How to Accurately Measure Your Compression Ratio

Carl
By Carl Heideman
Oct 11, 2021 | Shop Work, Compression | Posted in Shop Work , Drivetrain | From the Oct. 2008 issue | Never miss an article

[Editor's Note: This article originally ran in the October 2008 issue of Grassroots Motorsports, but we felt the method for measuring compression ratios was worth mentioning again.]

Once you’ve finished building an engine, the bombardment of questions soon follows: What camshaf…

This content is available for GRM+ members only.

You can read it for free in 10 days or subscribe to GRM+ to read right now.

Subscribe now

Join Free Join our community to easily find more Shop Work and Compression articles.
Comments
View comments on the GRM forums
akylekoz
akylekoz HalfDork
3/27/18 6:04 p.m.

Nice article, now we need one on how to degree a cam for dynamic compression with our new found data.

Please

te72
te72 New Reader
3/28/18 8:16 p.m.

I'll have to keep this article in mind if I ever have the Supra's engine out again. I really, really do not want to ever have to pull it. Not that it's overly difficult, it's just that an engine out usually means something went really wrong...

dean1484
dean1484 GRM+ Memberand MegaDork
3/29/18 12:20 a.m.

Actually you can just do a compression test and get the max psi and then from that calculate the actual compression of the motor. To me this is much more important than the theoretical max compression that a motor can make if the valves opened and closed at tdc and bdc 

wspohn
wspohn Dork
3/29/18 3:43 p.m.

Measuring and adding up various volumes is fine, but I prefer to do it in a more foolproof fashion.

Every engine I build goes on an engine stand with the ability to rotate the engine.

Rotate the engine so the plug hole is uppermost.  Bring the engine to TDC with both valves closed using a dial indicator on the top of the piston, through the plug hole.

Use a Burette to run light oil into the plug hole up to the bottom thread of the hole. That is your total combustion space, and needs to be adjusted only to account for spark plug tip (projected tip reduces total chamber volume, but the hollow around the tip increase it - usually close to a wash).

This means you don't need to worry about oddball piston crown shape with pop ups etc.  You get an exact reading.  Just remember to turn th engine upside down afterward to drain the oil out of the chamber.  I had a not so fast thinking friend that I showed this method to, roll it upright after doing the compression test, bolted on a starter, forgot one hole was full of oil, and cranked it over on the stand.  You might be amazed at how far the stream of oil travels as it comes out of the cylinder through the plug hole the first time that piston hits TDC. The one I saw went 10-20 feet over three adjacent cars.

snailmont5oh
snailmont5oh HalfDork
3/29/18 9:50 p.m.
dean1484 said:

Actually you can just do a compression test and get the max psi and then from that calculate the actual compression of the motor. To me this is much more important than the theoretical max compression that a motor can make if the valves opened and closed at tdc and bdc 

Do I remember something about cranking compression not necessarily being directly related to running compression?

dean1484
dean1484 GRM+ Memberand MegaDork
3/29/18 10:19 p.m.

Yes there are differences. I think it is due to the heat and expansion of gasses. And you also get better sealing of the rings from the pressures in the cylinder but all that being said I still use this as valve timing has a much greater effect than these things. I don’t know the percentages. That would be interesting to look at. 

snailmont5oh
snailmont5oh HalfDork
3/29/18 10:36 p.m.

In reply to dean1484 :

Also, ram filling of the cylinders. And maybe a little bit of "compression takes time to bleed off, and the faster stuff happens, the higher the compression will be". I remember hearing stories of circle track drivers drilling small holes in exhaust valves so that cranking compression was lower than running compression (especially at 8-9k). 

Torqued
Torqued New Reader
4/4/18 4:14 p.m.
wspohn said:

Measuring and adding up various volumes is fine, but I prefer to do it in a more foolproof fashion.

Every engine I build goes on an engine stand with the ability to rotate the engine.

Rotate the engine so the plug hole is uppermost.  Bring the engine to TDC with both valves closed using a dial indicator on the top of the piston, through the plug hole.

Use a Burette to run light oil into the plug hole up to the bottom thread of the hole. That is your total combustion space, and needs to be adjusted only to account for spark plug tip (projected tip reduces total chamber volume, but the hollow around the tip increase it - usually close to a wash).

This means you don't need to worry about oddball piston crown shape with pop ups etc.  You get an exact reading.  Just remember to turn th engine upside down afterward to drain the oil out of the chamber.  I had a not so fast thinking friend that I showed this method to, roll it upright after doing the compression test, bolted on a starter, forgot one hole was full of oil, and cranked it over on the stand.  You might be amazed at how far the stream of oil travels as it comes out of the cylinder through the plug hole the first time that piston hits TDC. The one I saw went 10-20 feet over three adjacent cars.

This method sure sounds easier but how can you be sure that you don't have any air bubbles in the combustion chamber since you can't see into it to check?  I can imagine that some combustion chamber shapes might be more problematic than others.

wspohn
wspohn Dork
4/5/18 11:23 a.m.

That method works perfectly on DOHC engines as the plug hole is right at the top - no air bubbles.

It also works on the OHV engines I've done it on, as you rotate the engine on the engine stand to ensure the plug hole is uppermost.  It wouldn't work on an enigine already installed in a car, with the plug hole usually on the side of the head.

Not saying that there aren't some oddball chamber shapes out there that might not work well, but the majority certainly should pose no problem.

BTW, when I calculated the volume using the old addition of all volumes method, the result was always at least 0.5  points of compression off the actual.measured figure.

volvoclearinghouse
volvoclearinghouse UberDork
4/5/18 12:53 p.m.
akylekoz said:

Nice article, now we need one on how to degree a cam for dynamic compression with our new found data.

Please

I developed an Excel spreadsheet for calculating DCR with given cam parameters and engine stats.  I can see if I can dig it up if you're interested.  

Screenshot (yes, this was a horrible combo, just for illustration.  Further work was done on 9 and 10:1 SCR.

Our Preferred Partners
bqwooqidFXbh5x6clddljmQFiQcoWNcRSBAbzuhYJYhMBcXNBUaMYohZFoYv41TM