Leakdown vs Pressure vs Vacuum

litefoot · Dec 7, 2008

litefoot said:
Chow and Tzed,
Great responses, both of you. What are acceptable numbers for a 2-stroke engine both in terms of % leakdown and time to leakdown? Thanks.

OK, back to my question please. What % leakdown is acceptable? Also, in regards to the "time to leakdown" test, is there a time that the rings should hold pressure...say something like leak down to no less than 50% pressure in 5 minutes? (Just throwing a number out there)

Lakeside53 · Dec 7, 2008

Time : It doesn't wotk that way.. you supply continous pressure.. it leaks continuously...

2 rings will "leak" less then one. What's acceptable... depends, and like compression, there's a wide range off acceptablily... ... Less than 5% is o.k... 2% would be great.

If I have time this week I'll check a bunch of my saws and see what they say.

chowdozer · Dec 7, 2008

litefoot said:
OK, back to my question please. What % leakdown is acceptable? Also, in regards to the "time to leakdown" test, is there a time that the rings should hold pressure...say something like leak down to no less than 50% pressure in 5 minutes? (Just throwing a number out there)

I've never leakdown tested a 2 stroke. All my experience has been with V8's. These would be ballpark for a V8 and by no means written in stone:

up to 4% - racing engine
5% to 7% - rebuilt stocker
8-10% - runable
11-20% - marginal
>20% - boat anchor

There are a lot of variables here. New P&C versus old, bore size, type of rings just to name a few.

I'm going to make a wild guess here and say over about 6-8% leakdown you will find a hp improvement with new rings or a P&C on a saw. A higher compression ratio will tolerate more leakdown before it becomes inoperable too. A leakdown test is normally done on a warm engine too, especially with an aircooled engine where the piston to cylinder wall clearances have alot of variance between warm and cool. Probably not possible with a saw as the mass of the P&C will cool rapidly.

I will be interested to see what Andy measures on the saws he checks.

litefoot · Dec 7, 2008

Lakeside53 said:
Time : It doesn't wotk that way.. you supply continous pressure.. it leaks continuously...

2 rings will "leak" less then one. What's acceptable... depends, and like compression, there's a wide range off acceptablily... ... Less than 5% is o.k... 2% would be great.

If I have time this week I'll check a bunch of my saws and see what they say.

chowdozer said:
I've never leakdown tested a 2 stroke. All my experience has been with V8's. These would be ballpark for a V8 and by no means written in stone:

up to 4% - racing engine
5% to 7% - rebuilt stocker
8-10% - runable
11-20% - marginal
>20% - boat anchor

There are a lot of variables here. New P&C versus old, bore size, type of rings just to name a few.

I'm going to make a wild guess here and say over about 6-8% leakdown you will find a hp improvement with new rings or a P&C on a saw. A higher compression ratio will tolerate more leakdown before it becomes inoperable too. A leakdown test is normally done on a warm engine too, especially with an aircooled engine where the piston to cylinder wall clearances have alot of variance between warm and cool. Probably not possible with a saw as the mass of the P&C will cool rapidly.

I will be interested to see what Andy measures on the saws he checks.

Thanks. And yes, I'd like to see what numbers Andy gets on some new and run-in saws.

chowdozer · Dec 7, 2008

One of the reasons to do leakdown testing, not pertaining to chainsaws of course, is the cam in a 4 stroke has alot to do with cylinder pressures.

A "lazy" cam will bleed off cylinder pressure at low rpm's. This causes a false reading with a compression test performed in the typical manner. My ol El Camino has the 350 horse 327 old skool cam in it. It has a lopey idle and bleeds of mucho cylinder pressure at low rpm. At about 2300 rpms, the time the cam can bleed of cylinder pressure is seriously shortened and it starts that "kick in the seat" feeling. At about 2900 rpms, it starts really making some power and by 3500 it really feels good. It's fun to drive between 2500-4500 rpms but off idle is is sluggish.

So a compression test at slow cranking speed in a 4 stroke can make the reading appear low and you would think the rings are shot. A leakdown test would show they are not. edit - I'll also add, if you change a cam you may not have any idea what to expect for compression readings. If you bought an engine that had a cam put in, you have even less of an idea. This is where a leakdown test really shines... the cam doesn't enter into the equation at all in a leakdown test.

To keep this about chainsaws, one of the uncontrollable variables that will affect compression readings is how fast you can pull the rope. The faster you can pull, the less time it has to bleed down.

stihl86 · Dec 7, 2008

For what it's worth, I have stopped doing a compression test or a leakdown on a 2 cycle. Too many variables. I have seen scored pistons and cylinders with relatively high compression reading and low leakdown. Why? I can't say. Remove the carb and muffler to inspect rings, piston and cylinder. I rarely get fooled by false test results that way. Vac/press test is something completely different.