ATV Drag Racer Forums banner
1 - 11 of 11 Posts

·
Registered
Joined
·
43 Posts
Discussion Starter · #1 ·
Question for the guys who are much smarter than i. I just recently got my cylinders back as well as new domes for my 4mil stock cylinder bike. This bike is a duner, not looking for a drag bike, just a good all around reliable dune bike. I do not know the port durations, but cylinders have "play port" set up for t5s and 34 pjs for 230 lb rider and 110 octane fuel. I do not know cranking compression as all parts are still in the boxes LOL, but my question comes as i was sent 19cc domes cut for 4mil stroke. I see it said on here quite a lot that you cant really rely on cranking compression for octane requirements, that UCCR is really a better factor to use. The cylinders are 65mm bore using 795 series pistons. If i have the correct info, the dome rise on that piston is .160" Now if im doing the math right assuming a .040" squish, the 19cc domes are going to put me at about 16.4:1 UCCR. Isnt this kind of high even for 110 octane on a duner bike. I was thinking i should have @ 21 cc domes. Again if my math is correct that would be closer to 14.3:1 Please discuss and thanks in advance for your help.
 

·
Registered
Joined
·
1,096 Posts
although mine is a drag bike, i am runnin 18cc domes with a similar motor setup and VP C12 fuel and mine runs relatively "cool"... does not run nearly as hot as it did as a stock motor trail bike on pump gas
 

·
Registered
Joined
·
81 Posts
i agree with 421ccub, .040 is pretty tight on a duner like he said .050-.055 maybe even a lil more depending on ur cranking compression, id be trying to keep it under 170psi but as close to 170 as i could with .050, one question i do have is do you know the dia. of your squish band? if its narrowed up some like 9 or 10mm itll be a lil less likely to detonate
 

·
Premium Member
Joined
·
1,953 Posts
I'm not at home so I can't add it all up but I think your math is off a bit. What's your elevation? That might be low comp for riding at 4000 ft. You haven't got it together and didn't mention what thickness base gasket, how far the piston sticks above the cyl. Those all factor in.



We just put together a gas 4 mil,65 bore with 19cc domes running shearer inframes a month ago with parts from FAST and its fine. The squish band on it was like 10mm wide. 220 lb rider.+5 timing, .043 squish

Honestly deal with a builder you trust to do it right. If your builder who ported the cyls, cut and sold you the domes,and told you where to set your timing at got you those domes, knows what pipes etc then that's what he should know will work well with his motor work.



My advice is trust the guy who got you the setup.

My motor is really wierd in timing, squish, dome etc.
 

·
Registered
Joined
·
767 Posts
.040 is not too tight for a dune motor if the MSV is where it should be. I have run .035 on my dune motors for quite a long itme with zero issues. I personally like to target .035 for bores 66mm and under. If the rest of the head is not correct or if the timing is jacked way up then you will have issues for sure.



My .02, I like also around 14-14.5:1 for race gas applications.
 

·
Registered
Joined
·
43 Posts
Discussion Starter · #7 ·
Elevation is sea level, sorry, that would have helped. As far as squish, i said assumed 0.04 just so i could do the math. Radar, my math could be off, i would appreciate a double check, and maybe if off you could tell me what i did wrong. I found info that the 795 series piston has a dome rise of .160" and i calculate that the crown then displaces 6.5cc. So i used the formula of swept volume + domes size - crown / domes size - crown. For my motor that would be 192.5 + 19 - 6.5 / 19 - 6.5 = 16.4 :1 Out of curiousity what did that 4 mil you put together kick out for compression? I agree there needs to be some trust between me and my porter. I didnt post this to prove anything to him or say he was wrong. I didnt list any names on purpose. I am trying to learn and am concerned only for the fact that this is my duner bike. Reliability is more important to me than squeezing every drop of power out. If i want to go fast, I have a Cub for that. Ultimately, I am the one doing the assembly so it is my responsibilty to check everything and i just really wanted some opinions. Perhaps i should tell the back story here, so you can see where i am coming from and why i have this question. I sent 2 sets of cylinders off to the same porter. First set was for my brother. 350 dune port with t5s and 160 lb rider 65 mm pistons. Got 19cc domes and told to run 5 degrees timing. Second set, my 4mil dune ported stroker. Im 230lb, and bike also has t5s and 65mm pistons. I was sent 19cc domes and told to run 4-5 degrees timing. So here in lies the question, why with 2 bikes set up almost exactly would you run the same size domes? The 4mil is obviously going to have quite a bit more compression. I realize that i am heavier and the compression is going to help the bottom end, but will it hurt on the top? Again, i would rather lose a little in favor of reliability.
 

·
Registered
Joined
·
43 Posts
Discussion Starter · #8 ·
.040 is not too tight for a dune motor if the MSV is where it should be. I have run .035 on my dune motors for quite a long itme with zero issues. I personally like to target .035 for bores 66mm and under. If the rest of the head is not correct or if the timing is jacked way up then you will have issues for sure.



My .02, I like also around 14-14.5:1 for race gas applications.




Bubba, could you explain to me what msv is? Also how do i figure it out for my motor. Part of the reason i posted this was that in an old PS thread when i was building my cub i remember a couple of guys mentioning to stay in that 14-14.5:1 range for gas in a dune bike. I realize that this new motor is totally different, diff pipes, port timing totally different cylinder etc, i just like to learn. Thanks for any information you are willing to share.
 

·
Registered
Joined
·
767 Posts
MSV stands for Maximum squish velocity, or some say mean squish velocity.



It is the maximum velocity of the charge moving across the squishband just before the piston reaches TDC. If it is too low then you dont burn the mixture effectively. Too high and you will detonate. You cant figure your MSV without a program though.



There is a good free program online. it is at http://www.torqsoft.net/squish-velocity.html

I couldnt get the link to work, so just paste it. Anything that changes your squishband width or angle, or your clearance will affect your MSV. Do some reading up on it, I think you will find it very useful.
 

·
Registered
Joined
·
43 Posts
Discussion Starter · #11 ·
bubba thank you. I see there is also a compression calculator on there, that will be handy. I have some reading and measuring to do. Im most certain i will be back with more questions. Thanks again
 
1 - 11 of 11 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top