compression

[stephen bebbington]

can anybody tell me what my compression reading should be on my tv series 3 175 fitted with rb200 kit should be cheers

[CHRIS in MARGATE]

Anything above 100 psi. But check your tester against another engine. My old 10,000 mile race tuned 225 iron was at 105 psi, still starting and running well when I took it out. My newly built Rapido 225 with 1.2mm squish is 135 psi.

[Fast n Furious]

You will get different readings from different types of compression testers, simply because, the volume in the connecting pipe and the gauge itself has to be taken into consideration as this volume effectively becomes part of the combustion chamber volume.
I abandoned this method of measurement years ago in favour of using a 16bar pressure sensor screwed directly into the sparkplug hole, which spits out a proportional 0-10v signal which I connect to a digital storage oscilloscope to read it. No compensation for combustion volume changes apply now, so what you read is accurate. Ok this kinda kit is not available to everyone but it makes all the difference when accuracy is required.
Static measurements up to 6:1 is normal at cranking speed bearing in mind that the exhaust port is open for nearly half of the compression stroke.
In reality, a 2 stroke is actually a variable compression engine. Your compression tester cannot compensate for the reflective actions of the exhaust chamber or the "Ram air velocity actions" of the carburettor. (Dynamic measurement)

[stephen bebbington]

[quote="CHRIS in MARGATE"]Anything above 100 psi. But check your tester against another engine. My old 10,000 mile race tuned 225 iron was at 105 psi, still starting and running well when I took it out. My newly built Rapido 225 with 1.2mm squish is 135 psi.


thanks chris mine is reading 65psi

[stephen bebbington]

You will get different readings from different types of compression testers, simply because, the volume in the connecting pipe and the gauge itself has to be taken into consideration as this volume effectively becomes part of the combustion chamber volume.
I abandoned this method of measurement years ago in favour of using a 16bar pressure sensor screwed directly into the sparkplug hole, which spits out a proportional 0-10v signal which I connect to a digital storage oscilloscope to read it. No compensation for combustion volume changes apply now, so what you read is accurate. Ok this kinda kit is not available to everyone but it makes all the difference when accuracy is required.
Static measurements up to 6:1 is normal at cranking speed bearing in mind that the exhaust port is open for nearly half of the compression stroke.
In reality, a 2 stroke is actually a variable compression engine. Your compression tester cannot compensate for the reflective actions of the exhaust chamber or the "Ram air velocity actions" of the carburettor. (Dynamic measurement)

thanks mine is only reading 65psi

[Fast n Furious]

Sounds about right to me.......65 to 75 psi depending on your exhaust port duration.