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91 ethanol free vs 93 with 10%
- Thread starter All Seas
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CReynoldsMIZ
Well-Known Member
Ethanol free is the way to be! List the mods, and maybe [MENTION=51824]Matt Braley[/MENTION] will be nice enough to tell you what octane you need. Might have to add a little bit of race gas to it, but no ethanol is the way to go
SabrToothSqrl
Well-Known Member
Ethanol free. Wish I could get that.
Spimothy Leary
Well-Known Member
I never really noticed any issues with ethanol.
But your mods might require 93 octane, especially if it has a high comp head so a list would help to determine the appropriate.
But your mods might require 93 octane, especially if it has a high comp head so a list would help to determine the appropriate.
problem is I purchased the ski used and do not know whats inside
shee list of what I found here: http://www.seadooforum.com/showthread.php?79288-What-mods-do-I-have
shee list of what I found here: http://www.seadooforum.com/showthread.php?79288-What-mods-do-I-have
Dr Honda
Well-Known Member
Stock engine??
I wouldn't bother with high octane. (89 is fine) AND... if it's more money... I wouldn't worry too much about the ethanol free stuff either. Higher octane burns slower, and in turn, doesn't push the piston as fast. Not to mention... since it's a 2-stroke... the exhaust port opens, and all that does is concentrate heat on the edge of the piston, and into the exhaust.
So, unless you have bumped the compression, or have added a few more degrees timing... you don't need it. (wasted $$$)
Yes... there are some issues with water, and ethanol. BUT... you are less likely to get pre-ignition on it.
Heck... the performance tunes for the spark can get an extra 15 hp using E85 over normal 91/92 octane. (stock 60 hp being pushed up to 112 hp with a tune and e85)
That's my story, and I'm sticking too it. Oh... and all my good running toys, are my back up on that. :thumbsup:
I wouldn't bother with high octane. (89 is fine) AND... if it's more money... I wouldn't worry too much about the ethanol free stuff either. Higher octane burns slower, and in turn, doesn't push the piston as fast. Not to mention... since it's a 2-stroke... the exhaust port opens, and all that does is concentrate heat on the edge of the piston, and into the exhaust.
So, unless you have bumped the compression, or have added a few more degrees timing... you don't need it. (wasted $$$)
Yes... there are some issues with water, and ethanol. BUT... you are less likely to get pre-ignition on it.
Heck... the performance tunes for the spark can get an extra 15 hp using E85 over normal 91/92 octane. (stock 60 hp being pushed up to 112 hp with a tune and e85)
That's my story, and I'm sticking too it. Oh... and all my good running toys, are my back up on that. :thumbsup:
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jimmaki
Active Member
Higher octane basically has more detonation retarders so it can be compressed to higher ratios without pre-igniting. Run a compression volume test to see if your mods include a higher compression ratio than stock. If you're at stock compression ratios than running higher than the recommended 87 octane just slows down your wavefront. If you experience engine knock, then up the octane ... or raise the heads.
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At the end of the day, the higher octane simply helps "pre-ignition" from taking place...otherwise known as 'pinging'. Some engines are extremely sensitive to this; for example the twin-turbo rotary in my '93 RX-7. While a piston engine may tolerate even fairly significant 'pinging' for a while, the Wankel engine will destroy its apex and side seals almost instantly.
Ask me how I know, $6,200 later.
There are some extremely knowledgeable Rotax experts on here...I'd listen to them. From what I can tell from the exposure I've had to the Rotax/Sea Doo world, it seems it takes some fairly significant modifications to make the higher octane REQUIRED. What those mods are and how each one plus the cumulative effect of all combined are important to understand.
Ask me how I know, $6,200 later.
There are some extremely knowledgeable Rotax experts on here...I'd listen to them. From what I can tell from the exposure I've had to the Rotax/Sea Doo world, it seems it takes some fairly significant modifications to make the higher octane REQUIRED. What those mods are and how each one plus the cumulative effect of all combined are important to understand.
jimmaki
Active Member
I think the stock compression ratio is 6.0:1 but doing a volume compression test is pretty involved and may be more work than you are willing to do so I won't explain the process at this point. More easily, you could try dropping down in octane and see when you get preignition and then back up to the last octane, or put stock heads on and run 87/88 octane and continue to listen for preignition as different pipe tuning stuffs more or less of the exhausted fuel charge back into the cylinder right before the exhaust port closes which has a supercharging effect.
Just to be clear, compression is not the same as compression ratio, the latter being the swept volume to the unswept volume at TDC.
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Often times it is the cumulative effect of the mods that end up being the deciding factor. Once again, going back to an RX-7 analogy, for example: Adding a freer flowing exhaust system is usually okay. Adding a freer flowing air intake system is usually okay. Adding a different intercooler is usually okay. Modifying the intake and exhaust ports (remember, only three moving parts in these engines) is usually okay. But when you start doing more than one together, it's known as "+1" mods, and gets you into the place where the stock ECU can't compensate and the requested injector duty cycles exceed what they are capable of, and you get a lean condition and a "boom".
I am certain that the really, really expert guys on here, people with extensive modification knowledge...maybe even race experience, are the guys who know can advise best on this, when it comes to the Rotax mods.
I am certain that the really, really expert guys on here, people with extensive modification knowledge...maybe even race experience, are the guys who know can advise best on this, when it comes to the Rotax mods.
jimmaki
Active Member
Unless they are blueprinting an engine, most tuners and racing competitors will push the engine limits using the highest rated fuel the rules allow and listen/watch for preignition which could be a knock, ping, or a hole in a piston. Deck height, pipe length, port height, rv timing and ignition timing requirements can change with the weather, temp, humidity, altitude, etc.
For someone who doesn't know what mods are on his engine and just wants to know what fuel to use, if it were me and I hadn't recently won the lottery or liked wasting money, I'd start with successively lower ocatane pump gas and if I heard preignition or saw signs of it, I'd either go back up to a higher octane gas or start putting stock parts on the engine. If the engine is doing 7k off the limiter with his impeller and weight, then he's pretty much getting stock performance anyway, so I'm not sure what mods he has and how much good they are doing him.
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jimmaki
Active Member
7k RPM is pretty much stock performance so unless the mods are covering up another problem, you're probably ok running regular gas. If you just want to run gas without alchohol in it and it only comes in 90/91 octane, then you might want to consider E10, use a stabilizer and turn it over more often than every 12 months. Even over that time it will lose some of its punch. So will non alchohol gas. Eventually non alchohol gas will be very difficult to find if the EPA has it's way.
Nike12000
Active Member
"twin-turbo rotary in my '93 RX-7. While a piston engine may tolerate" They are rotors in a rotary engine not pistons
"twin-turbo rotary in my '93 RX-7. While a piston engine may tolerate" They are rotors in a rotary engine not pistons
Uh, if you read the WHOLE sentence: While a piston engine may tolerate even fairly significant 'pinging' for a while, the Wankel engine will destroy its apex and side seals almost instantly.
Ask me how I know, $6,200 later.
I went to great pains to differentiate that a piston engine is tolerant of pinging while a Wankel (rotary) is not. There is a comma there...
You don't want to hear these Rotax engines ping. That is the sound of the perimeter of the piston coming apart and the head getting eaten up. You just don't get much detonation before devastation with these.
I told him in the PM to rum 93 unless it's gonna sit three months or more. The extra octane will help keep the deto at bay but this crap corn gas has no shelf life to speak of. If I had to fire up a ski with last years gas in it I'd rather the pure 91.
The 787's should be fine on pump gas up to 180 psi. As stated before, maybe not if there are a lot of other mods like advanced ignition timing and/or a tight squish band. At 7000rpm he should be way safe. I've pushed up to 7250rpm on 93(fresh 93) before the engines start to run too hot.
I told him in the PM to rum 93 unless it's gonna sit three months or more. The extra octane will help keep the deto at bay but this crap corn gas has no shelf life to speak of. If I had to fire up a ski with last years gas in it I'd rather the pure 91.
The 787's should be fine on pump gas up to 180 psi. As stated before, maybe not if there are a lot of other mods like advanced ignition timing and/or a tight squish band. At 7000rpm he should be way safe. I've pushed up to 7250rpm on 93(fresh 93) before the engines start to run too hot.
Nike12000
Active Member
Dr Honda
Well-Known Member
Old Air cooled VW's were the same way. Once you started to push them.... if you heard "Pinging"... then it was too late.
I remember (back in the day) a VW racing guru said... "The light pining heard, is the equivalent of hitting the tops of your pistons, with a 15 lb sledge hammer as hard as you can." (Bernie Bergman)
Seadoodoc
Active Member
Well worded Bajaman! Our race engines are experiencing a different kind of piston failure using ethanol fuel. Gone are the melted piston exhaust port side failures, in are slow blow by breaking down down piston cyl ring seal. Effectively we are seeing engines loosing compression from deto blow by. Good news is its a slow inexpensive death. One or two gallons of leaded race fuel per tank really is a radical $10 per tank insurance policy. Pay me now or pay later.
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