On the lovefab setup..the kit comes with either a 3inch or 4inch downpipe...but i wanted to run the stock exhaust so that my car would be quiet..is there anyway to keep it quiet with the 3 inch or 4inch downpipe? I already asked cody and he said to get his 3 or 4 inch testpipe to go onto the stock exhaust...if thats the only way how loud would it be?

 

Itll be quieter than stock; turbos are GREAT mufflers In all honesty, I would run the 4" downpipe if you plan on making any serious power. There is no extra work involved and the power gains should be excellent. In all of my spare time( ) Ill try to get a 3" VS. 4" downpipe comparison for all of you.
Also, our 3" exhaust is not very loud at all in conjunction with turbo. MUCH quieter than most of the NA exhausts out there. On the track it almost sounds like a stock S2000, just a bit more "throaty".

 

Test pipe with stock exhaust is not loud.
I have this setup running turbo. Sounds no different then stock.

 

I don't understand why you would want a 4" downpipe though if you are going to use the stock exhaust. Doesn't that defeat the purpose? Cody, if I am looking for 350whp do you think that the 3" DP is large enough? thanks -Michael.

 

Im in the same boat X. I want to keep the car quiet and retain my HKS exhaust. A 4" DP is probably worthless considering the exhaust diameter is 2.36" (I think)

 

Thermal expansion/contraction plays into the huge downpipe idea. I cant claim to fully understand it, but as the hot gasses out of the turbo expand, a larger chamber means less backpressure(because there is more room to expand), which means more power and quicker spool. The gasses will then begin to cool, then contract again.
This is the end of my understanding, Im unsure of how to model or apply this except to bolt it onto the car.
My primary reasoning for recommending the 4" downpipe is for "down the road". Its not "if", but "when" 90% of turbo owners begin playing with turning up the boost. With the 4" downpipe, you have the possibility to extract as much power out of the setup as possible. Thats all

 

This makes sense. I've never really put any thought into it until now. Due to the higher temps immediately after the turbine stage, the piping diameter required to maintain a certain loss coefficient will be larger than further down the system, when the gasses have cooled considerably. Essentially, 3" piping after the turbine is not the same as 3" piping at the muffler. For those of us who wish to retain our stock exhausts (or smaller diameter aftermarket exhausts...HKS), the gains with a 4" DP will not be nearly as noticeable as with larger diameter exhausts. Our bottleneck is most certainly not the DP. With the more free flowing exhausts (3" and up), a 3" DP would be a bottleneck, and therefore should be upsized to permit higher flowrates.