Stormtrooper

I have been arguing this with a buddy of mine for a while now...

With my Exhaust setup i will most likely be removing my cat entirely, now here is the thing, i have had people say you need "Back Pressure" I Disagree.. from my understanding of it, is all you need in a modern Fuel Injected engine, is proper "Exhaust Velocity" the idea behind it is that it will assist in "Pulling" the exhaust gasses from the cylinder when its correct, rather then the engine having to "Push" the gasses out.







Lets hear what you guys think.

wheel_of_steel

Exhaust gas is not a continuous stream. It's a series of pulses. If you had a psi vs time trace at the tailpipe, it'd jump around from slight boost to partial vacuum many times per second. As these bursts of partial vacuum reach the end of the tailpipe, they are sucked out into the atmosphere: that sucks the whole chain of exhaust pulses along the exhaust pipe. Now if you time it right, the exhaust stream will be sucked out of the exhaust pipe at the same time as the exhaust valve is opening. Bam. You got yourself some free power. Changing the diameter and length of the exhaust pipe will change how quickly exhaust gases flow through the pipe, thus changing how quickly the engine needs to be cycling to take advantage of this little gain. A similar-but-different principal applies to intake manifolds too, but that's a discussion for a different thread.



cliffs: your buddy is an idiot.

Stocker

Your friend needs to read this post: https://www.hyundaiaftermarket.org/f...20#entry666120 and you need to not take mechanical advice from him unless you already know what he says is correct!



Wheels, the diameter and length of the pipes affect scavenging only as far back as the collector. After that the main things are pipe diameter and internal smoothness of the bends. If you have a turbo manifold you will have basically no scavenging effect but you'll have boost so...

wheel_of_steel

I'd say that the exhaust pipe length after the manifold still matters, because the gases leaving the system still have an important effect on scavenging. Although you're correct that the manifold has a much bigger effect on scavenging.

i8acobra

Sport Compact Car busted the "backpressure myth" many, many years ago. They proved that modern cars produce the most power with a large diameter dump tube. However, they conceded that on a street legal car, this isn't possible, so they ended up suggesting a 3" exhaust for any car it will fit under. This is for both turbo and NA engines.

wheel_of_steel

link plox







You would lose power if you just removed the exhaust entirely and had an open block. So some exhaust ducting is necessary, a well tuned piece is best. Winning race cars all tune the ever loving sh*t out of their exhausts.

i8acobra

Link? To a magazine? Come by my house and I'll throw it in your general direction, that's the closest thing I have to a "link".

wheel_of_steel

So when you say modern cars, you mean cars with fancy new-age D-jectronic injection and 8 track hi-fi?

Whatnot

This is true....if by power you mean hp. But does he want a dyno queen that makes 'max power'? Or a fun drivable car or a racing car. Because if it is for anything BUT max power dyno queen, open dump is definatly not the best choice.



And I think I have heard if you have open headers that get really hot, and after you turn your engine off, since there is no massive slowly cooling exhaust, it can cool too quickly and warm/weaken your valves and head. Not positive on that one though.



But mostly what they said above, backpressure is always bad, and this "pulling" you spoke of, was the scaveging they spoke of.







But for N/A there is no perfect solution, you cater it to what you want. If you have a narrow pipe, when you are low rpm and low air flow, you can quickly begin scavenging. When you are scavenging or sucking the exaust out of the engine, the piston doens't have to push it out (or quite as hard anyway..), therefor robbing you or TQ. TQ is the key here. The more backpressure you have, the harder your piston has to push to get that exhaust out, robing that tq from your crank. So backpressure is bad, the more bakcpressure, the less tq to the crank and therefore wheels.



So though a tiny 1" dia exhaust is great for low rpm, you can scavenge, and have more tq, but when you hit high airflow and high rpm, you now have huge backpressure and huge tq loss at high rpm.



But lets say you have a massive 4", when you are at low rpm, this is essentially like an open dump, the airflow cna't even fill the entire diameter and kind of falls out, slowly falling down the pipe and out of it causing turbulence and slight backpressure, but providing no scavenging at all. But at the high rpm, when you have high air flow, you get enough exhaust velocity to begin scavenging, and helping reduce tq loss at high rpm and more tq at high rpm means more power. Which is why what I8 said is correct, for max power, open dump is the way to go...practically speaking anyway.



As also said, a manifold is where scavenging is affected the most. Though with most exhaust head ports are less than 1" dia, you go any bigger than that, and you run into turbelance issues. It is able to exit the exhaust port quickly and eaisily...utill it slowly fills the 2" tubing in which case you get turbulence and backpressure. This is why you don't see any tubular maifolds larger than the exhaust port. This is also why having an exhaust larger than the volume of combined ex man runners volume is unwise. Anything bigger and it is essentially an open dump with turbulence, and you get no scavenging. This is why open dump is max power. Technically if you could scavenge at the high rpm and air flow a massive pipe would be better than open dump, but only if you had a stupid impracticle engine with massive exhaust ports and massive ex man runners into that pipe. Which again is impracticle, and why an open dump will give you max power. Though you aren't getting any scavenging at max rpm, you are getting limited backpressure, as all the piston has to do is push out exhaust through that tiny exhaust port into a free much lower pressure atmoshphere.



This is why headers is the best choice (if money isn't an object), that is diameter and length matched to your largest power curve, generally 4-5krpm.



Then the math gets really complicated when you start talking about nozzles or combiners to go from 4 tiny holes to 1 large hole.



But the only way you are going to get zero backpressure is if you have scavenging to help out, and I am ont sure if that is possible to have enough scavenging to completly cancel out backpressure. Minimizing it is the key, since you can't eliminate it.



But all of that is for N/A engines....





With a turbo, you create 'purposeful' backpressure. Just like a SC uses drag power to make more compression power, a TC uses backpressure to make compression power, just much more efficiently.





I think is was Garett that claims on turbo cars, there is no scavenging behind the turbo, and an open turbine oulet is optimal. I disagree with that.

A turbine cleans up the pulses and puts them so close together basically merging them and making scavenging easier. And the better scavenging you have tbe, the faster/free'er it can spool. But that is just my theory on it. To which I really don't put much thought or effeort or money into, because by far the largest influence on backpressure on a turbo car is the A/R. Find a tiny A/R, you will have huge backpressure and instaspool with even more backpressure and 'choking' it up top. Have a large A/R and your backpressure is minimized at low rpm, though spool will take a while, and at high rpm your bakcpressure will still be minimal, but now you have massive boost to compensante. Focus on a correct turbo sizing rather than exhaust. 3" tbe is more than enough for any power levels below 750whp IMO.







TL;DR- Unless you have a dyno queen, get an exhaust diameter that gives you the most tq in the midrange 3-5krpm.

187sks

I have had a bunch of iterations of exhaust on my car. An open header with a turn down made the most power. Next most was a glasspack on the header collector and a turn down. The current 2-1/4" mandrel bent system doesn't lose much power, but it is enough less that I can feel the difference.