Pressure drop is a function of flow cross sectional area and length. The 2.5" outlet will restrict flow more than a 3" outlet but having a 2.5" pipe all the way back will create more pressure drop. Pressure drop is cumulative so total drop is drop through the outlet + drop through the pipe + drop through the resonator(s) + drop through the muffler. The lowest pressure drop would be direct from the outlet to atmosphere. As you either decrease pipe diameter or increase travel length, pressure drop increases.

 

The lowest pressure drop would be direct from the outlet to atmosphere. As you either decrease pipe diameter or increase travel length, pressure drop increases.

This makes sense to me, however, surely there is little gain to be had by further increasing the exhaust dimensions past that of the smallest diameter pipe used in the system.

The length can't be THAT bigger factor.

If you doubled the size of your garden hose wouldn't you be mostly limited to the flow rate of the tap (aside from the small losses through length, expansion of the hose etc).

A pipe with a diameter of 2.5 inches has an area of 4.91 square inches.

A pipe with a diameter of 3 inches has an area of 7.07 square inches, which is a 44% increase over 2.5inch pipe.

This seems like an unnecessary size increase to outweigh the pressure drop brought in by the systems components.

Do you agree that even larger sized pipe, like 3.5" (which is standard for a lot of aftermarket supra exhausts) is almost redundant if planning to forever stay on stock twins?

3.5" is nearly a 100% area increase over 2.5".

 

i can tell you that 4" exhaust with 2 resonators and a hks super drager tip, no cat, comes in at 94 dB at partial throttle. (with a ported head and cams moving quite a bit of air)

if i were you i would just install some quiet exhaust with a cutout. but that might be lame on stock turbos

i also think your onto something if were talking about stock turbos only.

i used to have stock turbos with decatted 3" downpipe/midpipe and 3.75" catback (hks super drager) and it was below 90 dB FOR SURE

it was so quiet compared to now its insane.

with my new setup ive been pulled over by the police and got a ticket forexhaust too loud, and had TWO neighbours come over and complain that my supra wakes them up when i fire it up in the morning.

so in my experienced opinion i think you will be OK for sound decibal meters with straight 3" decatted and a good catback. preferably one with a silencer tip you can keep in your trunk.

they dont make silencer tips for 5" outlets like my super drager because the catback is so quiet there is never a problem with it being too loud and creating illegal volumes.

however i somehow made mine illegally loud and i cant get a silencer for it now...... go figure.

especially if your running stock turbos in sequential mode. the bypass valve cuts off like half the volume at idle/low throttle so i think it would be damn hard to get the cops really mad at you with a 3" exhaust.

 

Thats some interesting stuff, cheers.

I found out today that exhaust laws have changed since I last modified a cars exhaust.

The law is (paraphrased):

Vehicles registered before 2008 - (which is mine) 95db when measured at a 45degree angle, at least 20cm off the ground and at least 50cm away from the exhaust tip

Vehicles undergoing an engine conversion, if the donor car was fit with catalytic converters the recipient car must have one or more catalytic converters.(bad for me)

How well does a good performance cat flow?

 

Just keep in mind that a 2.5" muffler, resonator or cat will not flow as well as a 2.5" stright pipe. A lot of time you increase the piping diameter of the exhaust simply to mount a larger diameter muffler or cat, there by allowing a 3" muffler or cat to flow as well or better than the 2.5" outlet.

Not that any of this matter. If you simply look at cars making approximately 300hp, they almost always make more hp with a 3" exhaust than with a 2.5" exhaust. Regardless of how theory explains this, this is the reality.

Later, Steve

 

high flow cats work fairly well i believe.

last i heard someone only lost 10hp or so by having a cat.

and hes making 550whp with a gt35r on a 7m engine with a quick spool valve

10hp isnt much considering his exhaust doesnt smell like burnt dog feces and peeled onions and make you cry when its idling.

if you arent required by aircare/emissions to have a cat, which i recall you arent, and neither am i here.. the only time anyone will know is if they force you to have the vehicle inspected.
besides that its basically just a matter of if its too loud you could get nailed.

or i guess maybe if someone recognizes the decatted smell and calls you on it. but i doubt that would/could happen.

then if that time comes you can throw in a test pipe/cat section of pipe.

be thankful of that 94 dB bylaw. around here it used to be 92 dB but they cracked down on motorcycles and made it 84 dB sound limit.
i beleive everywhere in the USA its 92 dB.

decibal rating is not linear so 94 dB is alot louider than 93 dB.
94 dB is pretty damn loud lol.

thats how loud my exhaust is so i can definitely tell you with confidence it will take some big pipes to make a stock turbo supra that loud.