Exhaust Scavenging

>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.

>>17683166
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.

>>17683170
>3sgte exhaust
For what purpose?

>>17683185
Converting power into noise.

>>17683202
I thought wider exhausts are quieter tho.

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 anon 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.

>>17683202
Replace your muffler with a hotdog?

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If I took an N/A engine and supercharged it, would it ruin the scavenging of the stock headers?

>>17683208
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.

>>17683226
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.

>>17683237
Well that seems nice.

>>17683135
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

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?

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>>17683135
dude..

>>17683252
>>17683248

>>17683248
Well that depends on the tune matie, map sensors don't always like changes in restriction.

>>17683252
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.

>>17683252
If tuned all de-restriction will increase power as long as you don't mess with pipe dia or length.

>>17683270
Worst case scenario you get a tune to adjust for it. Apparently Bosch ECUs self tune over several hundred kilometres of driving

>>17683135
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
https://www.youtube.com/watch?v=2CYMuzdyUk8

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.

>>17683248
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

>>17683294
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.

>>17683295
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.

Please stop using back pressure and scavenging interchangeably ITT.

>>17683295
That's not how it works on a 4t they don't have expansion chambers like a 2t.

>>17683300
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.

>>17683295
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

>>17683308
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.

>>17683310
see
https://www.youtube.com/watch?v=2CYMuzdyUk8

>>17683322
>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?

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>>17683332
His heart is in the right place, but he's still just not understanding.

>>17683316
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

>>17683332
Too busy watching JAV to read OP.

>>17683364
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.

>>17683223
Yes, but the increased combustion pressure fleeing the valves wouldn't give two fucks about scavenging.

>>17683170
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.

>>17683135

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.

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.

Why isn't it just a sealed surrounding leading into a large single pipe?

Max in, max out?

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Guys, what determines the loudness @ the exhaust tip? Volumetric flow rate? Pressure? Exit speed? I have no clue.