Forum

[AndyMc]

This may be a stupid question but. Can anyone explain (simply) how fitting a more free flowing exhaust can make an engine run leaner. Surely on the intake stroke, the exhaust valve is closed, the inlet is open and a measured amount of fuel air is drawn in. This is isolated from what has already gone out of the exhaust. Im considering one for my C5 but don't want the expense of a power commander or carb conversion (yet) Thanks

[Rattlebattle]

Look up valve overlap. In most 4 stroke engines the inlet valve actually opens at the end of the exhaust stroke, before the exhaust valve has shut. Therefore the incoming mixture is influenced to some extent by the exhaust valve still being open. Highly tuned engines tend to have more valve overlap. Try to view it as a continuous stream rather than as a single cycle of combustion as the influence of valve overlap increases with higher revs. Not a very technical explanation but I hope you get the gist.

[Alan R]

Hi ANDY Mc C ---- freer flowing means LESS BACKPRESSURE which in turn allows MORE charge air to flow over the standard jets which -- by comparison --- are now NOT passing enough fuel to maintain ideal mix ratio, hence the Weakness.......How's that ??...Typically, you might find that an increase in jetting between 5 and 10% is required....

[ric]

Providing you still have your lambda sensor fitted in the downpipe the ECU will adjust for the changes as a result of fitting any of the EFI suitable free flow exhausts sold here. BUT the bike will continue to run with an AFR of 14.7:1
If you want to take full benefit from a free flow exhaust you need to richer up the mixture which is quite simple on these bikes and details can be found within the tuning posts on the USA forum.

[PeteF]

To get the full benefit from a free flowing exhaust you really need to increase the inlet flow as well by fitting a free flow filter as well.

[Revband]

To put it very simply, any engine mod which improves performance will usually also require more fuel to get into the engine as well. You don't get anything for nothing and that is one reason so many home tuned engines give problems, some people think it's easy?, It's not it's far more involved than they think, one perfect example, look at the piston broke thread.

[Dan H]

I believe the lambda sensor may maintain an AFR of 14.7 : 1 but this will not be controlled across the entire rev range and throttle positions. Usually the ECU will make adjustments to the mixture at the lower end of the revs and at lower throttle positions using the signal from the lambda sensor.

At the higher revs and higher throttle openings the ECU will be running from its preset values and will not be effected by the lambda and this is the range that the engine can run leaner than originally intended.

With just a free flowing silencer a Power Commander should not be a necessity to fit and would not cause any problems as such but what it would do is help optimise the fuel mixture and ignition timing.

[Rattlebattle]

I concur with that entirely. These bikes don't have wide band O2 sensors; they don't operate throughout the entire rev range. The Hinckley Triumph I have is similar. The O2 sensor on that is known to operate only on throttle openings of up to 6%. Ideally you need an AFR of 13:1. The stoichmetric AFR is good for complete combustion, hence emissions, but not for maximum power. That is why people remove O2 sensors and then remap to suit the exhaust/inlet configuration.

[Alan R]

Forgot to mention that the bike's ( ie Engine's ) altitude will also have an effect on air/fuel ratio......The higher you go the less air ( Oxygen ) there is and the power output is reduced...

[Revband]

Buy a Triumph , they have a barometric pressure sensor built into the ECU.