patsbansh Posted June 18, 2011 Report Posted June 18, 2011 I've gotten 2 opinions from 2 well respectd builders on which domes to run in my 421 cub on Alky and they varied so much that i would like to get a few more before i put in either set of domes. Its going to be run on asphalt in the 1/4 . The 1st was 24cc and the2nd was 18cc so you see that is a big gap. Any experience good or bad would be extremely helpful. Thanks fellas Quote
SlowerThanYou Posted June 18, 2011 Report Posted June 18, 2011 I've gotten 2 opinions from 2 well respectd builders on which domes to run in my 421 cub on Alky and they varied so much that i would like to get a few more before i put in either set of domes. Its going to be run on asphalt in the 1/4 . The 1st was 24cc and the2nd was 18cc so you see that is a big gap. Any experience good or bad would be extremely helpful. Thanks fellas While there are many factors that go into the dome set-up. Your answer will probably lay in between the two extremes. It will still come down to you trying different dome configurations to see what works best for your set-up. Quote
camatv Posted June 19, 2011 Report Posted June 19, 2011 try both see which one you like better whats the indepth reasons for the diff cc's/ almost every alky dome i have gotten form diff builders has looked differnt heck some even say to just run the gas domes in them and "see how it works" the current gas cub domes look a lot like alky domes i have gotten in the past also some "alky" domes have ran killer on pump gas motors.. there is a LOT involved in a good head design. most of the best ones i ahve seen are from trial and error on a specific motor.. the ones i personally like are a small width squish band ( 5-9mm) in the 20-17cc dome range with a flat inner profile instead of a dome.. the dome shape, fuel quality are kinda going to make the timing matter the most. a big wide band i have only seen o-ring failure and deto and overheating of the dome. most "shelf" domes are shaped that way.. but thats just my personal experience with them Quote
SlowerThanYou Posted June 19, 2011 Report Posted June 19, 2011 patsbansh, 1/4 mile tuneups is a totally different than most set-ups. There are very few people that do it well and are fast. If it was as easy as everyone thinks; then everyone one would be doing it. Quote
patsbansh Posted June 19, 2011 Author Report Posted June 19, 2011 What is your deck height? [/quo Ok I looked into a couple things and i have 68mm cub cylinders that are as they come. i don't have them right now I am waiting on Kevin at Herr Juggs to do his thing. i should have them back in about 2 weeks. So i'm not sure, from what i read i'm assuming you are referring to the measurement between the base of the cylinders and where the head bolts on, if i'm wrong please school me. Quote
mailman Posted June 19, 2011 Report Posted June 19, 2011 When the motor is at TDC (top dead center) for either piston, you are actually measuring the distance from the top of the piston on the outside of the bore to the "deck" or top of the cylinder. A fairly normal reading would be .004 inches inside the hole (bore) which means that when the motor is at TDC, the piston doesn't reach the top of the cylinder. Some builders will run a thin base gasket, cut the base of the cylinder, or cut the cases to lower the cylinder which raises the piston out of the hole. I have seen some as much as .080 inches. Other builders will run a thicker base gasket which will put the piston much lower in the hole. This is important because it affects the actual trapped volume of the head. If you are .004" in the hole then your fpv (flat plate volume or volume of the dome which most people quote) must be added to the volume of the .004" in the hole. At .004", it is only like .37 cc's but if you were say .020" out of the hole then you would end up with about 1.8 cc's less. Deck height will affect port timings also. Once you find a setup that works well for you, then make sure to use the same thickness and material base gasket after each rebuild. A new one of course. Quote
patsbansh Posted June 19, 2011 Author Report Posted June 19, 2011 When the motor is at TDC (top dead center) for either piston, you are actually measuring the distance from the top of the piston on the outside of the bore to the "deck" or top of the cylinder. A fairly normal reading would be .004 inches inside the hole (bore) which means that when the motor is at TDC, the piston doesn't reach the top of the cylinder. Some builders will run a thin base gasket, cut the base of the cylinder, or cut the cases to lower the cylinder which raises the piston out of the hole. I have seen some as much as .080 inches. Other builders will run a thicker base gasket which will put the piston much lower in the hole. This is important because it affects the actual trapped volume of the head. If you are .004" in the hole then your fpv (flat plate volume or volume of the dome which most people quote) must be added to the volume of the .004" in the hole. At .004", it is only like .37 cc's but if you were say .020" out of the hole then you would end up with about 1.8 cc's less. Deck height will affect port timings also. Once you find a setup that works well for you, then make sure to use the same thickness and material base gasket after each rebuild. A new one of course. Thanks for the lesson i had no idea. but thinking about it all those factors would make sense. How finicky are these motors, so even the base gasket could change the port timing ever so slightly? After i get it back and on I'll be sure and check it let you know and write it down in my notebook. Thanks again Quote
J-Madd Posted June 20, 2011 Report Posted June 20, 2011 These motors aren't finicky at all, in the way they run. LOTS of room for error (at least in 300 ft dirt racing; less forgiving in the quarter.) You can change all sorts of things on my Cubs and they still run about the same (not saying that's a good thing.) Now a bigger motor - you can just have the air/fuel, timing, or domes a little off and sometimes they won't even run down the track. Quote
sleeper06 Posted June 22, 2011 Report Posted June 22, 2011 your at sea level like me ,4 the 1/4 i would go with 20cc domes from noss,you dont want your compression sky high it will slow you down on the big end Quote
SlowerThanYou Posted June 23, 2011 Report Posted June 23, 2011 It's not just about the CC size. Quote
J-Madd Posted June 23, 2011 Report Posted June 23, 2011 Yeah, some will run a different angle and squish band for longer runs. Keeps the MSV in check. Quote
patsbansh Posted June 24, 2011 Author Report Posted June 24, 2011 Yeah, some will run a different angle and squish band for longer runs. Keeps the MSV in check. If you could enlighten me about angle of what, what is my squish band and MSV, I would really appreciate it. I'm trying to get all the info i can. Quote
patsbansh Posted June 24, 2011 Author Report Posted June 24, 2011 try both see which one you like better whats the indepth reasons for the diff cc's/ almost every alky dome i have gotten form diff builders has looked differnt heck some even say to just run the gas domes in them and "see how it works" the current gas cub domes look a lot like alky domes i have gotten in the past also some "alky" domes have ran killer on pump gas motors.. there is a LOT involved in a good head design. most of the best ones i ahve seen are from trial and error on a specific motor.. the ones i personally like are a small width squish band ( 5-9mm) in the 20-17cc dome range with a flat inner profile instead of a dome.. the dome shape, fuel quality are kinda going to make the timing matter the most. a big wide band i have only seen o-ring failure and deto and overheating of the dome. most "shelf" domes are shaped that way.. but thats just my personal experience with them The 18cc was for more compression for a stronger hard out of the hole torquier motor the 24cc was lower compression for a smoother higher reving longer lasting motor. I'm running a Pro desidn cool head right now. Can you explain the squish band to me what it is what it does how to change it , Thanks all info is welcome and appreciated Quote
patsbansh Posted June 24, 2011 Author Report Posted June 24, 2011 The 18cc was for more compression for a stronger hard out of the hole torquier motor the 24cc was lower compression for a smoother higher reving longer lasting motor. I'm running a Pro desidn cool head right now. Can you explain the squish band to me what it is what it does how to change it , Thanks all info is welcome and appreciated Also are different head design domes interchangeable if so which has the flatter dome? If not what are others running that they like or would recommend and why? once again thanks Quote
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.