While trying to diagnose a missing cylinder a quick compression check was done, then across all four cylinders. My readings came out between 183psi - 195psi for my stock? (never touched more than the head since purchase) B18B. When I redid the head last spring was a little too much shaved off? The car runs great, but needs to be rich to prevent pinging.
Anyone have numbers to compare?
Thanks,
-Erik

http://www.georger.com/toad.htm On this page I devised a spreadsheet that takes the CR into consideration for a Volvo B20. I started with some measured values before I started the rebuild in hopes of arriving at the desired compression ratio. The first set of numbers are for a stock engine and to arrive with the same value as published stock, I had to ignore the "ring land" or the area between the top of the piston and the cylinder wall down to the first ring. The result gave me the 8.7:1 published number. The critical value is the head volume in CC's. This value is arrived at by measuring the amount of liquid that will fit into the combustion chamber with the valves closed. Turn the head upside down and place a flat piece if plexiglass with a a small hole in the top to "fill" it from. I had a glass cylinder from a science class with a stop-cock at the bottom. I measured that 57 cc's of liquid fit into the combustion chamber. Knowing the bore and stroke, I can tell what volume ( 496.38 cc's) is being compressed into the combustion chamber and thereby have the variables identified. Since I bored it out .030 I had more volume (504.93)and I knew that to still use "pump" gas I couldn't go over 10:1 so I determined that when I shave the head I will take a precise amount off to reduce the height and therefore reduce the volume of the chamber the piston is forcing the air fuel mixture into. By reducing the combustion chamber to 47.5 cc's I am achieving a CR of 9.7:1 and I did this be shaving the head. The performance is just what I wanted and I have driven this configuration 30,000+ in the past 2 years (93-94 OCTANE)without any issues. I have never checked the pressure build-up in the cylinder ("compression check") because it checks the condition of the engine as a machine ....the fit of interworking parts. The ability to hold pressure tells you about the condition of the "fit" of piston ring to piston and thus to the cylinder wall and also the valve seating to the cylinder head. You can have a good stock engine give a higher "pressure" test than a worn high compression engine. Compression rations are built into (designed) the component selection process in conjunction with the engine assembler. Engine condition is determined in the form of a "leak down" test resulting in a percent of lost compression. Check out the page listed above for my results.
PS I have the 9.7 engine in my 1800 but I also did the same to my 544 (virgil) but kept the stock cc in the head because I drive the 544 all winter and cold cranking has a lot to do with winter driving and starting.
Christopher Virgilthevolvo

Egggg-sellent post! Here's a little CR calculator app I found out on the internet: http://www.dunegoon.org/compression.html

Might be a good idea to put a timing light on it and see where your ignition
timing is set. Shouldn't be more than 24° BTDC at idle with everything
still connected. You might want to back it off to about 20 or even a little
less to see if it helps the pinging. Not a lot less though or you will suffer
poor performance and exhaust overheating.

I'd also check valve lift. I suffered for months and even did a valve job
on a car only to eventually find out that one cam lobe was GONE. That will
not affect compression. Sure does affect performance though!
If you replace your cam, get the best lifters you can find.
There's a guy named "Friendly Fred" who sells Sealed Power lifters on ebay
that are pretty good, like the ones IPD used to sell but a little longer
so that OEM pushrods work with them.
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma

Erik;

Those numbers sound on the high side, but very good, since they're within 8 points out of almost 200 (a mere 4%). I would be happy in your case, also, that you are able to keep your pinging under control. I don't have a correlation of pressure to CR (and comparing the actual to straight calculations would be subject to a lot of variables), but I would estimate around 10.5:1...maybe someone with a tight new engine (who knows the CR), can make a measurement and confirm...

Cheers

Well, in the bigger picture, I've heard a rule of thumb that 200psi is the target for pump premium, tight quench and aluminum headed v8's.
'All motor' Honda drag cars routinely run 300+ psi cranking pressure, and I've heard of super fussy guys getting 270psi to work on the street with everything ideal.

In our case, iron head and less than ideal quench/squish (unless you done some mods to make it so), I'm not surpised you are on the ragged edge. My 544 motor with nothing stock inside, caused me some worry- I'm getting 180 on the front cylinder and about 155 on the rear 3, that will eventually come up to 170 with way more rotations of the motor. (Note to self: Self! Run air cleaners next year). Anyway... point being, I thought it should be higher, and think I'm going to make it higher.

As far as more reading on the topic check out : http://victorylibrary.com/mopar/mopar-tech.htm#data

There is a good article with Mopar V8 examples somewhere linked off there, and there is a little applet-kinda deal (PhilS can correct my terminalogy) at the bottom, that seems to do all the math. I haven't used it yet.

I didn't download the calculator whatzis, but my belief is that there's no way to calculate CR from compression test readings. They're good for comparing one cylinder to another, but there are way to many variables involved to be able to state that 180 psi = 10:1, or anything of that sort. Cranking speed? Cam overlap? Valve lash? Air temp? Etc.

You're right on with the squish/quench being less than ideal in stock motors... tighten it way up, and combustion efficiency improves dramatically. Such "quick burners" need much less ignition advance and you can run fairly high CRs on 92-octane without any pinging at all.

**there are way too many variables involved to be able to state that 180 psi = **10:1, or anything of that sort. Cranking speed? Cam overlap? Valve lash? Air **temp? Etc.

Cranking speed doesn't affect the final PSI figure more than a fraction of a percent; it may affect how many strokes it takes to get there.

In the bigger scheme of the world's engineering, figuring out cranking compression is actually pretty easy. Rod ratio, air density, compression ratio, cam duration (specifically Intake Valve Closing) are about all it takes.

If I may ramble.. it's part of scientific experimentation to try to quantify these things. Doing such calculations should not be regarded as being beyond the average engine builder ( or rather, parts/specification selector).

Having a consistent, (if at the root empirically derived) rule of thumb for what might be the ideal intake and exhaust port cross sectional area per displacent and RPM range... or having a certain aim in mind for cylinder pressures achived are both examples of knowledge that can be used across various brands and families of motor, to compare & contrast.
In fact, doing so can be the best method to select areas for modification.

WOW ! what an explnation..........Now I've forgotten what the question was.

But going back to the basics The head was probably milled at one time therby reducing the combustion chamber at the top thus increasing the compression.Just enjoy the power increase you are probably getting and don't worry about it.