How much compression

[Edgeflyer]

I have acquired some second hand engines in good apparent condition but their compression is not what I'm used to with nearly new Irvines etc.

For example an OS 50SX which is ringed has some compression, but is relatively easy to turn through t.d.c just using the prop driver. With my Irvine (just run in) I can't do this without a prop!

Similarly, a good looking OS 52FS with a couple of hours running turns pretty easily over t.d.c. I've checked the valves. On compression I can hear pressure escaping through, I think, the air intake.

My last query is an OS Max 46AX. This one looks mint- again pretty easy to turn through compression with no prop.

Can anyone advise?

[Biggles' Elder Brother - Moderator]

Ringed engines often don't offer much compression when turned over by hand cold. The rings only really start working when there is plenty of oil and heat about. Having said that being able to turn it over by hand with no prop is very low.

 

TBH I wouldn't worry to much until you've run them. I've known a few engines that had seemingly very low compression but carried on running well and giving their full expected power.

 

With engines - the proof of the pudding is very definitely in the eating

 

BEB

 

PS A bit of air leakage passed the valves with the engine dry is also not at all surprising. Remember that again the oil is a major part of the sealing mechanism and in practice they only have to seal for fraction of a second when runing - they weren't designed to provide long term compressed air storage!

Edited By Biggles' Elder Brother - Moderator on 16/10/2011 22:39:01

[Edgeflyer]

Many thanks for that info which all makes good sense. I shall try them out and hope for the best!

[Steve Hargreaves - Moderator]

As BEB sez if it runs OK then it is OK......most ringed engines have very little compression when turned over by hand....this is because they rely on pressure above the ring to spread it into contact with the liner & seal the piston/bore & this pressure is usually only built up when the piston is moving quickly.

 

Non ringed engines (Irvine) rely on a very tight piston/liner fit so will have much more compression when turned by hand but even in these the presence of oil in the piston liner gap will enhance the seal & increase the compression.

 

See how they run.....poor compression is usually shown up by an inability to idle.....a worn motor tends to "fade away" as it idles....

 

But OS are excellent motors....the best available in my opinion....so they usually have a long & trouble free life......

[Ben B]

People talk about "compression" but that's not actually what they're assessing. To assess compression you need

 

a) a warm engine

b) a compression tester

c) a stated reading for that engine using a particular type of glow plug

 

I've never seen a model engine compression tester nor a stated reference range for an engine and I'm sure that 99% of the "compression assessment" is done on cold engines.

 

Turning the engine over and feeling the resistance on a cold engine as a proxy marker is in my opinion a waste of time because

 

a) ABC/ABN type engines have lots of top end nip (especially when cold) which might "feel" like compression but in fact isn't

b) Some engine designs have naturally high compression ratios whilst others have low CRs (like Saito)

c) glow plug choice can also have an impact on CR as it will modify the squish area

When you say you've checked the valves on the four stroke do you mean you've checked the tappets or have you had the valves out and checked them (and the valve seat) properly, including the gear train timing. Ingestion of a bit of grit leading to a nicely scored valve / seat would cause the above as would incorrect valve timing. Just because the engine only has a couple of hours running doesn't mean the previous owner won't have had it apart for a shoofty and put it back incorrectly or done the running-in at ground level (where the grit lives)....

 

I wouldn't bother "testing" for compression. If the engine runs okay, it's okay Turning over a cold engine in an attempt to derive the health of the engine like some form of aeromodelling seance isn't IMHO worth it and at worst will prematurely wear the engine because you're turning over a cold engine with only residual oil in it.

[Engine Doctor]

When the engine is warm you should feel some resitance when turning over even on a ringed engine as very low compression will mean poor performance and bad idling.Turning over smartly with aprop will give a better idea. Some of the cheap fs engines can have very poor comp due to bad valve seating and consequently will only run reliably at higher revs but will be very unreliable on tickover and pick up. The exception are engines fitted with a "Dykes piston Ring"as fitted to the older irvine engines .These had virtually no comp when warm and had to be started with a starter most times but could be started easily by hand when cold and well primed I dont know of this type of ring being used in any current engine though.As said earlier cold engines give a poor indication of actual compressiondue to thick oil residue . beware when bying second hand engines that are "well oiled" as this could be to boost the compression always ask to put some freshh fuel in the cylinder to rinse the oil out and you will get a better idea and avoid bying a pup.

[Edgeflyer]

Thanks all you experts, much excellent advice here and mostly I'm reassured! Ben B that's a good point re the valves, I only checked the valves movement without taking them out.

I need to test run these engines now and do some more delving if they don't perform.