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compression only adds bottom to mid......and trottle response, not a lick of topend.

incorrect

Imma go with Fuzzy on this one.

fuzzy wrote:

compression only adds bottom to mid......and trottle response, not a lick of topend.

incorrect

think about it...... you cannot turn an engine no faster than the gearing will allow you to go.... so how do you get more topend????? this is why you run high compression engines in drag cars, and low gears, its to get you there faster.............. the reason MORE compression eqals to more bottom to mid, is because you are compressing the air fuel more {PSI}, so when it ignites the fuel,air..... it actually blows the piston down with more force, creating more tourqe..... BUT i do have to eat my words when i said it didnt add a lick of topend......... because you will get a little topend, a few mph.. because you have more power to pull your gear

More compression makes more power everywhere regardless of quanitity of fuel charge. It's the simplest part of the combustion theory. You run the most compression you can for the fuel you are using to create the most power. Key things said here is compression matches fuel. If 120 octane fuel was what we had at the pump we would all have engines making more power with the same cam profile/fuel curve/etc....Basically free power. This is the same 4 or 2 cycle.

High compression necessity with a high lift/long duration camshaft in a 4-cycle gets into dynamic compression ratios/etc. Also, most drag cars will not run low (numerical) gears. They will run low or 'short' gears though....Depends what you are referring to as 'low.' For a drag car you want max engine RPM in high gear at the trap. With a high-rpm drag racing motor this means extremely low (numerically high) final drive ratios. Low=less gear, high=more gear. These cam-profiles will make big high-rpm power, and little power where the stock engine once did. IE a true 9000RPM v-8 will make crap at 5000RPM. This engine will undoubtedely be a high-compression engine, but lets not get into dynamic compression/cam duratin/etc. Depending on cam profile, high compression isn't a necesseity for this type of power output. Take your KZ1000. It's a 10k motor that makes little power under 6k, yet will run on 90oct. Reciprocating weight, blah, blah come into play here too.

On a 2-cycle this type of power output altering of the cam is done in the porting, and the pipe tailored to match. However, you can still have a 2-cycle making ridiculous top end at extremely high rpms (15+k) with no useable power anywhere else, and still have the static CR set to only require 93oct fuel. If you can up your fuel requirement, you can alter compression to match, and make more 'free' power.

In the case of this pump-gas head mod it's more of a squish profile deal (and of course SOME increase in compression), and that gets into things about a 2-cycle that I don't understand that well. In this case you're right the squish profile is being changed more than the CR, and this is creating more low-mid, and might only be helping very little at the peak. The slight increase in CR is due to the conservative factory head cut. A stock KDX will probably run on 'good' fresh from the refinery ethanol free 87oct gas stock with no detonation. We all run 93 so RB can get a little tighter while he's cutting.

i know about compression. you are right about what you said..... but what im am saying is that you cant take your kdx, leave everything the same, shave the head, and expect to pick up top end, and you shouldnt have to chage jetting. MY mustang street car{stock suspension class} runs 5.90 1/8 mile, and 9.40 1/4 . this is a 12.1 compression motor with 6 psi boost in a full interior, full body car on a 10" tire.. with a 3.55 gear. around here we say low gear because we know what were talking about. 4.10 is lower geared that 3.08..... also my kz1000 has 11.1 compression, its not stock, and my bandit isnt stock by no means, made 138 hp at the tire.... all the work on everything i have is done by me. this is what i do for a living. not trying to start a war here, so no hard feelings.

True to a point, if you're only talking about compression. Keep in mind, on a powervalve equipped 2 stroke, there are 2 different compression ratio's. One with the valves closed, and a 2nd lower compression ratio with the valves open. This is to provide more power on the bottom end using the higher compression ratio, without sacrificing power up top.

However, with a mod like RB's, there are other factors at play. Squish, dome configuration, and compression all play a part in providing the "more power everywhere" effect. The difference is enough that you should revisit your jetting after the mod.

with a 3.55 gear. around here we say low gear because we know what were talking about. 4.10 is lower geared that 3.08.....

4.10 is lower than 3.08 is correct, but you contradicted yourself. We're on the same page here, but you just take a rub the wrong way approach in your comments. I many have rubbed you the wrong way too, with the incorrect comment, but you were.

I didn't say anything about head cutting, and jetting....Just compression. To get picky about jetting you have to change it based of air density, and technically could do it twice or more in one day on some days. This is way more anal than anyone would usually care to get on a trail bike. Sooooo, who cares if a little squish cut requires a .1 change in AF.

Juliend is right on about the 'effective' compression ratio changing with the exhaust valve. Higher compression at lower rpm to increase power in that range, then changes to stop detonation. CDI alters timing here too.

......{quote} [fuzzy] ........ I didn't say anything about head cutting, and jetting....Just compression. To get picky about jetting you have to change it based of air density, and technically could do it twice or more in one day on some days. This is way more anal than anyone would usually care to get on a trail bike. Sooooo, who cares if a little squish cut requires a .1 change in AF.


head cutting is reducing the size of the combustion chamber, thus incressing compression...... which has everything to do with compression. thats how we get compression ratios on our drag motors. without the combustion chamber volume, ther is no way to tell what compression you have.

juliend wrote:Simply put; more or less air flow will require more or less fuel to match. Any change in compression is going to alter airflow characteristics.

Looks like I have ended up with 40 / 150 / CGK pos 3. I like it, a LOT.

Is that with an RB or stock carb?

Thanks

juliend wrote:Simply put; more or less air flow will require more or less fuel to match. Any change in compression is going to alter airflow characteristics.

Looks like I have ended up with 40 / 150 / CGK pos 3. I like it, a LOT.

what year is your bike?

sorry, 89 200, rb'd head, stock carb.

Cool, I just switch mine to match and it's great.

seems like a mid range hit I was missing?

Cool, glad it's working out for you. You might have to play with that needle position. I've dropped mine another notch, to the 2nd clip pos. I find that after the RB head, the bike is much more particular about the jetting.

I have a DGK needle which I put in sometimes too. The DGK offers NO midrange hit, which is really nice if it's slippery out, or if I am going to be riding for a long time. Much less tiring.....

Smoooooooooth is always good...IMHO...Unless you can really time the hit in conjunction with blasting out of bermed corners (that's about the only place I can see a 'hit' useful...Like in SX). They'll tie in a drag race.

Any jetting changes with the DGK? what clip position?

What effect does the 2nd pos have on the CGK needle?