Installed Exhaust now my car feels slow - Page 3 - Zilvia.net Forums | Nissan 240SX (Silvia) and Z (Fairlady) Car Forum

eek · 08-06-2008, 12:33 PM

If back pressure is such a good thing for NA cars, why do alot of the NA hondas run short pipes right off the header or run a e-cut out off the header? Not just hondas in general, but I've seen many cars on the dyno both na and boosted run e-cut outs right after the header or downpipe and get gains.

waltnuts42 · 08-06-2008, 02:41 PM

Those people who are saying we want back pressure are just very confused. It is not that people argue for the smaller exhaust because it creates more back pressure, it is because of the velocities, timing, and strength of the low pressure pulses, which is the technical part of scavenging.

But first an explanation of the exhaust basics so everybody has an idea what we are talking about because a lot of confusion is people having no idea what they are reading and then pass on their god awful interpretation of what they just read.

There are mainly two ways overall, through the theories and purposes of the exhaust, in which incomplete removal of exhaust gases reduce performance.

One is a little thing called volumetric efficiency. This little wonder decreases when residual gases take up space in the cylinder. The other is the higher cylinder pressures from incomplete exhaust blowdown will force burnt gasses into the intake port when it opens.

Blowdown occurs during the period between the exhaust valve opening and bottom dead center. This is where the exhaust gases are expelled from the cylinder, from higher cylinder pressure than the pressure in the exhaust, before the actual exhaust stroke begins (piston starts moving up). The left over gases not discharged during blowdown must be physically "pumped" out of the cylinder during the exhaust stroke, which lowers power due to pumping work.

The pressure wave of the exhaust leaving the cylinder is known as a compression wave. When this wave reaches the end of the header primary tube (the collector) it then expands, creating a suction wave (low pressure). This wave travels back up the primary tube and if tuned right it should just reach the exhaust valve just as the overlap is finished. What this does with the low pressure is evacuates all the spent gases and in turn will also create an effect known as inertia supercharging where it increases the velocity of the intake charge and actually creates higher intake charge pressure in the cylinder than it would be normally be and allows the ability to reach VE over 100% if done right. That, my friends, is scavenging.

Now all this explains how the exhaust system works. If you understand what you just read you should realize why we go after velocity (NOT DIRECTLY VOLUME). Just bigger pipes (more volume) reduces the velocity and velocity values below whatever the particular motor needs will not have strong enough suction wave to completely scavenge the cylinder which hinders the blowdown process and decreases volumetric efficiency.

Now finding the right header length and diameter is how you "tune" your headers. Having the right length times the suction pulse (at its optimum velocity) to reach the exhaust valve at its closing and the diameter which determines velocity and must be correctly sized to utilize all the strength of the suction pulse.

With all of this i mentioned NOTHING about obtaining back pressure and which few before have correctly stated WE DO NOT WANT.
This idea also goes along with the ports which is why if you want a truly good job on your ports you should leave it to the pros. NO exhaust can correct a poor port job.

I tend to explain things in which i understand but i think i did a decent job on this so hopefully this will clear some things up. Any further questions or if you do not understand part of this ask and i will try to explain it in another way or hopefully someone can.

waltnuts42 · 08-06-2008, 02:44 PM

Now i know alot of people are ignorant and lazy and will not read what i wrote because of its length so i am writing a short post for those people to read and go back and read the long post.

mystery_elmo · 08-07-2008, 09:37 PM

Quote:

Originally Posted by BigVinnie

Fuck you Tyler keep this up and I will fire you from work.... You even have a snap shot of one of your gay ass video games as your avatar!!!
I am going to kill your MX6 at the track today.

tsk tsk tsk....
hey that game is not gay!! ur gay! u will not fire me from work cuz im god aka now ur boss.... and no u will not kill my mx-6 cuz its alive and kicking!! unlike ur 240! hahahaha so yea whos ur daddy?!
and ur avatar is gayer then mine!! u actually have a gay person in it!! hahahahahahahaha

s14drifter69 · 03-18-2009, 07:35 PM

make the piping 2-2.5". i have it like that but im going turbo

The Chad · 03-18-2009, 08:22 PM

What inclines people to revive dead threads from the grave? This thread was a fail back in 2008...it is no better now, please close this or at least stop contributing. Thank you.

***S14DB*** · 03-18-2009, 08:46 PM

Quote:

Originally Posted by The Chad

What inclines people to revive dead threads from the grave? This thread was a fail back in 2008...it is no better now, please close this or at least stop contributing. Thank you.

To post shit that has been proven untrue years beforehand...

03-18-2009, 07:35 PM	#65
s14drifter69 Leaky Injector Join Date: Nov 2008 Location: Miami, Fl Age: 36 Posts: 141 Trader Rating: (1) Feedback Score: 1 reviews	make the piping 2-2.5". i have it like that but im going turbo __________________

08-06-2008, 12:33 PM	#61
eek Zilvia FREAK! Join Date: Feb 2004 Location: California Age: 43 Posts: 1,025 Trader Rating: (29) Feedback Score: 29 reviews	If back pressure is such a good thing for NA cars, why do alot of the NA hondas run short pipes right off the header or run a e-cut out off the header? Not just hondas in general, but I've seen many cars on the dyno both na and boosted run e-cut outs right after the header or downpipe and get gains.

08-06-2008, 02:41 PM	#62
waltnuts42 Join Date: Aug 2008 Location: Chicago Age: 35 Posts: 10 Trader Rating: (0) Feedback Score: 0 reviews	Those people who are saying we want back pressure are just very confused. It is not that people argue for the smaller exhaust because it creates more back pressure, it is because of the velocities, timing, and strength of the low pressure pulses, which is the technical part of scavenging. But first an explanation of the exhaust basics so everybody has an idea what we are talking about because a lot of confusion is people having no idea what they are reading and then pass on their god awful interpretation of what they just read. There are mainly two ways overall, through the theories and purposes of the exhaust, in which incomplete removal of exhaust gases reduce performance. One is a little thing called volumetric efficiency. This little wonder decreases when residual gases take up space in the cylinder. The other is the higher cylinder pressures from incomplete exhaust blowdown will force burnt gasses into the intake port when it opens. Blowdown occurs during the period between the exhaust valve opening and bottom dead center. This is where the exhaust gases are expelled from the cylinder, from higher cylinder pressure than the pressure in the exhaust, before the actual exhaust stroke begins (piston starts moving up). The left over gases not discharged during blowdown must be physically "pumped" out of the cylinder during the exhaust stroke, which lowers power due to pumping work. The pressure wave of the exhaust leaving the cylinder is known as a compression wave. When this wave reaches the end of the header primary tube (the collector) it then expands, creating a suction wave (low pressure). This wave travels back up the primary tube and if tuned right it should just reach the exhaust valve just as the overlap is finished. What this does with the low pressure is evacuates all the spent gases and in turn will also create an effect known as inertia supercharging where it increases the velocity of the intake charge and actually creates higher intake charge pressure in the cylinder than it would be normally be and allows the ability to reach VE over 100% if done right. That, my friends, is scavenging. Now all this explains how the exhaust system works. If you understand what you just read you should realize why we go after velocity (NOT DIRECTLY VOLUME). Just bigger pipes (more volume) reduces the velocity and velocity values below whatever the particular motor needs will not have strong enough suction wave to completely scavenge the cylinder which hinders the blowdown process and decreases volumetric efficiency. Now finding the right header length and diameter is how you "tune" your headers. Having the right length times the suction pulse (at its optimum velocity) to reach the exhaust valve at its closing and the diameter which determines velocity and must be correctly sized to utilize all the strength of the suction pulse. With all of this i mentioned NOTHING about obtaining back pressure and which few before have correctly stated WE DO NOT WANT. This idea also goes along with the ports which is why if you want a truly good job on your ports you should leave it to the pros. NO exhaust can correct a poor port job. I tend to explain things in which i understand but i think i did a decent job on this so hopefully this will clear some things up. Any further questions or if you do not understand part of this ask and i will try to explain it in another way or hopefully someone can.

08-06-2008, 02:44 PM	#63
waltnuts42 Join Date: Aug 2008 Location: Chicago Age: 35 Posts: 10 Trader Rating: (0) Feedback Score: 0 reviews	Now i know alot of people are ignorant and lazy and will not read what i wrote because of its length so i am writing a short post for those people to read and go back and read the long post.

03-18-2009, 08:22 PM	#66
The Chad Zilvia Addict Join Date: Feb 2007 Location: Richmond, VA Age: 43 Posts: 943 Trader Rating: (0) Feedback Score: 0 reviews	What inclines people to revive dead threads from the grave? This thread was a fail back in 2008...it is no better now, please close this or at least stop contributing. Thank you.