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I have a catless (cats replaced with 2.5” high flow resonators) milltek with pigges and am getting the H section changed to an X section.
While I was at the exhaust Shop I asked about changing to a 2.75” pipe size but we don’t get that size over here so it would be either 2.5” or 3”.
I don’t see the value in going from the milltek’s 60mm to a 2.5”(63.5mm) but have been thinking about the 3” increase.
Are there any pros or cons changing from a 2.35” to a 3”?

I would like a JHM exhaust but due to shipping and taxes the cost is more than I am willing to spend.

I have a JHM 93 tune as well.

milltek is basically a standard exhaust with less baffles, 2.5'' is good 3'' is too big and you will loose power, 2.75 has been tested and prooven to have the best flow and gains, if you want the power go 2.75 but it will be louder

btw you can get 70mm or 2.75 pipe on ebay

Do you think there would be any benefit going from the milltek to a 2.5”?

bhprs4 wrote:Do you think there would be any benefit going from the milltek to a 2.5”?

not massive but yes

Just wondering if the 2.75" exhaust is still the optimum if you have forced induction? As there will be more exhaust gasses to expel.

esp_mm-270 wrote:Just wondering if the 2.75" exhaust is still the optimum if you have forced induction? As there will be more exhaust gasses to expel.

With an NA setup exhaust/header pipe diameter is very very important.
Wide pipes obviously have less resistance however due to gas flow physics the gas speed through the pipe will be slower.
Conversely, narrower pipes cause resistance but the gas moves faster.

The gas speed is important as this is a big factor in determining the scavenging effect on the exhaust stroke - you want the exhaust gas moving as fast as possible as this draws in more ait than otherwise the natural vacuum effect of the piston could achieve. It one reason why an engine can be over 100% volumetrically efficient.

So like many things it's a compromise...
The "ideal" depends on what you want. If you want ultimate top end power then you want the pipes just wide enough that at the rpm you are running that the there is no resistance from the exhaust system. However at any rpm below this it's sub-optimal as the pipes will be oversized - there won't be any resistance but the gas speed would be slower reducing the potential scavenging effect.

If you want low end torque then you'd go for smaller pipes and accept at higher rpm the gas will have more trouble getting out of the system.
(as a tangent, if you can't fit equal length headers then you can compensate for length differences by changing the diameter of the pipes to get the same effect - there are some superbikes that use this effect quite pronounced).

For forced induction the the charger (what ever it is) will always be way more effective at increasing volumetric efficiency of the engine than what can be achieved with exhaust gas control. As such, I would expect the exhaust problem to be simpler - "bigger is better" as you just want the gas outta there asap.

Brill and ty Kinda where my simple brain was heading but nice to have some extra info as to why

Just stumbled on a nice article that explains a fair bit.
http://www.popularhotrodding.com/engine ... 505em_exh/

adsgreen wrote:Just stumbled on a nice article that explains a fair bit.
http://www.popularhotrodding.com/engine ... 505em_exh/

Thanks again