6 replies to this topic

[MFM]

Whats a healthy compression ratio for an LJ XU1 with standard engine unknown cam (lumpy). Just ordered a tester and want to know what the PSI should be. Can't seem to get a straight answer on google as some say depends on the cam overlap whatever that means?

[V-SLR5000-P]

A cam with a wide lobe centre angle will generate more compression than one with a tight centre angle due to lesser overlap. Since you don't know what cam you have this point is academic. The main thing is that compressions vary by a minimal amount between cylinders, say no more than 10%. Pull all six plugs with the engine warm and check compressions with the throttle blades open. At a guess I would expect  between 140 and 170 psi. Also have your battery fully charged so that cranking speed is consistant between cylinders.

Edited by V-SLR5000-P, 06 October 2020 - 09:23 PM.

[LXCHEV]

Agree - consistency across all cylinders is what you really want to see. If not, then you may have engine issues.

The actual dynamic compression reading that you’ll see on the gauge will absolutely depend on your specific combination. The static compression ratio and cam specs clearly having a big influence.

I just went through this myself on my 383 SBC with 10.5 comp and mild cam - the comp test produced a nice healthy 190PSI across all 8 cylinders.

There must be plenty of people with XU1’s or similar hot 6’s that can share typical readings from that kind of combo.

[claysummers]

Did a 186 recently that has been sitting around gumming up the last 30 years. Ranging from 159 to 178 but mostly in the upper range. Don't think it has been worked compression wise although it is a bitsa engine with 179 head.

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[MFM]

Another silly question,do the holden red engine heads including XU1 have valve stem seals?

[LC-GTR-1969]

A cam with a wide lobe centre angle will generate more compression than one with a tight centre angle due to lesser overlap. Since you don't know what cam you have this point is academic. The main thing is that compressions vary by a minimal amount between cylinders, say no more than 10%. Pull all six plugs with the engine warm and check compressions with the throttle blades open. At a guess I would expect  between 140 and 170 psi. Also have your battery fully charged so that cranking speed is consistant between cylinders.

 

This is not actually correct, but is an extremely common misconception- the overlap does not determine the compression/ psi that the engine sees on a comp test... 

 

The inlet valve closing angle and static compression ratio, as well as general ring seal, determine what reading you will get. 

 

A 110lsa cam with 240 degrees duration, installed with 4 degrees advance will have LESS cranking compression than a cam with 106lsa and 240 degrees duration, again installed with 4 degrees advance. 

 

The wider LSA will close the inlet valve later in the cycle than the same duration cam on a smaller LSA- the later closing inlet valve bleeds off the compression before it can be built in the cylinder, hence reducing the cranking compression reading. 

 

Hence, if you are running a compression limited setup, say a head / piston/bore combo that makes 9.5:1 compression, you can tighten the LSA to regain some cranking cylinder pressure (but cranking comp is a somewhat arbitrary number anyway). The tighter LSA cam will have more overlap but this is simply not what bleeds off the compression on a comp gauge.

Edited by LC-GTR-1969, 04 November 2020 - 04:41 PM.

[ozyozyozy]

Everyone has made good points.

Just to add a little, if your doing it for engine health, it is a good method, easy to do but a leak down tester is better again, this can be used to check if its valves, rings or head gasket that is losing pressure.