cool head dillemma

[370banshee]

the deal is that putting race fuel in a reg motor does nothing. if you build an engine to run on race fuel ie; shaved head, small cool head domes or ecked height shaved then you will run into detonation problems. the way it works is that the higher the octane the slower the burn you get. so if say a 250r has a totally stock top end and it's supposed to run 93 and you put in 110 torco then the slower burn actually wil give you less power because your piston is already slightly past where it would normally get the "push" down on the power stroke. the slower burn will let the piston past where your "push" starts and you will get less are of travel with the power from the power stroke. now if you do mods like milled heads smaller domes. things that raise compression then you need the higher octane for the higher compression. the higher octane fuel actaully burn faster with high compression because of increased cyl pressure. increased pressure helps with combustion. all compression is really is just the pressurizing of air fuel mix. normal air pressure at just above sea level is 14.7:1. higher elevations get a lower air pressure also. the compression ratios afor 2 storkes work differently than taht of 4 strokes (i know about 4 strokes becasue i build hot rods that is also how i know these tidbits of info on octane and how they burn and compression ratios and so on.) now you get the added horse power with the higher compression and the benefits of the higher octane fuel like lower operating temp sand things. now low octange fuel (87 octane) on that same all stock 250r will detonate because the faster brun of lower octane fuel the "detonation" is actually the "push" happing to early and it fights the rotation of the piston. so likewise if you have your YZ engine built i't may run good on 93+ booster and beat guys running race gas in a less modded YZ. you might actually benefit from high octane fuel. because like TZ says the booster isn't very accurate. sometime you may be lower octane or higher octane. try using like 100 octane fuel playign around and see what you get. do a few runs and see if it helps. you can always do a compression test and see what kinda of compression you have take the total compression and divide it by noraml air density of 14.7 psi. then you know you baseline to start your figureing for fuel type. i hope this info helps you out.

[TZ375]

Hey guys, i didn't mean to freak you out. As i said my world is high rpm road race motorcycles, and i wandered into your world checking on cool heads.

the last post is a pretty good way to think of things. in fact gasoline based fuels contain more or less the same amount of energy and have more or less the same potential to make power. Some of the latest VP fuels actually make more power and oxygenated race gas typically makes more pwoer as long as the motor needs it.

As was correctly pointed out, some race fuels burn more slowly - but not all. Increased compression and the turbulence in the combustion chamber from the right squish clearance makes for a faster flame propagation rate, so if you get it righ it all balances out - more or less.

There is a whole lot of numbers you can calculate for squish velocity and the main vairiables other than RPM are squish clearance and width of the squish band.

Back to race gas - VP now makes a whole slew of different fuels and some of them are designed specifically for 2 strokes. I would think that one of the motocross blends would work best for general running and maybe a road race one for drags where the motor is wide open for longer and probably running higher rpms.

When someone says that they can beat the pants off the guys with race gas, we do have to ask the questions of how much of that is the rider? MAybe you have faster reactions or are better at keeping the motor in the meat of the power band.

Alternatively, it's possible that they have gone too far on their motors and things don't match or they may need a session on the dyno. Another possibility is that their motors are really close to stock and don't need the octane ( knock resistance). in that case they are just throwing money away.

Could be they are jetted all wrong or their timing is not optimized or amyriad of other things.

They may also have their motors set up so that they only make power over a very narrow rpm range and that way they lose out because they can't keep it in the power band.

Could also be that they are heavier and that will lose them time.

Could also be that they are using something like Cam2 which is a slow burning gas that is not necessarily OK in your two stroke.

Just for the record we tested our honda CB160 based vintage racer (yes i build 4 strokes too) and on the dyno we got an extra 0.5 hp on street gas versus Cam2. our motors rev to about 12,000 but compression is fairly low.

We find race gas is easier to jet at the track, but on the dyno we can do the exhaust gas analysis and really see what is going on with jetting. That way we can set up the righ slides and needles and air jets. We have even had to use taller or shorter spray noxzzles to tip the fuel slope slightly, but that was an extreme case.

What i would recommend if it's practical, is to run your quad on a dyno with your usual gas mix and with a good race gas. Set the ignition timing with each to get the best power curve after you dial in the jetting and let us know what happens.

My guess is that it will be the best 100 bucks you ever spent. Of course you may be lucky and talented and have the best set up already, but since every motor is different i would hope you'd be able to find a few more horses in there and be able to get a nice wide powerband and beat the rest of them by an even larger margin.

good luck - have fun and stay safe.

[370banshee]

TZ

when i put the engine back together for my banshee would it be ok if i picked your brien about a few tuning tips. i'm pretty good at these things but i have definatly been unseated now. i'd just like to try setting it up your way to see what i can get out of it in the dunes. and since a banshee engine is more or less a RZ350 enigne minus powervalves you might be able to help me out quite a bit. would that be ok???


oh yeah i was also wondering something about the RZ engines. seems like that most guys who build banshees seem to punch them .080 out to make a 370cc have you sen much of this in road racing or is that just a dirt/ sand type thing