elwilliams13 Posted January 4, 2009 Report Share Posted January 4, 2009 Just calculated my compression ratio at about 8.50:1 on a 4mil with 24cc domes +4 timing stock carbs and an agressive trail port running around 187 degrees of duration I was told by the builder. I also have some 21cc domes I plugged into the calculator and it put me at around 9.60:1 with 185compression. My question is how the ratio is related to octane requirements. I was running c12 with the 185 compression and ran out of fuel once and used avgas and it worked fine for the rest of the trip even tore it down once I got home and the piston crown was perfect. Just looking for some recommendations related to ratio numbers and not so much on cranking compression. Thanks. Quote Link to comment Share on other sites More sharing options...
hercalmighty Posted January 4, 2009 Report Share Posted January 4, 2009 I think your off on your figures for the ratio. Did you add in your piston dome and if you did, did you make it - the amount or did you just put it in. Make sure you put it in as a - to get the right numbers. Quote Link to comment Share on other sites More sharing options...
bansheefreak Posted January 4, 2009 Report Share Posted January 4, 2009 if you have 24cc domes.. which i have never seen stock is 22cc.. anyways if you have porting and 24cc domes you be lucky to have 100lbs compression u need to be under 20cc or 19cc to run race gas.. Quote Link to comment Share on other sites More sharing options...
hercalmighty Posted January 4, 2009 Report Share Posted January 4, 2009 They are 24cc because they are stroker domes I'm pretty sure. I remember Kevin from Herr Jugs saying 24cc domes when I asked about a stroker motor. Elwilliams13 said he had 185 psi with 21cc domes Quote Link to comment Share on other sites More sharing options...
bansheefreak Posted January 4, 2009 Report Share Posted January 4, 2009 ohh ok.. well my stroker domes are 20 cc and i have bout 175lbs.. but they are custom cut so i dont know all the numbers as he is speaking... Quote Link to comment Share on other sites More sharing options...
hercalmighty Posted January 4, 2009 Report Share Posted January 4, 2009 Yeah I dont know. I just remember Kevin metioning 24cc domes to be able to stay on pump gas. Quote Link to comment Share on other sites More sharing options...
elwilliams13 Posted January 4, 2009 Author Report Share Posted January 4, 2009 Let me measure again just to be sure. It said a negative value needs the "-" symbol. I got wiseco795 pistons in place I can't find specs on the piston measurements. Quote Link to comment Share on other sites More sharing options...
hercalmighty Posted January 4, 2009 Report Share Posted January 4, 2009 I think for the piston dome it's -6.5cc. You are changing everything to inches also right? Quote Link to comment Share on other sites More sharing options...
elwilliams13 Posted January 5, 2009 Author Report Share Posted January 5, 2009 No I did it in millimeters. Maybe that is where I screwed up Quote Link to comment Share on other sites More sharing options...
2001Stroker Posted January 5, 2009 Report Share Posted January 5, 2009 Yea, you have to convert everything into inches. And yes, dome size is a negative number. I did alot of research on this topic, and I contacted a few oil companies about it. I finally got a chart for it. Contrary to popular belief, cranking psi has nothing to do with what octane you need. Port timing (or cam duration on 4-strokes) changes so much, with the static compression-vs-cranking compression. You can actually have 200psi of cranking compression, and only need 104 octane. On the other hand, you could have only 155psi, and need 110 octane. What really matters is the compression ratio. Quote Link to comment Share on other sites More sharing options...
deckheight Posted January 5, 2009 Report Share Posted January 5, 2009 I punched your numbers into TSR... With a 4 mil crank at 8.5:1 corrected compression ratio and a 187* duration it is outputting 110 octane. At 9.6:1 it is over 120 octane (TSR will not output that high). Also, at 187* duration it is saying your exhaust opens at 86.57* or 31mm ATDC. If the opening timing is not correct, octane estimates will change obviously. At any rate, something is not computing... I suspect either the exhaust opening is wrong or the CC/R is. Or both maybe? If you truly are setup for 110, there is no way you would have survived a high rpm run on 100??? If your exhaust timing is something other than above let me know and I'll run it again. Like they say with TSR and the rest of the programs... Garbage in = Garbage out Quote Link to comment Share on other sites More sharing options...
elwilliams13 Posted January 5, 2009 Author Report Share Posted January 5, 2009 Deckheight you know what you are talking about. Throw me a bone and let me know what numbers you need to compute this stuff. I got the top end apart now and will be able to measure it out 1st thing tomorrow. Really would appreciate a hand. Quote Link to comment Share on other sites More sharing options...
deckheight Posted January 5, 2009 Report Share Posted January 5, 2009 Bore size, trapped volume and exhaust opening ATDC would be perfect. Sounds like it may be time to invest in a buret? :biggrin: Shoot me a private for contact info. Sure save a lot of time typing... :biggrin: Quote Link to comment Share on other sites More sharing options...
blowit Posted January 5, 2009 Report Share Posted January 5, 2009 (edited) One of the things to check is how you are calculating the value. We will use nothing but corrected compression ratio here or CCR. This is also how pretty much al metric OEMs calculate because it is much more relative to actual engine conditions. This will only assume the "trapped" volume and ignores total stroke for the calculation. For a base line, our OEM calculations are a 30mm exhaust port from zero deck, Piston comes to full TDC, 22cc head, and .8CC for the head gasket. That brings us to 6.35:1. This is pretty much why we can get away with a pretty small head chamber from stock and still run pump fuels. Detonation is NOT entirely based on CR. Other factors, especially in two strokes, are squish zone, squish velocity, clearance, dome lofted surface contour, etc. Many other factors will play in which is why engine testing is generally used for final determination of fuel requirements. I agree with above comments. Something in the calculation is high. I recommend just using the good old calculator and the mentioned burette to learn actual volume data. You did not say, but I assume you are using a billet head? Brandon Edited January 5, 2009 by blowit Quote Link to comment Share on other sites More sharing options...
blowit Posted January 5, 2009 Report Share Posted January 5, 2009 (edited) Yea, you have to convert everything into inches. And yes, dome size is a negative number. I did alot of research on this topic, and I contacted a few oil companies about it. I finally got a chart for it. Contrary to popular belief, cranking psi has nothing to do with what octane you need. Port timing (or cam duration on 4-strokes) changes so much, with the static compression-vs-cranking compression. You can actually have 200psi of cranking compression, and only need 104 octane. On the other hand, you could have only 155psi, and need 110 octane. What really matters is the compression ratio. I agree with most of what you said. Some people think we are nuts for running 190psi in a Banshee with 91oct fuel but if you calculate the CR, it tells a much different story. Other engine configurations are on a totally different playing field too. I could not totally agree with you on the effects of port timing or cams in a thumper. Those components relate directly to CRR and are very valid and realistic numbers that show how a bigger cam can actually cause your engine to run worse. Most 2-stroke tuners that know much will usually recommend a head mod with porting to "get back" compression loss from porting. One consideration with porting is cylinder fill efficiency. A better flowing head or cylinder will "fill" more efficiently and will cause a higher dynamic CR at higher rpm. It seems rather elementary to calculate the "ideal" CRs on paper but getting to real world dynamic CRs get very confusing because it starts to relate back to the variable of time for open/close of valves but where the tendencies and properties of air simply do not change with rpm. Example might be to blow air into of 2L bottle with a .25 hole drilled in the bottom. If air is applied slow, there is no pressure build but if you hit it, you can still explode the bottle, even with a leak in it. I guess where I was going with that was a table that relates CR to octane simply cannot be applied to all applications in a general fashion. Good for an ideal baseline though. Guess I could blab all day about these issues. :biggrin: Brandon Edited January 5, 2009 by blowit Quote Link to comment Share on other sites More sharing options...
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