bigboybanshee Posted September 15, 2005 Report Posted September 15, 2005 I know that once you start running high compression you need to run race gas, but my question is how do you know what octane will be enough? Is there a "standard" or anything stating if you run "x" amount of compression, you should run "x" octane gas? I found a hookup on some AV gas, it's supposed to be rated at 103 octane. The stuff I put in there for the first tankful was 110. I'm just curious how to find out what method you guys use to determine what octane is necessary for your application. Is there a formula or something for it? Thanks!!! Quote
ellison445 Posted September 15, 2005 Report Posted September 15, 2005 Thats a good question Travis, Im curious on this one as well I know that once you start running high compression you need to run race gas, but my question is how do you know what octane will be enough? Is there a "standard" or anything stating if you run "x" amount of compression, you should run "x" octane gas? I found a hookup on some AV gas, it's supposed to be rated at 103 octane. The stuff I put in there for the first tankful was 110. I'm just curious how to find out what method you guys use to determine what octane is necessary for your application. Is there a formula or something for it? Thanks!!! 415312[/snapback] Quote
locogato11283 Posted September 15, 2005 Report Posted September 15, 2005 whats ur compression travis?? my bike is about 170 pounds, and i ran 100 octane for a long time with no problems. im assuming since yours is a drag bike you arent any higher than about 170 or 175 pounds of compression, otherwise your motor wont turn the rpms that a drag motor should. what size dome do u have?? i just switched tio vp c12 because i dont wanna run oxygenated fuel any more and this new shit is 108 octane, which i dont really need... by the way, if u want a professionals opinion, dlnoss can answer it for u...i think he makes domes and shit and knows a little about compression Quote
RNBRAD Posted September 15, 2005 Report Posted September 15, 2005 I doubt you could make a standard, too many variables between engines, timing, heat, loads, elevation, ambient temps etc... I think a general rule of thumb is anything above 160 compression requires at least a 50/50 mix and above 165 is straight race gas. I use 110 octane to definitely play it safe. I know there is ways to test what octane an engine requires but I believe it's engine and modification specific as well as other variables to make it somewhat accurate. Quote
bigboybanshee Posted September 15, 2005 Author Report Posted September 15, 2005 whats ur compression travis?? my bike is about 170 pounds, and i ran 100 octane for a long time with no problems. im assuming since yours is a drag bike you arent any higher than about 170 or 175 pounds of compression, otherwise your motor wont turn the rpms that a drag motor should. what size dome do u have?? i just switched tio vp c12 because i dont wanna run oxygenated fuel any more and this new shit is 108 octane, which i dont really need... 415330[/snapback] I'm not 100%, I was going to check the compression tomorrow when I lean out the jetting/change plugs. I'm pretty damn sure I'm in the 170 range, I'm running 18cc domes now. I was just curious if there is a method to find out exactly what octane to run, mainly to see if this 103 octane AV gas would be sufficient. LAST thing I wanna do is detonate! Thanks Tyler and thank you Brad! Quote
dlnoss Posted September 15, 2005 Report Posted September 15, 2005 It would probably be best to ask GRR since they did the porting. Like Brad said, there are lots of variables. Your timing, porting, and compression all play into it. When in doubt though, run high octane. My gut response would be that 103-104 would be okay at 175psi or less. Quote
locogato11283 Posted September 15, 2005 Report Posted September 15, 2005 I'm not 100%, I was going to check the compression tomorrow when I lean out the jetting/change plugs. I'm pretty damn sure I'm in the 170 range, I'm running 18cc domes now. I was just curious if there is a method to find out exactly what octane to run, mainly to see if this 103 octane AV gas would be sufficient. LAST thing I wanna do is detonate! Thanks Tyler and thank you Brad! 415332[/snapback] travis, you may find that those 18cc domes might be killin your top end. when fast did my bros cylinders he had 18cc(about 185lbs) domes. my setup which was nearly the same had 21cc domes.(about 170 lbs) there was a very noticeable difference in the top end of the bikes. mine would simply outpull his. might be something to think about when you get a tester and see what yours has..i guess with high compression you can gear high to compensate. with low you could gear low...just something to think about... Quote
bigboybanshee Posted September 15, 2005 Author Report Posted September 15, 2005 It would probably be best to ask GRR since they did the porting. Like Brad said, there are lots of variables. Your timing, porting, and compression all play into it. When in doubt though, run high octane. My gut response would be that 103-104 would be okay at 175psi or less. 415405[/snapback] That's not a bad idea, I'll check with Andy and see what he thinks. travis, you may find that those 18cc domes might be killin your top end. when fast did my bros cylinders he had 18cc(about 185lbs) domes. my setup which was nearly the same had 21cc domes.(about 170 lbs) there was a very noticeable difference in the top end of the bikes. mine would simply outpull his. might be something to think about when you get a tester and see what yours has..i guess with high compression you can gear high to compensate. with low you could gear low...just something to think about... 415406[/snapback] It's definitely something I will try, I have a set of 21's, 20's, and the 18's that are in there. I figured I would let the track be my determining factor; I'll run it with the 18's first, then move to the 20's, and 21's as necessary. I have a feeling I'll be changing my setup quite a bit over the next few months trying to see what works best. I did notice that when I get to the very top of the powerband the power drops off dramatically, maybe it's because of those 18cc domes. I appreciate the advice, every little bit helps!!!! Quote
bkabina Posted September 17, 2005 Report Posted September 17, 2005 If you are unsure about what octane to use, start out higher.But if the bike seems too rich on bottom and you can't realy fix the jetting then the octane maybe a little to high.I ran into this while playing with domes. Quote
RNBRAD Posted September 17, 2005 Report Posted September 17, 2005 That's not a bad idea, I'll check with Andy and see what he thinks. It's definitely something I will try, I have a set of 21's, 20's, and the 18's that are in there. I figured I would let the track be my determining factor; I'll run it with the 18's first, then move to the 20's, and 21's as necessary. I have a feeling I'll be changing my setup quite a bit over the next few months trying to see what works best. I did notice that when I get to the very top of the powerband the power drops off dramatically, maybe it's because of those 18cc domes. I appreciate the advice, every little bit helps!!!! 415411[/snapback] Yea you need to do a compression check Travis. I'm sure your not running near as much compression as everyone thinks because your exhaust port height has probably been increased signigicantly. This decreases compression and you have to run smaller domes to compensate. Just a thought. Quote
Big Blue Posted September 17, 2005 Report Posted September 17, 2005 It's definitely something I will try, I have a set of 21's, 20's, and the 18's that are in there. I figured I would let the track be my determining factor; I'll run it with the 18's first, then move to the 20's, and 21's as necessary. I have a feeling I'll be changing my setup quite a bit over the next few months trying to see what works best. I did notice that when I get to the very top of the powerband the power drops off dramatically, maybe it's because of those 18cc domes. I appreciate the advice, every little bit helps!!!! 415411[/snapback] I was at 170 to 175 with those domes travis, Yours in theory should be lower due to yours being a ported for drag, mine was ported for dunes. As for the top of the powerband you could be reving past the point where it makes power I didnt notice it much with my 350 but with the cub I notice myself doing it quite a bit, A habit ill have to break myself from. Quote
locogato11283 Posted September 17, 2005 Report Posted September 17, 2005 Yea you need to do a compression check Travis. I'm sure your not running near as much compression as everyone thinks because your exhaust port height has probably been increased signigicantly. This decreases compression and you have to run smaller domes to compensate. Just a thought. 415953[/snapback] very true...compression tester will tell it all...but, stephens bike was drag ported and it still had 180 something poounds..not sure how much more travis's are hogged out... Quote
bobbybonilla Posted September 17, 2005 Report Posted September 17, 2005 Dont the newer dirt bikes come with compression in the 200lbs? What do the dealers reccomend to run in them? Quote
sandman121383 Posted September 17, 2005 Report Posted September 17, 2005 (edited) i got 160 in my cyclinders with 19cc domes , stock bore n stroke, unported, i ride between 600 and 1000 feet usually and i run a 50/50 AV fuel with 93 or 94 mixed 50:1 with amsoil. run a compression check n get back to us Edited September 17, 2005 by sandman121383 Quote
bigboybanshee Posted September 18, 2005 Author Report Posted September 18, 2005 Yea you need to do a compression check Travis. I'm sure your not running near as much compression as everyone thinks because your exhaust port height has probably been increased signigicantly. This decreases compression and you have to run smaller domes to compensate. Just a thought. 415953[/snapback] Doing it today! I didn't have the time to check it yesterday but did lean her out 2 main sizes and oh man did it help, she was pretty rich I still have a full tank of 110 that I'm running in it now, I'm gonna check the compression on it this afternoon and I'll get back with you guys; something tells me by the feel of the kicker it doesn't seem any more than 170 but I could definitely be wrong, might be less LOL, each time I make the slightest adjustment on this beast it opens up a whole nother can of whoopin my ass, this shit is fast! Taking it to the track next weekend Andy @ GRR Thanks for all the help guys!!! Quote
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