Sparky2263
11-16-07, 11:35 PM
Back-pressure, flow ratings, restrictions, etc, blah, blah and blah.
Considering most of us have V6's, all those fancy flow ratings won't mean a lot. If somebody want's to get with the math, that'd be cool. Something like cu. in./2/2 x rpm's x .8/1728? The 2 being duals, the next 2 being a four stroke engine (2 revolutions for 1 exhaust valve opening), .8 being efficiency and 1728 being the number of cubic inches in a cubic foot.
Comes out to a staggering 182 cfm at 6000 rpm's for our 262. But that's just a distraction. More on that later. This was about backpressure.
The year is 1978. My 1974 GMC 1/2 ton just got home from the car lot and I'm headed straight for the hacksaw. 3 hours and 4 blades later, I'm done. Stock exhaust from the y-pipe back is laying on the ground, cherry bomb is installed so it ends right under the passenger seat, and I'm blazing a trail down the street as the neighbors run out in horror and my parents go hide. Massive horsepower and sound increase. Suhweeet.
3 things happened that night. My g/f’s feet got hot (slide over here real close), I saw a glowing red exhaust system for the first time, and I found that even though my truck was louder than Zach’s 429 Ford 1/2 ton, I could not outrun it. Yet.
My first experience with backpressure led to my first experience with carburetors. Yes, I had gained a lot of power but in the process had leaned out the fuel mixture so badly it resembled Chernobyl by the time I got to town. The next few weeks were spent learning carburetors and distributors and I probably doubled my horsepower gain. Not bad for the price of a $10 cherry bomb, 3 hacksaw blades, a $1.99 dist spring kit, and a $7 carb kit. And I still couldn’t outrun Zachs 429 Ford 1/2 ton. Yet.
My $49 Blackjack headers arrive and on they go. Damn, that paint don’t last long with a glowing orange header. So off comes the top of the Quadrajet, again. Go hunt down Zach again. Cool, at least he doesn’t beat me as bad, again. Did I mention the excessive noise ticket for the open headers? I stay out of town for a few days ‘cause I’m really enjoying ripping down the dirt roads with open headers. The power is ungodly (I’m assuming it’s capable of a low 15 second ¼ mile by now) gotta be at least, what, 3-400 hp? ;)
Anyway, g/f decides she’s tired of my loud truck and staying in the woods all the time (I didn’t see the problem). So I went and got me 2 Thrush turbo mufflers. WTF!?! How could I lose so much power? And now I’m blowing black smoke at full throttle!
Backpressure.
Backpressure is bad. Period. End of story. No debate. Think you need “x” amount of backpressure for low end torque? You already got it the minute you bolt anything to the exhaust side of the cylinder head. And I mean anything. All this backpressure crap started in the late 70’s when instructors at automotive schools started teaching about backpressure EGR valves and how ANY modifications to the exhaust would relegate the customers engine to the scrap heap. Some major “Gloom and Doom” prophecies going on here.
Somehow it spread to the armchair racers. Guess a few of ‘em made it to those classes and needed to sound important and words like “low speed torque”, “cam overlap”, and (gasp) "backpressure", made ‘em sound important. Well, by golly, the gospel of backpressure was born and spread like wildfire.
I’ll say it again, BACKPRESSURE IS BAD!
You guys have it easier. We have MAF’s now. Very easy test. Get your exhaust where you can drop it at the y-pipe. Drive to the local track. Make a pass, then, drop the exhaust at the y-pipe. No muff, no cat, nothing. Go make another pass. You’ll pick up between .3-.5 seconds in the ¼ and you’ll notice something else while driving it around the track (besides the folks looking at your raggedy sounding ass). It’s responsive and peppy on the bottom end like it’s never been. Gee, how could that be? Uncle John Cockbite said if I didn’t have a bunch of backpressure, I’d kill the bottom end! Blah, blah, ****in’ blah.
Anyone disagree? Post away………..
;)
Oh yeah, as for flow ratings, I got time slips that say flow ratings ain’t so accurate. We’ll save that for another time.
Considering most of us have V6's, all those fancy flow ratings won't mean a lot. If somebody want's to get with the math, that'd be cool. Something like cu. in./2/2 x rpm's x .8/1728? The 2 being duals, the next 2 being a four stroke engine (2 revolutions for 1 exhaust valve opening), .8 being efficiency and 1728 being the number of cubic inches in a cubic foot.
Comes out to a staggering 182 cfm at 6000 rpm's for our 262. But that's just a distraction. More on that later. This was about backpressure.
The year is 1978. My 1974 GMC 1/2 ton just got home from the car lot and I'm headed straight for the hacksaw. 3 hours and 4 blades later, I'm done. Stock exhaust from the y-pipe back is laying on the ground, cherry bomb is installed so it ends right under the passenger seat, and I'm blazing a trail down the street as the neighbors run out in horror and my parents go hide. Massive horsepower and sound increase. Suhweeet.
3 things happened that night. My g/f’s feet got hot (slide over here real close), I saw a glowing red exhaust system for the first time, and I found that even though my truck was louder than Zach’s 429 Ford 1/2 ton, I could not outrun it. Yet.
My first experience with backpressure led to my first experience with carburetors. Yes, I had gained a lot of power but in the process had leaned out the fuel mixture so badly it resembled Chernobyl by the time I got to town. The next few weeks were spent learning carburetors and distributors and I probably doubled my horsepower gain. Not bad for the price of a $10 cherry bomb, 3 hacksaw blades, a $1.99 dist spring kit, and a $7 carb kit. And I still couldn’t outrun Zachs 429 Ford 1/2 ton. Yet.
My $49 Blackjack headers arrive and on they go. Damn, that paint don’t last long with a glowing orange header. So off comes the top of the Quadrajet, again. Go hunt down Zach again. Cool, at least he doesn’t beat me as bad, again. Did I mention the excessive noise ticket for the open headers? I stay out of town for a few days ‘cause I’m really enjoying ripping down the dirt roads with open headers. The power is ungodly (I’m assuming it’s capable of a low 15 second ¼ mile by now) gotta be at least, what, 3-400 hp? ;)
Anyway, g/f decides she’s tired of my loud truck and staying in the woods all the time (I didn’t see the problem). So I went and got me 2 Thrush turbo mufflers. WTF!?! How could I lose so much power? And now I’m blowing black smoke at full throttle!
Backpressure.
Backpressure is bad. Period. End of story. No debate. Think you need “x” amount of backpressure for low end torque? You already got it the minute you bolt anything to the exhaust side of the cylinder head. And I mean anything. All this backpressure crap started in the late 70’s when instructors at automotive schools started teaching about backpressure EGR valves and how ANY modifications to the exhaust would relegate the customers engine to the scrap heap. Some major “Gloom and Doom” prophecies going on here.
Somehow it spread to the armchair racers. Guess a few of ‘em made it to those classes and needed to sound important and words like “low speed torque”, “cam overlap”, and (gasp) "backpressure", made ‘em sound important. Well, by golly, the gospel of backpressure was born and spread like wildfire.
I’ll say it again, BACKPRESSURE IS BAD!
You guys have it easier. We have MAF’s now. Very easy test. Get your exhaust where you can drop it at the y-pipe. Drive to the local track. Make a pass, then, drop the exhaust at the y-pipe. No muff, no cat, nothing. Go make another pass. You’ll pick up between .3-.5 seconds in the ¼ and you’ll notice something else while driving it around the track (besides the folks looking at your raggedy sounding ass). It’s responsive and peppy on the bottom end like it’s never been. Gee, how could that be? Uncle John Cockbite said if I didn’t have a bunch of backpressure, I’d kill the bottom end! Blah, blah, ****in’ blah.
Anyone disagree? Post away………..
;)
Oh yeah, as for flow ratings, I got time slips that say flow ratings ain’t so accurate. We’ll save that for another time.