Backpressure: The MYTH and Why It's Wrong
10-14-2011, 06:06 PM
#50
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I have Borla true-dual H-pipe + Berk's HFC and HPS silicone intake tubes and the car gets hot very soon after the spirited driving! And it is getting into "pre-limp" mode, loosing power the more it gets hotter.
So the right tune with the right AFR could solve my problem, couldn't it?
Last edited by harp00n; 10-23-2011 at 06:07 PM.
02-21-2012, 10:38 AM
#51
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Guys, how do you think can it be my issue too?
I have Borla true-dual H-pipe + Berk's HFC and HPS silicone intake tubes and the car gets hot very soon after the spirited driving! And it is getting into "pre-limp" mode, loosing power the more it gets hotter.
So the right tune with the right AFR could solve my problem, couldn't it?
I have Borla true-dual H-pipe + Berk's HFC and HPS silicone intake tubes and the car gets hot very soon after the spirited driving! And it is getting into "pre-limp" mode, loosing power the more it gets hotter.
So the right tune with the right AFR could solve my problem, couldn't it?
10-16-2014, 10:57 PM
#52
Just joined the board and have to comment.
1) It would have been nice to attribute the author.
2) I laughed at the Civic reference. This misunderstanding goes back to early last century, before Mr. Honda had any dreams of building motorcycles.
3) For normally-aspirated engines, the back-pressure myth came about because of the way carburetors work. Fuel is pulled into the air stream by the venturi effect. If you change the mass flow for a given throttle opening by changing the exhaust, you have to rejet to maintain an appropriate differential pressure (dP) across the jet face to pull fuel out of the carb bowl. Reducing back-pressure increases mass flow which increases dP at the jet. While the higher dP will pull more fuel, it's not enough at WFO to maintain stoichiometry. The engine goes lean and burns valves and piston tops. Since most modders didn't understand carbs, including "mechanics", they just aimed to maintain back-pressure because that prevented them from having to figure out how to rejet the carb.
Fuel-injected engines with with MAP sensors (think Honda) suffer the same problem. Higher air flow without changing the fuel map in the ECU causes the same lean running. MAP stands for manifold absolute pressure. The ECU senses the intake pressure and calculates air mass flow using pressure and temperature. Ricers/poor kids manage by running higher fuel pressure regulators. The better solution is a remapped ECU.
Thankfully, Nissan and others decided to use the MAF, mass airflow sensor. Usually a variation of a heated wire. A given mass of air has a given heat capacity. As the air flows past the heated wire, it cools the wire by a given amount for a given mass. As the ECU has to increase power to the wire to maintain its temperature, it looks up the power change correlation to heat loss to determine air mass flow. Most MAF ECUs have maps with broad enough range to manage ambient temperature and humidity changes. These broad maps also accommodate back-pressure changes without hiccup.
BMW among others use/used a flap in the intake air stream to measure mass air flow. As the air flow (mass) increased, the flap would be pushed farther open. The ECU measured the flap position and used that to determine mass rate. With a broad enough ECU map, back-pressure changes were managed just fine.
4) Any change in pipe diameter causes a reflection pulse. These pulses could affect VE if they make it back to the exhaust valve. If Infiniti uses constant diameter exhaust, it behooves you to do the same when purchasing after-market exhausts. The reflected pulses are damped by mufflers and cats, so the problem is less of a concern the closer you get to the exhaust exit.
5) The best possible NA performance would be to exhaust into a hard vacuum.
1) It would have been nice to attribute the author.
2) I laughed at the Civic reference. This misunderstanding goes back to early last century, before Mr. Honda had any dreams of building motorcycles.
3) For normally-aspirated engines, the back-pressure myth came about because of the way carburetors work. Fuel is pulled into the air stream by the venturi effect. If you change the mass flow for a given throttle opening by changing the exhaust, you have to rejet to maintain an appropriate differential pressure (dP) across the jet face to pull fuel out of the carb bowl. Reducing back-pressure increases mass flow which increases dP at the jet. While the higher dP will pull more fuel, it's not enough at WFO to maintain stoichiometry. The engine goes lean and burns valves and piston tops. Since most modders didn't understand carbs, including "mechanics", they just aimed to maintain back-pressure because that prevented them from having to figure out how to rejet the carb.
Fuel-injected engines with with MAP sensors (think Honda) suffer the same problem. Higher air flow without changing the fuel map in the ECU causes the same lean running. MAP stands for manifold absolute pressure. The ECU senses the intake pressure and calculates air mass flow using pressure and temperature. Ricers/poor kids manage by running higher fuel pressure regulators. The better solution is a remapped ECU.
Thankfully, Nissan and others decided to use the MAF, mass airflow sensor. Usually a variation of a heated wire. A given mass of air has a given heat capacity. As the air flows past the heated wire, it cools the wire by a given amount for a given mass. As the ECU has to increase power to the wire to maintain its temperature, it looks up the power change correlation to heat loss to determine air mass flow. Most MAF ECUs have maps with broad enough range to manage ambient temperature and humidity changes. These broad maps also accommodate back-pressure changes without hiccup.
BMW among others use/used a flap in the intake air stream to measure mass air flow. As the air flow (mass) increased, the flap would be pushed farther open. The ECU measured the flap position and used that to determine mass rate. With a broad enough ECU map, back-pressure changes were managed just fine.
4) Any change in pipe diameter causes a reflection pulse. These pulses could affect VE if they make it back to the exhaust valve. If Infiniti uses constant diameter exhaust, it behooves you to do the same when purchasing after-market exhausts. The reflected pulses are damped by mufflers and cats, so the problem is less of a concern the closer you get to the exhaust exit.
5) The best possible NA performance would be to exhaust into a hard vacuum.
The following 2 users liked this post by slartibartfast:
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stickman587 (10-28-2014)
03-28-2016, 05:49 PM
#53
Hopefully I'm in the right thread to post this...
I recently changed converters. The new converter outlet is 2.5" but the stock Y-Pipe inlet is 2.25". I want to change my Y-Pipe to have a 2.5" inlet. The issue encountered is any pre-made pipe I've found with 2.5" in also has a 3" outlet. Would there be any harm in mating the new 3" outlet to the existing mid-pipe? Would it even fit? It is listed as a direct fit on the manufacturers site but I'm trying to be optimal. Thanks!
I recently changed converters. The new converter outlet is 2.5" but the stock Y-Pipe inlet is 2.25". I want to change my Y-Pipe to have a 2.5" inlet. The issue encountered is any pre-made pipe I've found with 2.5" in also has a 3" outlet. Would there be any harm in mating the new 3" outlet to the existing mid-pipe? Would it even fit? It is listed as a direct fit on the manufacturers site but I'm trying to be optimal. Thanks!
The following users liked this post:
el_donks (03-28-2016)
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