In your last quote, you said if I had read into it i'd know it was a side exhaust issue, which is wasn't, you said right after it was a wheel problem, no fault of the exhaust, had the wheels not been faulty then the exhaust wouldn't matter.

You can run open headers on an EFI motor and be fine, as long as there is an O2 sensor on the headers themselves, but running open heads isn't good if ran like that for an extended period of time.

Also I meant to say that I have ran Magnaflow for 10 years, not my setup. It first came with stock 2.75" pipe with a rotted/gutted cat, 24" Magnaflow and dual 2.5" tails with those tips out back like this:



I didn't like the hollow sound the gutted cat made so I put a pipe in it's place. Then later I wanted it louder so I put in new 3" piping, a Thunderbolt highflow cat and an 18" Magnaflow, and since the tails were rusted and looked like swiss cheese, I had the guy just point them out the side sinc eI was too cheap to have new tails made. It's louder, has barely any drone and I love it. No problems, no power loss and I consistently get 18mpg in the city or better, and that's with a tired old 305.

 

Most OEM exhausts are sized the way they are for maximum efficiency and (key phrase here) LOW END TORQUE. Not stump pulling torque, but enough to get the vehicle moving and into higher speeds where the TCC can engage for maximum fuel economy. Ask anyone, anywhere, from any generation...changing the OEM exhaust diameter more than 1/4" will sacrifice some low end torque, while gaining a few ponies up top. Any dyno will confirm that, I dont care what you run. And since most of us want the best MPG and dont take our trucks to the strip, OEM size is best. The OP clearly wants more power, as he is already up to 270. For him, the best choice would be a large exhaust. See what Im getting at? Its a case by case basis, and anyone who starts putting bolt ons on their vehicle should know that there really is no such thing as free power...go for the high HP mark and low end will suffer.

And Ill probably get flamed for this but Ill say it anyway. Modern engines are designed to have a certain amount of backpressure. This is directly related to the performance of the engine in its stock parameters. In every engine, there is a certain amount of valve overlap (where both the intake and exhaust valves are open simultaneously). Now, you would think thats bad right? Everything coming in from the intake is being dumped right out the exhaust? No. With the proper size exhaust, enough restriction (aka backpressure) is created to help preventing dumping and actually participate in filling the cylinders to their ideal efficiency. Go too large with your pipes, and you lose that restriction. This is why many (not meaning any of you) people get O2 sensor codes after a large exhaust is installed. A great deal of raw fuel is not ever entering the cylinders, its just being dumped out the exhaust, where your O2 sensor sees it as a rich condition. So there is a method to all that, guys.

And side exit is cool to some, not to others. But for our trucks, its illegal. So its not really an issue. Personally, I like it. But for the 1 HP gain that you wont feel, is it worth it?

I really dont see why this was turned into a pissing contest about backpressure and Ford side exit recalls when a guy just wanted to know what everyone thought about his idea. Un-effing-necessary.

 

Changing pipe size alone isn't going to affect the amount of fuel being evacuated from the cylinders before it's burned. That has to do with scavenging which is largly affected by the manifolds. Throw on some headers, especially longtubes and they significantly increase scavenging and then you lose some fuel, in which case you need an ECU tune to compensate. You can also change the cam and timing so there is less overlap, which also makes the most out of the exhaust upgrade as well.

Engines aren't simple to tune to run at optimum efficiency, everything has to be in sync so if you change one thing, more than likely something else must be changed in order for it to work correctly. Going to slightly larger pipes helps reduce restrictions but largely doesn't affect power, it's mainly for sound. Without removing all restrictions anyways just new mufflers and pipes don't do much besides sound, the manifolds and y-pipe are the biggest exhaust restrictions anyways, aside from the stock muffler.

 

Well, youre right about the manifolds. However, when most people upgrade their camshaft, they usually increase the amount of overlap, which is what largely affects idle quality and lowers vacuum. Thats why those big *** V8s have to constantly blip the throttle at stop lights...and also because it sounds badass.

I remember seeing this back when I was in school at Baran. Probably needs to be moved, but since we're on the subject of HP, I thought it was relevant (and fun).

One Top Fuel dragster 500 cubic inch Hemi engine makes more horsepower than the first 4 rows at the Daytona 500.

* Under full throttle, a Top Fuel dragster engine consumes 11.2 gallons of nitro methane per second; a fully loaded 747 consumes jet fuel at the same rate with 25% less energy being produced.

* A stock Dodge 426 Hemi V8 engine cannot produce enough power to drive the dragster's supercharger.

* With 3000 CFM of air being rammed in by the supercharger on overdrive, the fuel mixture is compressed into a near-solid form before ignition. Cylinders run on the verge of hydraulic lock at full throttle.

*At the stoichiometric 1.7:1 air/fuel mixture for nitro methane the flame front temperature measures 7050 degrees F ( 3900 degrees C ).

* Nitromethane burns yellow. The spectacular white flame seen above the stacks at night is raw burning hydrogen, dissociated from atmospheric water vapour by the searing exhaust gases.

* Dual magnetos supply 44 amps to each spark plug. This is the output of an arc welder in each cylinder.

* Spark plug electrodes are totally consumed during a pass. After 1/2 way, the engine is dieseling from compression plus the glow of exhaust valves at 1400 degrees F. The engine can only be shut down by cutting the fuel flow.

* If spark momentarily fails early in the run, unburned nitro builds up in the affected cylinders and then explodes with sufficient force to blow cylinder heads off the block in pieces or split the block in half.

* In order to exceed 300 mph in 4.5 seconds dragsters must accelerate at an average of over 4G's. In order to reach 200 mph well before half-track, the launch acceleration approaches 8G's.

* Dragsters reach over 300 miles per hour before you have completed reading this sentence.

* Top Fuel Engines turn approximately 540 RPM's from light to light! but stop & ponder the fact that the engine is only used for apx 4 to 5 seconds.

* Including the burnout the engine must only survive 900 revolutions under load.

* The red-line is actually quite high at 9500 rpm.

* The Bottom Line; Assuming all the equipment is paid off, the crew worked for free, and for once NOTHING BLOWS UP, each run costs an estimated $1,000.00 per second. The current Top Fuel dragster elapsed time record is 4.441 seconds for the quarter mile 10/05/03, (Tony Shumacher).

The top speed record is 333.00 mph (533 km/h) as measured over the last 66' of the run (09/28/03 Doug Kalitta).

*Putting all of this into perspective:

You are riding the average $25,000 Honda MotoGP bike. Over a mile up the road, a Top Fuel dragster is staged and ready to launch down a quarter mile strip as you pass. You have the advantage of a flying start.

You run the RC211V hard up through the gears and blast across the starting line and past the dragster at an honest 200 mph (293 ft/sec). The tree' goes green for both of you at that moment. The dragster launches and starts after you. You keep your wrist cranked hard, but you hear an incredibly brutal whine that sears your eardrums and within 3 seconds the dragster catches and passes you.

He beats you to the finish line, a quarter mile away from where you just passed him. Think about it, from a standing start, the dragster had spotted you 200 mph and not only caught, but nearly blasted you off the road when he passed you within a mere 1320 foot long race course.

That, folks, is acceleration.

 

Old skool hit it right on the head. The term backpressure is being used incorrectly.

mierze,
Overlap in these engines is not that high to where you would end up with wasted fresh air/fuel blowing out the exhaust, nor is that really a problem with most naturally aspirated engines until you get up into engines designed for high RPM operation where overlap is generally increased. The 4.3L is just not built for turning high RPMs. The heads are too restrictive in and of themselves. Add an artificial lung (turbo or supercharger) and then overlap gets exaggerated and fresh air and fuel do get pumped out the exhaust

Your bit about low end scavenging is true. Exhaust systems designed to increase low end scavenging tend to be smaller than exhaust systems designed for higher rpm scavenging. It is all a balance with the rest of the system (heads, cam, etc). A system built for low end scavenging will have more restriction as the RPMs rise therefore reducing your peak power potential. A system built for high end scavenging will garner as much of a benefit in the lower RPMs affecting low end torque. By that comparison, the restriction isn't necessary, it is a byproduct of the design. There are trade-offs for everything.

 

Ok I was unclear with previous post. Maybe wrong, even. Im not saying anyone of us was right or wrong. But hopefully this will clarify some things.

Backpressure is one of those terms that will almost always be used erroneously, and we just have to accept it. Backpressure does exist, but it has nothing to do with actually exhaust PIPING. Backpressure is the force that the rotating assembly must overcome after the power stroke, when beginning the pistons ascent, but before the exhaust valve actually opens. When a piston hit BDC after a combustion event and begins traveling upwards, the exhaust valve is not yet open. This creates a POSITIVE pressure in the cylinder, known as backpressure. Backpressure causes power loss due to the energy required for the piston to defeat it and continue traveling upward.

Scavenging is the suction of the air/fuel mixture into the combustion chamber by the exhaust from the negative pressure (vacuum) the exhaust creates in the combustion chamber AFTER the exhaust valve opens. ***When all that positive pressure (backpressure) is relieved when the exhaust valve opens, there is a sudden absence of positive pressure, resulting in a vaccuum, which not only clears the spent gases, but also is applied to the intake when the intake valve opens (this occurs at overlap). Whats truly interesting is what I just read. It states that in a Chevy 350, more vaccuum is applied to the intake via the exhaust scavenging process than the downward movement of the piston on the intake stroke. This means that your exhaust provides you with more intake potential than your actual air induction/intake system does!

They key here is velocity. The biggest pipes will diminish your exhaust velocity and decrease scavenging abilities. The best exhaust is one that provides you with opitmum velocity and the most scanvenging. But remember, there is over-scavenging. It is that moment at overlap when the exhaust vaccuum is actually pulling the air/fuel mix directly through the valve train, taking it out of the comustion equation altogether. It happens in all engines, and is a fact of life in four strokes.