning and running the car a little more.  Then Ill do as you suggested and keep records of periodic tests.

By the way, I live in southwest Virginia and the 49 makes the 8th Cadillac that I have owned over the past 33 years - I still have 6 of them.  I bought both a 55 and a 67 when I was 18 and kept the 55 until only a few years ago (oops, I just told my age didnt I?).  Two 56s (a convertible & Sedan deVille), were inherited from my father.  I purchased two more 56s a couple of years ago (Eldorado Seville & Std. 4-door) for parts cars.  I obviously have the affliction and dream of the day when I complete the convertible.  Its not just Cadillacs either.  I also have two old Ford 1-1/2-ton trucks (47 & 52) and a 64 VW convertible - the 52 Ford and 64 VW are drivers.  I have so many things going on now  with construction of a new house and a large (30 x 70 2-story) shop building, thus the 49 to drive in the meantime.

Sam Moore
North Tazewell, VA