compression pressure throttle closed/wide open?

[fairdeal]

difference due to throttle open/closed is "night and day"

now, caveat: only tested "cold" (if you non-Floridians accept 80F as "cold")
and with a cobbled-together tester (Irma ruined my tester - waiting on a replacement)

so the max values, with throttle wide open, I was getting were only slightly over 100 psi

but with throttle closed, max reading was NMT 40 psi


I'll give it another try next week with a "real" tester, and having run the engine so its "hot"

[boscoe99]

If air can't get into the cylinder then there is nil air to be compressed.

I can see where it would affect a four stroke more so than a two stroke.

Going to be interesting IMO to see the values for your motor when tested hot. I have some ideas but am keeping them to my self. I might be wrong. Don't want to embarrass me.

Here is what Yamahammer has to say

Always use full throttle with ignition disabled when testing compression. This lets as much air as possible enter the cylinder so the cylinder condition, not the throttle restriction, may be tested. This is especially important when testing the compression on a high performance 4-stroke engine like the F50, Testing the compression on the F50 with a closed throttle may produce readings as much as 80psi lower than normal.

[99yam40]

interesting.
Do you think a carbed motor would be different than a Injected motor?

Since multi cylinder injected motors share a common intake/throttle body there should be a constant vacuum being pulled with the throttle plate closed.

I am not sure if multi cylinder 4 stroke carbed motors have multiple carbs , one for each cylinder like 2 strokes or not.
I would see no need for that since the valves are there, so a single carb could handle things well causing the same result for single throttle plate on it also

I would think that a single cylinder motor would grab some air with each intake stroke as the plate is not closed completely, so would let a little air in and there are not a bunch of other cylinders pulling it away.

[boscoe99]

Don't think a carburetted motor would be any different from an EFI motor. The throttle valves need to be open to allow the maximum amount of air to flow into the combustion chamber.

There are multi-cylinder models that use one throttle valve (carburetor or EFI) per cylinder and then there are multi-cylinder models that use one throttle valve for all cylinders. Each valve, how ever many are used, needs to be fully open to allow the most air to flow in.

I recall you saying that your two stroke has the same compression pressure with the throttle opened or closed. This surprised me. The two stroke takes in a gulp of air when the piston is coming up and that air is then pressurized when the piston goes downward, eventually making its way into the combustion chamber when the piston opens up the intake port. I would think that with the throttle valve being closed the gulp of air would not be as great as it might be if the throttle valve was open. All of the air getting into a two stroke is via the throttle valve. Seems that anything that restricts the flow of that air would affect the compression pressure. Two stroke or four stroke. More so in the four stroke.

[99yam40]

Running a compression test usually means the motor is out of the water, so the exhaust is open to get air also on the 2 stroke.

I did not believe it at 1st when I was told that, so I tried it.
I was amazed.
being the 4 stroke has valves that only let air in when the intake valve is open, it depends on if there is air in the intake to suck in.
If only one cylinder is doing that sucking the the cracked open throttle valve should have a little for it to suck in on every intake stroke because the cracked open throttle plate will let air in and the intake will not stay in a vacuum.

If there are multiple cylinders sucking there will be a lot less for each cylinder to suck in.
So a 1 cylinder should build up compression but slowly.
multiple cylinder motors maybe so little it would not be noticed.

Just my thoughts

[rodbolt17]

now tell me how to open the throttle while testing an f250 or the v8.
that is why I simply grab my leakdown tester on 4 strokes, and it is so much faster than a compression test.
even on motors with no flywheel index marks . that is where counting teeth an basic long division comes in.

[99yam40]

now tell me how to open the throttle while testing an f250 or the v8.
that is why I simply grab my leakdown tester on 4 strokes, and it is so much faster than a compression test.
even on motors with no flywheel index marks . that is where counting teeth an basic long division comes in.

But what are your thoughts on a closed throttle during a compression test on a 2 stroke and a 4 stroke?

[boscoe99]

This thread is about compression tests. Not differential pressure tests.

[rodbolt17]

on a four stroke it can make a difference,however if there is valve or piston damage you will see it as now one will be much lower than the rest.
quite a few Yamaha motors are going to a single electronic throttle valve.
so does most all Volvos since about 2010.
on a two stroke I have neve seen a difference mostly cause all the two strokes I have ever seen fail it was catastrophic enough that it was easy to spot.
at 100 bucks an hour I can do a more accurate test in about 1/2 the time.

[zenoahphobic]

Compression tests seem to be something that appear to be productive. Gives you some peace of mind after screwing out the plugs, putting in the compression stem, cranking it with a bit of noise, maybe some air farting past - the real busy professional act if you like.
But the reading you get has no meaning other than comparative between cylinders. Accuracy of gauges and the conditions of the test often does not correlate with published figures.
That number will also not correlate with the calculation of what it might be using cylinder volume and compression ratio. It is also influenced by pulses adding or subtracting to that peak pressure. And also the cranking speed is below any operating speed designed in the engine. And then there is the open or closed throttle argument.

It makes little sense to crank the engine with WOT, as this situation rarely occurs at idle speed, and then only for a very short time (tramping flat to the floor from a standing start).
One of the reasons why compression tests were done with WOT was to slowdown or reduce the amount of fuel entering the cylinder and washing away oil.
Now that fuel injection is common this is no longer necessary when injection can be temporarily disconnected.
So you have a choice when fuel injection is turned off : nearly closed or fully open.
Your readings will always be a bit different between the two, if only for the slighter slower cranking from the difference of the load on the starter between the two situations.