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Discussion Starter #1
Subject says it all, i've heard this being a worry of people for years...im a thinker, i try and figure alot of things out and although im reasonably mechanicaly informed and inclined i dont see how that would make you lose power. so fire away...
 

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S-Clusive's Jester
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it just does. if theres not enuf back pressure the exhaust cant flow freely and efficiently out. the exhaust lingers in your exhaust or slowly creeps out the end. sooner or later you'll backfire. if you have a bigger exhaust then your engine can fill with exhaust you'll notice a loss of low end torque. but at high speeds you'll probably notice a difference due to the engine using up more fuel and making more exhaust thus causing lots of back pressure. its not something you really need to think about. someone was talking anbout the venturi theory or something? personally i dont really want to know cuase it will probably make my head hurt more than what i just wrote. im thinking what i said is rite. Anybody? second opinion
 

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S-Clusive's Jester
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i would go to your local high school chemistry or a car shop teacher
 

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Always Dreamin Big
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It's not about the backpressure, it's about the exhaust velocity and the cylinders sacavanging from one another. With too large of a pipe the exuast gases will travel very slowly, which hurts power. As gases leave each cylinder they do so in a "pulse", since the exhaust valve is only open a split second. With the right exhaust velocity each pulse will work to pull gasses from the next cylinder that opens, meaning every cylinder gets a fresher, more complete charge for the next round. If you slow this pulse down you lose a lot of the scavanging effect and therefor leave a lot of old charge in the cylinder, which means the next time if fires the mixture won't be ideal.
 

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it is not a loss of backpressure. this topic has been beaten to death and everybody goes back and forth, but here is my $.02. this is a post that came of a website for a local truck club. the poster is very knowledgable, as is evident...:


"No, you absolutely cannot have too little backpressure. That is one of the biggest and dumbest fallacies that exist in relation to 4 stroke IC engines. Why? I'll tell you.

Pumping Losses The concept of backpressure means that there will be a high pressure area at the exhaust port. When the valve opens, the escaping exhaust gases have to push against that high pressure area. How can a parasitic loss be good for your engine? The only engines that NEED backpressure are various small engine designs, mostly 2 stroke. The problem with them is they have the tendency to be too effective at driving the exhaust out, loosing compression. I assure you that on a big 4 stroke engine such as an SBC you want the exhaust to have the free-est path possible. In fact, it would be ideal to have a low pressure zone at the exhaust port. A low pressure area would help to pull the exhaust gases out. Freeing up more power that your engine would have wasted pushing the gases out...which brings up the next topic beautifully.

Scavenging We've heard this before, but what does it mean in relation to our discussion of exhaust. Easier for me to use an example. So your engine fires and now it's on the way up on the exhaust stroke. Exhaust doesn't come out at a consistent rate, it comes out in pulses. Each pulse is a high pressure area, and as it moves, it leaves alow pressure area behind it. Aha, there's our scavenging. You want that low pressure area to be at it's peak when the exhaust port opens on the next exhaust stroke. Thats another reason why headers make better power than manifolds. besides just flowing better, instead of all the pulses being dumped in a log fighting with each other, the tubular runners allow the exhaust pulses to stay seperate and create a nice low pressure area behind it. This is also where tuned and equal length headers come into play. Tuned headers are sized such that the length of the tube corresponds the speed of the exhaust pulses so that the low pressure area is maximized at certain rpms. No surprise that short headers are better for high rpms than longtubes.
Problems can surface if you use too large of a primary diameter, loss of torque. The morons are quick to spout 'you lost backpressure and thus torque.' Next time you hear that you will smile and know that that person failed physics in high school. The problem with using too large of a primary is this. The exhaust pulse only has so much gas and energy in it. If the tube is too large, the pulse expands to much, losing energy and thus velocity. When it loses velocity, it can potentially stall and stop moving in the tube, or at least slow down. aha! Too large of a header actually CAUSES backpressure, and thus lost power. We feel this power loss as a loss of torque because usually this effect is much more pronounced at low rpms as much less gas is moving. "
 

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footbraker
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If loss of backpressure slows your truck you probably need to do some work on the intake side of things. If you needed the backpressure every pro racer would have to change their exh. setup. And yes, primary tube diameter is an important consideration for every motor.
 

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Discussion Starter #8
the post of bristol makes the most sense to me. i've heard before that that it doesnt matter on a 4 stroke, but i only heard it a couple times, i've heard you'll lose too much back pressure a thousand times...danke
 

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Project: Sabriel
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yeah. 3" is too big for a 4.3L engine, you only need 3" i believe if your hp is over 350-400.
 

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especially stock, not if you had an s/c or turbo
 

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The scavenging also effects your intake charge.Before the exhaust valve seats the intake is opening(overlap).The exhaust creates suction in the intake port drawing fuel in.Too much backpressure and exhaust gas will travel up the runner and pollute your intake charge.Too big of a header and you'll lose the pulse and the pull on the intake.

This is why big cams with tight lobe seperations sound so lumpy they just can't get the exhaust out at idle.But,work great up in the rpm.
 

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tight ass S10
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MuddyS10 said:
yeah. 3" is too big for a 4.3L engine, you only need 3" i believe if your hp is over 350-400. [/QUO


my dads 1990 454ss 1500 chevy full size has a 454ci 245hp 405 footlbs torq and it has 3in all the way back o yeah straight pipesounds mean as hell and its stock pipe just cut mufflers and cats off and put flex pipe in place of them
 

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THE ONE THE ONLY
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the easyest way to explain it is to look at water pump(aka your engine) if you have a huge tube and a small pump not a lot of water is going anywhere and if you have a huge pump and a tiny hose...well you can see why that won't work
 
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