engine modfications

[pekasdam]

hey there guys, can anyone recommend any engine mods for a 2003 400ex. im not into racing but i would like to go a little faster up hills. thank you, any help will be appreciated.

[pekasdam]

anyone!

[EXevan91]

if u dotn have one already u might want to get a 14 tooth front sprocket and or u can add a aftermarket exhaust and and air filter.

[pekasdam]

i have that stuff on there already, i was thinking more along the lines of another piston, i just dont know how to go about it.

[Wheelie]

Buy a service manual if you plan on tearing into the engine.

A high compression piston and drop-in cam would definitely help.

[bansheemorphine]

it all depends on how much money u want to spendif u take your motor to a shop they will tell u what to do

[pekasdam]

how does the high compresion and drop in cam work? and i would like to keep using premium gas.

[krt400ex]

Originally posted by Wheelie
Buy a service manual if you plan on tearing into the engine.

A high compression piston and drop-in cam would definitely help.


if u have an aftermarket pipe it will help. u won't feel much if u do that without a pipe.

[pekasdam]

i have the whit bros r4 pipe on it right now. i just dont get the whole compressiong thing.

[GPracer2500]

Originally posted by krt400ex
if u have an aftermarket pipe it will help. u won't feel much if u do that without a pipe.

Not true. A piston's compression is essentially unrelated to the exhaust's ability to flow gas out of the engine. Exhaust matters when you change the bore, because then you're changing the volume of gas needing to be moved out. But even a big bore is still going to make a noticable difference with completely stock exhaust. You'd be forgoing some performance w/ stock exhaust, but to say you won't feel much without a pipe is off base.

Anyway, he already said he had a pipe.

[GPracer2500]

Originally posted by pekasdam
i have the whit bros r4 pipe on it right now. i just dont get the whole compressiong thing.

The compression rato is a measure of how much the piston squeezes the air/fuel mixture at the top of the piston's stroke within the cylinder. Its the ratio of cylinder volume when the piston is at he bottom of its stroke vs. the cylinder volume when the piston is at the top of it's stroke. The more compression an engine has the more energy it can extract from the air/fuel mixture and the more torque it will produce. The stock compression ratio an a 400EX is 9.1:1 (9.1 total units of volume get squeezed into 1 unit of space when the piston is at its highest point).

But you don't really want too much compression because then 1) it really starts to put stress on all of the engine compenents and reliabilty suffers, and 2) once you reach a certain compression ratio it becomes mandatory to use high octane racing fuel to prevent the abnormal combustion phenomenon called detonation. In a 400EX, 11:1 generally as high as you can go and still use 93 octane pump gas. Any higher than 11:1 and you'll start having to buy special gas.

I agree with Wheelie that a piston and a camshaft are worthwhile choices. As far as which piston (i.e. bore size and compression ratio) your choices are many. You can go with a stock bore higher compression (10.5:1) piston and a camshaft for nice gains. And it just goes up from there depending on what you want to spend and how complicated a build you are prepared to tackle.

I had a 86mm (that's 1mm oversized, 406cc vs. the stock 397cc) 10.6:1 piston and a cam in my 400EX for several years. I liked it (at that time) and you may want to consider something similar. If your going to go with an overbore, there's a good argument for going up to a 87mm piston (416cc) but at that point I'd recommend heavy duty head studs which is an extra expense and step.

Good luck!

[krt400ex]

Originally posted by GPracer2500
Not true. A piston's compression is essentially unrelated to the exhaust's ability to flow gas out of the engine. Exhaust matters when you change the bore, because then you're changing the volume of gas needing to be moved out. But even a big bore is still going to make a noticable difference with completely stock exhaust. You'd be forgoing some performance w/ stock exhaust, but to say you won't feel much without a pipe is off base.

Anyway, he already said he had a pipe.

i was talking more about a cam. the purpose of a cam is to flow more air, and with a stock exhaustu won't feel it that much because the stock exhaust restricts air flow

[pekasdam]

thank you very much everybody. that was very helpful. these are by far the best forums with real knowledgeable people. thank you once again.