Triumphrat

Big_Guns · 05-28-2005

In one of the magazines Alan Cathcart states the 1050 motor in the '05 Sprint ST pops on decel because its running too rich. I thought a lean engine, not a rich one pops on decel. It also makes more sense to me that our bikes are running lean as delivered to pass emissions testing. I'm just wondering because it seems adding a freer flowing aftermarket exhaust would flow more air causing a leaner mixture. This would be a good thing if Cathcart is right, a bad thing if I'm right. Opinions?

Martin_R · 05-29-2005

Quote:

On 2005-05-28 20:02, Big_Guns wrote:
I'm just wondering because it seems adding a freer flowing aftermarket exhaust would flow more air causing a leaner mixture. This would be a good thing if Cathcart is right, a bad thing if I'm right. Opinions?

If you fit an aftermarket exhaust go and have your dealer download the Triumph off-road can fuelling map to your bike and see if that will work with your new cans. Should be better than the stock map at least.

But to your actual question: no, I have no idea if the S3 is running a bit on the lean side by default.

[ This message was edited by: Martin_R on 2005-05-29 02:10 ]

mjw930 · 05-29-2005

The following is something I saved a few years back, it's one of the best decriptions of why we get deceleration popping I've seen.

Quote:

“Popping” (backfire) is occurring NOT because the engine is lean, but because during overrun, when the throttle is shut but the engine is still spinning above idle speed, the air-fuel charge in the combustion chamber is at extremely low pressure (high vacuum), much more so than during idle. This causes extremely slow flame propagation across the combustion chamber. This, in turn, has two combustion-critical effects:

The first effect is that, during each combustion cycle, a small amount of fuel burns over a long period of time. This results in “quenching” on the cylinder walls. Quenching is the process whereby the flame inside the cylinder is not releasing enough energy to sustain combustion because of the cooling effect of the cylinder/combustion chamber walls. That is to say, the flame is being cooled faster than it can release heat to sustain itself. This leads to the flame being extinguished before all of the fuel is completely burned. This results in fuel and oxygen present in the cylinder, unused and unburned.

The second effect is that the low-pressure flame (away from cylinder walls and other cooling surfaces) is so slow in consuming the remaining air-fuel charge, that when the exhaust valve opens, there is still some fuel burning in the cylinder. Put another way, there was not enough time for the low-pressure, slow-moving flame to consume all of the fuel before the exhaust valve opened.

So, under the condition of overrun, once the exhaust valve opens, you now have (a) quenched, unburned fuel being pumped into the exhaust port and header, and (b) fuel that is still on fire being pumped into the exhaust port and header. Guess what happens. The lit fuel ignites the unlit fuel and you have a backfire. It becomes completely obvious that this is what is happening, if you have ever witnessed ANY high performance engine run with the exhaust manifold/header removed. You will see (and hear) the high-speed gas come out of the port, and then you will see a flame leap out AND IGNITE the earlier-released gas that is already out of the port/engine. On two-stroke engines, at low speed, this is so severe that you are left with the impression that more gas burns OUTSIDE the cylinder than inside it, which partially explains the high fuel consumption of two-strokes.

Furthering this effect, when the throttle is shut (and the engine is supposed to be idling), the engine designers greatly retard the timing to allow full compression of the air-fuel charge before igniting it, which promotes smooth idling and reduces misfiring. During overrun, this results in the fuel being lit LATE, at a time when you wanted it lit EARLY. This is also why opening the throttle a little during overrun helps reduce backfiring: when you open the throttle, you are at least slightly advancing the timing from its idle setting (see explanation above for smooth idle), as well as increasing the pressure in the cylinder (again, see above for the effect of low pressure on flame propagation).

I have driven some very expensive engines with state-of-the-art fuel management systems. These engines have had literally hundreds of hours of set-up and testing to set the mixture perfectly, and THESE CARS BACKFIRE during overrun. It is not a problem…it is what engines do…a natural occurrence. High-vacuum-induced slow combustion speed is the reason that, on any reasonably sophisticated engine, as you begin to close the throttle, the computer or engine management system (electronic or mechanical) starts ADVANCING the ignition timing. During high vacuum (throttle near closed), the fuel must be lit earlier and earlier as the engine spins faster and faster, or, otherwise, the above-described combustion lag and flame quenching takes place. Eventually you will always get to a point (RPM vs. vacuum) where you can’t light the fuel early enough (as discussed above) to complete the combustion process before the exhaust valve opens. This is why your engine pops.

Lean engines exasperate this phenomena because lean mixtures burn more slowly than mixtures that are stoichiometrically correct (or even just rich of the correct ratio). But EVEN ENGINES THAT HAVE PERFECT MIXTURE FORMATION, in the correct ratio and which are NOT LEAN, will still backfire during overrun (though a good muffler will keep you from hearing it). Race cars, that cost MILLIONs of dollars, do this. You don’t hear it on most passenger vehicles because of the stock/EPA-approved exhaust system. Also, all passenger cars from about 1985 on TURN THE FUEL INJECTORS OFF during overrun conditions. That is to say, when you come down from the crest of a hill or down a mountain, when the engine is spinning above idle but the throttle is shut, your engine is essentially shut off, with the fuel injectors turned off. The reason they must do this is to protect the catalytic converter. If the unburned fuel and air generated during overrun (see above) get into the catalytic converter (for any sustained period) the catalyst will be ruined (either by overheating or fuel-poisoning).

If you set your bike so rich that the backfiring stops during overrun, you have set it far richer than soichiometric, and you are probably extinguishing the flame propagation (see above) with a rich mixture, in a way analogous to welders who use a rich flame to consume all of the oxygen around the weld joint, and thereby prevent oxidation within the weld. This overly rich mixture could also be the cause of fouled spark plugs (not oil fouled plugs, only the rich mixture fouled).

My advice is to NOT set your fuel mixture based on backfiring during overrun, because backfiring during overrun is NOT necessarily an indication that there is something wrong. The mixture could be fine, even perfect, and you could still have backfiring.

[ This message was edited by: mjw930 on 2005-05-29 09:33 ]

xSpeed3x · 05-29-2005

***** MJ,

That is one indepth explanation!

Thx

crashmasterd · 05-29-2005

Some of the best information I've read here. Very nice.

Thanks for sharing.

Also, for anybody that is interested in internal combustion engines in general, and performance motorcycles in particular, pick up the Sportbike Performance Handbook by Kevin Cameron. It explains just about everything concerning combustion chamber design, intake harmonics, suspension design, and more and more and more.

What's so great about it is that it doesn't tell you how many clicks to set your rebound damping (for instance), but what it does do is tell you how to determine that for yourself for your particular bike.

Highly recommended. Don't let some of the reviews put you off.

Sportbike Perfomance Handbook at Amazon.com