Exhaust and AFR

[Pinhead]

How does the presence of "backpressure" change the AFR coming out of a carb? I've always thought that backpressure was unnecessary, while velocity and tuning lengths in the header/collector is the important part...

The mufflers rusted through and fell off my '79 CB650 motorcycle the other day. When I posted pics on www.forums.sohc4.net and mentioned that low RPM torque had drastically improved but high-RPM power had suffered, I almost got beaten to death about the importance of backpressure in the exhaust for carb tuning. They said I will have problems ranging from a loss in power (which I have noticed above 4000 rpm on an engine with a 9500rpm red-line) to burning exhaust valves and ruining exhaust valve seats. Strangely enough the engine will now idle smoothly at 900 rpm whereas before I had to have it set to 1300. Also, I can pull away in 1st at idle, whereas before there was absolutely no possible way to take off below 1500.

The way the mufflers are designed on my 650, it seems that the "collector" extends into the muffler about 6 inches, and then is dumped into a termination box (a large sudden change in CSA and volume).

So where does the "backpressure myth" come from, and why did I lose power when the mufflers came off?

[FlowSpecialist]

Exhaust pulse tuning effects influence the inlet system during the cam overlap phase at TDC between the exhaust and inlet strokes when both valves are open. Any change in an exhaust will always have some influence on the inlet system pressures and therefore fuel mixture with carbs. Obviously fuel injection systems are more immune to this.

It's not that backpressure is ever needed or desirable in an engine, it isn't, but if the carbs have been calibrated to suit a certain exhaust system they'll go out of calibration if it changes. A perfect exhaust system always has zero backpressure. Nothing that causes a flow loss can ever be actually desirable in an engine.

Dave

[Pinhead]

That is exactly as I suspected, but I suppose I still don't understand one thing regarding how the exhaust effects the carbs. I always thought that the flow of fuel is more-or-less controlled by the velocity of the air going past the jet in the venturi. If the exhaust is more "open" and allows more air to flow, why doesn't that increased air volume cause more fuel to flow as well, keeping the air/fuel ratio close to the same?

The consensus seems to be that an opened exhaust will burn exhaust valves; in my experience that means the carbs went lean. If there is less backpressure there should be more airflow, more of a depression on the carbs, and therefore more fuel flowing. I'm still confused....

[FlowSpecialist]

It's not just about airflow, it's about pressure waves propagating up and down the inlet tract that are created in the exhaust system and transmitted during overlap.

Dave