400 hp RB25DET restriction problems?

Hi guys!

I have a bit of a dilemma.

I am swapping an RB in an MX5 NB.

I am planning it around 400hp and i would like to run a 3 inch exhaust, but a 15-20 cm part of the downpipe has to be 2.5 inch equalent oval.

I have to shrink it down to fit through that hole in the attached picture.

How much would it hurt to have that "small" senction 2.5 inch insead of 3?

If i can help it i would not want to cut the chassis or hammer it. With the 3 inch equalent oval i would have 5-5 mm clearance at all sides which is a bit close to my liking. Best case i can mount the end of the downpipe to the chassis and hope it would never touch, and put a flexible part between the turbo and the restrictive part so the top half of the downpipe could move a bit with the engine.

The manifold is 40mm ID and i have a twin scroll setup.. I am not planning a perfect back presurre-less setup, but i also don't want to be on the high end with it.

Any opinion or experience how much that restriction with the flex pipe would hurt? (i guess i should also mention that the biggest oval flex pipe i could find is also 2.5 inch equalent sooo... you could say that most of the downpipe would be 2.5 instead of 3... also no cat)

Assuming you have access to another car with that engine and a 3" exhaust. Consider building a temporary downpipe with your restriction in it and testing it. Then you will know for sure.

If it costs 20 hp, then you'll need to plan on building a 420hp engine...

Well, all the parts on it are more than capable of even 600HP except my conrods, thats why i want to limit it to a safe 400, so i could up the power and still be okay with it.

I just wonder if it would have some significant effects throughout the power range, spool-up, heat, wastegate management.

There will be heat insulation on it from the exhaust port to the end of the downpipe.

It's still stock cams so im not that worried, but when i saw in real life how small a 2.5 inch equalent oval pipe is, i started to have some second thoughts about it.

I was hoping someone had some similar issues or experience in this area. When im going to make the downpipe, its going to be a final version first try because i have to bend it in every single direction to go around certain things, and since its going to be heat insulated, messing with it after the installation is kind of out of the question.

I think you might be ok but its not optimal for sure. Depends on what turbo, cams and fuel that you are going to run to try to make that power.

Stock cams, 98 octane and and a 3076 gen 2, I played with it and I might be able to fit the 3inch oval if I hammer it here and there to shape, but the flex pipe is still fixed 2.5 inch so I don't know if there's any point suffering with it.

There's a saying that a pipe's diameter is it's smallest point but I don't how much truth is there to it based on the length of the restriction.. well, I might cut the oval and make it 2.75 inch or something and make the diameter change as smooth as possible.

Gonna come back in a couple months to report how much hp I could get out of it on 1.3 bar boost with a safe ignition timing.

I won't be able to measure the back pressure but a Dyno curve could say something to someone

Are you running a divorced wastegate pipe? That would help. Cut and shut oval is probably your best bet. Is there any gains from offsetting the engine and gearbox?