278 Posted April 1, 2009 Report Share Posted April 1, 2009 Just did a comp. test on the newly rebuilt motor, it's 150 in both cylinders, but that was when it was 27 degrees outside, Im running +4 timing, and the premium gas here is 90 octane, what should I run as far as fuel, Im thinking maybe mix premium with some 110 or 112 octane in a 50/50 mix?! Quote Link to comment Share on other sites More sharing options...
GrMeyer Posted April 1, 2009 Report Share Posted April 1, 2009 im thinking right around 95oct... just my .02 I think when you hit around 160+is when you start running 100. but some builders will be able to help out better then I can... Quote Link to comment Share on other sites More sharing options...
2001Stroker Posted April 1, 2009 Report Share Posted April 1, 2009 You really need to find out what your compression ratio is. That's the only way to find out what octane you need. You can have 145psi, but 12:1 ratio, so you need 110 octane. On the other hand, you can have 165psi, but have 10.5:1, so 93 will work for you. It all depends on ratio, timing, port duration, temperature, humidity, and elevation. You can't base octane needs on cranking compression. You need to know the ratio. Quote Link to comment Share on other sites More sharing options...
GrMeyer Posted April 2, 2009 Report Share Posted April 2, 2009 You really need to find out what your compression ratio is. That's the only way to find out what octane you need. You can have 145psi, but 12:1 ratio, so you need 110 octane. On the other hand, you can have 165psi, but have 10.5:1, so 93 will work for you. It all depends on ratio, timing, port duration, temperature, humidity, and elevation. You can't base octane needs on cranking compression. You need to know the ratio. how do you do this...I know most people would like to know Quote Link to comment Share on other sites More sharing options...
2001Stroker Posted April 2, 2009 Report Share Posted April 2, 2009 I've probably got these links posted in about 30 different threads man. But, here ya go. The Blaster pistons use approximately -1.5cc Piston Head Volume. That number changes slightly with different bores. It has to be a negative number for domed pistons. Stock style pistone are approximately -1.2cc. Metric Conversion Calculator Compression Calculator Quote Link to comment Share on other sites More sharing options...
GrMeyer Posted April 3, 2009 Report Share Posted April 3, 2009 Damn...I didnt think I would have to figure out these kind of numbers...Thanks for the links!!! Quote Link to comment Share on other sites More sharing options...
Rodneya Posted April 8, 2009 Report Share Posted April 8, 2009 If using the compression ratio link, what is the standard gasket thickness to use. I don't have one with me right now to measure and want to try the calcs. Quote Link to comment Share on other sites More sharing options...
TwistedSheeRida Posted April 8, 2009 Report Share Posted April 8, 2009 thats what i run for compression 93 pump your all set Quote Link to comment Share on other sites More sharing options...
MILO Posted April 8, 2009 Report Share Posted April 8, 2009 hey 2001stroker, can you help me with this calculator? i'm not sure what numbers to plug in the fields. i'm not exactly sure what the field descriptions actually are. i can tell you this.... hot rods 4 mil long rod stroker crank wiseco 795 pistons bored .020 over custom cut 20cc stroker domes HJR aggressive trail port / dune port .050" squish Quote Link to comment Share on other sites More sharing options...
2003LimitedBanshee Posted April 8, 2009 Report Share Posted April 8, 2009 In reference to this calculator, wouldn't stroke be calculated from the time the exhaust port closes? I.e. much shorter than the total stroke of the engine? I'm assuming this, since this appears to be a four stroke caculator and all the valves would be closed for the "entire" length of the compression stroke, whereas on a two stroke you are still "losing" compression until the exhasust port is closed. But, I also know you have a backpressure wave coming from the pipes that help to contain some of this loss. For instance I just plugged in a stock bore (64mm converted to In.)/stroke(54mm? converted to in.) with 18cc domes. Now, I didn't have a gasket/o-ring available to measure, so i assumed the gasket bore was the same the cylinder and a gasket thickness of .020" (not sure how accurate that is?). So, the numbers may not be exact and I understand that my static compression would change as a result of this assumption, but a minor amount. It calculated this to be a static ratio of 9.3:1 That can't be right can it? EDIT: I thought BMEP (which I'm still trying to fully grasp) was the main calculation as far as octane requirements are concerned? I understand that there is a correlation between BMEP and compression ratio, but I don't have that formula... Quote Link to comment Share on other sites More sharing options...
blowit Posted April 8, 2009 Report Share Posted April 8, 2009 I've probably got these links posted in about 30 different threads man. But, here ya go. The Blaster pistons use approximately -1.5cc Piston Head Volume. That number changes slightly with different bores. It has to be a negative number for domed pistons. Stock style pistone are approximately -1.2cc. Metric Conversion Calculator Compression Calculator You are on target but I did want to find out how you got your numbers for dome displacements? We did all of ours with a CMM and came up with vastly different numbers. Obviously that number can make a huge difference in calculating ratios. Brandon Quote Link to comment Share on other sites More sharing options...
BansheeJohn Posted April 9, 2009 Report Share Posted April 9, 2009 I'm trying this and I'm not getting it one bit :geek: Can someone please tell more about this? I'm running 65mm pistons 4 mil long rod crank 19cc domes 13 degree's 173lbs compression I'm getting -14.474 for numbers I have no idea what that means Quote Link to comment Share on other sites More sharing options...
Rodneya Posted April 9, 2009 Report Share Posted April 9, 2009 You are on target but I did want to find out how you got your numbers for dome displacements? We did all of ours with a CMM and came up with vastly different numbers. Obviously that number can make a huge difference in calculating ratios. Brandon What is a CMM, and what numbers did you get for stock bore pistons? Quote Link to comment Share on other sites More sharing options...
FireHead Posted April 9, 2009 Report Share Posted April 9, 2009 You are on target but I did want to find out how you got your numbers for dome displacements? We did all of ours with a CMM and came up with vastly different numbers. Obviously that number can make a huge difference in calculating ratios. Brandon You may want to define CMM for the masses......... (I have found the same thing by the way. I figure out my ratios in a Catia model. ) :geek: Quote Link to comment Share on other sites More sharing options...
FireHead Posted April 9, 2009 Report Share Posted April 9, 2009 What is a CMM, and what numbers did you get for stock bore pistons? Just to give Brandon a hard time............ CMM = Cajun Monkey Mattress (not really) :biggrin: Quote Link to comment Share on other sites More sharing options...
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