Changing side pipes and how that changes engine tune

[lewma]

All, my Type65 coupe ran and was tuned ( HP Tuners ) with the standard Factory Five pipes.

I swapped them out for some quieter pipes ( don't want to get into which brand etc as I don't want this to turn into a bashing thread ).

Took the car for a drive then saw that the engine is roughly 7-8% leaner across the board.

I'm struggling to see why adding muffler would cause the engine to run leaner ?

Anyone got a simple explanation of this ? Backpressure / scavenging / flow etc ? I'm assuming those are the components that affected the tune ?

I expected to re-rune but just wanted to know why lean ?

thanks!
mark

[Skuzzy]

Just thinking out loud for a moment......

A change in scavenging might cause the engine to run richer or leaner. More restrictive exhaust could reduce scavenging which might leave higher pressure in the cylinder after the exhaust valve closes. Scavenging also pulls in the fuel mixture before the piston starts down. Reduce scavenging and less fuel mixture gets pulled in before the exhaust valve closes.

Although I should think this effect would be reduced at lower engine RPM's.

[J R Jones]

Your post suggests you assume quieter is more restrictive. That may not be the case. You would have to do a flow (static pressure) evaluation of both mufflers to know the difference in restriction. You may have picked a winner.
Has the performance changed? Are these mufflers quieter? Again, measured evaluations are required.
BTW straight through (glass pack) mufflers do not attenuate sound like a chambered muffler. In many cases both work well together.
jim

[lewma]

I was told that these pipes reduce sound by increasing backpressure a little. I'm assuming increased backpressure means more restrictive overall.

Too early to tell if performance has changed.
Idling sounds around the same.
Highway driving has less drone.

[Skuzzy]

You are correct J R. I did assume a more restrictive muffler. I am also assuming the engine control unit is the stock GM system and not an aftermarket system.

The thing that still has me scratching my head is this engine is an electronic fuel injected engine. The O2 sensor should have detected a lean condition and adjusted accordingly, unless the adjustment is outside of its resolution.

[lewma]

The O2 sensor should have detected a lean condition and adjusted accordingly

My original post wasn't clear. O2 is detecting lean and adjusting correctly. I'm looking at the STFT+LTFT readings showing that they are adding 7% fuel.

I'm just puzzled why a new side pipe, with more muffler, less air flow, results in a lean condition. I would have expected a rich condition.

Perhaps this new muffler has better air flow ?

[J R Jones]

In many cases a chambered muffler is better at reducing drone. The baffle(s) also send a stronger pulse back to the combustion chamber.

[Skuzzy]

My original post wasn't clear. O2 is detecting lean and adjusting correctly. I'm looking at the STFT+LTFT readings showing that they are adding 7% fuel.

I'm just puzzled why a new side pipe, with more muffler, less air flow, results in a lean condition. I would have expected a rich condition.

Perhaps this new muffler has better air flow ?

The opposite is true. The more the exhaust flow is restricted the leaner the engine may run due to scavenging being suppressed. I am being deliberately simplistic here. With a variable timing valve train the optimal scavenging window can move up and down the RPM range, and can also vary in width. Anything which can impact the velocity and/or mass of the exhaust flow can impact that window.

Augmenting what J R said, that pulse sent back to the combustion chamber can be varied depending on where it starts in the pipe. That will be fixed and will impact a specific RPM range.

[lewma]

The opposite is true. The more the exhaust flow is restricted the leaner the engine may run due to scavenging being suppressed. I am being deliberately simplistic here. With a variable timing valve train the optimal scavenging window can move up and down the RPM range, and can also vary in width. Anything which can impact the velocity and/or mass of the exhaust flow can impact that window.

Augmenting what J R said, that pulse sent back to the combustion chamber can be varied depending on where it starts in the pipe. That will be fixed and will impact a specific RPM range.

Thanks for the info. Great stuff !!

[Dave Tabor]

...adding 7% more fuel...will that make 7% more power?

Dave

[lewma]

...adding 7% more fuel...will that make 75 more power?

Dave

We'll see :D

Now I'll likely need externally mounted fuel tanks to make the run from Lund to Hiko :)

[Skuzzy]

...adding 7% more fuel...will that make 7% more power?

Dave

I think you may be having some fun here, but just in case someone reads it and thinks "WOW! Cool way to increase HP".

No. The change in the ratio is to compensate for the lack of scavenging. Think of scavenging as a way to preload the combustion chamber with an air/fuel mix. This allows the rest of the incoming charge to be leaner. So with less scavenging, the richer the incoming fuel/air mix needs to be to compensate. At the end of it all the fuel/air mix, in the combustion chamber, should be nearly identical. If anything it will need to be a little richer to help with cooling when scavenging is compromised. A richer mix burns a bit slower, so it can reduce power.

How much? Probably not noticeable until you at the limits.

[lewma]

I think you may be having some fun here, but just in case someone reads it and thinks "WOW! Cool way to increase HP".

No. The change in the ratio is to compensate for the lack of scavenging. Think of scavenging as a way to preload the combustion chamber with an air/fuel mix. This allows the rest of the incoming charge to be leaner. So with less scavenging, the richer the incoming fuel/air mix needs to be to compensate. At the end of it all the fuel/air mix, in the combustion chamber, should be nearly identical. If anything it will need to be a little richer to help with cooling when scavenging is compromised. A richer mix burns a bit slower, so it can reduce power.

How much? Probably not noticeable until you at the limits.

Again great info! Learning a ton here :)