Relative Compression Amplitude to PSI calculator

I created this the other day for some guys in my company, because I couldn't find anything like it on the inter-webs.. Next time you do relative compression on a scope, choose a cylinder and input the Amplitude of it. Take a manual gauge reading on that same cylinder. Input the data into this sheet, and then just make yourself a personal note of the scale you used.

As long as you use the same Amperage/voltage scale every other time you scope relative compression, you can use this, and will know actual compression PSI just by typing in a cylinders' amplitude in the designated box, without having to break out that compression gauge anymore.

This way, your "relative" compression checks are no longer relative. If all cylinders have low compression, this will show it.

Please click "file" and "Make a copy"for yourself, since anyone with this link can edit it.

docs.google.com/spreadsheets/d/158W4qBxq...w-I/edit?usp=sharing

Interesting --- I will try it out in the New Year

Nevermind. I reread the post

Andy.MacFadyen wrote: Interesting --- I will try it out in the New Year

Andy,

Have you come across an opportunity to try this yet? I was never an algebra wiz (or Microsoft Excel for that matter), but I am pretty confident that I got the algorithm/formula correct.

The reason I made this was because I did a relative compression test on a "no-start" that showed even compression on all cylinders. While cranking, it sounded normal to me. Because of these 2 facts, I ruled out a compression problem. I was wrong... All cylinders had low (but some) compression, so the "humps" looked uniform as normal. I would have caught that if I saw the actual PSI instead of just looking at the amplitude of the "humps". This tool solves that dilemma.

I am thinking about making a demonstration video on it. I think this tool could be a real nice enhancement to those that already utilize relative compression testing.

I haven't had a chance to look at it yet, not sure if it would be helpful with ultra low compression on all cylinders. I have only encountered non-starting due low compression on all cylinders on a Jag XJ8 although looking back to my younger years it might also have been the issue with a 1970 Lotus Elan that wouldn't start after being parked up over the winter.
With the Jag it was a well known issue and an internet search provided the direction needed to get it running,
Iin the case of the Lotus the solution was a very long tow start.

The cranking RPM and wet plugs were the only real clues with the Jaguar but any vehicle that has had starting problems the battery is likely to have issues. You would need to know what was in the normal range for a particular engine type and ambient temperature.

I have been Facbook befriended by Peto Petkov the developer at AutoDitex who developed the i-Scope relative compression tester and the relative compression tests on the Ditex scopes. He always willing to talk and really worth contacting.


Added --- I came across an interesting bit of info on the internet a 2 litre 4 cylinder requires more starting power from the starter and battery than a 3 litre v6-