sandman81 Posted March 9, 2009 Report Share Posted March 9, 2009 all i seem to read about is cranking compression and not compression ratio. yamaha's banshee spec sheet reads 6.5:1 compression ratio and noss machine is 130 cranking (stock at sea level). but how about a worked up engine for ex. has dune porting with 66 mm pistons and 18cc domes, or a stock banshee bore with 21cc domes. so what i want to know is how to get compresion ratio for something that is not stock? Quote Link to comment Share on other sites More sharing options...
Snopczynski Posted March 9, 2009 Report Share Posted March 9, 2009 rod length, stroke, bore size, and exhaust port height, etc......... Then I plug them into a program I have and it tells me the compression. Quote Link to comment Share on other sites More sharing options...
2001Stroker Posted March 10, 2009 Report Share Posted March 10, 2009 For domed pistons, use a negative cc number. Dished pistons, use a positive cc number. Talk to your porter to get the duration. Compression Calculator Conversion Calculator Quote Link to comment Share on other sites More sharing options...
sandman81 Posted March 10, 2009 Author Report Share Posted March 10, 2009 rod length, stroke, bore size, and exhaust port height, etc......... Then I plug them into a program I have and it tells me the compression. stupid question. how do you figure out exhaust port height? what are the etc, and the formula you use? Quote Link to comment Share on other sites More sharing options...
Snopczynski Posted March 10, 2009 Report Share Posted March 10, 2009 stupid question. how do you figure out exhaust port height? what are the etc, and the formula you use? You have to have your builder tell you, or at least have the head off and measure it yourself. Quote Link to comment Share on other sites More sharing options...
sandman81 Posted March 10, 2009 Author Report Share Posted March 10, 2009 You have to have your builder tell you, or at least have the head off and measure it yourself. that's the thing i don't want some one to tell me i want to be able to do it on my own. maybe it wasn't a stupid question. HOW DO YOU FIGURE OUT EXHAUST PORT HEIGHT? what are the etc you mentioned and what is the formula? i don't think a formula can be that hard to be figured out by writing it on paper. so if someone can help me i would be greatly appreciated. Quote Link to comment Share on other sites More sharing options...
2001Stroker Posted March 10, 2009 Report Share Posted March 10, 2009 For domed pistons, use a negative cc number. Dished pistons, use a positive cc number. Talk to your porter to get the duration. Compression Calculator Conversion Calculator You must have totally missed this. Click on this links that I put in here for you. Quote Link to comment Share on other sites More sharing options...
sandman81 Posted March 10, 2009 Author Report Share Posted March 10, 2009 You must have totally missed this. Click on this links that I put in here for you. thaks, i did see it. my last post was mostly about finding out how to get exhaust port height and wanted to know the etc. Snopczynski was talking about. since i still don't know how to figure out exhaust port height i can't come up with a ratio. see i know the formula for four stroke, built alot of high performance small blocks, and that the reason for my ratio question. cranking compression really doesn't play a part in building those type of engines. since i'm new to the two stroke world. the whole exhaust port height has me dumb found going of cranking compression. there has been a lot of post i've read on here were ALOT of guys have a high cranking compression then what they should. so, there trying to figure out why, even there engine builders don't have a answer. that make me think that cranking compression is not a good number to go by, unless i'm wrong. Quote Link to comment Share on other sites More sharing options...
Snopczynski Posted March 10, 2009 Report Share Posted March 10, 2009 thaks, i did see it. my last post was mostly about finding out how to get exhaust port height and wanted to know the etc. Snopczynski was talking about. since i still don't know how to figure out exhaust port height i can't come up with a ratio. see i know the formula for four stroke, built alot of high performance small blocks, and that the reason for my ratio question. cranking compression really doesn't play a part in building those type of engines. since i'm new to the two stroke world. the whole exhaust port height has me dumb found going of cranking compression. there has been a lot of post i've read on here were ALOT of guys have a high cranking compression then what they should. so, there trying to figure out why, even there engine builders don't have a answer. that make me think that cranking compression is not a good number to go by, unless i'm wrong. You just measure from the top of the exhaust port to the cylinder deck. Quote Link to comment Share on other sites More sharing options...
2001Stroker Posted March 10, 2009 Report Share Posted March 10, 2009 That calculator will give you a static compression without the duration. You only need the duration to figure out your dynamic compression. Which does help for finding out what fuel to use. But, you will get a good start without it until you get those ports measured. Quote Link to comment Share on other sites More sharing options...
deckheight Posted March 10, 2009 Report Share Posted March 10, 2009 (edited) IMHO, I do not think cranking compression should play much of a role in 2-stroke engine tuning either. But a lot of 2-stroke people seem to think pretty highly of it for whatever reason. Measure your exhaust port height with a caliper from the cylinder deck to the top of the exhaust port and subtract the deckheight. In this case a Banshee with OEM cylinders and base gasket has a dechheight of .002". Then just figure your "corrected compression ratio" using this number. Exhaust and transfer port durations are referenced in degrees of crankshaft rotation very similar to a cam in 4-strokes. The geometric shape of the ports could be compared to a cams lift... kinda... Sort of... The reason for the corrected compression ratio on a 2-stroke "ported" cylinder as opposed to a full stroke or 4-stroke ratio should be pretty obvious. thaks, i did see it. my last post was mostly about finding out how to get exhaust port height and wanted to know the etc. Snopczynski was talking about. since i still don't know how to figure out exhaust port height i can't come up with a ratio. see i know the formula for four stroke, built alot of high performance small blocks, and that the reason for my ratio question. cranking compression really doesn't play a part in building those type of engines. since i'm new to the two stroke world. the whole exhaust port height has me dumb found going of cranking compression. there has been a lot of post i've read on here were ALOT of guys have a high cranking compression then what they should. so, there trying to figure out why, even there engine builders don't have a answer. that make me think that cranking compression is not a good number to go by, unless i'm wrong. Edited March 10, 2009 by deckheight Quote Link to comment Share on other sites More sharing options...
2001Stroker Posted March 10, 2009 Report Share Posted March 10, 2009 IMHO, I do not think cranking compression should play much of a role in 2-stroke engine tuning either. But a lot of 2-stroke people seem to think pretty highly of it for whatever reason. I totally agree with this statement. I've been stressing it for a while now, and nobody wants to hear it. You can have 130psi, but have 13.5: ratio and need 110octane. On the other hand, you can have 200psi, but have 9.5:1 ratio, and only need 98octane. It all depends on your porting. Quote Link to comment Share on other sites More sharing options...
sandman81 Posted March 10, 2009 Author Report Share Posted March 10, 2009 I totally agree with this statement. I've been stressing it for a while now, and nobody wants to hear it. You can have 130psi, but have 13.5: ratio and need 110octane. On the other hand, you can have 200psi, but have 9.5:1 ratio, and only need 98octane. It all depends on your porting. SWEET! i'm not the only one wondering this mystery of using cranking compression over compression ratio. maybe one of the many members that builds engine on here can in light in us. thanks, stroker, deck and snop. Quote Link to comment Share on other sites More sharing options...
deckheight Posted March 10, 2009 Report Share Posted March 10, 2009 Yup, I have often wondered what a compression gauge reading made at a couple hundred rpm has to do with what is happening in a combustion chamber when an engine is actually running and making near max rpm... Nothing I suspect! On the other hand, I can understand why a lot of folks rely on them. Compression gauge numbers are easily obtained, the data is easy to understand and they do provide something of a reference point. Problem is, it is an inherently inaccurate reference point... Compression gauges do sell a lot of parts though... LOL! Not saying that corrected compression ratios are 100% accurate for 2-strokes either... Things like extremely efficient tuned exhausts thrown in the mix sometimes make it real interesting... But CCR's are what I use anyway :geek: Me thinks you probably won't hear from any of well known engine builders posting on this site defending their compression gauge tuning abilities either :ermm: Quote Link to comment Share on other sites More sharing options...
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