Stock M20B25: 2" exhaust large enough?

[phenryiv1]

I am about the swap out my B27 for a B25. The stock eta exhaust is on the car, but this is not a street car- rallycross only. For that reason, I can run a straight pipe from the factory eta "y" to a muffler, but the stock pipe (after the collector) is only 2" before where I'd remove the cat.

With a stock B25, is a 2" exhaust large enough, or do I need to get a 2x1.75" to 2.25" or 2.5" and go that route?

Car is on a tight budget, so if I can re-use the 2" eta "y" section i'd like to do so.

Thanks!

[nrubenstein]

Worst case scenario, you lose a couple hp.

[phenryiv1]

I was thinking the same. I can always go bigger later. It isn't like the 2" muffler and some bent pipe is that much to lose if I do upgrade in 6 months.

[nando]

Stock is much larger than a single 2"

But it will work until you replace it with something better.

[phenryiv1]

Stock is much larger than a single 2"

But it will work until you replace it with something better.

Stock "I" or stock "E" is larger than 2" all the way back? I searched and the results showed that the eta OEM exhaust is 2" at the exit of the collector pipe. I never measured it myself.

I have a full "I" exhaust but it is heavy and I don"t need the cat. I'd like to use the dual 1.75" stock I downpipes and collect them into a single 2.25" or 2.5" pipe toa muffler and then out the side or back. I guess I could temporarily use a 2" to whatever adapter and go ahead with my permanent muffler & pipe assembly then swap out the downpipes and collector later. Less waste that way.

[nando]

stock I is dual 1 7/8"

single 2.25" is way too small and 2.5 is slightly smaller overall than the twin stock B25 exhaust.

[Bearmw]

My question is whether the stock dual exhaust necks down
in any location enough to make them flow significantly less than if it had all mandrel bends? In other words is it accurate to make exhaust calculations by pipe diameter when the pipes may not maintain that diameter for their entire length?

[nando]

There aren't any crush bends in the stock exhaust that i've seen.