compression vs displacement

[cals400ex]

right now i am currently running a 10.8:1 ross piston 406. i was wondering if it would be beneficial to go to a little more compression or a little more dispacement? i want this bike for drag racing (not anything too irratic, mainly dragging friends). i don't need low end power or torque. right now, my bike revs well past the stock rev limiter and that is fine. i am wondering if it would be more beneficial to run like a stock bore piston with a little higher compression piston?? i am going to be using 93 octane. however, with a stock bore i figured it would run a little cooler so possibly i could go a LITTLE higher on the compression? i was thinking it would be beneficial since all i am worried about is power at high rpms for dragging? i know a new sleeve would need to be installed since it is already a 406. maybe what i am saying makes no sense. i don't know because i have never tried it, so its just a possibility. thanks

no replacement for displacement.


I think more displacement with a little less compression is better.

[cals400ex]

Originally posted by Jingle
no replacement for displacement.


I think more displacement with a little less compression is better.



you may be totally correct. however, i can't see that being the case when all i am worried about is high rpm's. i don't need the displacement or low end power. you could be right though, thats why i asked. :) we will see what others say. thanks

[bmw500hp]

fcr 39 :devil: & something along the lines of a stage 2 cam

[Colby@C&DRacing]

More displacement creates more torque and low end power due to the extra weight of the piston. You do want low end power to get the jump off the line not in top speed unless you and your buddies are seeing who has the top speed. most drags are 100 yards. going back to a stock bore with a higher compression piston I feel would not be beneficial. the most important engine temp is the temp of the piston itself,being on the stock bore would have little to no effect on internal cylinder temps.

[Juggalo]

just go with a 416 or a 426. no new sleeve is needed. 406 is the first bore. 426 is the last bore on the stock sleeve.

[cals400ex]

sorry, i forgot to mention i am running a stage 2 cam. i plan on getting a fcr 39 or 41 sometime, i just haven't researched it enough yet. sorry colby, i forgot to mention that we race through 5th gear. 5th gear is probally more important for myself and the type of riding i do (yeah, we don't have any tracks around here:D) than 1st gear. i am not saying your wrong, i was just wondering how it would run or if it would give me any more punch in the mid to upper gears.


i have been told that the je and ross pistons are cut for the xr so their compression ratios are incorrect for the ex. does anyone know if that is true?? if so, how low are the ratings? i was just wondering if it would be beneficial to get a high compression piston and get it cut down to a desired ratio? maybe this all sounds rediculous, but i always try to find ways to tweak the bike. thanks

[cals400ex]

Originally posted by Juggalo
just go with a 416 or a 426. no new sleeve is needed. 406 is the first bore. 426 is the last bore on the stock sleeve.


yeah, i was thinking about it but i don't know if it would be beneficial with my desires. i kind of explained what i was looking for in my previous post. also, i only have about 6 or 7 tanks of gas through my 406, so it is really fresh.

if i was told that i would definately notice a power gain going from my 406 to a 416 or 426, i would really consider. however, i don't know if i jumped from one bike with a 426 for example, to a bike with a 406 with the exact same mods, compression, etc that it would be noticable. i know i can't go too wild on 93 octane either, which i want to run.

[wilkin250r]

Any reason you are set on running 93 instead of race gas? Is it just cost?

Displacement equals power. All other things being equal, burning 440cc of fuel/air will create more power than burning 400cc of fuel/air. However, there are also some drawbacks to bigger displacment, like piston wieght and RPM range, especially if the bigger displacement is and aftermarket addition.

More displacement and torque isn't necessarily a bad thing, because you can gear your bike taller to take advantage off the added torque. I did a similar thing with my 250r. When I put my 340cc engine in, it didn't rev out as high. However, after gearing it properly, I had much more top speed than the 250cc engine because I had so much more torque.

So, I'm not trying to change your mind, but merely presenting the other side of things. Bigger displacement equals more fuel, and thus more power. You would need at least a 13:1 compression in a stock bore (which you can't run on 93 octane) to get the same power as a 426 running a 10:1 compression.

[chad502ex]

Originally posted by wilkin250r
Any reason you are set on running 93 instead of race gas? Is it just cost?

Displacement equals power. All other things being equal, burning 440cc of fuel/air will create more power than burning 400cc of fuel/air. However, there are also some drawbacks to bigger displacment, like piston wieght and RPM range, especially if the bigger displacement is and aftermarket addition.

More displacement and torque isn't necessarily a bad thing, because you can gear your bike taller to take advantage off the added torque. I did a similar thing with my 250r. When I put my 340cc engine in, it didn't rev out as high. However, after gearing it properly, I had much more top speed than the 250cc engine because I had so much more torque.

So, I'm not trying to change your mind, but merely presenting the other side of things. Bigger displacement equals more fuel, and thus more power. You would need at least a 13:1 compression in a stock bore (which you can't run on 93 octane) to get the same power as a 426 running a 10:1 compression.

well said.
chad502ex.com

[KASCHAK]

hrmm... very well said. im gettin a 416 10.8:1 with a hotcam 1 for drags hills trails and xc racing. should be a pretty good runner.

[cals400ex]

Originally posted by wilkin250r
Any reason you are set on running 93 instead of race gas? Is it just cost?

Displacement equals power. All other things being equal, burning 440cc of fuel/air will create more power than burning 400cc of fuel/air. However, there are also some drawbacks to bigger displacment, like piston wieght and RPM range, especially if the bigger displacement is and aftermarket addition.

More displacement and torque isn't necessarily a bad thing, because you can gear your bike taller to take advantage off the added torque. I did a similar thing with my 250r. When I put my 340cc engine in, it didn't rev out as high. However, after gearing it properly, I had much more top speed than the 250cc engine because I had so much more torque.

So, I'm not trying to change your mind, but merely presenting the other side of things. Bigger displacement equals more fuel, and thus more power. You would need at least a 13:1 compression in a stock bore (which you can't run on 93 octane) to get the same power as a 426 running a 10:1 compression.


are you kidding me??? holy shi$!! there will be that much difference between a stock bore and a 426?? i don't mind not revving it as high but i just want it to be overall faster in a drag race. i have no interest in power coming out of a turn on a track or anything like that. i just want power through the gears for a drag. which i understand the torque will help in a drag too. i understand your not trying to change my mind. i was just trying to think "different" than most people. you may be right, i don't have any experience with anything besides the stock piston and a 10.8:1 406. i don't want the bike to over heat and i want to run the highest compression possible with pump fuel. overall, top speed is more important than power in first gear. wilkin, i want to stay with the 93 for a couple reasons. the price is part of it. also, i can't get it around here that i know. the closest place i can get it from is St. Louis and that is a 45 minute drive. also, i dont go through that much fuel and i don't know how long i can let it sit before it goes bad. i will probally go through 5 gallons in 2 months. thanks for the help

[wilkin250r]

Displacement isn't everything, but all other things being equal, more displacement will win every time. In stock form, a 400EX is faster than a 300EX, a banshee is faster than a blaster, and a Raptor is faster than a Warrior. Why? More displacement equals more fuel, which equals more power.

Now, there ARE ways to get around that. A 400EX bored out to a 440 with high compression and and aggressive cam will outrun stock Raptor, even though the Raptor is 660cc.

However, if you do the same thing with the Raptor, throw a high compression piston and aggressive cam, basically equal the playing field, it will be faster than the 440EX. Again, we get back to, all things being equal (compression, cam, carb, ect) bigger displacement will have more power.

Some of the most powerful internal combustion engines made today (like the kind that power huge cruise ships) are about 22,000,000cc and only run at about 100 rpms, but they have HUGE amounts of torque. I think they generate over 100,000 horsepower.

Basically, if you really want the fastest possible, get the MOST of everything. Biggest bore possible, stroker crank, and highest compression possible, biggest carb, and then gear it accordingly.

[chad502ex]

i really like the way ^ thinks. I'd like to think his words sounds like something I'd say...... except for 100,000hp at 100rpm comment. When Torque goes to infinity, hp approaches zero, but somewhat depends on the torque curve. Torque at 100 rpm is near max (1 rpm should be max) and hp should be low. HP is located in upper rpms. It's possible to have hp at 100rpms but only where torque decreases the most on the curve. The steapest decrease on the torque curve is where hp is max.

hope this helps.

chad502ex.com

[duneittilludie]

cals400ex-If you are mainly concerned with a drag quad in a four stroke i would keep in mind that your choice of pipes will also play a big role in what rpm range it performs best at.As an example my wifes 440 with (11:1PISTON-MIDRANGE CAM-STOCK CARB-POWER BOMB HEADER-FMF-Q MUFFLER)will beat a stock raptor in a drag race of 300 feet by about 3 quad lengths with the fairly rsetrictive q muffler.I mention the q-muffler because we chose it initially because of the fact that it was so quiet(90 db as tested) but it turned out to perform very well on the 440 in drag racing even with or probably in fact because it has more backpressure.I am not sure i am making my point correctly but what i am trying to say is that my wifes fairly mild built 440 with its fmf that builds most of its power down low seems to be a good choice for drag racing as we have seen many times.I also agree with some of the others that say there is no replacement for displacement because i have seen this proven many times.I dont know how well some of you others do in a race against a raptor or 700 kawi but i see what my wifes 440 does against them and similar quads and i think we made the right choice of displacement and pipes so at least this is another point of view.I thought i should also mention that my wifes 440 with the fmf has raced a stock raptor in a drag race of over a quarter mile and outran it there by a considerable distance as well.Top end power is a good thing to have but if you are not going to be in a situation where you can get into the rpms where it is at it may not be the thing to be most concerned with. Torque and low end power also play a big role in drag racing when geared appropriately.Sorry to ramble on but as you can tell i am impressed with my wifes 440 and i think you would be happy with a similar set up for drag racing.

[cals400ex]

i am running an x-6 pipe, so i shouldn't have a problem there.

basically, your saying that displacement is more important than compression to a certain extent?

so you can say it would be more beneficial to run a 10.8:1 426 than it would be to run a 11.5:1 406? this is assuming everything else is the same.

thanks guys, your really helping me out

[wilkin250r]

Originally posted by chad502ex
except for 100,000hp at 100rpm comment. When Torque goes to infinity, hp approaches zero, but somewhat depends on the torque curve. Torque at 100 rpm is near max (1 rpm should be max) and hp should be low. HP is located in upper rpms. It's possible to have hp at 100rpms but only where torque decreases the most on the curve. The steapest decrease on the torque curve is where hp is max.

hope this helps.

chad502ex.com

Not quite.

It's true that torque curves and HP curves can look very different, in fact they can even seem opposite. Generally, torque is high at low RPMs, and low at high RPMs, while the opposite is true of horsepower. But infinite torque does not equal zero horsepower. Horsepower is dependent on torque.

Work is force times distance. Power is the rate at which work is done. In rotational terms, force is torque, and therefore power is torque times speed. With some conversion of units thrown in, horsepower is basically torque times RPM. However, due to the nature of piston engines, generally as RPMs increase, torque will decrease. However, the basic formula still applies.

So, if everybody is sufficiently confused, I'll sum it up plain and simple. Power equals torque times RPM. If you can increase torque, you increase horsepower.

[cals400ex]

that sounds good to me wilkin. i guess i may just bore this hog out and see if it runs better.

[chad502ex]

Originally posted by wilkin250r
If you can increase torque, you increase horsepower.

if you have a horsepower dyno graph (aka function) of a motor, your can calculate the derivitive of that hp function to get torque. This is what dyno's actually do (use calculus to calculate torque). But basically you can increase torque but not increase hp that much or none if the slope of torque increase is small (1>x>0). Calculus.

Between both of our interpetations, this about covers the lesson, Don't you think Wilkin? I don't think most here want to do this anyway,....

:)

Chad502ex.com