compression 57 3 hp Johnson

[scubachuck]

ok i’m going to try a new head gasket.
how is your easy way to lap the cylinder head?
Thanks Chuck

[crosbyman]

circle 8 the head on 150 sand paper over a piece of glass or any dead flat surface … (ceramic tile) circle 8 till all sections of the head a shiny

remove all crud before starting check powerhead with a straight edge

Joining AOMCI has priviledges 🙂

• This reply was modified 4 years, 6 months ago by crosbyman.

[outbdnut2]

If your compression gauge has a rubber hose on it, and the checkvalve is at the gauge end of the hose, the reading will be low due to the volume of the hose, but the difference you see between cylinders indicates a problem with the one reading 40 even if the gauge is reading low.

[billw]

I’ve heard this many times about the long hose and I want to start a conversation about it. To me, it doesn’t matter if the hose is 100 feet long. You keep pulling the engine over until the gauge doesn’t go any higher, and that’s the compression. The check valves in the compression gauge basically make the engine into an air compressor and it will KEEP compressing air in the hose until the cylinder reaches it’s max. Am I not right? Not to be a nit picker but I have heard this so many times and I don’t get it.

Long live American manufacturing!

• This reply was modified 4 years, 6 months ago by billw.

[outbdnut2]

You are right if the check-valve is at the engine end of the hose, then the hose will build up compression pounds with each stroke. If the checkvalve is at the gauge end of the hose, the compression in the hose is lost each time the piston backs up. I have a gauge like that and it gives me about 20 pounds less on a 3 HP Johnson. The amount or error will vary from gauge to gauge depending on the volume of the hose. On bigger cylinders, the volume of the hose becomes less significant.
Dave

[fleetwin]

OK Bill, let me try to relay how I have attempted to make sense of this issue. Let’s just say an engine showed 100PSI on a compression gage after cranking it vigorously with an electric starter.. Now, let’s just say we added another head gasket to the block and put the head back on, then rechecked compression….Can we agree that the compression reading would go down when measured with two head gaskets installed?
Now let’s relate the above example to the little JW powerhead. The compression gage hose adds some volume to the JW’s cylinder/combustion chamber when the check valve is up by the gage, kind of like our example of adding another head gasket to the engine and rechecking compression..
So next let’s try to understand why this hose is such an issue for the JW powerhead, but not a big deal on larger powerheads….Let’s just imagine we are checking compression on a 350 V8 engine with this gage/hose set up. We probably wouldn’t see any difference in compression readings with a gage with no hose vs a gage with the long hose and valve up by the actual gage. Theoretically, there would be a difference in readings, but it would be so small we wouldn’t notice it on the gage. The volume lost in the hose is so minor when compared to the large combustion chamber explaining why a change in compression readings would be just about undetectable.
On the other hand, the volume in that hose is quite a bit when compared to the size of the JW combustion chamber, which explains why we see the lower readings…
Perhaps my explanation/interpretation is incorrect, but it makes sense to me….

[aquasonic]

I’m going to refer to a law of Physics known as Pascal’s Law.

Pascal’s principle, also called Pascal’s law, in fluid (gas or liquid) mechanics, statement that, in a fluid at rest in a closed container, a pressure change in one part is transmitted without loss to every portion of the fluid and to the walls of the container. The principle was first enunciated by the French scientist Blaise Pascal.

This explains why the length of the compression tester hose makes no difference. In this case, the source of compression is the piston compressing air within the combustion chamber.The hose is not part of the source of compression, but is part of the sealed system. The sealed system will pressurize to the maximum pressure generated by the source, no more, no less.

Because air is compressible, the only effect of the longer hose, which creates a larger sealed system, is that it will require more strokes of the piston to reach maximum pressure.

By adding two head gaskets, as fleetwin mentioned, you have changed the characteristics of source of the compression by increasing the volume of compressible air within the combustion chamber. This will lower the final compression readings, but only because the source has been altered. The hose itself is the recipient of the compression built up in the combustion chamber, and is not part of the source.

Another example in this hobby is the dual line pressure tanks. The crankcase generates a fixed amount of pressure, say 5 PSI, and this is the pressure source. Both four gallon and six gallon tanks will pressurize to that pressure. The only difference being that the six gallon will take a few more pulses of the crankcase to reach that pressure.

• This reply was modified 4 years, 6 months ago by aquasonic.

[outbdnut2]

Hey Aquasonic. Let me try another way of explaining. I am very familiar with Pascal’s law, and you are right in that the pressure generated against cylinder combustion surfaces is the same as that in the hose, but please understand that with a hose on the gauge and the checkvalve up at the gauge, a piston stroke is pushing all that air into a larger space, consisting of the volume of the upper combustion chamber + the volume of the hose, so that air can’t compress to as many pounds per square inch in the first place because it’s being squeezed into a bigger place. Exaggeration may help understand. Imagine if the hose was two inches in diameter and you are pushing one piston stroke of air into both it and the combustion chamber. Would you get much of a reading?

To use your words you used describing an added head gasket, the same applies to the added hoseby adding the volume of the hose “you have changed the characteristics of source of the compression by increasing the volume of compressible air”. The only difference with adding the hose is that while the air in the hose is not in the combustion chamber – it is still part of the equation, adding to the volume of air being compressed. So compression pounds will be less, just as adding a thicker headgasket does.

I know it can be confusing, but I have two each of both kinds of compression gauges, and the results are very repeatable. I make sure I don’t use one with a gauge-end checkvalve on small cylinder engines. If the checkvalve is at the engine end of the hose, the readings are accurate. When I test my gauges for accuracy by applying a continuous pressure with my air compressor, they all read alike
Dave

[scubachuck]

Good news!!!!
I lapped the cylinder head and installed a new gasket and now both cylinder are 60 psi and running on both cylinders
Thank you all for the help

[fleetwin]

Well, I remember being exposed to Pascal’s law in school, but was too young/immature to understand….Seems like now I am too old with too many dormant brain cells to understand its definition…
But, I don’t think the pressure tank explanation is quite the same as what is going on in the combustion chamber….Yes, the engine will continue to pump pressure into the tank until the desired pressure is achieved…But, keep in mind that the engine is capable of over pressurizing the tank, the tank’s check valve maintains the proper pressure…..And yes, it will take any given engine a few minutes longer to pressurize a larger tank, or one with little/no fuel inside, but the pressure will be eventually achieved….
This is not quite what is going on in the cylinder though. The pressure built up in the cylinder is relieved during every exhaust stroke, whereas the pressure tanks have check valves to keep pressure from escaping. Let’s look at it another way. What would we expect to find if we opened/loosened the tanks cap once during every stroke? We would only see a reading of whatever pressure the tank was able to achieve with one stroke, which would be much less than the 5PSI we have spoke of. But, given this scenario , we would sure see a higher pressure reading on a tank that was nearly full vs one that was almost empty.
The other issue that confuses this discussion is that the compression gage itself has a check valve in it that “captures/retains” the highest pressure achieved while cranking so we can actually read the gage afterwards…Other wise, the gage would drop to zero after every compression stroke.
So, Aquasonic’s terminology is a bit confusing to me. True, the hose is not a “source of compression”, and yes it is sealed once the check valve closes, but the long hose (with check valve at the gage end) will definitely affect compression readings. Let’s look at it another way, how about on a 57 35hp bigtwin with the compression relief mechanism….Consider the relief chambers on the head in the same light as the compression gage hose, and we can equate the compression relief poppet valve in the head in the same way we think of the check valve on the compression gage….All things being equal, let’s just pretend we could pull the recoil as fast as the electric starter cranks the engine/for a given amount of revolutions….
Can we agree that the compression readings taken while cranking the engine with the recoil (same speed and number of revolutions) will be lower than the readings we would see if cranking the engine with the electric start??
If we can agree on that, then the question is why are the compression readings lower when cranking with the recoil? And the answer is that the compression relief system is activated by the recoil, which lowers the compression readings….The compression relief system is not activated while using the starter motor, so there is no pressure loss/transfer to those chambers on the cylinder head.
So, am not sure who is arguing which point here, but it would seem as though I agree with Dave….The long hose on the compression gage with the check valve up by the gage will definitely affect compression readings, especially on engines with small combustion chambers. The combustion chamber can’t quite be equated with some sort of pressure pump, because the pressure is relieved during every stroke when the exhaust ports are opened.
Finally, forgive me if I have misinterpreted/misread things that have been written, perhaps we are all saying the same thing….

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