1.3l engine compression tolerance

[RicardoM]

Skoda factory service manual says that a new engine has 10 - 14 bar compression for carburetor engines and 10 - 15 bar compression for injection engines. The same manual says that wear limit is at 8 bar.

 

I am stupefied how big is the tolerance of compression value for a new engine! We're talking about 40-50% tolerance!! What life will have an engine that has 10 bar compression right out of the factory compared with another one having 15 bar?! Did Skoda have such low quality standards??

[Tamber]

Yikes, that's quite a bit of variance. Probably make some of those engines feel a little bit weak; but perhaps they figured they'd get away with it, since they were being designed for economy rather than performance.

[RicardoM]

In fact, where could come that pathetic compression variance from??

In a factory that had a QC department.

[Tamber]

QC work with the tolerances you give 'em; if you give 'em a stack of really loose tolerances, your product will be all over the place while being to spec.

[RicardoM]

Yeah, but I was asking for a technical justification. I mean they didn't manufacture and assemble the engines in a dusty blacksmith barn. They used German tools and VW technology. I don't see why at the end of production line the engine compression was all over the place. I don't think that cousin VW Polo from that period had such compression variation.

[Breezy_Pete]

Interesting topic.

I suspect some of the variation may be to do with the effect of engine/oil temperature on the results.  As far as I can see, just oil temp >30°C is the pre-requisite before testing, but that leaves quite a wide range.

I've not done a pair of cold/hot comparison tests to see what differences come out on the same engine on the same day, but it's something I could do out of curiosity.

 

The current VW testing info for my (2003) BBY Polo engine gives the same 10-15 Bar range as you've seen documented, but with an even lower wear limit at 7 bar, and a between cylinders max of 3 bar.

[RicardoM]

The current VW testing info for my (2003) BBY Polo engine gives the same 10-15 Bar range as you've seen documented, but with an even lower wear limit at 7 bar, and a between cylinders max of 3 bar.

Unbelievable... it is even worse than Felicia. Allowable difference between cylinders is 1 bar.

 

So let's say I install new piston rings on a well maintained engine. Cylinder sleeves are in specs, pistons too, honing marks still visible, valves sealing properly, etc. Theoreticallly the engine will be like new. What compression should I be satisfied with? 10 or 15 bar? It's mind boggling...

Edited 27 August, 2015 by RicardoM

[RicardoM]

A fellow member from Egypt (ehabmobarak) has proof that compression value varies quite extensively with valve clearance.

 

Could this be the reason for compression variation right from factory?

[RicardoM]

Here is a quote from another forum. One guy asked if lower compression on some cylinders could be attributed to incorrect valve timing.

 

Valve timing and compression can be directly related. If you do not have sufficient gap between the cam lobe and the valve stem, then it is considered tight. If it is so tight that the valve never gets a chance to fully seat against the head, you will loose compression back through the valve that is not fully seating. If the valve does not seat against the head long enough between cylinder firings, the heat keeps building up in the valve and it can "burn". The time a valve seats against the head, it is loosing heat to the head. The head is a heat sink for the valve. There is a critical amount of time the valve needs to release heat to the head so it will not eventually "burn". The newer valves can handle alot more heat (sodium filled), but you don't want to leave a valve "tight". Unfortunately, you cannot hear a tight valve. The loose valves warn you that it is time to adjust the valves...but if they sound good , you could still have a tight valve and be loosing compression as well. Usually though, a tight valve that isn't seating all the way will be prone to either backfiring through the exaust if it is a tight exhaust valve and backfiring through the intake if it is a tight intake valve.

 Ask your mechanic to write down which valves were tight (by how much) and which ones were loose (by how much). Keep this info. for the next adjustment to see if there is a pattern of valves that get tight or loose. This long term documentation can be helpful in future diagnosis.

Edited 1 September, 2015 by RicardoM