the reaper Posted May 27, 2011 Report Share Posted May 27, 2011 Just put on my noss head with 21cc domes... I started at 120psi stock without the heads, now with the 21cc domes I got about 145-150... So I'm exactly at the psi that the chart on noss' website gives... So now my question is, is it safe to go to 20cc domes and still be able to run pump gas?? According to noss' chart that'll put me at 156 psi range... And is there gonna be a performance difference between the 2 domes?.......my mods are t5's k&n air filter with lid removed, v-force 3's and gonna be advancing the timing +4 within the week or 2.... Sorry if this question has been asked b4 but I couldn't find it in the search Quote Link to comment Share on other sites More sharing options...
Infamous039 Posted May 27, 2011 Report Share Posted May 27, 2011 I believe anything over 170psi requires you to move up to race gas. Some people run 19cc domes safely on pump fuel, just do a compression test after you install them. Quote Link to comment Share on other sites More sharing options...
KlotzBanshee Posted May 27, 2011 Report Share Posted May 27, 2011 I'm at 165 with 19s and run 50/50 per Jeff at FAST Quote Link to comment Share on other sites More sharing options...
BLK'N'YELLOW Posted June 1, 2011 Report Share Posted June 1, 2011 they say 180+ run 110 plus, 160-170 av gas or mixed. 140-155 mixed will make it run cripser, Quote Link to comment Share on other sites More sharing options...
mikechief Posted June 1, 2011 Report Share Posted June 1, 2011 P.s.i doesn't have anything to do with octane . Your compression tester is just a tool to tell you how healthy you top end is . I read a thread on another site that was about exactly this same situation . It sounded to me like 19cc *NOSS* domes are safe for pump gas . The way it sounded , noss domes are a better design than others . It also sounded like 19cc was boarder line safe . If this guy had a 64mm bore and you have a 66mm bore ...you may wish you would have stuck with 20cc Quote Link to comment Share on other sites More sharing options...
Burt Reynolds Posted June 1, 2011 Report Share Posted June 1, 2011 You should be fine on pump gas,maybe mix 1 gal of race with 4 of regular gas. but with 21cc domes you should be fine Quote Link to comment Share on other sites More sharing options...
Jereme6655 Posted June 1, 2011 Report Share Posted June 1, 2011 P.s.i doesn't have anything to do with octane . Your compression tester is just a tool to tell you how healthy you top end is . I read a thread on another site that was about exactly this same situation . It sounded to me like 19cc *NOSS* domes are safe for pump gas . The way it sounded , noss domes are a better design than others . It also sounded like 19cc was boarder line safe . If this guy had a 64mm bore and you have a 66mm bore ...you may wish you would have stuck with 20cc ugh.......what??? octane rating and compression go hand in hand with each other when it comes to what is safe for a motor with smaller domes.... Quote Link to comment Share on other sites More sharing options...
sleeper06 Posted June 1, 2011 Report Share Posted June 1, 2011 155 and under pump anything over i run race fuel its not worth pre igniting your motor,stay with the 21s on that motor its safe and power increase from 21s to 20s will be very hard to feel,static comp is only 1/2 the equation Quote Link to comment Share on other sites More sharing options...
gotta_goatsfast Posted June 1, 2011 Report Share Posted June 1, 2011 ugh.......what??? octane rating and compression go hand in hand with each other when it comes to what is safe for a motor with smaller domes.... So if you kick over 155psi on a cylinder with 200 degrees of exhaust duration, it will run best with a lower octane than a cylinder which reads 165psi on a 182 degree duration? Compression readings at <100rpm (unless you have a really strong leg) vary vastly compared to cylinder pressure during the powerband. This article is fairly simple to follow compared to some (and it saves me time)... http://www.2strokeheads.com/tech2.htm Quote Link to comment Share on other sites More sharing options...
Jereme6655 Posted June 2, 2011 Report Share Posted June 2, 2011 So if you kick over 155psi on a cylinder with 200 degrees of exhaust duration, it will run best with a lower octane than a cylinder which reads 165psi on a 182 degree duration? Compression readings at <100rpm (unless you have a really strong leg) vary vastly compared to cylinder pressure during the powerband. This article is fairly simple to follow compared to some (and it saves me time)... http://www.2strokeheads.com/tech2.htm haha....i get what your sayin gofast.....im just also sayin that compression can be an enemy and can create situations requiring a higher octane fuel to keep it stable at certain compression levels... sorry if it sounded different than what i was tryin to get at haha Quote Link to comment Share on other sites More sharing options...
armyvet Posted June 3, 2011 Report Share Posted June 3, 2011 yeah compression is only part of it.timing also plays a key in the equation as well.Just like everything alse reults may vary.. Quote Link to comment Share on other sites More sharing options...
zach15 Posted June 3, 2011 Report Share Posted June 3, 2011 I have 15cc domes in my pro design cool head and im at 175psi in summer in winter im at 212psi an i dont have to run race gas or av gas because the compression is still only 8.5:1 now this winter im decking the cylinders and im gonna try to get 9.5:1 or 10:1 and then i might start runing av gas. Quote Link to comment Share on other sites More sharing options...
SkinInd_Banshee Posted June 3, 2011 Report Share Posted June 3, 2011 note: Running in the sand is a bigger load on the engine, especially after 4th gear, I would recommend high octane in the dunes, Your squish is going to make a HUGE difference also. Quote Link to comment Share on other sites More sharing options...
mopar1rules Posted June 6, 2011 Report Share Posted June 6, 2011 So if you kick over 155psi on a cylinder with 200 degrees of exhaust duration, it will run best with a lower octane than a cylinder which reads 165psi on a 182 degree duration? Compression readings at <100rpm (unless you have a really strong leg) vary vastly compared to cylinder pressure during the powerband. This article is fairly simple to follow compared to some (and it saves me time)... http://www.2strokeheads.com/tech2.htm nice link. kelsey says what i've been saying to members on here for a long time already. Quote Link to comment Share on other sites More sharing options...
muggzy Posted June 6, 2011 Report Share Posted June 6, 2011 nice link. kelsey says what i've been saying to members on here for a long time already. OK, so I've got a basically stock ported engine with 65mm pistons and 21cc domes and just for shits and giggles, my cranking compression is 158psi. How then do I know what fuel I should be running? I'm using 93 octane pump gas and want to stay that way. Quote Link to comment Share on other sites More sharing options...
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