Importance of 3inch exhaust for FI
 

Hey All

Sorry again another exhaust question, i did some searching but it always ended up with people's builds and not about the potential power gain.

Fact 1: as we all know the pipes are not true 3inch designs anyhow with the ends tapering down to the stock 2.5inch diameter

Now the question
Just how much of a gain is going from 2.5inch to 3inch apply in the case for Forced Induction.

Reason for asking is, my car is now supercharged Daily Driver

It has in terms of exhaust works, 2.75 headers, 2.75(soon to be 3.0inch BHR) hi flow cat system 100 cel. and Borla exhaust with 3inch piping.

There is a new exhaust system made by A'pexi Japan (so 2.5inch not 3inch diameter) Hybrid Megaphone which is QUIET, and QUIET is what i want, i think i am beginning to get older and the loud exhaust noises is getting a bit of pain, but at the same time i dont want to sacrifice the power as i am chasing new figures soon.

So will i "really" loose that many HP from going a borla (3inch) to a A'pexi (2.5inch)

Input with discussion will be highly appreciated

Isnt that potentiallly dangerous? Just my opinion but dont you need the 3 inch because of the amount of volume throwing through it and a 2.5 could cause a bottleneck and maybe back pressure?

I dont know. I am just throwing out my theories.

pettitracing,com has a good read on this ask them

I would think it'd cost you a lot of power, hell, if we'd got even another 20-30hp out of our N/A engine I'd be considering moving to a 2.75"

PhilipM. Is your 268BHP from a dyno? curious what you got under the hood

See this is the part i am concerned with as well, due to the design of all the exhaust parts there for the rx8, there seems to be always 2 points of bottle neck, which is the 2 ends of the cat section. no matter what diameter of piping, as they are all reduced to 2.5.


Where abouts on their website, had a quick look but couldn't find it

See that is where the issue lies ultimately, from Brettus's post he acheived his 300whp using the greddy turbo system and yet still maintains the original exhaust work (found in the get the most bang from your greddy post)

When the air goes to the back, it is cooler (more dense?) and less speed flow, so will it actually affect the the hp that much?

Mazda spent alot of RnD the exhaust system too, and i believe the compromising point is the 2.5 inch stock, so that leaves me to wonder.

Doing my head in ey

Yeah, from when it was mapped on the old engine, don't know what the new one is putting out yet though.
That's flywheel, obviously :lol:

I found it on Pettit. Its basically talking about the back pressure causing meltdowns and serious engine issues. Im not going to try and find out:)

O.K. your scaring me. Old engine, new engine. What happened and what mods did you have dare I ask?

Cracked a seal on the old one due to trying to finish a rally with a misfire...

Some details Here to try to keep this thread on topic.

thanks, just was curious.

Very good question OP subscribed

more air ou t= more air in. more air out = more power. 3" = more air out than 2.5 inch

My supercharged car is very loud with a 3" system. I have the following, extractors, SMB mid pipe with 100 cell highflow cat and resonator, 3' rear section with resonator, custom cat back exhaust.

Adding the additional resonator and changing from the Hymee exhaust has made it less raspy however, it still needs a lot more done to tone it down.

I am now thinking of splitting the pipe past the mid pipe and running two resonators. This will also allow me to reduce the pipe size down perhaps.

What are your thoughts?

I did not like the SMB mid pipe, i ended up changing it with BHR hi flow cat + resonator combo, as it was quiet enough that i managed to take out the second resonator on my borla.

Hope that helped

Regards
Jason