rev limiter?

[moe7404]

i have a jato 3.3, i just wiped out the piston skirt, by over reving to long. is there a rev limiter for such engines? it would sure pay for its self.

 

[RC Addict]

Were you revving it up under load? Or with the wheels off the ground?

 

[2revo1maxx]

Yeah you can not over-rev a nitro when it's under load. If it's running free it can break, but usually because it's lean or not moving and then it will over-heat.

 

[olds97_lss]

is there a rev limiter for such engines?

Yes, there is. Almost everyone has one (except for uncle stumpy...). Look at your left hand. The finger closest to the thumb that controls the throttle on the remote is how you limit the RPM's.

Really though, if you find you don't know when too much is too much, then I'd suggest adjusting your radio's EPA's (if it has that) so it doesn't open the carb all the way. Have it max out at 3/4 throttle. Even then though, WOT off the ground even at 3/4 throttle will cause damage over a period of time.

General common sense is really the only thing to limit you though. Now that you know the ramifications of running wide open all the time, you may be less likely to do so. If not, I'd suggest looking into electric. However, then your going to bust axles and blow tires apart as well.

 

[RC Addict]

And melt ESC's and motors.

 

[moe7404]

to: addict it was on the ground but it has a low gear ratio. 16/58 i bought some higher gears the stock one for sure, maybe the higher gear will keep the rpm down. also i thought about adjusting the throttle so it only opens half way. what think?

 

[RC Addict]

olds is by far the guru here--I would do as he suggested and set your EPA so that you only get about 50-75% total throw. Of course if you are using the stock transmitter you won't have that option. Barring that you could set up some kind of physical limiter that would only allow your throttle to open up so far. Since you have a rotary carb that shouldn't be a huge obstacle--not nearly as difficult as having a slide carb!

 

[olds97_lss]

The jato is an easy rig to blow an engine in. With the 2.5R, it's still overpowered. It's so light that on a not too long flat stretch of ground, you can really get the engine to sing. With a 3.3... that's just stupid IMHO. I can't believe traxxas did that. All those years of running .15ci engines in even their MT's being completely underpowered, then they crank out the 3.3 and put it in everything even though it's a horrible idea... in an MT, it's great. In a 5lb 2wd 2 speed stadium? C'mon! That's just silly!

Gearing taller will help, but seriously, it's still going to be over powered due to lack of traction. This will cause high rev burnouts and high rev long stints to still be a dominant part of your RC'ing.

Since the carb is a slide carb and the stock remote doesn't allow for EPA adjustment, you can always just move the collar on the throttle wire so it engages later and causes less throttle movement. If you really can't tell what "bad" is, I'd suggest that or your engine burn up problems will continue.

 

[moe7404]

i was thinking of adjusting it to only open 50%-80%. its good to know its ok to do it like that. thanks guys i may not get it running for 4-5 days.

 

[RC Addict]

I thought they were rotary not slide. Hmmm... My bad. Just looked at my Maxx and guess what--slide carb! I'm an ass. Looking at the setup olds is correct--loosen the collar and slide the wire farther in to limit the amount of overall throw.

See, there is a reason he is the Nitro Guru and I am the King of BS *er* the Internet...

 

[moe7404]

to: RC Addict no trouble your advice is always welcome.

 

[Rolex]

You can also move the linkage to a hole farther in on the servo arm.
The farther out the linkage is, the longer the stroke.

 

[neobart]

God how I hate radios that don't have EPA adjustment.

 

[Rolex]

I won't EVER be without computer radios, both for air and ground.

 

[moe7404]

throttle adjust

to: rolex the jatos servo arm only has one place to put the throttle, with out moding it. huuuum modding, my most favorite thing to do. also i put extra head shims on the 3.3, will i be ok?

 

[moe7404]

ok guys can i ask an other question? i have a o.s. 11k carb comming. should i break in the new piston on the stock 3.3 carb? or use the o.s. carb? if you say the new carb then do i set it different the how it comes to me? or use it as is? thanks a lot.

 

[olds97_lss]

Extra head shims are only necessary if your increasing your nitro content of your fuel above a certain percentage. I think the 3.3 is safe to run with 15-25% nitro in it's stock form. Possibly higher. Adding more shims increases the combustion chamber which lessens compression, power and retards the timing unless you have the extra nitro in the fuel. More nitro makes the fuel more volatile, so it burns easier which causes it to fire too early in the stroke. Adding shims adjusts the timing of the burn since compression is lessened.

Even if you do mod the servo horn, I don't think you have enough room to do it. You would need to move the servo/wire mount closer to the center of the horn and I think it's already almost in the center if it's like mine. Sliding the collar a bit is as easy as loosening a set screw.

I'd break the engine in with the stock carb, for two reasons. One, warranty. Keeps you from having to lie to traxxas in case the engine craps out on you. Two, you an tune it/break it in via traxxas specs/settings. Then you know what the engine is doing and whether or not the new carb even improves anything.

EDIT:
Just checked the 3.3 manual and apparently it can run 10%-33% nitro without the need of more shims.

You may use 10%, 20% or 33% nitro-content fuel. Try to use the same percentage all the
time, avoid switching back and forth between fuels. We recommend that if you break in
your engine on 20% fuel that you stick with that percentage. If you do move to a higher or
lower percentage, make sure you readjust your fuel mixture to compensate....

In reality, there are practical limitations. Engines are
designed to run best within a range of nitro percentages. How the engine is ported, the size of
the combustion chamber and other factors determine how much nitro can be efficiently used
in the engine. TRX Racing Engines respond exceptionally well to a maximum of 33% nitro,
returning cooler temps, more power, and a smoother throttle response. For those who want to
run higher nitro, 33% Top Fuel is the optimum nitro percentage for TRX Racing Engines.
Increasing the nitro beyond 33% can introduce the need for engine modifications (ports, head
shimming, etc.) to avoid starting and tuning difficulties. There are limits to how much nitro an
engine can effectively use to make more power. Lower nitro percentages have their own
advantages. Nitro is an expensive component in the fuel so 10% nitro blend is more economical
for the sport user. 10% also provides greater latitude with the needle settings for easier tuning.

 

[moe7404]

thanks olds, what you say makes sence. i think ill start putting it back together, i dont like having it apart to long i forget where things go. thanks.