1930 Chrysler 77 Compression ???

[RoadsterRich]

What compression (my gauge is in PSI) would one expect on a 1930 Chrysler 77, with a 268.4 Silverdome Flathead L6 engine? If it matters it has 3 3/8" bore, 5" stroke. I tried to look up the information in the manuals I have but found no reference. I could find no reference to compression, or to the compression ratio.<P>Rich

[ply33]

The <span style="font-style: italic">Pictorial History of Chrysler Corporation Cars</span> published by Chrysler Corporation in 1975 has the following specifications for a 1930 Chrysler 77:<P>Production began July 1929, ended May 1930.<BR>Engineering Code W-77, advertised name: Chrysler "77"<BR>Wheelbase: 124 9/16<BR>Cylinders: 6<BR>Bore and stroke: 3 3/8 x 5<BR>Displacment: 268.9<BR>Compression Ration: Std: 5.0, Opt 5.8 (Redhead engine)<BR>Advertised BHP: Std: 93@3200, Opt: 102@3200<BR>Advertised Torque: Std: 180@1000, Opt: 196@1000

[RoadsterRich]

Well not being a physicist or a rocket scientist, or anything remotely related, here was what I was thinking...<P>As I understand it the compression ratio is the ratio of the displacement of a single cylinder to the 'open' space when the piston as at TDC. In my case this would be (3.375/2) * (3.375/2) * PI * 5 (Volume of a cylinder = (Diameter/2)^2 * PI * Height), or the square of the radius of the piston times PI times the stroke. This gives me 44.73 cubic inches per cylinder. The amount of 'open' space when the cylinder is at TDC then must be 44.73/ 5.0 (compression ratio) or 8.95 cubic inches.<P>Well I just realized I didn't need that, I could have just used the compression ratio to start with. Oh well, it at least explains a little about compression ratios and what they mean, hopefully.<P>Now for the ASSumption part <P>Assuming the initial pressure were atmospheric, and that an even compression with constant temperature it would be roughly 5.0 * 14.7 or 73.5psi. This of course does not take in to account, the temperature change, the fuel mixture, the effects of the exhaust/intake and valves on the initial pressure, etc. nor the poor seals on valves, rings, etc. I would assume the overall effect of the 'other' factors would be an increase in pressure over this rudimentary calculation.<P>That well exceeded my physics and chemistry expertise. So I'm sure I'm full of flaws in my logic, but I really would like to have some rudimentary understanding of the correlation.<P>Anyone who can further my understanding (or at least lessen my ignorance) please let me know. Maybe my assumptions, guess work, and memory of physics and chemistry from 2 decades ago aren't too bad, who knows, but I'm not holding my breath.<P>Rich

[Guest De Soto Frank]

Rich,<P>This is an educated guess, based on my 6 cylinder Chevys: I took the "recommended" compression for a 235 Chevy six (135#) and divided that by the compression ratio (7.5:1),and came up with a "factor": 18.<BR>I then multiplied that by the CR for your engine (5.0:1), and came up with 90# . <P>(The physicists & engineers will probably flunk me for my logic here, but the final number seems reasonable.)<P>My pre-1950 MoToR's manuals don't list "compression pressures"; they maintain that "eveness in pressure between the cylinders is more important"- they suggest that if all the readings are within 5-10# of each other, you're in pretty good shape.<BR>They also suggest squirting some oil in a cylinder which reads "low" and re-checking(seals the rings); if the compression improves by 10# or more, that indicates broken or worn rings. If not, then there's probably leakge past the valves or head gasket.<P>When checking compression, rig the throttle wide-open, and you should reach max. pressure within 3-4 revolutions.<BR>Traditionally compression is checked at operating temp.<P>I'm sure there is a way to mathematically determine compresion pressure based on displacement and all that; when I checked the compression on my '41 De Soto (228 flathead), it ran around 90# per cylinder (88,000 miles).<P>Hope this helped!

[Guest De Soto Frank]

RdstrRich,<P>How 'bout this:<P>instead of either of us risking brain injury from thinking too hard, can you run a compression check on this dinosaur and post results?<P>The fact that there seem to be no compression specs. in the early service literature suggest to me that other than eveness between cylinders, it's not so important? (That statement oughta draw fire!)<P>Next!

[Tom Deering]

Roadster Rich,<P> Here are some excerpts lifted from MOTOR'S Auto Repair Manual, 20th edition: <P>Compression Pressures and Ratios<BR> "Just to satisfy the reader's curiousity, below is a table showing the approximate relationship between compression ratio and compression pressure at cranking speeds: 6.5 ratio= 110#PSI, 7.0 ratio = 120#PSI, 7.5 ratio = 130#PSI, 8.0 ratio = 140#PSI.<P> Various design factors affect the compresssion pressure. Therefore this table may apply to some engines but not to all."<P> Notice the values are linear. This leads me to conclude through extrapolation that your 5.0 :1 compression ratio should translate into 80# PSI. This also validates your calcuated 73.5#PSI result.<P> I have to agree with DeSotoFrank regarding a consistent reading among cylinders being the most important consideration.<P> This reference source I'm quoting has a lot of info on the subject. They claim a variation of 20#PSI may be encountered on the same engine due to combustion chamber variations. I find that number awfully hard to swallow. They also state a cylinder that reads 25#PSI lower than the highest reading cylinder, probably has leaking valves.<P> I'm kinda sorry I read the book. Really liked my old rule of thumb that said there should be no more than 20% variation between highest and lowest cylinder reading without regard to the compression ratio. Oh well, we live and learn.<P> Tom<p>[ 01-15-2002: Message edited by: Deering ]

[RoadsterRich]

Tom,<P>Thanks for the information. This is all academic at this point, I don't have a working starter as of yet. I was hoping to have one start of the week, but alas it didn't work out. I have hand cranked the engine over and tested that there is compression in each cylinder. With my feeble hand cranking the compression (very dry) ranged from a low of 38psi to a high of 42psi. I measured this with all the plugs removed so I could get a better run on it. Of course these numbers are likely pretty much meaningless, other than the fact that I do have compression in each cylinder, and assuming I crank steadily they are all similar. I tested each one three times and the numbers were allways within the 2psi range of each other, as well as across all 6 cylinders.<P>The engine has not been run in over 30 years, in late August I put about an ounce of Marvel Mystery Oil in each spark plug hole. Most of which poured out through the intake and exhaust ports, forgot to consider that most of the valves would be in same state of opening/closing. <P>Here is my 'shaggy dog' story of how I got to where I am in the compression questions... and please bear in mind when you read this I am new to all this!<P>At that point (Late August) I couldn't turn the engine over (I didn't have a crank, the starter didn't work, and the radiator was still in place.) Frustrated and worried I took on tasks I could handle, the water pump, cleaning the water jackets, the carburetor, removing the radiator, etc. Finally when I could get to the front of the engine after removing the radiator, I put a good heft on the crankshaft via the pulley, strained my arms, but nary a budge. Finally a week or two later I resorted to a slight rocking of the car, success! The pulley moved with the car in gear. Tried again to turn the crankshaft by hands on the pully, no luck. Frustrated and concerned I went back to working on other aspects of the car. Over Christmas I decided to see just how 'stuck' the engine was. I put a torque wrench on the crank nut at the front of the engine and started to pull, right at 40ft lbs it started to move, it was a little stiff, but it moved. In my 'excitement' I decided to spray a little penetrating oil in each cylinder to see if it would help, I grabbed the wrong can and ended up with a squirt of carburetor cleaner in each cylinder instead. Another of those oopses crops up. At this point I tried turning the engine over by hand, I could just barely turn it with the pulley. I tried the torque wrench again and found that at 28ft lbs it turned easily. After two full rotations, with a bit of effort, but no straining I could turn the engine over by hand using the hands on the pulley. Last Friday (Jan 11 2002) I got a crank for the car, with it in the crank hole I can turn the car over quite freely with no plugs. With all the plugs in I can barely turn it over (more a testament to my weakness than its stiffness at this point I believe). In the meantime I picked up a rebuildable starter and got that part of the project in motion.<P>Well thats the story of how I got where I am in the compression testing. I was hoping to have information on what to expect to coincide with the working starters installation. Looks like I have information before the starter.<P>Well, now I figure I'll hear the shoulda and coulda's, and maybe a few shouldnta's too. As long as I keep on learning, without destroying the car, and it keeps on getting closer to running, I am happy. Besides I have learned to ask here first now, that is a major improvement!<P><BR>Rich

[ply33]

Under the would of/could of comments:<P>It would be a good idea to drop the pan and clean out any crud that may be there and make sure the screen is clean.<P>While you have the plugs out you have or should have almost no load on the bearings. That would be a good time to put some engine oil down the cylinders and to fill the crankcase with clean oil (after the above cleaning). Turn the engine over, by hand or by the starter motor, until you have some oil pressure indicated on the gauge.<P>This will assure there is some oil in the bearings and on the cylinder walls before you actually try to start it.

[RoadsterRich]

Hey, I did something right! I just drained the pan last week, and got a nice new gasket for it from Olson's. Getting ready to drop the pan. I'm going to start a new topic with my questions on this one.<P>Rich

[Guest De Soto Frank]

Rich,<P>If you've got a source for gaskets, may I suggest acquiring a head gasket and pulling the head off to see what the cylinder bores look like?<P>If she was "bound-up", there's probably rust or corrosion on the cylinder walls above where the pistons were positioned while she "slept", and spinning her with the starter or actually firing it up might score the cylinder walls if a piston ring has broken, etc.<P>I'm sure you're anxious to get it going, but it might be better to dig into it a little bit and make sure you won't unintentionally do more damage by running an engine with possible internal damage.<P>Engine parts and people(machine shops) who are qualified to work on this machine are getting scarce & expensive.<P>I say this because I once had an expirience with a Chevy 216 that was "stuck", and I finally got it loose, and thought I was "golden", so I put a battery on it, plugs & such and tried in vain to start it, and discovered that it wouldn't turn because<BR>of stuck valves, and in "getting it free", I bent two push rods and finally stripped the fibre cam gear! Yeah, the crank spun, but the cam, distributor, fuel pump, etc. didn't.<BR>(It eventually did run, but if I'd dug a little deeper, I would've been ahead of the game.)<P>My 2 cents worth; sounds like you've got a nice project going- I'm only suggesting caution to avoid more work for yourself in the long run. <P>Keep us posted!