Hmmm, personally I think you would burn a hole in your piston if you were running lean regardless of compression ratio. I ran an 11:1 piston for a year with 92-93 pump gas & the piston looked brand new when I recently replaced it. I started out using VP red 105 octane but it ran like it was too rich. Then I tried mixing it with same results. Then I ran straight high octane pump gas & it ran the best. I even pm'd a few people who get away with running high octane pump gas on a 12:1 piston.

I recently upgraded to a 12:1 piston & I am mixing about 1/3 VP red to 93 octane pump gas & I will let you guys know if it burns any holes, maybe holes in the ground from roosting, but I doubt any piston holes. Most people agree you only need an octane high enough to prevent pinging & detonation which will also make the most power. Since I can run 92-93 octane with an 11:1, then I should only need a few octane more to run a 12:1, so I end up mixing VP red & pump gas to get an equivalent of about 97 octane.

You west coast guys seem to run richer than us east coast guys. You must be close to sea level to run such big mains. I just found out that the mountains we ride in are around 1300'. Even with recent head work, the largest mains I can run is 165-170, any larger & she bogs on hill climbs etc. I can jet up to 175-180 & it runs fine on flat ground, but it definitely bogs when riding with any elevation changes.

I do have a pesky valve tap that is not related to octane. It seems my left exhaust valve resonates like a church pipe organ even when the tappet is adjusted on the tight side. You can even wiggle it with your finger with tight tolerances to the point that it barely has room to move & it rings like crazy. Can't figure it out, the others valves are fine. Maybe the tappet is hitting the valve at an angle such that the contact point is small & sharp & acts like a resonator. I took the tappet out & its ok. Maybe the recent sun storm radiation has magnetized it in the wrong polarity, LOL.....
Hey 14 mins, I never did let you & Bob hear it, my lawn mower that is, LOL.....

With recent head porting I had some jetting problems only to find out the damn floats were out of adjustment & there was too much gas filling up in the bowl.
I really hate when that happens, makes jetting a pain in the a...ss. I had that happen a couple of times now & its always the last thing I check.
Maybe its caused by the few times that I've flipped on a mountain top too? LOL....
Actually, I had the problem before my last tumble down Mt Everest in Tower city PA. You should have seen it, just like in the hill climbing videos. I got to the very steep part of the top when I lost the wheel spin & she grabbed & got good traction & it wheelied & flipped out from under me & away we both went. I really wish I had someone videoing it, was frickin' spectacular......

 

Chimmer- Bob and i waited patiently by the phone awaiting a call that never came. You can get buy with less octane the further away from sea level you get.

While i was high in the mountains this summer i filled my truck up with 2 dollars a gallon 91 octane not thinking and my truck ran like crap until i burned it out. That cold mountain air up high does not need the octane to get the job done.[img]i/expressions/face-icon-small-smile.gif[/img]

 

Can someone please tell me if it will be ok to run an 11:1 compression piston at an altitude of 4700 feet? We only have 93 octane fuels here and it is illegal to purchase race fuel. I was also thinking about getting the piston and head ceramic coated.

I phoned up the fuel company and asked them about the fuel we have here. They told me that the 93 octane fuel at my altitude is the same as running 98 octane at sea level.

 

chimmer we are at sea level and it's the long runs drag racing up the dunes is where little problems turn into big ones. The same set up would probably be fine doing allmost anything else.
Freez at that altitude you'll probably be fine.

 

I agree, higher elevation equals less oxygen(thinner air) equals richer jetting equals less octane requirements.
At 4000 plus feet I would think you wouldn't have any problem running pump gas with an 11:1 piston.
Bottom line is, if she pings under heavy load, then you need to run higher octane, else enjoy the power. [img]i/expressions/face-icon-small-smile.gif[/img]

14 mins, I apologize for not calling back Friday, I was knee deep in trying to get the jetting spot on & trying to eliminate the pesky valve tap & a few other things to do before I had to get ready to work the night shift.
I enjoyed the time talking to you guys, your friendlyness & concern is a valuable asset to the atv community. Its a shame we don't live on the same coast, it would be fun to hang out & take a ride with you guys & share some hill climbing stories.

 

I can get VP red for 30 dollars for a 5 gallon drum,that's pretty cheap and is at 105 octane.

 

Klotz does have an octane booster, You can buy it by the gallon for allot cheaper then the 16 oz cans. We are using this in 12.2/1 and 12.7/1 motors mixed to 103-108 octane according to the can. One once is suppose to raise the octane by 2.5 points. We mix 4-5 ounces with the lower comp. and 5-6 ounces for the higher one with no problems. This is at sea level, in the winter with the cold air we sometimes get a little ping on real cold nights. We run 11/1 on pump gas in the summer and add two ounces to a gallon in the winter. I have tried VP red, 105 octane and have heard no pinging with it at all, so I'm assuming the Klotz mixing claim may be a bit high? The klotz equals out to like 2.75-3.00 a gallon plus you can carry it with you for filling on the trail. Hope this helps.

 

you also have to be careful if you have a Dyna ignition box. advancing the spark can cause problems if you do not increase octane as well.

my bike runs fine with 91 octane at sea level and I have run 87 at altitude but only on the stock Dyna ignition setting. Last weekend at Glamis I didn't have any race gas so I jetted up and tried the advanced spark curves. It ran good but when I pulled the plug it looked lean and I am fearful I burned a bit of my piston off [img]i/expressions/face-icon-small-shocked.gif[/img] I blame the 30 pack of Bud Light I drank

 

My stealership sells VP Red and C12 for $40/5 gal metal can. The local race shop sells a gal of the same stuff for $5.50 if you have your own containers. The distributor sells 30 gal of VP Red for $130, barrel included. With your build, I'd run a step colder plug and try 93 octane. Keep a very close eye on the plug, if you see any flecks on the porcelain, add some octane booster or race gas and try again. Don't run lean.

Regarding carbs, I think the stock carbs, with a few mods, have a better top end than the 35mm FCRs. Either that or my FCR jetting is way different than everyone elses and my spark plug doesn't know it. I don't know if you'll find the difference with a lower compression piston and the Stage 1 cam, I have the 12:1 and Stage 2. I'm going to investigate the 39s and Bob Craycraft's new FCR intake. In another thread, a guy talks about swapping some parts on the 35mm to make them 39mm. If you can afford the 35mm FCRs, you'll love the throttle response and if they are upgradeable, you'll be set for future mods. If you are on a budget, the stock carbs can be modified to perform better.

 

I had my stock carbs modified by Sparks and it made a big difference then I went to the FCR's about a month later and made another BIG difference. I believe modding the carbs does allot but the 35 FCR's does alot better. I am just glad I had good luck with my bike cuz I am running this thing hard and it doesnt give.