New Yamaha F150XB compression test

[99yam40]

So, after reading that is the 9 to 1 a static or dynamic compression ratio?

[rodbolt17]

the advertised compression ratio is based on a mechanical ratio.
Kennesaw actually says to do a compression test then if further tests are indicated do a leakdown cause it is much more accurate.
a compression test costs 25-40 bucks at 100/hr.
a compression test AND a leakdown will cost you 40-60 bucks.
a leakdown costs about 25 bucks.

steps for a compression test on an F150.

disable the ign OR hook up the spark tester.
remove the plugs.
inspect the electrodes very carefully.
hookup some type of remote starter OR get another man(on your ticket) OR start climbing from the engine to the helm.
install tester in #1 plug and crank until the needle stops paying close attention to the first 2-3 needle jumps.
interpret the needle jumps.
repeat for cylinders 2,3 and 4.

steps for a leaakdown.
remove the plugs.
rotate the engine to #1 TDC compression stroke.
( I install the tester adapter hose so I can hear the air ruhsing out as I approach TDC compression stroke).

apply the tester,record the reading.
move the test hose to #3,rotate the flywheel 180* to the other TDC mark.
test and record 3.
install the tester on 4 and rotate 180 to TDC test and record.
install the tester on 2 rotate 180* and test and record. undo the equipment and your done.

the leakdown will spot small leakage due to stuck rings,leaking valves or a minor head gasket leak that a compression test wont.

and it is way faster.
at 100/hr minutes add up quick.

[boscoe99]

please explain briefly how you get 175psi with a 9:1 engine while just cranking over....me not understand that...thanks

Via a calculation you don't. Because calculated compression does not take into account the temperature rise with compression. Pressure also rises due to an increase in temperature. Higher temperature results in more pressure. So, you have a mass of air that is both compressed and that has an increase in temperature at the same time. It is a compounding effect.

If following compression the cylinder would not leak, I would expect the temperature to reduce back to ambient and the pressure would decrease along with it.

Another thing that seems to stick in the back of my feeble mind is that the smaller the cylinder bore the greater the temperature rise will be. Not sure if that is really true and, if so, to what extent the temperature rise will change.

Now when it comes to a leak down test, putting X psi of air into the cylinder of a big bore motor (F350) having more leakage paths (four large valves) will have vastly different results than say putting that same air into an F2.5 having just two tiny valves. Yet Yamaha does not offer any guidance with respect to what the leak down allowances should be. The gauge does not know what motor it is connected to. Should it?

[boscoe99]

I look at it as a compression will give one a general indication if the motor is healthy or not. The leak down will tell you where the unhealthiness arises. A bore scope is another tool that can be used, but is rarely talked about here.

Both are just tools that should be used together to try and determine the healthiness of the motor. Both should not be taken as the gospel. They need to be viewed in context with a bunch of other variable parameters.

If an F300 makes 300 HP at 5500 RPM, uses little or no oil, starts, idles and runs well, has no blowby, but a leak down indicates an excessive amount of leakage via a valve or ring, should the motor be torn apart to correct the issue? Is it not possible that a valve could leak at 100 psi (leakdown input) but not at 1000 plus psi (cylinder pressure while running)?

[99yam40]

Another question.
on loop charged motors what does that mean.
is it something to do with air moving thru the intake that kind of pushes a little extra air/fuel charge into the cylinder because of the way it was designed?

[rodbolt17]

nope
if it leaks it simply leaks.
does not matter if it is an F350 or an F2.5.
Yamaha states 15%, industry standard is 20% max.
however any leakage past a valve must be corrected,same for a head gasket, due the the severe erosive effects of hot combustion gaskets.

cannot say how many exhaust valves I have replaced where part of the valve was simply missing.
cant say how many blocks I have scrapped due to a groove cut between the cylinders due to combustion gas.
it can and will act like a cutting torch.

a perfectly sealed cyl will show 0%.
however we have piston ring gaps on most engines.
so we know it is not perfect.
so we inject a known air pressure in this sealed cylinder.
then monitor how much make up air it takes to maintain the known pressure.
that is called the percentage of leakage.
works the same for most all piston enginges including diesels.
anything over 20% indicates an issue regaurdless of how well it runs.

it is why you have engine surveys done.

[panasonic]

So, after reading that is the 9 to 1 a static or dynamic compression ratio?

static....calculated by stroke, bore size...etc was not thinking about it proper...lol

[panasonic]

See what a good informative conversation I got going from my ignorance.!

Never stop learning...

[99yam40]

still looking for the meaning behind loop charged

[boscoe99]

still looking for the meaning behind loop charged

Patience grasshopper.

[boscoe99]

The best choice when high performance is demanded is “loop charge scavenging.” As we see here, the transfer ports are positioned on the sides of the cylinder and directed toward the intake side. This causes the fresh air/fuel charge entering the combustion chamber to collide with one another at the intake side and then loop around toward the exhaust port, pushing the burnt gases into the exhaust pipe — hence the term “loop charge scavenging.”
Because the piston is not used to deflect the fresh charge in the desired direction, as with cross scavenging, this loop-charge design allows the use of a relatively flat piston crown.

The advantages of this design are a smaller combustion chamber with higher compression ratios. Loop charging translates into greater power output from the same size power plant than one using a different design. There’s also less potential for pre-ignition or detonation occurring due to hot spots on the top of the piston.

[99yam40]

thanks for that info.
I thought there was some kind of pushing or pulling between the intake and exhaust gases.
Just thought it helped some how to get more fresh air and fuel in to the cylinder almost like a boost from the gases moving.
just realized this is only on 2 strokes , right

[TownsendsFJR1300]

thanks for that info.
I thought there was some kind of pushing or pulling between the intake and exhaust gases.
Just thought it helped some how to get more fresh air and fuel in to the cylinder almost like a boost from the gases moving.
just realized this is only on 2 strokes , right

I had an Evinrude 140 V4, two stroke, looper many moons ago. (carbed).

It was built on a late Friday, spark plug cross threaded from the factory, one of the trim wire connectors NOT crimped...

I opened the crate and rigged it myself... Other than that, pretty dependable..

Evinrude gave me a new head and gasket for the repair...

[Seanic08]

Good day all. Sorry it took so long, but it’s been a busy summer. I finally got a chance to do the compression test. The test gauge was bought from Harbor Freight. None of the adaptors that was in the kit fit spark plug thread so I ordered an adaptor kit from Amazon. Well, it’s approximately 160 PSI on all four cylinders.

I warmed up motor for about 15 minutes prior to starting the test.
Check out the video on YouTube.

https://youtu.be/Cou2p_pOd6o