Increasing rev limit my another 1000rpm on 98 Carbed engine

[knight]

I have a 98 carbed B.bird engine.....I'm interested to know if any of you know if its possible to increase the rev limiter by another 1000RPM using the standard ECU....either modified, or with an electronic add-on gizmo?.....I'm not interested in junking the standard ECU, as in the race series I run they stipulate that I must use the standard ECU......Ideally I'm looking for some electronics boffin who can re-program the ECU.....does anyone provide such a service?.....

[xxxbirdxxx]

I am not sure why you would want to do that?

That would be taking a big chance at over rev and Popping the motor, NO?

The rev limiter keeps you from doing just that right, so why change that? I really don't think you will gain anymore speed out of her, and if you did, it would only be in each gear and not over all speed. Or maybe you have a Turbo/NOS on it?

I can't help you I am sorry. But I just had to know why you would want to do that?

[Pete in PA]

Unless you've changed the engine drastically, power has fallen at 10k rpm already and it's time to change gears. I think the limiter cuts in at 11.5.

The rev limiter is there to stop valve float and subsuquent fragging of the engine.

[pace]

Well, there are several approaches to increase horsepower:

- Increase capacity.

- Improve volumetric efficiency.

- Raise RPMs (since power is a function of torque and RPMs).

However, as others have mentioned you will need a stronger (heavier) valvetrain and also will need to tune the motor to shift the torque peak further up the RPM range. Cams and porting can both help to achieve that. On an otherwise stock Blackbird, you'd gain nothing.

-Pace

[pace]

We're in agreement, elixxir. I personally feel that for a street-ridden bike especially, increasing displacement or VE gives preferrable characteristics than raising the power band. In addition, most shadetree mechanics/builders lack understanding of the relationship between increasing RPMs and the implication for tensile forces on the rods and pins.

-Pace

[RodeRash]

If I remember correctly, Knight uses his bird motor to power a race car. The rules prohibit modifications, so he is trying to find a way to get a little more competitive without being caught. One way to do this, would be to change lengths and diameters of the exhaust, and then degree the cams to work with the new pipes and make more power at higher RPM. (at the expense of low end /midrange). More than likely, the overdesigned Honda motor can deal with a little extra RPM. Now he needs a way around the rev limiter, but he still needs to keep the stock ECU.

I have never heard of anyone that can hack the code of the stock ECU. I only know of replacement units (I think Vance and Hines made one at one time) or add-on units (Like the Power Commander)

[pace]

It seems like it should be possible to piggyback off the Honda ECM; possibly intercepting and modifying the coil/pulse signal, and thus spoofing the computer into allowing a higher rev limit. There are devices that perform this kind of function for automobile ECMs. The question is whether the Honda ECM uses the RPM signal for anything else on the carb'd 'bird. Adaptive timing? Knock control? I can't imagine the 'bird computer is very sophisticated compared to modern high-performance autos, and it's possible that this signal is only used to implement the limiter??

-Pace

[tomek]

Higher rev limit or overrev could be very usefull on the race track.

It could mean couple less gear changes per lap,like letting the motor rev instead of shifting up and down or riding rev limiter before corners.

Race motors get inspected often,get fresh valve springs,etc,1000 more revs are not going to a kill it.

[pace]

1000 more revs are not going to a kill it.

I guess that depends on how over-engineered the valvetrain is. My experience with autos is that higher performance motors generally do not have much margin for error, since they are already pushed close to the limits. Both the Subaru WRX and Mitsubishi Evolution valvetrain for example, cannot take more than a few hundred RPMs over the factory limit before they start to float. And if you have an interference setup, you're looking at an expensive experiment.

I have no idea what the 'XX valvetrain is capable of. But I suspect on a machine that is oriented primarily toward performance, the manufacturer put the RPM limit in place for a good reason. Definitely an interesting question though, and I'm curious to see what the motor is capable of withstanding.

-Pace

[tomek]

Pace -you have to keep in mind that valve springs wear or get weaker during the life of the engine so safe rev limit for the valvetrain becomes lower and lower.

Rev limit on production engine has to take those worn,weaker parts into account and production tolerances too.Race engine with parts inspected ( replaced) frequently can rev much higher.

[knight]

"If I remember correctly, Knight uses his bird motor to power a race car. The rules prohibit modifications, so he is trying to find a way to get a little more competitive without being caught"

you know me to well!!!!.......degreeing in the cams and increasing the rev limit sounds the way to go......would be great to find someone that can hack the standard ECU.......your help much appreciated