3" exhaust, N/A Edition
 

Emilio, Andrew, anyone who has spent time around "high" powered n/a BPs:

Have any of you tested 3" exhausts? If so, what were your findings? (Besides probably sounding horrid)

No direct experience but I dont see why it wouldnt make more power and more torque on the whole rpm band. Now if the power difference is enough to offset the extra weight (or even be noticed in the dyno variance) is the question. I'm interested in real numbers.

I am interested in this as well. I know the f20 guys get big gains from going 3 inch wonder if this holds true for the bp as well.

My theory: Won't see much on a stockish/mild motor. On something like yours, i'd suspect real gains.

Interesting point to note, E used to sell a 2.75 hytech exhaust back when they were doing that. His claim was OTS exhausts were too restrictive.

I think @OneTwo ended up with one of those. I'd need to stick calipers on mine, but i'm fairly certain mine is a 2.5" version.

Confirmed I have one. I need to stop being a puss and put it on the car, but I do not want to drive to the track with something that loud.

I want one.

I want more to get an unexpected present and realize that mine is actually a 2.75" on the car already, and may explain why it's so ear-splittingly loud.

I have a 3" exhaust currently on my car. It is not that loud actually, it does however drone like hell. It's connected to a way too big runner (1.875) 4-1 header I made from an ARH vette header.
I don't have any direct comparison between any other exhaust unfortunately.

Can confirm major droning. 2600-3000 is pretty annoying. I'd imagine it's the same way on a 1.8

My car is absurdly loud to begin with, not worried about drone in the slightest. Performance only.

This is not an exaggeration.

dunno if this helps you out at all?

Forgot to say it's just a vibrant 18" resonator and a dynomax 16" long muffler.

I run a 3" straight pipe exhaust. Its loud.

A 3" straight pipe with a short 2.5" section straight after the header merge point makes more power than 2.5" straight through. It also makes lots more noise.

If you are worried about weight use 2mm wall thickness aluminium tube. I ran an aluminum exhaust for 18 months until I crushed the thin walled mandrel bend section bouncing off kerbs. It sounded so much nicer than my current stainless exhaust so I'll go back to it once I up my welding skills. I also found the aluminium V band flange deformed over time. Next time I'll lathe up a custom flange with a larger overlap flange so its held in place.

Dyno? My gut feeling is that you are way past the point of diminishing returns running a 3" exhaust on a normally aspirated car. I'd love to be proven wrong.

I built my car with a 3" so I don't have a before and after. The engine builder I bought my motor from said they spent days on the dyno tweaking yhe exhaust and this combo netted the most amount of power. We might be talking 2-3hp on a 180whp+ engine but thats enough in a race car to justify it.

The headers I'm using have large and long diameter primaries into a 4 to 1 collector with around 300mm of 2.5 pipe before converting to 3". The length of this section is fairly important I believe.

Yeah, if you're making 180HP normally aspirated, I think you've got the flow to justify it. What RPM are you going to? And what displacement? That 2.5" section must be important for scavenging to make the headers work. Interesting.

"A Megaphone is used, especially with a reverse cone, for maximum peak horsepower and power bandwidth. A Megaphone with Reverse Cone has the added dimensions of; taper, Max OD, Outlet OD, and Overall Length."

Burns Stainless LLC Header Collectors | Merge Collectors

There are formulas floating around for exhaust diameter vs bhp. We have found that the BP isn't particularly sensitive to exhaust diameter or design but have not conducted extensive or controlled testing.

A few scattered data points

-Tested a JDM NB1 engine with stock ECU, straight pipe vs Racing Beat midpipe and muffler. Straight pipe was like 120db, the loudest Miata I have ever heard. Made 3whp less than the full RB system.
-Deviate, which we made 300whp running a C30-74 Rotrex at about 10psi, ran through a RB resonated midpipe and muffler (2.375") and was reasonably quiet. I suspect it would have made a few more hp with 2.5" but looking at the plots, I doubt it would have any more with a 3".
-Our NASA enduro cars, Crusher in particular ran a resonated 2.5" with truly massive 2.5" offset I/O muffler in back. No lack of top end power on those and they were quieter than OEM.
- Quiet exhausts are better than a few whp extra and deafness.

My take; 3" is way too big for any N/A BP. We switch to 2.5" from the 2.375" RB only for turbos above 220whp. If it is street driven, I'd stick with the RB or equivalent quiet system even if its 2.375". For a race car, I'd bump to 2.5 above 160whp. In our world, power is almost always capped
so taking a few whp hit to run an exhaust that is quiet and lasts forever. The few extra pounds from a burly exhaust is best added down low anyway, provided you have a class min weight.

We have used and raced on just about every brand of OTS exhaust and many different muffler brands in custom systems. We have seen premature failures on every private label OTS exhaust we have ever had in our shop. Premature in that an RB that was 3 years older was still intact and muffling when the OTS system cracked, blew out its packing, broke internally or something. This is properly installed, not smashed. We have settled on the RB exhausts because they are the easy button. They last just about forever, are quite and don't break. Not the lightest but we learned long ago that chasing light weight in an exhaust only makes sense where min weight is unrestricted in your racing class and the car see very annual race hours.

I have a 94 with the standard VVT/Flatop/MS3/header combo. I have a full 2.5" exhaust with 2 resonators and the giant magnaflow muffler with 2.5" inlet and outlet. Thing has to be at least as quiet as a stock exhaust, if not quieter. I honestly was surprised by it, as "common" saying was 2.5" with naturally aspirated will always be to loud. I am 100% in agreement with you that having a quiet exhaust is worth it and is doable with 2.5".