I4 with Milltek, hp bump?

[Andrew25]

I’m sorry if this has already been discussed somewhere in the forums, i did a search but could not find anything.
In this video, the youtuber says that the i4 emira equipped with the Milltek exhaust has a power boost of around 10%.
I’m no expert so this might sound dumb to some of you but…is this actually true? Can anyone give some evidence? I checked the Milltek website but there’s no hint about it. 10% on the ‘24 i4 means nearly 400hp with “just” (…) the new exhaust.

 

[geoffbland]

When I spoke to Milltek (although about something else) we did briefly touch on this. The guy I spoke to said there is a performance boost from removing the cat' and having the valves open but it was not something they want to guarantee and may vary from car to car so they will not state any figures on this. 10% sounds too high to me, I would expect just a few percent. I guess you need to find someone that has dyno'ed this mod.

 

[docron]

highly doubt 10% power gain...maybe 10hp gain (if that)

 

[ManuelB]

The i4 Emira has a GPF in the exhaust, so there’s no way a cat-back alone gives anywhere near 10% more power. That’s seriously optimistic. Real gains will only come once it’s possible to remove the DPF and tune the ECU (which isn’t doable yet because of the MG1). Until then, the exhaust is mostly about sound, not power.

 

[Emira Fan]

[the exhaust is mostly about sound, not power.]​

Well said, and I agree. With the stock exhaust, and exhaust valve fully open it's basically an open system after main Cat off turbo.​

 

[Kaz]

In my US I4, Miltek valve (open) with an exhaust and hi flow cat - we don't have a separate GPF: the exhaust I agree with both above the exhaust is for sound which is great, deeper, a little louder. However, once I added the sports cat/dp throttle response opened up and even louder. Butt dyno says more power but it's probably just the torque curve moving slightly to the left and just being more responsive. Do both!

 

[ManuelB]

In my US I4, Miltek valve (open) with an exhaust and hi flow cat - we don't have a separate GPF: the exhaust I agree with both above the exhaust is for sound which is great, deeper, a little louder. However, once I added the sports cat/dp throttle response opened up and even louder. Butt dyno says more power but it's probably just the torque curve moving slightly to the left and just being more responsive. Do both!

Absolutely, makes total sense. Over here in Europe, we’re stuck with the GPF right after the turbo, which kills most of the sound and seriously restricts the flow—even with a hi-flow cat or aftermarket exhaust, the GPF is the biggest limiter for both sound and throttle response.

Honestly, if we could retrofit a US-spec ECU, ditch the GPF, and unlock the full 400hp, it would be a game-changer. Throttle would feel way sharper, and the car would finally sound like it should. Over here it’s more about fighting restrictions than actually enjoying the mods… but we keep pushing anyway!

 

[Kaz]

your GPF is between the turbo and cat? ! I think usually in the US in other cars the GPF is the last in the sequence, and usually does NOT have a sensor on it. All we have here in the states for the I4 is the cat, but it's huge and my shop suspects the unit contains and cat and a GPF...maybe. I just know moving to hi flow really opened up the breathing. Have you tried the TDI unit for tuning the I4?

 

[Andrew25]

Indeed here in EU is more about fighting restrictions, i agree. And in the meanwhile, i’m fighting against an incoherent evergrowing number of trucks that produce way more CO2 than a small bunch of sports car here and there.
That 10% bump sounded so unrealistic to me, everyone would be swapping their exhaust if that was true.
Unfortunately, taking the risk of voiding the warranty and paying huge fines (or even car seizure!) to actually upgrade the car (and then being forced to retrofit the stock parts) makes it so much worthless.
What a shame EU.