Infiniti Q50 Forum banner
21 - 28 of 28 Posts

·
Registered
Joined
·
4,585 Posts
Untrue.

Cross section of a 2.5" pipe allows for 509cfm theoretical max flow. A 3.5" pipe allows for 1029cfm, for example.
My comment was a generality. It's significantly easier to run 2 smaller diameter exhaust pipes than a large single and you can get plenty of flow over a large single. Quoting the actual science:
Volume flow rate = π X pressure difference X pipe radius 4 X liquid viscosity / 8 X viscosity X pipe length.

F = πPr4 / 8nl

To put this relationship into words: At a given temperature, flow rate through a tube or pipe is inversely proportional to the length of the tube the viscosity of the liquid. Flow rate is directly proportional to the pressure gradient and the fourth power of the radius of the pipe.
 

·
Registered
Joined
·
829 Posts
Yes, two smaller pipes are easier to route usually. also, are you insinuating what i provided isnt correct? lol. though the number i provided are not perfect, they are close enough to rely on.

regardless, the rule of thumb for the past....i dunno...100 years or so for exhaust diameter tubing size is 2.2 cfm per hp to keep the system under 1psi. 475chp x 2.2 = 1045cfm. divide that by 2 because V6 and you get 523 (522.5)cfm. 2.5" exhaust flows right at 500-530cfm depending on how you wanna use ID or OD.. if youre making much over 450whp, a 2.5" exhaust is a restriction at higher rpm. a single 2.75" pipe will flow around 620-650cfm, which is a restriction for a stock red sport but not really enough to matter.

additionally, a 2.5" pipe has an area of 4.49sq in., 3" is 7.10sq in., 3.5" is 9.60sq in, and 5" (because you seem to think 2x as big only means 2x as much potential?) is 19.62(!)sq in. i cheated and used a calc for those lol. i cant do that kind of math in my head. i kinda think were saying the same thing here ultimately, id also much rather run twin 2.5" vs a single 3.5" under most cars, although its really not an issue with a Q50.

anyhow, long story short: 2.5" duals for < 500chp, 3" duals for > 500chp to about , and no Y pipe bullcrap. they ALL neck down to a single 3" or less.
 

·
Registered
Joined
·
4,585 Posts
Yes, two smaller pipes are easier to route usually. also, are you insinuating what i provided isnt correct? lol. though the number i provided are not perfect, they are close enough to rely on....
No I wasn't, not sure how it came across like that. The math was so someone could calc for themselves.
 

·
Registered
Joined
·
829 Posts
No I wasn't, not sure how it came across like that. The math was so someone could calc for themselves.
Cause nuance is sometimes lost on the internet and we're different people. 🤷‍♂️

Easiest way to solve this issue is to through on a low pressure gauge in place of the rear O2 sensor and do a run. When it goes over about 1.5psi, it's really starting to cost hp/tq.

Plus, with any turbo engine, the largest exhaust that doesn't get you a ticket or that you can stand will always be the best exhaust. There is no such thing as the ideal exhaust size, it's more "does it fit?" and "Will old man Jones down the road call the police if I start it at 6:30am to go to work?". Lol
 

·
Registered
2019 Q50 Luxe
Joined
·
2 Posts
If anyone is interested, I went ahead and got the Tomei for my Q50 and I am pretty happy with it. Sounds good, definitely curious to hear how it will sound with AMS Lower Downpipes which I will find out soon enough because I ordered those as well. If you want to see how it sounds and looks, I made a YouTube video of the install and review of the exhaust.
 

·
Registered
Joined
·
3 Posts
This exhaust will become a restriction on a tuned VR30 at higher rpm. The main problem will be that it relies on the dumbass 3" Y pipe merge that's so common with these systems. None of them are good for a tuned VR30 and only barely adequate for a RS.

That exhaust system pipe will flow a little over 800cfm. 3" flows like 770cfm +/-. A dual 2.5" exhaust pipe will flow a touch over 1000cfm. Neither of those take into account muffler type or bends, both of which reduce flow to varying extents. A 2.5" system starts being a restriction past 500hp at the flywheel.

It does weigh less at least. It will also be loud as hell and most likely sound like bees and rocks in a coffee can at around 3,000rpm.
Will it run efficient with my 3" IRS Y pipe
 
21 - 28 of 28 Posts
Top