rev box yes or no

[Jayson_406]

Hey

I was woundering if i should get a rev box for my 400 ex ive heard that thier useless and ive heard their not just woundering what you all think and weather or not you like them and if so what is the best . Thank you

spend you money on something else

i would tell you why, but i am tired of typing it out, and i am tired of looking for where i have posted it before...

[Rip_Tear]

There are so many variing opinions. And it kind of depends on what you want it for?

Here are a few threads about rev boxes...

http://www.exriders.com/vbb/search.php?s=&action=showresults&searchid=1227199&sortby=lastpost&sortorder=descending

[wilkin250r]

They are both. A hi-rev CDI is helpful and it's useless. I depends on your engine setup.

A stock 400EX makes about 30HP at 6500rpm, at it's peak. After that, power starts dropping off. It's down to 25hp at 8000, and less than 20hp at 9000, and falling off very fast. If it could rev out to 10,000 rpm, it would only be making about 8hp. So why would you spend money to rev out that far? It's useless, you can't even use that power.

However, if you have a modified engine, designed specifically to make more power at high RPM, a rev box is certainly useful. You wouldn't peak out at 6500rpm, you would peak much later, like 8000RPM. At 9000, you're only down 5% from peak, and at 10,000, you're only down 20%. If you peak at 45HP, you still have 42HP at 9000. That would still be decent, usable power, you certainly don't want to chop it off right there. You would extend your rev limit to 10,000, in order to take advantage of that power at high RPMs.

Stock engine, a hi-rev CDI is useless.

Modified engine, a hi-rev cdi is worthwhile.

[mountaineer]

My mechanic told me not to use my rev box with a high compression piston. He said the valves are close enough (to the piston) with the stock rev box.

[Chino886]

Originally posted by mountaineer
My mechanic told me not to use my rev box with a high compression piston. He said the valves are close enough (to the piston) with the stock rev box.

I maybe be wrong, but the CDI has nothing to do with the "squish" I think its called.....

Wilkin, where you at on this one?

[underpowered]

Originally posted by mountaineer
My mechanic told me not to use my rev box with a high compression piston. He said the valves are close enough (to the piston) with the stock rev box.

Your CDI has nothing to do with valve timing, it changes your ignition timing and raised your rev limiter. The only thing i have noticed is by advancing teh ignition timing, you run a greater risk of spark knock, i have just experienced with my CDI i just recieved. It cause the spark to come closer to TDC which should give youi a slight power increase, but it also increases heat and causes the spark knock.

[wilkin250r]

Originally posted by Chino886
I maybe be wrong, but the CDI has nothing to do with the "squish" I think its called.....

Wilkin, where you at on this one?
Yes and no.

Just to clear the first issue up, "squish" is the distance between the piston and head. It's slightly related to valves, but not absolutely concrete. You can alter valve clearance by using a high-lift cam (valves will be closer to piston), or you could cut deeper valve-relief pockets in the piston to get larger clearance.

As for the high-rev CDI, the mechanic isn't completely full of crap. The danger doesn't come from the spark timing, it comes from the higher rev limit.

There is a term called "floating a valve". In ordinary operation, the camshaft pushes the rocker arm, and the rocker arm pushes the valve to open it. When the camshaft stops pushing the rocker arm, the rocker arm falls back and the valve closes. The valve is always in contact with the rocker arm.

"Floating a valve" occurs at extremly high RPM. It happens when the camshaft stops pushing the rocker, but the valve doesn't close fast enough. It's "floating" because it's not in contact with the rocker anymore, it's supposed to be closed but it's not. (by the way, this is half the reason that aftermarket camshafts need heavy-duty valve springs. The stiffer valve springs close the valve faster).

This is obviously a problem. If the valve is open when it should normally be closed, the piston could hit it. It doesn't happen at low rpms, because the valve springs have more time to react. It only happens at high rpms, and a close valve/piston clearance increases the danger.

It doesn't have anything to do with spark timing, it's about higher rpms. The solution would be to get stiffer valvesprings, or custom cut deeper relief pockets in the piston.