Dirt Rider . Net Text Version Home
Pages: 1
A better rev limiter?
(Click here to view the original thread with full colors/images)
Posted by: Matt Fisher
On a post a couple weeks ago, Rich mentioned that holding a motor wide open and hitting the kill switch or hitting the rev limiter was a good way to grenade the motor due to plastic deformation of the small rod end.
What I'm wondering, is what would be a better way to make a rev-limiter, since cutting spark or fuel has such possible consequences. Would retarding the ignition do the trick, or would that have the same basic result as cutting the spark off?
Posted by: steve.emma
probably best and easy way is for a manufacturer to build a circuit into the cdi that intermittantly kills the spark so the bike basically missfires but doesnt completely cut out. with a carb there isint any simple way to cut the fuel so it has to be done with spark somehow.
really though any rev limiter wont stop a rider from over revving an engine due to accidental downchanges etc.. also most guys dont normally need the engine protection a limiter provides as generally the engine has fallen out of its powerband before the point a limiter would need to kick in. smart racers would be shifting cogs while still in powerband to provide best accleration, not just revving the hell out of the motor till it pops. 
Posted by: _JOE_
On some bikes that is true. On my crf however it smacks the limiter like brick wall while still pulling. On most bikes I've ridden they do sign off a bit before the limiter hits. Mine did a bit before I rejetted. It was quite lean on the main stock. I have been looking at the Vertex ignitions systems, but it's not in the budget right now. http://www.tokyomods.com/honda/99-0...250%20RX.html#G
I would agree that avoiding the limiter is best.
Posted by: Matt Fisher
I'm hoping Rich will chime in here, but from what I understood, cutting either fuel or spark even for a moment while at high rpm will have the same effect- no combustion resuting in no "cushion" for the piston. This can lead to the piston trying to rip itself out of the connecting rod's grasp from the inertia.
I'm thinking that by the time you retarded the timing enough to massively cut down on the rpm's increasing, you'd effectively have the same basic problem as not having spark.
The only way I can think of that would totally limit throttle induced over-rev would be along the lines of BMW's double vanos system. Change the effective cam profile to something that would not support high rpms.
Something else I just thought of- if it's bad to cut spark or fuel at high rpm's due to the piston having no combustion event to cushion it, why doesn't the exhaust stroke have the same issue?
Posted by: cujet
Just what do you think happens at the end of the exhaust stroke when reaching TDC. There is minimal cushion on the piston there too.
Are you talking about running an engine wide open in neutral or with the clutch in? If so, one could have a lower rev limit for this, like drag racers do.
Most cars today modulate the throttle valve (throttle by wire cars, that is) to achieve a rev limiter.
Chris
Posted by: DieselTech
Simple solution. Engine brake ("jake"). It holds the exhaust valve slightly open (of course, fuel or spark would have to be cut as well) so that the compression is released. This will slow the engine down, as well as provide the "cushion". Of course, this may not be entirely practical for a dirtbike.
However, another solution exists, in my mind, and that would be to reduce the fuel entering the cylinder. This would require fuel injection, and would actually be easiest to implement with direct injection. Technology that is way out in the future for consideration in dirtbikes, I'm sure. The engineers are just starting to use this in cars (direct injection - for gasoline anyway).
Just my thoughts....
Text Version Home
vBulletin Copyright ©2000 - 2008,
Jelsoft Enterprises Limited.
vB Easy Archive Final ©2000 - 2008
- Created by Stefan "Xenon" Kaeser
