Is bigger always better?

[K2e2vin]

, Hence why I said the header makes most of the difference. Though, I've seen smaller suggested(for D-series, around 2" seems to be good, something not too much larger than the runners). I'll measure my Bisimoto header later and see what it is on there(though it has a 2.5" exit).

If you think about it, the gasses in the exhaust(after the header) should have minimal pulses if the header is set up to time the exhaust pulses correctly. In other words, the pulses should be so close together after the header that it would really be just a continuous flow.

 

[badbreedchiney]

so having a 2" 4-2-1 header straight to the cat would be the best choice and afterwards i can go up to 2.5" straight out without any powerloss at low and high rpm then right? it wouldn't have any effect on performance at that point out?

 

[K2e2vin]

Shouldn't.

Just to clarify we're actually talking about two parts of the merge collector; the throat and the outlet. Consensus here, from a performance standpoint, a smaller throat diameter expanding into a larger exhaust is optimal. This is all concerning the sizing on the header/merge collector itself.

 

[pooksy64]

With N/A builds, Too big of an exhaust can hurt cylinder scavenging. Exhaust "pulses", and if you have the right size exhaust and a well designed header, the pulse from one cylinder will actually help suck the exhaust out of the next cylinder when the valves open. Too big and you lose all scavenging, too small and you create a bunch of pressure, which is bad.

You want to aim for both a high flow AND high velocity exhaust system.

With turbo builds, from what limited knowlege I have of turbo stuff, Bigger is just plain better. Up to a certain point, then it doesn't matter.

well said

basically you still need back pressure frm the exhaust pipe. If your turbocharged you want to go bigger more free flow exhaust turbocharger cannot have much back pressure. or you will back up the exhaust stream overheating the turbo

 

[SiCRX88]

2.25 at most. & keep your cat-converter, must have back-pressure for n/a

 

[K2e2vin]

That is absolutely WRONG. You never want back pressure. Any time you have back pressure the engine has to work to push it out.

Do not confuse scavenging/velocity for back pressure.

 

[phunky.buddha]

That is absolutely WRONG. You never want back pressure. Any time you have back pressure the engine has to work to push it out.

Do not confuse scavenging/velocity for back pressure.

 

[SiCRX88]

Maybe I did? I didn't mean back pressure as in you have a fully built motor w/ stock piping, not that kinda back pressure; but back pressure from a cat converter oppose to straight pipping, maybe i'm confusing that w/ "scavenging/velocity", if so, i stand corrected haha.

 

[caboose]

2.25 at most. & keep your cat-converter, must have back-pressure for n/a

myth.

 

[pooksy64]

u do need back pressure from the exhuast. you need some resistance were not talking back pressure as being 5psi. its very minimal. snap-on sells a back pressure gauge. you slip into the tailpipe. @ idle it reads amountt of flow

 

[pooksy64]

"A running gas or diesel engine produces a lot of exhaust and heat that needs to be removed from the engine to allow it to run properly. A blockage of exhaust in the engine can cause a back flow of exhaust into the engine, causing the vehicle to be sluggish. Some back pressure is needed in an engine and the catalytic converter and muffler provide that; too much back pressure, however, can cause problems. Measuring the back pressure is fairly easy and requires a few tools." courtsy of YAHOO car info. we have the same description but more detail off the nissan support manuel

 

[K2e2vin]

Read up on teamintegra, they have a couple of advanced exhaust articles.

As stated before, do not confuse back pressure with velocity. There's exhaust pressure that comes from the exhaust pulse, but this is not the same as backing up/blocking the exhaust to increase this pressure.

If that statement is true, why do race cars, who's goal is to try to extract every bit of power they can, lack a catalytic converter and muffler(most drag cars run open header/megaphones)? Even racing mufflers advertise high flow for a given diameter and usually feature "free flow" or straight-through designs.

 

[pooksy64]

1.race motors do not go through emissions. Ther for thats why they delete the cat("straight throught design"/ "free flow").
2 Race motors are flowing alot more than stock engines (compression). Whether it be forced induction, or a n/a motor @ a (higher compression ratio).
**I was refering to stock engines. Because of the fact that most ppl on here(honda swap) use there cars on the street and are not running majorly bored or stroked motors on C16 race fuel. Yes race motors drag/track/rally etc. want the most out of the engine perfomance wise @ WOT. They dont care how the engine performs @ idle or cruise speeds.

Stock engines have a more round about in drivability. The backpressure keeps the exhaust valves running @ a cooler temp roughly around 1100-1300F temp (this temp is taken @ exhaust port)depending on the engine. Different lay outs of stroke/bore/crank=compression ratio can change this.

EX. if you were to run "open header" and dump the exhaust out before the cat. You will over heat the exhaust valves. This will not happen right after you do this (disconnect exhaust), but overtime with a constant higher exhaust temp. The valves will start to warp, causing "ticking sound". Scavenging does help keep the flow of exhaust from building up @ the exhaust port (hence EGT- Exaust gas temps) thats what an EGT guage is for, so you can watch EGT's on a live reading while the engine is running @ WOT

 

[K2e2vin]

What do you think the whole argument is about? If you're looking for the best "stock" power, keep your shit stock. Most of the people are looking to tweak their cars, hence why people get their engines tuned and even swapping out their stock motors for more power. A street car does not mean your engine has to be stock.

Also, you do NOT need an overly built motor to take advantage of a better exhaust system. Even stock engines can see some gains up top with a freer flowing exhaust.

Warped exhaust valve? Really?

EGT is not for back-pressure or scavenging tuning, it's use for similar purposes a AFR monitor is used; though most commonly used for individual cylinder tuning/trim. Tell me, how exactly can you figure out the exhaust flow through EGT? EGT changes with AFR and timing.

Fact is, as stated previously before, back pressure is bad; do not confuse this for velocity or exhaust pulse. Here's a pic I took from TeamIntegra(definitely check it out!); that illustrates the exhaust pulse we're talking about:

 

[pooksy64]

LoL