Poor man's compression test?

[sundance]

Good way to go. Compression ratio is more critical that high compression. Using a 162 husqvarna as an example. The squish at .031 calculates to 13.7 : 1. With the squish at .021 it's 16.2:1. Anything over 14:1 is deisel territory and gas will ignite from the compression. Which is why gas in a deisel engine will grenade the engine. Most saws stock are 11.5 - 12.5 for a reason. They don't want to reach the deisel threshold. I was taught 45 years ago to never assume that the engineers don't know what they're doing. I'd be willing to bet that 70-90% of ported saws on the internet have less power than stock if they were all dynoed. There's a placebo effect in loud saws.

I'd be curious to see the compression ratio math to support your example. Doesn't seem plasible.

 

[Dennbb]

I'd be curious to see the compression ratio math to support your example. Doesn't seem plasible.

Of course it doesn't seem plausible because it goes against what youtube followers are led to believe. The math is pretty simple. It's not my math. It's the universal excepted math for calculating . compression ratio. Add the swept volume and trapped volume. Divide by the trapped volume. The swept volume on the afore mentioned saw is 43.8 cc and the trapped volume is 3.4 cc which adds up to 47.2 cc then divide 47.2 by 3.4. The math for finding the trapped volume is more complicated and I don't have the time or inclination.

 

[Bob Hedgecutter]

He told me the easiest way for him to do a quick test is to pull the spark plug and turn the crank over. If he can hold his thumb over the plug hole, he knows compression is dead. If air is pushed out, he knows compression is good.

Does that same Guru have the ability to jam his todger up a dirtbike exhaust and be able to tell if it is running rich?

 

[Ted Jenkins]

What is wrong with just pulling the starter rope. After fifty years of pulling ropes I think one would kind of get the idea when compression was weak. Thanks

 

[kevin j]

Of course it doesn't seem plausible because it goes against what youtube followers are led to believe. The math is pretty simple. It's not my math. It's the universal excepted math for calculating . compression ratio. Add the swept volume and trapped volume. Divide by the trapped volume. The swept volume on the afore mentioned saw is 43.8 cc and the trapped volume is 3.4 cc which adds up to 47.2 cc then divide 47.2 by 3.4. The math for finding the trapped volume is more complicated and I don't have the time or inclination.

Using the total cylinder volume is a nominal compression ratio, like 4 strokes are calculated. But the actual compression ratio for a two stroke is the volume when the exhaust port closes. Clearly the actual CR is not 15-18:1. More like 6 or 8 to 1.

 

[Dennbb]

Using the total cylinder volume is a nominal compression ratio, like 4 strokes are calculated. But the actual compression ratio for a two stroke is the volume when the exhaust port closes. Clearly the actual CR is not 15-18:1. More like 6 or 8 to 1.

You need to study more.