Okay i have a question out there for the Engine builder or exhaust experts.....

I have a 914 with a WRX 2.0 turbo in it, it flys! I am basically making it into a very streetable quiet car. One of the first things i did during my build is put this aftermarket very shiny equal length chrome header on it and took off the stock heavy cast manifold for more power(never tuned or ran with stock).

I had the engine tuned with this header on it, so its perfectly tuned with it. Now.....I have the stock heavy manifold back on with all the heat reflectors,etc to keep the heat in the manfolds......its is Noticeably quieter than with the equal length, lower to the ground, crappy shiny header and... power seems almost just as great.

So....
New heaver manifold before turbo, smaller diameter exit than aftermarket so more backpressure.....do i need any changes on my fuel map? (i have programmable Link storm ecu.)

(I did get one backfire through exhaust....thats really why i am asking)

(I want to know also....)

going back to stock exhaust, your turbo will spool slower. so you'll actually have a bit less air than your fuel trims are set for. So you'll run a bit rich. Not an issue for reliability, not at all. But, you're probably not getting as much power or mileage as it's capable. The biggest change is that if you're running the stock up-pipe (USDM, anyway), there's a cat between the engine and turbo, that really slows down the spool!

The issue is going the other way, if your turbo is spooling quicker, you're going to run lean. and that's when you risk detonation.

thats exactly what i thought. thankyou. I have the JDM version so no cat, and my up pipe is custom made by me, so its huge compared to the exit of the stock manifold.

So it sounds like, actually, since i am running a little rich now, i should pick up a little more mileage too if at tuning day i reduce the fuel trim to match my perfect AFR.

Dyno day here i come, this time its a little easier and cheaper since all the bugs are worked out.

Hey Budman,

What kind'a muffler are you running? Have any sound clips?

I too want to run a live-ably semi-quite system, so I fitted the biggest muffler that would fit my situation - though it's a Magnaflow, so not quite sure how it's going to sound.
My header is sort of a copy of the factory exhaust layout - short headpipes joining together right out of the head, and both sides connected with a Y section, feeding the turbo after that.
I suspect it will sound a lot like your typical WRX with an aftermarket exhaust/muffler.

Jeff

so far just an el cheapo Autozone Factory looking muffler. Not even a turbo muffler. Not sure of the brand really. I bet the magnaflow will sound good. I heard those are good at idle and just off idle, and they are LOUD when you lay it down! which might be perfect!

Ok, so it's not quite as extreme a change compared to a USDM stock exhaust.

But yep, you're on the right track. safe to drive, but some power and mileage to be had by leaning it out a bit.