How Automatic Compression (ACR) works

[squonk 41,151]

There was a lot of talk about Automatic Compression Release and Automatic spark advance camshafts in my makes no sense thread. With both styles of cams on my bench, here's a quick rundown on how they work:

 

Early engines used Auto Spark Advance. Utilizing a 2 piece cam. The lobe for the points was separate and engaged two spring retracted weights. At low RPM (starting) The weights were held by springs and positioned the lobe in a spark retarded position. As the engine sped up. The weights were forced out by centrifugal force and the dogs on the weights moved the points lobe to an advanced position.

 

 

 

 

 

With the ACR cam on later engines, at rest and below 600 RPM the cam has a raised arm that the tappet rides on holding the exhaust valve open. Finger is pointing to raised arm.

 

 

In this picture my finger is pointing to the tab on the weight keeping the arm raised.

 

 

Valve will be held open about this amount.

 

 

In the next picture, I am holding the weight out simulating RPM over 600. You can see the tab as moved away from the raised arm.

 

 

In the final pic the weight is out of the way of the raised arm and the arm is now allowed to drop from the pressure of the exhaust valve spring and tappet. The tappet now rides on the main exhaust lobe.

 

 

 

 

 

 

[WheelHorse520H 708]

Nice post, I’d never heard of this but now I know how it works, it’s pretty cool.

[953 nut 55,310]

Here is a little more information to add to Mike's great post'

Gapping the ignition points at .020 has always been the standard answer to how points should be set. That probably will get you into the ballpark where the engine will run, but with a bit of additional effort you can improve the engine’s power and performance.

The Kohler engine manual in the Red Square files section covers two methods for setting the ignition timing, Static Timing and using a Timing Light. This manual is a relatively new manual and it overlooks the fact that many of our engines were built prior to the ACR (automatic compression release) camshaft.

Earlier engines (mostly 1965 and earlier) had a Spark Advance camshaft that can not be timed using Static timing. At rest (and very low RPMs) the timing is retarded to fire slightly after TDC. The timing mark (SP) on your flywheel is at twenty degrees before top dead center but at rest the points on these engines break about ten degrees after top dead center. The only reliable way to check or set the timing on these engines is with a timing light.

There are a couple ways to determine what camshaft you have. Presuming the camshaft in your engine is the one it was born with the data plate on the engine has a suffix that can tell you what camshaft was used. The table below will tell you the suffix applicable to your engine. The other way to determine what camshaft you have is to remove the cam gear cover and take a look. If you see a mechanism attached to the cam gear it is the ACR cam.

 

 

The following engines have the spark advance camshaft;

K-141, Suffix prior to “C”

K-161, Suffix prior to “J”

K-181, Suffix prior to “D”

K-241, Suffix prior to “D”

[squonk 41,151]

There is no cam cover on either K181's I have.

[c-series don 8,721]

Question: how does one check the compression one these ACR engines if the valves are slightly opened? 

[squonk 41,151]

5 hours ago, c-series don said:

Question: how does one check the compression one these ACR engines if the valves are slightly opened? 

One way would be to connect a strong enough electric motor to the pulley and turn the engine above 600 RPM. The compression release lowers the compression ratio to about 2:1. A better way to test is do a cylinder leakage test. At TDC  the tappet is past the raised arm and is resting on the cam lobe. It is also the position the engine needs to be to adjust the exhaust valve lash.

Edited August 31, 2021 by squonk

[c-series don 8,721]

@squonk Thank you sir ! That makes perfect sense! 

[953 nut 55,310]

10 hours ago, c-series don said:

Question: how does one check the compression one these ACR engines if the valves are slightly opened? 

With ACR engines a Cylinder Leak Down Test will be your best diagnostic tool. Perform the test with the piston slightly past TDC and hold the crankshaft with a pipe wrench. On a Kohler engine the key-way on the PTO end of the crankshaft will be up at TDC.

Mike @wallfish did a great thread on the subject a few years back.

You can also find lots of Youtube videos on the subject.

[Wheelhorse#1 1,667]

Good stuff .thanks for the Education