Tuning a 450cc single cylinder engine with plenum and restrictor

[dpapazach]

Hello,

My name is Constantine and I'm from the University of Hawaii FSAE team. We are currently trying to tune our Yamaha 2012 YFZ450R motor with a
carbon fiber plenum. We are using a Vortex X10 ECU and a standalone A/F monitor. Our maps are all based off tps and we have control over fuel,
ignition timing btdc, and end fuel timing. It starts pretty good and we have been able to get it to idle well at around 2200 RPM and it revs up nicely
with good throttle response without any surging or RPM holding.

The only problem is that it dies after revving from idle on the way down. In other words at a steady idle when throttle is blipped up to ~50% tps momentarily
it will rev up to ~8000 RPM and then die when revving back down to idle. It won't even hold more than a fire or two at idle RPM before dying unless throttle
is blipped again by catching it from falling below idle RPM.

We're thinking that this is because when the rpms are falling as the throttle is nearly closed it creates a vacuum in the plenum chamber and not enough air can
be supplied to support the fuel being injected at that rpm. We tried reducing fuel around 2k-4k RPM but that didn't seem to help much.

Could we just need a bigger plenum? What are your guy's thoughts or experiences with tuning a similar setup?

Thanks in advance.

1310

[mech5496]

Disclaimer; NOT an ECU/engine guru, just aware of simple principles/concepts by experimenting in my own car.

Has your ECU the ability to control idle advance per RPM delta? If yes, try to use ignition advance to keep the idle steady.

https://www.youtube.com/watch?v=kiPow6aeb_0

BTW car looks smart.

[Owen Thomas]

Wouldn't consider myself an expert either and not familiar with that ECU, but have dealt with similar problems before.

Check for an "over run" trim that changes fuel based on TPS rate and play with it, you are probably right that there is too much fuel after the throttle snaps closed.

Also look at your ignition map and see if there are any timing walls that are keeping the revs lower (or higher) than wanted. Sometimes people put in a sort of idle pit where the spark is retarded a few degrees and a steep jump in spark advance just above that, if this area is misplaced it can cause your engine to hesitate or bog down. I'm also bringing this up because 2200 rpm idle seems a bit high so this might be something you want to look into to control that.

Good luck!

[Pat Clarke]

Constantine,

What size is your throttle body?
Teams often get into tuning and driveability trouble if they use the standard throttle body with the restrictor.
In order to get good resolution for the TPS signal, the T/B needs to be matched to the restrictor.

Pat

[MCoach]

Hello,

My name is Constantine and I'm from the University of Hawaii FSAE team. We are currently trying to tune our Yamaha 2012 YFZ450R motor with a
carbon fiber plenum. We are using a Vortex X10 ECU and a standalone A/F monitor. Our maps are all based off tps and we have control over fuel,
ignition timing btdc, and end fuel timing. It starts pretty good and we have been able to get it to idle well at around 2200 RPM and it revs up nicely
with good throttle response without any surging or RPM holding.

The only problem is that it dies after revving from idle on the way down. In other words at a steady idle when throttle is blipped up to ~50% tps momentarily
it will rev up to ~8000 RPM and then die when revving back down to idle. It won't even hold more than a fire or two at idle RPM before dying unless throttle
is blipped again by catching it from falling below idle RPM.

We're thinking that this is because when the rpms are falling as the throttle is nearly closed it creates a vacuum in the plenum chamber and not enough air can
be supplied to support the fuel being injected at that rpm. We tried reducing fuel around 2k-4k RPM but that didn't seem to help much.

Could we just need a bigger plenum? What are your guy's thoughts or experiences with tuning a similar setup?

Thanks in advance.

1310

Watch your A/F monitor as you rev and un-rev the monitor. Does it show anything very rich or lean on rev?
Is there a chance you are losing crank sync loss on rev? Note: This is common with Megasquirt (I know you're not using it) and single cylinders.
If you slowly rev the motor does it still die?

I'm going to guess that you don't really have a way to catch the idle as the engine winds back down.
If you're tuning using "Alpha-N" or in your words, based on TPS, then there is a little trick that can help.
If you choose an RPM you want to idle at, such as your chosen 2200 RPM point, make that your minimum spark advance point, 4 to 10 degrees is probably in the ballpark.
Bump the spark advance all around this point in TPS and RPM grid a little higher than that point. It will give a little "valley" for the idle to settle into. If it tries to drop below this point the spark advance will help increase torque to push it back up into the valley.
Play with fueling as well to hit your air:fuel targets and stabilize the idle.