BOOOM fire in my exhaust

[papa roached]

posted by JustinB on that board


It all started back in the day when the government started regulating vehicle emissions. The first catalytic converters were incredibly restrictive, and most cars were still runing carbs.

The cats were so restrictive that at low engine rpm, air would sometimes pass through the carb twice! Carbs were then re-jetted to compensate for this - to keep the air from getting a double-dose of fuel.

When you take one of these cars and remove the restrictive cat/exhaust to replace it with a free-flowing system, the stock carb will not work right anymore - you will not be getting enough fuel at lower rpms and will get a loss in torque. The solution - change the carb!

Modern catalytic converters are free-flowing compared to early pellet-filled versions. Modern fuel injection systems are not designed to compensate for super restrictive catlysts. Modern fuel injected cars do not need backpressure.

The truth - exhaust gas velocity

Here's the real deal about choosing an exhaust that is too big. I'm not a physicist, my understanding of fluid dynamics is very limited.... but here's my understanding.

Exhaust gases moving out of your exhaust can be used to pull other exhaust gases out of your cylinders. This is why headers are sized and the tubes matched the way they are. This effect is called scavenging. The faster the gases move, the more you benefit from scavenging.

The larger the tubing, the slower the gases move, the less scavenging occurs. (but, more ultimate flow with less restriction is possible)

The smaller the tubing, the faster the flow. (but obviously, the resistance can quickly overcome the benefit of scavenging if you start reducing your tubing size... a drinking straw simply can't flow enough for a motor)

Notice I didn't say anything about backpressure. Backpressure is just a measure of the restriction in the system. It can be caused by bends, narrow tubing, badly flowing mufflers, etc. Backpressure will be present in any system - but the less that is there the better. A large diameter system with a restrictive muffler (and therefore a high backpressure) will not gain any velocity. So now you will have to worst of both worlds - reduced scavenging and a restriction to free flow at higher rpms.

This is the reason that apexi made the N1 dual. The two pipes can support the flow of a larger pipe, but maintain better gas velocity and scavenging.

Some people claim to lose low end power when strapping on a big bore exhaust. This may be true, but it's because of a decrease in the scavenging effect, not because you don;t have enough backpressure.

[Black Magic]

na engines need backpressure for optimal performance, on turbocharged engines the backpressure is deliverd by the turbo.

Bastian

[RaverChankoMX3]

That's the link I posted Papa R. but that's cool It goes through the same Back Pressure Myth you quoted. Reptition helps the memory. Read this again:

Modern catalytic converters are free-flowing compared to early pellet-filled versions. Modern fuel injection systems are not designed to compensate for super restrictive catlysts. Modern fuel injected cars do not need backpressure.

[johnnyb]

why would you take the cat off in the first place? On our small engines you wont gain enough hp for it to even matter. IMO a hi-flow cat is the best way to go, not only will your emissions be cleaner but you wont lose much hp. Has anyone tested on a dyno an mx with a cat and without. I'm guessing 1 hp increase with it removed, which is pointless on a daily driver. Its not like that 1 hp will get you anywhere faster

[RaverChankoMX3]

Don't be so quick to judge. When my cat was fried, I was offered a test pipe instead because it was 100.00 cheaper. My decision was financial.

[neutral]

na engines need backpressure for optimal performance, on turbocharged engines the backpressure is deliverd by the turbo. Bastian

Not quite. NA & turbocharged engines both require optimal exhaust velocity, not back pressure. Papa Roach's post is dead-on.

Turbo'd engines do not deliver back pressure. They deliver a greater exhaust gas volume at higher velocity than NA engines. That's why turbocharged set-ups benefit from larger bore exhausts - they can fill up that fat piping AND keep it moving at high velocity - a beautiful thing. Backpressure is not needed but high enough exhaust velocity is def a must.

[papa roached]

That's the link I posted Papa R. but that's cool It goes through the same Back Pressure Myth you quoted. Reptition helps the memory. Read this again:

Modern catalytic converters are free-flowing compared to early pellet-filled versions. Modern fuel injection systems are not designed to compensate for super restrictive catlysts. Modern fuel injected cars do not need backpressure.

i said i posted it from that link cause some people dont like to click on to other pages

[hgallegos915]

I did this for finantial situations, my original kat is clogged.. like I said I am going to get another kat..cant compare 10 dlls to 45 -80 dlls or even more for a freeflow at the moment.