FLTRI2--question about reversion / over-scavenging

[vanwill]

Bob, this question is carrying over from the old site.  You mentioned reversion and over-scavenging in a way I didn't understand.  Which of these applies to a SuperTrapp Supermeg 2:1, 95", Andrews 37 cam build?  Still having trouble getting smooth operation at very light TP (less than 10%) and light load in the 1750-2250 RPM range.

[FLTRI]

"Still having trouble getting smooth operation at very light TP (less than 10%) and light load in the 1750-2250 RPM range."
This a classic sign of reversion. This can be from cams, exhaust, and/or porting.
You might try adding 5-10% fuel in the low TP(<20%) @ 2000-2500 range to see if that takes care of it.

Bob

[Sonny S.]

Bob,

Would you typically expect to see reversion using a ST w/closed cap, and TW37 cams ?

Sonny

[whittlebeast]

People often confuse reversion and scavenging.

Think of everything in terms of getting as much spent air out of the combustion chamber as possible so that we have room for fresh air and then best predict how well we did this to get in the matching amount of fresh fuel from the EFI.

Mis-guess on the reversion and or scavenging and you end up with the wrong amount of fuel.

Reversion is air flow (on the intake side or on the exhaust side of the combustion chamber) going in the wrong direction. Normally due to sound waves on miss matched parts (length) compared to the frequency of the sound waves coming at it.

Scavenging is the use of the momentum of the air flow to help the air flow to continue in the direction that you desire. You can use the momentum of the other cylinder to help suck on the cylinder in question. With a wobble fire motor, this is all a mess to get correct over any sort of a wide RPM band.

Listen to a 2-1 and how the sound changes with RPM and you will get a clue on what you are up against.

Remember that only fresh air needs fresh fuel. This is the very basics on the definition of th VE number.

AW

[Sonny S.]

Reversion is air flow (on the intake side or on the exhaust side of the combustion chamber) going in the wrong direction. Normally due to sound waves on miss matched parts (length) compared to the frequency of the sound waves coming at it.


Correct, but I wouldn't expect that with TW37's ( early intake close/ low overlap )and a ST with a closed cap....would you ?

Sonny

[Sonny S.]

I always seem to ask the hardest questions..... 

[1997bagger]

Question on this subject, while running a Wego tuning system on deceleration, the reading will go to 19.1. I'm pretty sure this due to reversion filling the exhaust with air causing the excessive lean reading, want to know for sure incase it comes up in a wateringhole conversation.

[FLTRI]

Go to the bar, you are correct

[ToBeFrank]

Question on this subject, while running a Wego tuning system on deceleration, the reading will go to 19.1. I'm pretty sure this due to reversion filling the exhaust with air causing the excessive lean reading, want to know for sure incase it comes up in a wateringhole conversation.

Monitor the injector pulse width. Some (do all of them?) calibrations cut off the fuel in certain RPM ranges during decel.

[whittlebeast]

Frank

My Sportster does not kill the fuel in over-run.

AW

[FLTRI]

Not sure what overrun means as usually it refers to a turbocharged engine still boosting intake pressure after the throttle has been closed.

Decel backfiring is influenced by increasing or decreasing fuel introduced during 0 throttle operation.

The first SERT calibrations actually made decel backfiring worse till they figured out what was actually happening then they fixed it pretty well, allbeit not perfect.

We have found, depending on exhaust system design adding fuel works better than reducing fuel during 0 decel. The exception to this rule is the stock "Y" pipe system that reacts totally differently than other 2into1 system and respond better to decreasing fuel during decel to mitigate backfiring.

[ToBeFrank]

Not sure what overrun means

Google "engine overrun"

[FLTRI]

Not sure what overrun means

Google "engine overrun"

Everyday I learn something new. I've been in the automotive and motorcycle racing industries for over 35 years and only never heard the term overun refered to a normal decel of a naturally asperated engine.
Thanks for the info

[whittlebeast]

Overrun may be a term used mostly in the EFI coding world.  In the racing world it is known as the "trailing throttle part of the fuel map".  It's all the same thing.  Suck-Squish-BANG-Blow and predicting how much fresh air wound up in the combustion chamber.

[whittlebeast]

I wrote this for a different forum

<<<<<<  snip  >>>>>>

Why does adding fuel in over-run change the backfiring?

The answer to this one is not what everyone has been led to believe. Hang with me on this one.

It takes three things to get backfiring in the exhaust. Heat and quite a bit of it, unburned fuel and air containing 02. Leave out any one of these things and the backfiring will not happen.

So how do each of these happen at the same time. The first one to address is unburned fuel in the exhaust. There is a couple of ways that this can happen. First is that for some reason, the the spark failed to light the compressed fuel and air in the combustion chamber. This can be either the spark plug failed to spark, the mixture was too lean to light or the mixture was too rich to light. All of these combinations will result in 02 and raw fuel ending up in the exhaust just waiting for an excuse to blow. The easiest way to get a source of heat is the next time the spark does manage to get the mixture to fire and when the exhaust valve does open a small bit of this following flame finds/catches the mixture left over from the last miss hit and BANG you get a backfire out the exhaust. this is kinda rare but can happen.

The more likely thing is that the mixture was too rich and when the spark happened, the fire ran out of air before it ran out of fuel and the left over fuel ended up in the exhaust just needing heat and a little o2 from somewhere. The place that is most common to find a little frest air is from the exhaust mannifold gasket that is leaking or from the joint between the head pipe and the muffler. At this point there is plenty heat floating around for the mixture to relight and BANG, you get a backfire.

The last possibility that I had not considdered was to fix this entire issue by flooding the motor in over-run with way too much fuel and and killing the heat in the exhaust and forcing the temps low enough to kill the possibility of a backfire. It turns out that there is apparently a 1ms minimum Pulse Width in the a Sporty code and in some situations this is still too much fuel to get all the way down to 14.7.

Remember that if the motor is at 14.7 or leaner in over-run there is just no fuel left in the exhaust and there is now way a backfire can happen. The problem is with the 1ms min PW you may never get down to 14.7

The way some of the Harley codes get around this is to kill fuel entirely in overrun. The problem with this solution is you can often feel the injectors suddenly turn back on at some point right before idle takes over or the throttle rolls back on. All fun and games of EFI tuning.

A big thanks to Steve at www.nightrider.com for pulling all this together. The same guy that does the XIEDs.

[ToBeFrank]

The way some of the Harley codes get around this is to kill fuel entirely in overrun. The problem with this solution is you can often feel the injectors suddenly turn back on at some point right before idle takes over or the throttle rolls back on.

Amen to that. I get a small (but very noticeable) surge when the fuel kicks back in after the decel cut. It's worse on some maps... I switched from the 105HB map to the 105LK map because it was really bad with the HB map. It's especially annoying when cruising around town where I'm constantly in and out of the throttle because of traffic.

[nc-renegade]

It's especially annoying when cruising around town where I'm constantly in and out of the throttle because of traffic.

This is why I do not enable the decel feature on T-Max.  It is annoying in the above scenario.