blowit Posted April 27, 2010 Report Share Posted April 27, 2010 Here is the deal. A 421 with .050 thou squish and 20cc domes using blaster profile pistons gives you an UCCR of around 14:1. Im not sure you really want to run less than 110 octane on that. I dont. My bike with the 20 cc domes kicks 180 psi at sea level on a know accurate Snap-on gauge. It is a run of the mill cleaned up 421 cub. Now lets say i take the motor send it off and have a full blown drag port done and change nothing else. Assuming they change the duration fo the exhuast i will more than likely see a drop in kicking compression. Does that mean i can now run lower octane fuel? I dont believe so. Im not saying you are wrong, that has just been my experience. if what you are doing is working cool. To the OP, i would consider better fuel if i were paying the bills. I agree with most of this post. By looking at the actual ratios, you can see that there are some concerns at 20cc on pump fuels. However, the trapped volume does play in. I tend to agree strongly that looking at trapped volume is much more realistic. However, you must consider the air/fuel density when trapped to get an accurate estimate of octane. It gets damn tricky and I still think it is more or less a guess. When we do head work, we look at both numbers, pipes, timing, etc to arrive at what should be used. Ie, a 14:1 engine with poor fuel/air density when trapped will have no problem with pump fuels... Density is key but calculating it ties us in knots all the time. Mull Engineering Quote Link to comment Share on other sites More sharing options...
okbeast Posted April 27, 2010 Report Share Posted April 27, 2010 Here is the deal. A 421 with .050 thou squish and 20cc domes using blaster profile pistons gives you an UCCR of around 14:1. Im not sure you really want to run less than 110 octane on that. I dont. My bike with the 20 cc domes kicks 180 psi at sea level on a know accurate Snap-on gauge. It is a run of the mill cleaned up 421 cub. Now lets say i take the motor send it off and have a full blown drag port done and change nothing else. Assuming they change the duration fo the exhuast i will more than likely see a drop in kicking compression. Does that mean i can now run lower octane fuel? I dont believe so. Im not saying you are wrong, that has just been my experience. if what you are doing is working cool. To the OP, i would consider better fuel if i were paying the bills. Been doin it for 2 years and I ride the hell out of it and it's been fine. I'm fixing to tear it down in a few weeks and we'll see what it looks like. So wtf does one need to do to get the 'best' idea of what my compression is? I'm not changing anything, just putting in an override but the motor hasn't been apart in 2 years so I figured I'd see if it could use a re-ring. Quote Link to comment Share on other sites More sharing options...
blowit Posted April 27, 2010 Report Share Posted April 27, 2010 Been doin it for 2 years and I ride the hell out of it and it's been fine. I'm fixing to tear it down in a few weeks and we'll see what it looks like. So wtf does one need to do to get the 'best' idea of what my compression is? I'm not changing anything, just putting in an override but the motor hasn't been apart in 2 years so I figured I'd see if it could use a re-ring. That is a good example of "technically" not going to work, but does... I would say at 2 yrs, you would have found if you had an issue. If you want to know your actual CR, give me a buzz just before you tear down and will get you a tool list so you can check it out. Or you can just put it back to bed and ride.... Quote Link to comment Share on other sites More sharing options...
okbeast Posted April 27, 2010 Report Share Posted April 27, 2010 That is a good example of "technically" not going to work, but does... I would say at 2 yrs, you would have found if you had an issue. If you want to know your actual CR, give me a buzz just before you tear down and will get you a tool list so you can check it out. Or you can just put it back to bed and ride.... Ya if it hasn't blown by now I figure I'm good. I thought about going to a .012 base gasket like most cubs run, but I'd honestly rather be able to run 91. It's not a full blown drag bike yet so might as well be as ride-able as possible right now. I'll holler at you when I get ready to tear down, the engineer in me is curious about this. Quote Link to comment Share on other sites More sharing options...
blowit Posted April 27, 2010 Report Share Posted April 27, 2010 Ya if it hasn't blown by now I figure I'm good. I thought about going to a .012 base gasket like most cubs run, but I'd honestly rather be able to run 91. It's not a full blown drag bike yet so might as well be as ride-able as possible right now. I'll holler at you when I get ready to tear down, the engineer in me is curious about this. I would be happy to walk you through it. Just bring your pencil. B Quote Link to comment Share on other sites More sharing options...
rubberneck Posted April 30, 2010 Report Share Posted April 30, 2010 Yeah, i dont get it, how can 2 motors that are basically the same behave so drastically different. Perhaps it is the design of the domes? I dont know, that is for somebody way smarter than me to figure out. My motor did run the .012 base gasket and the squish was .050 thou. The domes were from FAST, and im not sure if they were run of the mill cut by noss or if jeff has them cut to his specs. I belive the squish band was 10mm wide if i remember, its been a while since i looked at it. Quote Link to comment Share on other sites More sharing options...
doug Posted June 2, 2010 Author Report Share Posted June 2, 2010 Update: The jugs have been sent down to Millenium - The word is that the plating failed as a number of you had suggested. They do not feel that it is a detonation issue. If it was detonation they informed that the following would have occurred: 1) Pitting around the top of the head 2) Piston would show signs of melting on top Having said this, I'd like to run at a very safe compression level. What do you guys suggest in terms of a very conservative setup? 22cc domes? Also, does anyone run exhaust temperature gauges? A buddy recommended I do this as he uses it on his sleds. Thoughts? Thanks. Quote Link to comment Share on other sites More sharing options...
252wheelieking Posted April 25, 2012 Report Share Posted April 25, 2012 Update: The jugs have been sent down to Millenium - The word is that the plating failed as a number of you had suggested. They do not feel that it is a detonation issue. If it was detonation they informed that the following would have occurred: 1) Pitting around the top of the head 2) Piston would show signs of melting on top Having said this, I'd like to run at a very safe compression level. What do you guys suggest in terms of a very conservative setup? 22cc domes? Also, does anyone run exhaust temperature gauges? A buddy recommended I do this as he uses it on his sleds. Thoughts? Thanks. What happen with this rebuild? How was it set-up, domes, fuel & etc.? Quote Link to comment Share on other sites More sharing options...
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