Reply To: compression 57 3 hp Johnson

[Don]

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….