Okay, so I watched some videos on youtube and I think I understand exhaust scavenging now. It is inadvisable to enlarge the pipes of the exhaust collector pipes (headers) because the stock diameter was chosen by the manufacturer to achieve a usable torque band.
But why not enlarge the diameter of the down-pipe AFTER the headers? I have a high-revving NA engine and I want it to get a larger diameter down-pipe and exhaust, because it is hard to find exhausts that are as narrower as my stock down-pipe (BEAMS SW20). Would it harm my torque band to choose a fatter exhaust?
>beams sw20 Should have a fairly fucking good exhaust manifold from the factory, be very wary fiddling with the exhaust because they are tuned from the factory for the stock exhaust and the ecu doesn't necessarily adjust well to changes in air restriction (map sensor).
But to answer your question most of the scavenging occurs in the headers and it's not uncommon for an exhaust to expand out with a cone after the final merge collector.
David Clark
Yeah I have no intention of touching tne headers. But after the flex-pipe I would like to transition to a wider exhaust flange so I can use the same exhausts that the 3S-GTE dudes run.
Kevin James
>3sgte exhaust For what purpose?
Jace Reed
Converting power into noise.
Parker Richardson
I thought wider exhausts are quieter tho.
Aiden Lopez
Save your money op. Unless there is a documented restriction on your car then there would be no point changing an exhaust for any reason other than noise.
As user said, headers do the bulk of the scavenging. After that the pipe is generally about an inch and a half wider than the individual runners. On a stock exhaust you can be assured that any restrictions will be from mufflers or bends not from the size of the pipe.
Robert Campbell
Replace your muffler with a hotdog?
Jose Evans
If I took an N/A engine and supercharged it, would it ruin the scavenging of the stock headers?
Nicholas Hill
I'm not trying to make more power though. I just want to go from a single tip exhaust to double tip, to reduce the noise it makes. But living in burgerstan, I can't just go buy a Beams exhaust.
Brody Miller
I don't think a larger turbo exhaust would reduce the noise.... Although it may change the note from a harsh raspy sound to a warmer deeper roar.
Wyatt Hernandez
Well that seems nice.
Ethan Morales
Engine Masters already proved that back pressure or whatever is a meme and that more flow is always better, you make more power throughout the entire rev range
Got race headers for my car, gutted the rest of the exhaust, and de-restricted the intake and my butt dyno tends to agree, though I'll really need numbers to prove it
Carter Evans
Felt like this thread would be a good place to ask. What about back pressure? Considering a software tune would be done afterwards, could switching to a cat converter with a lower cell number be actually detrimental?
Austin Phillips
dude..
Ryder Fisher
...
Henry Gray
Well that depends on the tune matie, map sensors don't always like changes in restriction.
Jace Johnson
I'm not sure I know what your question is (?).
Backpressure is always bad. But engineers haven't found a way to reduce it to zero, so all engines have some degree of backpressure.
Liam Reed
If tuned all de-restriction will increase power as long as you don't mess with pipe dia or length.
Samuel Edwards
Worst case scenario you get a tune to adjust for it. Apparently Bosch ECUs self tune over several hundred kilometres of driving
Luis Thompson
Back pressure is a meme for 4t. You want a free flowing exhaust because there is 5% of the cylinder that needs to be flushed out that is not achieved by the piston. The cam opens both the exhaust and intake valve for a short period of time to flush out the 5% due to emissions they can only do this in a narrow rpm band but if you change the volume of the exhaust it will flush out the 5% the whole rpm range. see youtube.com/watch?v=2CYMuzdyUk8
Xavier Wood
Look at you faggots.
Velocity/flow/wave scavenging/inertial front.
There's a decent amount of maths that can go into headers/exhausts, but truthfully it all relies on the ECU and what you're tuning for/tuned at.
Christopher Moore
To a point, then you run into the issue where the exhaust cools and slows in the pipe and the engine has to activly push it out with the next exhaust pulse and continuing that cycle rather than the last pulse creating a vacuum to suck out the next
That's partially where the back pressure meme of old came in, the other half is dumbass boomers running significantly larger exhaust pipe size in their carbed or early efi engines which run lean because they don't auto compensate for the major increase in flow which means more exhaust gets pushed out and more fresh air taken in but not more fuel because as said above
Jackson Thompson
Meme for 4t. Fluid dynamics aside the 5% of the cylinder head being cleared is more beneficial. Stock exhausts are very good but there designed for noise and emissions.
Jayden Jones
This. I don't see where people get off at the BIGGEST TUBES R BETTERSTEST.
It's like they don't know anything about how gasses work.
Kevin Barnes
Please stop using back pressure and scavenging interchangeably ITT.
Easton Powell
That's not how it works on a 4t they don't have expansion chambers like a 2t.
John Roberts
It's not a meme. Headers can be built to work for a very specific range or to work 'ok' across a wide band. Inertial scavenging is still a very important aspect of design parameters, and they need to be tuned specifically to that engines fuel/flow characteristics.
Fuck your 5% and fuck your 'there'. Especially fuck your fluid dynamics aside, engines are literally a fluid/gas pump.
Jaxson Foster
No one is telling you to run massive 4 inch pipes all the way through
And even when they ran over sized pipes in their test they noticed no drop in power and concluded that it was just a waste of money and made your car marginally heavier
Brody Rogers
Stop posting.
They don't have expansion chambers LIKE a 2t, but they don't rely on scavenging to 'charge' a cylinder with the pressure wave. They still experience it and benefit from it but for the opposite application.
>no mention of scavenging >doesn't seem to cover headers at all Why are you replying to a bloke talking about headers and scavenging with a video talking about exhaust and backpressure?
Sebastian Hughes
His heart is in the right place, but he's still just not understanding.
Gabriel Edwards
Yeah dual 4 inches on a v8 is going to do nothing vs the 3.5 or whatever they tested but I'm talking ricers putting diesel pipes on their Hondas and wondering why their torque drops to the single digits
Alexander Young
Too busy watching JAV to read OP.
Lincoln Ortiz
Inertial scavenging/velocity versus volume.
If the pulse is carrying on like a bawss and the space between the ex valve turns into vacuum, that's great. It's really shit though when it turns from a vacuum into 'ambient' when the ex valve opens and the exhaust gasses are like 'oh shit we missed the party, wait up niggers'.
IMO, I think the biggest problem is people changing out headers/downpipes/mufflers and not getting a dyno/tune done to compensate for the altered pulses fucking up the head flow.
Like if I take the headers off my GSF250 (i-4 250cc screamer), it wont even idle, I can barely get it to tick over WOT.
Alexander Howard
Yes, but the increased combustion pressure fleeing the valves wouldn't give two fucks about scavenging.
Jordan Jackson
Unless you're gonna put tuned extractors on it there's zero point in fucking around with your exhaust. You'll see fuck all gains per $ either way, unless you're just trying to get a nicer sound out of it.
Henry Hill
Find headers with the same diameter of your exhaust manifold?
Get them jet hot coated if you want to keep exhaust velocity meme up.
Even still, there are a lot of variables in place you cant attribute all to one specific vehicle.
Just a thought.
Christopher Bailey
Where you guys getting 5% from? Does everyone here run 20:1 compression engines, good lord. Exhaust scavenging is basically having an exhaust sized so that it creates a vacuum effect on the exhaust side of the exhaust valve, so that when the valve is opened the gases rush from the cylinder side of the exhaust valve to the exhaust side of the exhaust valve and after swept volume is cleared of exhaust vacuum continues to suck out the exhaust gasses from the compression chamber (and if you have dished pistons that volume as well), so that the unswept volume is filled with fresh air/fuel mix instead of exhaust gases.
I'll assume flat top pistons for this example and a 10:1 compression ratio. Say you have a 6.0 chevy, 364ci. That's swept (by the piston in the cylinder) volume and does not take into account the compression chamber, gasket thickness, etc. 10:1 compression means that you're taking 10 parts air/fuel mix and compressing them to 1/10 of that volume, or 10%. Your unswept volume will be 10% of your swept volume, in this case 36.4 ci. With proper exhaust scavenging you are well on your way to making your 364ci engine act like a 400.4 ci engine. Of course you can scale these calculations to any compression ratio and ci that you want.
Julian Howard
Why isn't it just a sealed surrounding leading into a large single pipe?
Max in, max out?
Oliver Russell
Guys, what determines the loudness @ the exhaust tip? Volumetric flow rate? Pressure? Exit speed? I have no clue.
