cool head dillemma

back to basics

We run TZ750s and RZ350s here and in Australia and there are some issues that remain the same.

First off, porting and pipes change the EFFECTIVE compression ratio at different points in your power curve.

Second, you have to get the squish clearnace right before you do almost anything else. if say your base gasket is too thick or the barrels are taller than spec ( or at the upper limit) then your squish band may be too wide. A tall squish band tends to increase the probability of detonation, so it pays to get it right. there are posts all over the internet on what "right' is, so speak to your favorite tuner.

Increased compression improves the bottom and mid range but tends to hurt the top end because of increased pumping losses. The trick is to find a compression ratio that works best for your type of riding. You may want to try a couple of different inserts to get to what works best for you.

Also keep in mind that different head shapes work at different parts of the RPM curve. Narrow squish works at the top end and broad ones for lower down, so feel free to experiment with different domes and see what works for you. Ignore all the hype and what other people do. We all ride differently and what works best for your riding style is the way to go - not what someone else uses.

Race bikes aren't quite the same as quads. We run long straightaways so we can't take as much compression as you guys, so we lose out a little of that mid range. We also can't use as much ignition advance at the top end, so we use ignition boxes that run more advance down low and less as the revs go up. Dyna do a programable CDI box for Banshees. WOLF probably do one as well. Now we're talking bigger dollars, so get the basic set up right first.

On TZ's historicaly stock bikes set up very carefully run faster and longer than all the trick ported high comp, super doodad machines. Make usre that what you have is working perfectly with the right jetting and compression and iginition advance. jetting two or three sizes off costs more HP than most accesories. Timing is the same. get it right.

A worn chain will cost you 1 to 4 hp at the rear wheels and may wear out sprockets or leave you stranded.

VP and Sunoco both do a range of fuels. We use VP C12 for most motors if we can get it. We also do a mix of Sunoco 100 unleaded and their 115 Octane, but my preference is to not mix anything other than oil and fuel and for that I use an Accu-Mix jug, available at www.accu-products.com

Avoid octane boosters like the plague. they are hard to mix accurately, add nothing in the way of power and really are just an expensive form of paint stripper.

Checkout any of the replacement dome guys for estimated compression pressure and the fuel you'll need, but try C12. If you are feeling rich, get a drum of the latest VP race gas MR8 or ultimate and feel the extra half a horsepower.

Check fuel:air ratio by readig the plugs or better still pull the heads and check. Unleaded leaves carbon and is impossible to read unless you are an expert. It also needs hotter plugs to burn off the soft carbon film.

So with race gas you should see a light gray or brown (depends on oil) and be prepared to go harder (colder) on plugs - say B9EVX in platinum or b10 if the plug shows signs of overheating. You can also use B9EGV or B10, but try to stay away from regular plugs with 1.3mm electrodes.

Life is a learning experience - feel free to experiment - you never know what you will learn - and keep reading too.

Woa there Hillclimber

Sorry that a different perspective upsets you.

I mentioned the TZ to let you know that I don't run quads, but that I was offering some advice that applies to all racing two strokes. The TZ is the water cooled production race motorcycle. I wasn't bragging about them - I was just letting you know that they are different in some ways.

As for what the heck is a TZ? Well, the first TZ came out in 1972 and was similar to the RD350 road bike of the time. Over the years Yamaha have produced them in 125, 250,350 and 750 versions. The 750 was a 4 cylinder with 66.4mm bore, 54mm stroke, 34mm Mikunis on reed blocks and made about 140hp. The current TZ250 weighs about 250 pounds and makes almost 100hp at the rear wheel.

The RZ, as your more tolerant and open minded buddy, *** Banshee, mentioned is the motorcycle version of the Banshee with the same motor but with powervalves as stock.

What else would you like to know?

In answer to the other question I don't work as a mechanic, but I am an engineer and I have been crew chief and mechanic for a couple of motorcycle road racers with Aprilia AF1, RSR125, RS250, Honda RS125, RS250 and Yamaha RD350,RD400, RZ350, TZ125, TZ250 & TZ350. Hope that helps. I try to be open minded and to keep up with two stroke technology in all areas. Banshee parts often work on our RZ's for example.

If you want to use an octane booster go right ahead, but please share with us the scientific results of your tests. The reports I have read all say the same thing - no improvement in performance.

Sure if you are stuck in some small town and can't get good gas, then go ahead and mix some in as a get you home trick, but it is not the smartest way to protect your motor. The closer you take a motor to the edge of detonation, the more imortant it is to be consistent with fuel. Street gas varies considerably and summer blends are particularly bad.

If you run a high compression output, high compression motor, do not wreck it on cheap gas and octane booster. test it with different gas and see what it needs and if it needs race gas, then spend the money and schlep the gas drum around - it's cheaper than meltdown.

But, hey, it's your motor, you do what you want to do.

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.

Good description

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.