VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1929-1932 commpression

I know from past posts that the commpression on a '31 motor should be 85-90 lbs. but is that with the motor running or with all sparkplugs out and turning over with the starter? I only get 35-42 lbs. with the later even when I added some marvel mystery oil. :confused: Thanks. jerrygolf

Hi jerrygolf,

Compression is taken with all plugs removed and the throttle propped wide open. The results are established as uniformity rather than the amount. When you add the MM you have then just conducted the 'wet' test and when compared to the 'dry' test gives an indication of the condition of the rings.

Why would you need the throttle popped wide open when when you are not using it as all the plugs are removed? Also, don't you need a certain amount of commpression to get significant combustion (spelling)? :confused: :)

Hi jerrygolf,

While true that the throttle doesn't absolutely have to be propped open it is a good practice when doing compression checking.

Due to many variables, accuracy of gauge, combustion chambers not being exactly uniform, possibility of carbon buildup, cranking speed, and other variables makes it impractical to establish an exact pressure that must be achieved on a compression test. However, expected numbers based on compression ratios run as follows: 6.5 = 110, 7.0 = 120, 7.5 = 130, and 8.0 = 140. If any one cylinder is 25 lbs or more lower than all other cylinders it is PROBABLE that the valves are leaking in that cylinder. "Wet" testing gives an indication of ring leakage. A more accurate method for leakage is the leakage test. This is done with compressed air.

Thanks Ray, all pistons are fairly even so I should be o.k. Think I'll get the clutch fixed, new coil and condensor and give it a pull, may start better. That will be after the golf season is over. Then to start restoring. jerrygolf

Morning Gang,

It takes about 5:1 compression ratio for an Otto Cycle engine to run. Our old Chevies are about 6.5:1 or so, the later <pre-war> cars are probably noticably higher. My Ferrari is at about 10:1.

The general rule on a compression test is 'uniformity within 10%'. That is, if you 'assume' the highest pressure reading represents a 'perfect' seal, no cylinder should be more than 10% below that value.

(1) Dry test: This is a test for rings AND valves seal. Set up this way: all plugs out, throttle open 100%, no liquids in the cylinder, compression gauge in the plug hole of the cylinder being tested. Spin the engine on the starter and count the approximate number of compression strokes. There will be a pronounced difference in the sound of the cylinder with the gauge connected. That is your clue. Allow 8-10 compression cycles then read the pressure. Take your time doing this. The starter should be allowed to cool between tests. Remember, it is not designed for continuous operation. Repeat the dry test for each cylinder in sequence record and compare the results.

Evaluation of (1): If any cylinder(s) is below 90% of the maximum pressure, a leak is implied but you don't know if it is rings, valves or even perhaps a blown head gasket leaking! If all cylinders are within 10%, you are done and your engine is probably healthy. If not, go to (2).

(2) Wet test: Add about one teaspoon full of oil to the cylinder being tested. A squirt-type oil gun works great but do the squirting slowly. The goal here is to put an oil seal into the rings but NOT to get it up on the valve seats! Repeat the tests on each cylinder just as with the dry tests. All pressures may be a little higher but again, compare to the highest wet pressure -- calling that reading 'perfect'.

Evaluation of (2): If all cylinder pressures are now withing 10%, the pressure leak found in test (1) is caused by a poor ring seal (broken compression ring?) as the oil has effectively sealed the ring allowing the pressure to build to within spec. If the cylinder in question STILL is low, you don't know for sure that the rings are good but you can bet that the valve(s) are not sealing (burnt). The head needs to come off for a valve job.

I hope this helps. A number of us wrote most of this in various bits but I thought it might be nice put it all into one note. Enjoy!
Rick

Hi Rick! Thanks for your excellent overview on compression testing. Your facts regarding the subject about says it all, and I think that your posting answers Jerry's question perfectly......as well as some questions that I had too. Again, an excellent posting on compression testing!!!! :cool2:

Thanks again Ray and Rick, I now have something to go on. It is very easy to follow your instrctions.

The compression for the early 6 cylinders is around 5 : 1. Stroke and head design changed in '33 to begin the increase increase toward modern "high" compression engines. Availablility of Ethyl gas made this possible.

Yes Skipper, the '32 has a bit higher compression than the '31 but only a bit.

How would anyone know how many times their cylinder head might have been milled and how much the compression might be raised? How much can you mill off a '31 or '32 head, what happens if you up the compression to 7 or 8:1? Does it run rough, overheat, etc?

d2d2 wrote: "How would anyone know how many times their cylinder head might have been milled and how much the compression might be raised? How much can you mill off a '31 or '32 head, what happens if you up the compression to 7 or 8:1? Does it run rough, overheat, etc?"

I add: You are getting into the area of great interest for me. I just love this nerd stuff and design (and build) high performance race and rally engines.

(Is this the correct forum for this conversation? I believe that it is. Tell me if otherwise. I can add lots of automotive engineering and design and others like d2d2 can add gobs of practical experience bringing my thoughts to applicability -- not to take one cent away from d2d2's technical knowledge.)

The way to understand how much has been milled from a head is to measure the combustion chamber volume and compare that number back to the original specs. Of course, the head has to be OFF the car to do that correctly. Amount_milled_off*Pi*((bore/2)^2)will equel the volume of the DIFFERENCE in combustion chamber volume. The new <static> compression ratio is then calculated from the bore, stroke and new combustion chamber volume.

Happy Monday! Rick...

Rick, I can't think of a better place for these ideas. For the calculation to work you have to have the original spec. Does that still exist? Maybe I could use it to figure out the compresssion ratio for a mystery cylinder head that I have. It has Buick valves and larger ports.
I'm also interested in what I call exhaust gas analysis. If I called it emissions testing you probably wouldn't want to hear about it. But as I have said before, it can be a tool for diagnosis and for precise tuning. If anyone wants to know what they are, I'll post the next CO and HC readings from my '31.

Have at it d2d2 and Rick! I, for one, would like to see more! Go for it!! Ya!

I don't think it was very common to mill a valve in head on a stock engine was it? or am I wrong? Some have been trued up I suppose. We always tried to find a new head or a low milage one if it become warped, because of the expense and having to re do the valve seats, I think it cost about 3 or 5 dollars per seat, Wow!, that could run into real money!

One major consideration is the head bolts. If you add too much compression then you will have problems with the soft head bolts. Studs and nuts will be a necessity to hold the head down. It may also be necessary to add a few additional studs.

Because of the rectangular depression where the spark plug screws in, the compression gain is very limited. Can the depression be filled and the spark plug relocated? Is it better to use a later model head? How about a new head with more symetrical design?

The original design leaves the plugs in a lean burn environment and rich mixture in most of the rest of the combustion chamber. Bad for performance!!!

Then there is the problem of only 3 main bearings. Can you amagine the crank twist and deflection if you up the force (read compression, horsepower) too much.

It sure looks like fast distruction to me!!!

I think ChevyChip is right on the mark. We shouldn't try to make these engines into something that they are not. Kinda reminds me of the kid on the corner who asked me if I was going to "...hop it up." I replied, "Dude! This guy IS hopped up." :-P

The head on "Stove Bolt 6" is really massive and I don't know whether that leads to more or less liklihood of <heat> warpage. My experience is mostly with alloy heads where one has to be really careful of overheating. I do agree that these heads were probably never milled to increase the compression ratio but rather to resurface the head for a better head gasket seal. Any combustion chamber volume calculation made on a head today would probably be pretty close to factory spec. As for compression ratio, I would suspect to find that many of our engines will have higher compression ratios caused by overboring.

One source lists the compression ratio for the 1930 engine at 5.2:1. If we do the math,

Bore = 3-5/16" = 3.3125

Stroke = 3-3/4" = 3.75

Compression ratio = CR = 5.2:1

Combustion Chamber = CC = Stroke*(pi*(Bore/2)^2) / (CR-1)

CC = 7.6945 cubic inches

Adjust the bore (read: bored oversize), rearrange the equation and one can find the new compression ratio. Also remember that the combustion chamber includes the valve pocket within the head, the depression around the plug as mentioned earlier, the volume from the crown of the piston to block deck (which may be a negative number with domed pistons) and the short little cylinder formed by the head gasket.

Most books on automotive engineering state that about 5:1 compression ratio is the minimum that will allow an engine to run. That tells me that a largely overbored Stove Bolt 6 might just start and run a tiny bet better even
if the head isn't mildly milled.

0.040" (forty thousandths) overbore results in an increase in compression ratio to 5.3:1.

Best Tuesday!

rick

I had never thought of a bore increase having the effect of raising the compression.
I read that much of what was then known about combustion chamber design came from research by Ricardo, Midgley and others using side valve engines, a pancake shaped chamber for a low volume to surface area ratio and the spark plug in the center. On the valve in head engine they had to bend the pancake 90 degrees on one side and locate the spark plug way off center and is why the ignition timing is 12 (or 18) degrees BTDC. Compare that with an Alfa Romeo 2000 (4cyl) engine with hemi combustion chamber, double overhead cams, spark plug over the top center of the piston, and basic timing at TDC.

d2d2 wrote, "On the valve in head engine they had to bend the pancake 90 degrees on one side and locate the spark plug way off center and is why the ignition timing is 12 (or 18) degrees BTDC. Compare that with an Alfa Romeo 2000 (4cyl) engine with hemi combustion chamber, double overhead cams, spark plug over the top center of the piston, and basic timing at TDC."

Exactly! Timing is all about the propagation rate of the flame front so that all of the 'work' is done before the piston is in an inopportune position. That is balanced against the problems of too much spark advance and pressures building before TDC. In short, everything is a trade-off. Proper tune happens at only one engine speed. In an hemispherical combustion chamber with the plug centered (read: double overhead cam designs) the flame front is just about symmetrical. On the old bathtub heads, frequently there are 'squish zones' where the charge is squeezed sharply causing turbulence which results in a quicker advance in the flame front and a minimization of likelihood of the end gasses flashing.

In Formula One engines, the valves are pneumatically operated with the timing being dynamically adjusted according to rpm, load and demand. Same with the inlet runner lengths! Great stuff.

With our cars given the 2700rpm redline, there seems to be ample time for the flame front to do anything it pleases! Not so in my 3 liter, V8 Italian car at 8000rpm.

Ok bear with me, Question is the compression ratio a volume to volume ratio C.I/C.I?, or, The volume of the cylinder and combustion chamber at TDC / The volume of the cylinder and combustion chamber with the piston at the lowest point? with the combustion chamber being common to both sides of the equation, and also the bore common to both sides of the equation? or is it the ratio of the combustion chamber to the volume of the cylinder ?

Jeepers, I ask a simple question and Hee Hee. Really, alot of unknown information brought to light. Very informative even if I don't understand it all. :confused: jerrygolf

Ask us what time it is and we'll draw you a blueprint for a clock.

Something like the wife's cooking. I ask for a sandwich and she sets out a banquet. Now that ain't all bad neither. :p

I think it has become pretty obvious that we are all very interested in many aspects of our cars; the technical issues, the history, the restoration, etc.. I am particularly interested in the engineering practices that were used before modern day 'best practices' were standardized <by convergent evolution>. For example, 'splash lubrication'.

I also love to rant-on about technical topics and can easily loose the thread and go off into techno-weenie-land. Should that happen, I won't know it so someone please send me an e-mail asking that I come back to Earth -- or perhaps just shut up. I am also sensitive about sounding like a know-it-all -- which I am not. I am just very interested in these matters and have studied <and done> a lot. Somehow, my family tolerates me. :-]

MrMack asked about how the compression ratio is determined. Here is the scoop: The compression ratio, as usually quoted, is the STATIC compression ratio. That is, it is not a calculation of how pressures change in a running engine (that's why you can't just multiply the CR times 14.7psi and get cylinder pressure). Rather, it is a theoretical value found by,

CR = (combustion_chamber_volume + displacement) / combustion_chamber_volume

...of course, displacement and CCVolume are the displacement and CCVolume of one cylinder. In English, it means, &#8220;What is the ratio of the entire volume at BDC to that at TDC.&#8221;

In reality, the actual compression found within an engine is a function of many influences. The valve timing is usually such that overlap causes one valve to be still a little open when the other one starts to open. At that point, there is an open circuit from carb to muffler! Fortunately, the charge (air+fuel vapor) has a mass and a viscosity. We don't experience that much at the speeds that humans move but at engine speeds, the charge moves more like a thick fluid. This is kinda hard to imagine but the dynamics of the fluid flow define the behavior of much of the engine -- and visa versa. Put your hand into the stream from the garden hose and feel the pressure. That is the same pressure that causes 'passive supercharging' when an engine's induction tract is 'tuned'. The charge, when at engine speeds, is like that jet of hose water! Then, when everything seems to be under control, the pressure curve takes a huge swing upward as the charge ignites. YeeHaa!

Happy Wednesday,

Rick

Skip,
What does the Chevrolet Engineering Book gives for the compression for the early Chev 6?

The 1932 Chevrolet Engineering Specifications Manual shows a head compression ratio of 5.2 to 1. And, on the compression itself, I have a chart that lists the compression for 1930-31 (uses the same cylinder head) at 76 pounds, and the compression for the 1932 at 80 pounds.