Compression check

[Matt Mordja]

Does anyone use a gauge like this for monitoring an engines health though out the season? Baseline a freshly broken in piston and liner, and check it regularly after heats for example.

http://www.horizonhobby.com/products/model-engine-compression-gauge-DYN2514

 

[moparbarn]

+1 - a few years back, mike z. told me about the bump test. as the engine wears, you can feel the difference in both the bump & how many times you can roll it through before it fades. one thing i didn't see in marty's notes was that you only move the crank far enough to go JUST past tdc, DO NOT uncover any ports or you will need to start the test over. you want to squeeze the same slug of air, over & over.

 

[SayMikey]

The most important thing is the seal,when you move the piston past the boost ports going up does it seal good.And will it hold that seal with just a light amount of oil on the piston.

 

[julianconde]

You can have no bump on the engine that does not mean it's worn out. People get that confused all the time. Remember when the piston heats up it will grow and like mike said as long as it has a good seal the engine is still good. When I ran my 80 twin that dick Loeb owns currently. Those engines had no bump at TDC but that twin hauled ass and that's because the engines had a good seal that's the most important thing to me is the seal.

Julian

 

[moparbarn]

when i said "bump", i did not mean pinch. bump (to me) is the amount of pressure it takes to compress the air in the chamber, & to "bump"/pop over tdc. pinch can be felt without a glow plug in, "bump" cannot. pinch is the amount of taper in the sleeve & how tight the piston fits that taper, "bump" is the seal of the piston to the sleeve. i've had some engines that had VERY little pinch out of the box, but had excellent "bump" & ran like a striped azz ape. then i've had some that had great pinch & didn't seal well ("bump"), & ran not so great.....

back to duke's question - i like to feel the bump & seal by rolling it over by hand. spinning the engine with a starter to run a compression check could lead to a false/possibly high reading. to be done properly, the engine should be a operating temp. our engines lose heat too rapidly to make me believe you could get accurate or consistent readings with the gauge. i've watched a club member's telemetry on a nomadio radio. from wot to a SHORT slow down in to shore, the engine lost almost 30* of head temp.

 

[anthony_marquart]

let's not confuse bump and pinch,.. the pinch is when there is no plug in the engine and the piston sticks a bit at TDC,. bump is the compression bump with the plug in and a bit of oil in the engine.

 

[moparbarn]

let's not confuse bump and pinch,.. the pinch is when there is no plug in the engine and the piston sticks a bit at TDC,. bump is the compression bump with the plug in and a bit of oil in the engine.

B) i think we were typing at the same time anthony!

 

[Grimracer]

Yep... Bump in this case.. Rod G taught me this years ago..



Pinch is somewhat different BUT.. pinch goes away with the break in and when that does the motor is ready for a bump test!

Clean everything (I spray the motor our with break clean).. allow to dry

place a drop of light oil in the plug hole and spin the motor over by hand to distribute

Put in a plug

Rotate the motor over TDC but do not allow the piston to go back down and enter the ports.

Rock the piston back over TDC

Repeat



You should be able to compress the same air AT LEAST 10 times.. the more the better.. I have seen lesser motors run OK but there time in doing so was limited.

Grim

 

[Duke Mordja]

Yep... Bump in this case.. Rod G taught me this years ago..

Pinch is somewhat different BUT.. pinch goes away with the break in and when that does the motor is ready for a bump test!

Clean everything (I spray the motor our with break clean).. allow to dry

place a drop of light oil in the plug hole and spin the motor over by hand to distribute

Put in a plug

Rotate the motor over TDC but do not allow the piston to go back down and enter the ports.

Rock the piston back over TDC

Repeat

You should be able to compress the same air AT LEAST 10 times.. the more the better.. I have seen lesser motors run OK but there time in doing so was limited.

Grim

I think part of the answer what Matt is looking for has been answered by Grim. Compressing the same air at least 10 times will produce an air pressure for the guage to read.

To know if you are losing compression over time, the baseline pressure needs to be measured. As an engine wears, it will still compress the same air, but how much??

That is what the guage will do. Determine the air pressure and not rely solely on our brain to convince us the engine is still good or not. (Like I am trying to convince myself on a new P/L for my scale boat.) Runs great, but is a new p/l needed?

From the posts so far, I don't think anyone uses a pressure gauge.

 

[Grimracer]

Nope. not me.. Had one of those and it went into the lake..... hype man..

NOW.. if you had a multi cylinder engine that might have some use... maybe.. MAYBE!

Grim

Bty.. I can gauge my motor by how many bumps.. I have 12 bump motors and 25 bump motors.. I have had 13 bump motors become 6 bump motors.. easy gauging!

 

[Duke Mordja]

Grim uses the bump gauge. Couldn't get any easier.

KISS principle rules!

Mike, what issues caused you to throw it in the lake? h34r:

 

[anthony_marquart]

I have had this method be misleading. I have had motors that would have a good 10-15 bumps cold, but when hot seemed to loose compression.. Now these were .12 motors.. all the bigger ones followed the rule very well. This also doesn't tell you is the bearings are bad and causing lean runs.. but still a good gauge for compression.

 

[Marty Davis]

You can have no bump on the engine that does not mean it's worn out. People get that confused all the time. Remember when the piston heats up it will grow and like mike said as long as it has a good seal the engine is still good. When I ran my 80 twin that dick Loeb owns currently. Those engines had no bump at TDC but that twin hauled ass and that's because the engines had a good seal that's the most important thing to me is the seal.

Julian

Julian:

The smaller the engine the more critical the seal is. With a .21 if you don't have a great seal, the boat will not launch with a fast props.

With the larger engines, there is what I call "excess horsepower" that allows those engines to run even with little bump or seal.

The roll over trick is a great way to test to see how well you engines seals. This is a trick that Ed Hughey told me about when I was first starting the hobby.

The bump over trick is all you need. Sometime when you swallow a plug element, try the bump test after that. You will find that it leaks down immediately.

 

[Grimracer]

Grim uses the bump gauge. Couldn't get any easier.

KISS principle rules!

Mike, what issues caused you to throw it in the lake? h34r:

I was upset cus I wasted my money....lol

Grim

 

[Terry Keeley]

You can have no bump on the engine that does not mean it's worn out. People get that confused all the time. Remember when the piston heats up it will grow and like mike said as long as it has a good seal the engine is still good. When I ran my 80 twin that dick Loeb owns currently. Those engines had no bump at TDC but that twin hauled ass and that's because the engines had a good seal that's the most important thing to me is the seal.

Julian

Julian:

The smaller the engine the more critical the seal is. With a .21 if you don't have a great seal, the boat will not launch with a fast props.

With the larger engines, there is what I call "excess horsepower" that allows those engines to run even with little bump or seal.

The roll over trick is a great way to test to see how well you engines seals. This is a trick that Ed Hughey told me about when I was first starting the hobby.

The bump over trick is all you need. Sometime when you swallow a plug element, try the bump test after that. You will find that it leaks down immediately.

Got an element scratch between the crown and wrist pin hole on my CMB 21 once, ruined it!

 

[julianconde]

The seal is important on both little engines and big block engines as well. But some people worry more about pitch than the seal.

 

[anthony_marquart]

I have been lucky I guess.. My NR21 5p's have eaten lots of plugs and do not end up with scratches. I changed the P/L on the last one just cause I had so much fuel through I thought I should.. It still doesn't run as well as it did before I fooled with it!!!

 

[DickJones]

To "Pinch" or to "Bump", that is the real "Question"?

Try this one, take the water jacket and the button of the engine, hold the engine in one hand and use appropriate size C clamp to lock the liner down putting the screw clamp at the bottom of the engine and the flat part of the C clamp on the liner area where the button sits, snug the clamp down, do not over tighten, grab the fly wheel with your empty hand and rotate the crank until just before the piston goes over top dead center and then snap it thru as fast as U can, do this several times to see how much resistance there is going over top dead center. This procedure will give U a good indication weather the engine is capable of a good seal.

JM2CW

dick