registered user Posted September 4, 2017 Report Share Posted September 4, 2017 thats not how u do it. its full swept volume+head volume/ head volume. but u cant use 21cc because thats flat plate volume. thats why i said pour oil in. otherwise u have to know piston deck height and how much dome radius u got Quote Link to comment Share on other sites More sharing options...
SlowMoe Posted September 4, 2017 Author Report Share Posted September 4, 2017 So you're saying trapped volume will be significantly less than 21cc?Sent from my iPhone using Tapatalk Quote Link to comment Share on other sites More sharing options...
blowit Posted September 4, 2017 Report Share Posted September 4, 2017 Registered is giving accurate advice. We are more a fan of using math than dumping in oil, but it can work the same. Question- Are your piston domes flat or called domes for a reason? IE, that works into the math, and something that needs to be accurate and that data is not published. A 4mm with drag timing exhaust, 21cc dome, and pump fuel is asking for it. This right here is why we recommend users to roll timing back, then bring in timing 1* at a time. For a quick test, you should try rolling your timing back to 0* or even less, and see if you have overrev. Nevermind the math, I see multiple issues here and think just throwing race fuel at it is a reasonable solution for now. Brandon Mull Engineering Quote Link to comment Share on other sites More sharing options...
SlowMoe Posted September 4, 2017 Author Report Share Posted September 4, 2017 Brandon: What size domes do recommend for pump gas?Recommended user:What kind of measuring device is typically used to measure the amount of oil poured on top of the piston, and you stop at the spark plug hole, correct?Sleeper06:What trapped volume and displacement did you use in your calculations?I'm just confused, as my builder is not a newb. Has clients bikes in his shop from two states over and assures me my bike is safe to run on pump gas.If indeed I have a problem on my hands I would rather just buy new domes than be stuck running race gas.FWIWI ran the piss out of it today at the dunes on pump gas with no issues. Mainly because I have faith in my builder. However you guys have me curious and I would like the peace of mind if doing the displacement calculations myself and coming to my own conclusion before moving any further.Sent from my iPhone using Tapatalk Quote Link to comment Share on other sites More sharing options...
Ayesully810 Posted September 4, 2017 Report Share Posted September 4, 2017 how come you ported the motor and had someone else assemble it? Quote Link to comment Share on other sites More sharing options...
registered user Posted September 4, 2017 Report Share Posted September 4, 2017 4 hours ago, blowit said: This right here is why we recommend users to roll timing back, then bring in timing 1* at a time. For a quick test, you should try rolling your timing back to 0* or even less, and see if you have overrev Brandon Mull Engineering this is good advice for everybody not just noobs. set timing at 0, get engine running good then adjust timing accordingly. other than maybe losing alittle power, theres really no ill affects of too little spark lead . on other hand, to much spark lead can cause all kinds of problems. i guess it just sounds cool to say you have boat loads of spark adv measuring with oil was always easiest for me. its almost idiot proof as long as u can find tdc. i might do it a different way if i were better at math 1 Quote Link to comment Share on other sites More sharing options...
SlowMoe Posted September 4, 2017 Author Report Share Posted September 4, 2017 how come you ported the motor and had someone else assemble it?The crank wasn't fully assembled with all the gears and stuff and I just didn't feel comfortable for some reasonSent from my iPhone using Tapatalk Quote Link to comment Share on other sites More sharing options...
SlowMoe Posted September 4, 2017 Author Report Share Posted September 4, 2017 65.5x65.5x58x3.1416¿4000= 195.4346163+21 then divide the total of that by 21 which is the cc of the dome equals 10.30 compression which is safe for pump gasSent from my iPhone using Tapatalk Quote Link to comment Share on other sites More sharing options...
SlowMoe Posted September 4, 2017 Author Report Share Posted September 4, 2017 ^this is what the builder sent meSent from my iPhone using Tapatalk Quote Link to comment Share on other sites More sharing options...
Ayesully810 Posted September 4, 2017 Report Share Posted September 4, 2017 6 minutes ago, SlowMoe said: The crank wasn't fully assembled with all the gears and stuff and I just didn't feel comfortable for some reason Sent from my iPhone using Tapatalk not hating just curious. Quote Link to comment Share on other sites More sharing options...
SlowMoe Posted September 4, 2017 Author Report Share Posted September 4, 2017 It's cool. I've built like 5 motors from scratch but it just didn't feel right this timeSent from my iPhone using Tapatalk Quote Link to comment Share on other sites More sharing options...
blowit Posted September 4, 2017 Report Share Posted September 4, 2017 28 minutes ago, SlowMoe said: 65.5x65.5x58x3.1416¿4000= 195.4346163+21 then divide the total of that by 21 which is the cc of the dome equals 10.30 compression which is safe for pump gas Sent from my iPhone using Tapatalk That right there is largely concerning to me........ Its actually nagging a hole in my head that someone with the word "professional" in there would accept this as correct arithmetic! It ain't right! 1 Quote Link to comment Share on other sites More sharing options...
SlowMoe Posted September 4, 2017 Author Report Share Posted September 4, 2017 That right there is largely concerning to me........ Its actually nagging a hole in my head that someone with the word "professional" in there would accept this as correct arithmetic! It ain't right! How would your math look?55.5mm bore58mm stroke21cc dome And at what UCCR is pump gas no longer safe?Sent from my iPhone using Tapatalk Quote Link to comment Share on other sites More sharing options...
Keno Posted September 5, 2017 Report Share Posted September 5, 2017 Probably the fact that first we need to convert mm to cm since cc=cm^3. Also bore is diameter and we need radius for our equation. so, Bore = 6.55cm Bore radius = 6.55cm/2 =3.275cm Stroke = 5.8cm (3.275^2)*pi*5.8= 195.43 cc I'm assuming the area of the top of the piston above the deck can be determined with the formula for the area of a partial sphere? If so then. h= height of top of piston above deck a= bore/2 V= pi*h/6(3(a^2)+(h^2)) Nevermind, Probably not correct might get you close if the edge of the piston crown is equal with the deck. I'm kinda curious now the proper way to calculate this. Quote Link to comment Share on other sites More sharing options...
registered user Posted September 5, 2017 Report Share Posted September 5, 2017 im sure Brandon is a lot better at math than us. atleast a lot better than me. imo this method leaves to much room for error for the normal joe unless your efficient with numbers. not only that but finding deck height accuratly isn't the easiest thing, especially if the pistons in the hole. and getting the piston dome angle or height can be a challenge as well. now if your familiar with the particular engine components and know all their relevant values then its just simple arrithmatic but starting from scratch on a unfamiliar engine can be a real pain to figure out this shit. ya its less messy than oil but if you get a couple numbers wrong then u spend forever going in circles to sort it out lol 1 Quote Link to comment Share on other sites More sharing options...
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