Cold Air Intake, Headers, and Exhaust

[TOOSTUBBORN2FAIL]

Originally Posted by JohnnyGrey

This is definitely NOT a myth. An increase in exhaust diameter results in a reduction of exhaust velocity and therefore scavenging. Yes, a larger exhaust will open up your top end because there is a limit to exhaust gas velocity, but when you lose that velocity on the low end, you lose power. The same principle applies to the intake runners.

i think were basically talking more about restrictions here, then using larger pipes, say a mutli baffled muffler vs a straight through glasspack

[99LeCouch]

A smoother warm air intake I can see having benefit on a particularly restrictive OEM intake.

Other breathing mods can hurt fuel economy if you over-size them. Particularly the exhaust. Your truck needs some backpressure at low RPM's to generate torque.

Headers are good if they're sized well. Too big and you lose the low end to benefit the top end.

Nail the sizing and you should see gains.

[IsaacCarlson]

WHY??? I guess i just don't quite understand it.
I know why on a 2 stroke but on a 4?

[Christ]

4 stroke engines don't need back pressure to make torque, that part is a myth. It comes from the fact that people notice that by using smaller diameter pipe, you can increase low-RPM torque.

The reason this actually happens (as supported by fluid dynamics) is that when the pipe's diameter is too large to maintain attached flow inside the pipe, the fluid in the pipe will begin to swirl back on itself, creating it's own form of restriction. A smaller pipe will have higher velocity, and since, when a fluid is moving faster, it is under less pressure, the smaller pipe will have LESS backpressure (provided it is properly sized to allow the fluid to pass efficiently) than the larger pipe.

The term back pressure is used incorrectly in most applications - back pressure is never a good thing, except (there is always an exception) when flow restriction is required to prevent over flow, as in a 2 cycle engine.

Exhaust temperature and volume are the major contributing factors when considering a given diameter. You always tune for slightly larger than the median volume/temperature's required diameter, then tune length for the required RPM, which produces harmonic scavening (by length) and heat/pressure scavenging (by temperature/volume).

[99LeCouch]

That's the most coherent explanation of exhaust sizing I've heard! That makes a lot of sense!

Throw in a term genuinely used for another engine application and there's an instant recipe for confusion.

Isaac, on our cars a ported and polished lower intake manifold is supposed to help out quite a bit all over the rev range. I have one I'm installing in a month. Seeing how I'll be doing a lot of driving this fall and winter, I'll have results to report back.

Some cars come from the factory with adequate exhausts, but junk it up with a restrictive muffler. Often changing the muffler to a less-restrictive one is as good as changing the entire exhaust system. Again, you don't want to over- do the free-flow.

[IsaacCarlson]

or did you do it yourself? I have heard some about the intake on our engines. These things can get you from A to B so fast you won't have tome to do any testing
Can you get pics of it so i can see? me curious...

[Christ]

Intake streams apply the same fluid dynamics as exhaust streams, only smaller... the air hasn't been super heated, thus, has less volume (same or slightly less mass, less volume).

Anything after the TB is where you tune your intake for flow velocity. Here comes a problem, though. If the flow velocity is too high, it could create an influx wave as it passes the valve, increasing pressure at the valve's face, and restricting airflow. (in theory, I've never seen it in practice, but this is one of the reasons that swirl induction came into play on high-speed engines, not excluding the homogeneity (yes, that's a word) of the air and fuel mixture.), so once again, it's best to tune for slightly above median airflow, then tune the length for harmonics. Ideally, you'd want the 1st harmonic frequency (the strongest) to land exactly where your RPM range normally falls while driving (Harmonics only affect a very small range of RPM), which means a very long intake pipe. You can always suffice with the 3rd or 4th frequency, though, and still get a net decrease in pumping losses at whatever your tuned RPM is.

By tuning both the intake and exhaust as a system within each other, and not by themselves, you can optimize your engine for it's best efficiency in a given RPM range.

Remember, though. If you tune just the exhaust, your intake will be improper. If you tune just the intake, your exhaust will then be improper. If you tune one, then the other, they'll work together, but they won't work as well as they could. If you tune BOTH, TOGETHER, they become complimentary to each other.

Of course, if you tune anything and change your BSFC for the better, your gearing will now be incorrect for maximum efficiency, and your engine's overall displacement will be higher than necessary for the same amount of work. Go figure. I guess ya can't win 'em all, right?

[99LeCouch]

I bought it off a guy who did it for a project. You can roll your own with time, patience and a Dremel.

I'll have some A-B data later this fall when it's my turn to drive in the 140-mile roundtrip carpool I'm doing with a classmate. Of course, the car is also getting a pretty major going-over before the B testing, so that will pollute the data.

[JohnnyGrey]

Yes, you cannot think of an exhaust system as static, garden hose type flow. People use the term "backpressure" incorrectly. Backpressure added by say, putting a restriction on the muffler tip will always cause a power loss.

Ever see a bowler fly into the lane after his fingers get stuck in the ball? That's how you should think of exhaust side scavenging. The exhaust pulse compresses hot exhaust gases and sends them out the exhaust port at a high rate of speed. That's the bowler's arm putting energy into the ball.

The next time the exhaust port opens, there's going to be a vacuum right at the valve because the last pulse is still moving and creating a low pressure wave behind it. When the exhaust valve opens, this pulse actually sucks the exhaust gas out of the cylinder and brings in fresh air through the intake port. That's like the bowler being dragged into the lane by the ball. The energy he previously imparted onto the ball is now being returend to his body and he goes flying.

[TOOSTUBBORN2FAIL]

Originally Posted by JohnnyGrey

Yes, you cannot think of an exhaust system as static, garden hose type flow. People use the term "backpressure" incorrectly. Backpressure added by say, putting a restriction on the muffler tip will always cause a power loss.

Ever see a bowler fly into the lane after his fingers get stuck in the ball? That's how you should think of exhaust side scavenging. The exhaust pulse compresses hot exhaust gases and sends them out the exhaust port at a high rate of speed. That's the bowler's arm putting energy into the ball.

The next time the exhaust port opens, there's going to be a vacuum right at the valve because the last pulse is still moving and creating a low pressure wave behind it. When the exhaust valve opens, this pulse actually sucks the exhaust gas out of the cylinder and brings in fresh air through the intake port. That's like the bowler being dragged into the lane by the ball. The energy he previously imparted onto the ball is now being returend to his body and he goes flying.

haha, thats a great way to explain it to other people, ive alwasy had a hard time figuring out how to explain to people how backpressure isnt needed on a car. I'll have to use that one next time