Dear all,

A question which has been at the back of my mind ever since I bought my 405 mi16: WHY is the exhaust manifold 8 to 2 to 1, taken that it's supershort and NOT equal lengths?
Why bother going this no doubt more complex and expensive route and not doing a simple 4-2-1 non-equal?

I'm about to send a query to Peugeot - it just can't stop bothering me...

Hi rodionski,

Don't forget to take the downpipe into consideration too, as it has a centre divider in it & makes the exhaust system actually 2 4-2-1's!

Hi there - fraid you're not quite right.
The 8 runners connect to a 2-fold chamber where from they go straight to the downpipe which is also 2chamber and then in the end part of downpipe they connect to 1 = which makes the system 8-2-1.
But it is the manifold that bothers me - WHY 8 runners?!

WHY NOT

for one - production-wise it's much more complex and expensive.
for two - why do such a complex manifold and not make it equal lengths?

The 309 GTI-16 have the same engine, but another exhaust manifold - they have all equal lengths.
Click to view attachment
The 8-2-1 exhaust was for 405 and BX. I think the 309 exhaust manifold is better, but 405 and 309 have the same power (on paper).

Yes, you are absolutely right. Worth noting that the 309 one yields +1Nm torque at peak though.
And also, almost no difference in power is most likely due to stock intake which is quite restrictive.

Just for more torque in the low RPM .
you know the wight problem and flat diagram torque.

Could it be that the 8 ports were made in order to somehow equal out the flow - say 1 tube of 1st cylinder is shorter, and the 2 tube of the 1st cylinder is longer etc. that could approx make the flow close to equal?