Ign Advance -Cranking RelativeCompression & In-cylinder

A really good question was asked about a recent SD Premium Video www.scannerdanner.com/scannerdanner-prem...sting-continued.html

"How many degrees of ignition advance can you expect to see on a cranking relative compression test ?"

The simple answer is somewhere between the static ignition advance and the advance at idle speed.
Static advance the initial advance set in the computer before the crank starts to rotate is generally about 5 degree BTDC any earlier than this would result in combustion pressure rise starting before TDC and the engine trying to start in the wrong rotation direction fighting against the starter .

At idle speed for example 800 rpm the ignition advance will be controlled by the computer somewhere between 10 and 15 degrees.

You can look at this from scan data --- this is a Land-Rover Freelander KV6 2.5 engine from Key On Engine Off to about 4,500 RPM and back to zero rpm. 5 Deg BTDC Static advance rising rapidly to around 14 BTDC at normal idle



To make interpreting both Cranking Relative Compression Tests and In Cylinder Compression tests easier and avoid doing maths a hard and fast rule is you always expect the ignition sync firing line to be about 2 to 2.5 ms before peak compression pressure.

Andy.MacFadyen wrote: To make interpreting both Cranking Relative Compression Tests and In Cylinder Compression tests easier and avoid doing maths a hard and fast rule is you always expect the ignition sync firing line to be about 2 to 2.5 ms before peak compression pressure.

Good stuff right here.