Cascade Crew

aaron_sK

Paul's comment the other day that a free exhaust system is going to decrease mileage got me thinking.

What I learned from my dad, and other assorted rednecks who taught me about cars was that running an open exhaust, especially headers, is going to increase mileage by way of reducing the effort of the engine on the exhaust stroke. Since you have the same cam, your valve is opening for the same amount of time, and the same amount of exhaust will exit, so you're not creating more HP, you're just increasing efficiency.

However working back-to-back split shifts two days straight ( Mad

) gave me some time to think about this, and it occured to me that if your exhaust was tuned well enough, pressure in the cylinder would drop to the point that more A/F mix would be drawn in on the intake stroke. That in turn would obviously cause more fuel to be used.

Paul, Rodney, is this even close to realistic, or am I way off here?

84transam383 · Banned Joined: 25 Jan 2004 Posts: 431 Location: bend, or

think of an engine as an air pump....

whatever you do to get air/fuel in faster,and more quanity of,you will increase horsepower,same with exhaust,the faster you can get the spent fuel/air out,the more power you make,and the more effiecent the motor becomes.

and i didnt read pauls comment about the mileage reducing,but when i put headers on my ram i saw a solid 1.3 1.5 mpg increase,along with the xpipe and flowmasters

it all varies thoughon a bonestock motor say,an LG4,with the horrible exhaust,you will see in increase in power/mpg no doubt,but on a complete race built motor,youll see tons of power,but probably wont see the mpg increase.

Dewey316 · Posted: Sun May 21, 2006 9:33 pm Post subject:

Yes, the engine has less load on it, when it is more effecient. You would really have to do some test, because even though there is more volume that CAN be filled, it may not be, because there is less load. Really i think it would depend on the circumstances, the driver, the injection system, and how it fuels, etc.

The real question is at light cruise, are the headers actualy more effectiant, since there is less exaust flow, it could be that the stock manifolds are more effecient and scavaging the exaust.

Ok, so the truth is, without testing it out, I don't have a good answer.

BigDaddyVu · Posted: Sun May 21, 2006 11:13 pm Post subject:

everbody knows you need back pressure to create HP and torque equal length headers and a cross over pipe you can always tell by the "scavanging" effect at and just off idle. free flow headers only work on High Compression "redneck blasters" monster trucks and alcohol funnies high revving enigines that go past the 14k rpm mark. I just read that article in car craft mag a couple of months ago.

Xophertony · Posted: Sun May 21, 2006 11:17 pm Post subject:

thats all fine and good on paper. i am sticking with my 3" cat back and headers.

Twilightoptics · Posted: Mon May 22, 2006 8:41 am Post subject:

Dewey316 · Posted: Mon May 22, 2006 8:48 am Post subject:

Twilightoptics · Posted: Mon May 22, 2006 8:49 am Post subject:

aaron_sK · Posted: Mon May 22, 2006 9:39 am Post subject:

Alright. so i wasn't as completely off as I thought. Very Happy

Xophertony · Posted: Mon May 22, 2006 5:51 pm Post subject:

great writeup by urbanhunter44 on TGO about it. i saved the link. knew somone would need it.

http://www.thirdgen.org/techboard/exhaust/338421-more-than-you-ever.html

aaron_sK · Posted: Mon May 22, 2006 7:32 pm Post subject:

Read through that a couple times before. Good stuff, but doesn't touch the mileage question.

wanarunhard · Posted: Sat Jun 24, 2006 12:47 pm Post subject: exhaust

some motors especially stock ones are designed to work with backpressure becuz of valve diameter and injector sizes otherwise they run to leand and burn a valve usally exhaust valves so if you have stock motor then consider this also

QwkTrip · Posted: Sat Jun 24, 2006 5:52 pm Post subject:

There's no such thing as back pressure. The exhaust is going to flow in the direction of high pressure to low pressure. The further downstream you travel in the exhaust system the lower the pressure until you reach atmospheric pressure at the exhaust tips. The more restrictive a zone, the higher the pressure drop across that zone.

Given all other things equal, I would suspect that adding a well designed exhaust system to a car will increase power output and efficiency. You get the best of both worlds. If your fuel economy goes down... then you probably have a poorly matched exhaust system.

Haven't really given it a lot of thought. Just my intuition on the matter.

wanarunhard · Posted: Sat Jun 24, 2006 9:26 pm Post subject: exhaust

then why have exhaust at all just run straight pipes right? then theres no pressure wouldnt that be best? why have tuned headers then like hooker and everybody else does?

QwkTrip · Posted: Sun Jun 25, 2006 10:08 am Post subject: Re: exhaust

Any time you consider air flow think in terms of pressure differential causing air flow, not air flow causing pressure differential. There's going to be a pressure drop between the cylinders and the exhaust outlet (atmospheric pressure) regardless of how the exhaust is constructed. Otherwise there wouldn't be any air flow at all. So there's pressure drop in the straight pipe too. But one type of exhaust isn't the best for all engines. The idea is to use an exhaust system that is best suited to the characteristics of your engine. But that's where things get complicated. If the engine had constant air flow then the answers would be easy to come by. But the camshaft timing and air flow characteristics of the engine add a lot of variables that make it nearly impossible to have one right answer. And if the answers were easy to come by then all exhaust kits would be identical.

Engines create pressure waves that cycle through the exhaust system each time a valve opens and closes. Tuned headers take advantage of these pressure waves to help draw air out of the cylinders. The length of each primary tube is carefully thought out so that as each exhaust valve opens there is a high pressure drop between the cylinder and the exhaust primary tube (pressure in the primary tube is as low as possible). This pressure drop helps evacuate exhaust from the cylinder. If you didn't have tuned headers then it is possible that a pressure wave from one cylinder could travel up another exhaust primary tube and hit the exhaust valve at the same time the exhaust opens. The pressure drop would be less and the exhaust wouldn't flow out of the cylinders so readily.

So what happens when the engine changes speed? The timing of the exhaust pulses changes but the speed at which those pulses travel doesn't change (It will travel at the speed of sound through that medium, and the medium didn't change - it's still exhaust gas.). This will alter the timing at which the exhaust pressure waves hit each of the other exhaust valves. Even tuned headers are only good for a particular engine speed. So again, one answer isn't necessarily good for your engine setup. You can install tuned headers but if those headers were designed to work best between 6000 - 8000 rpm then it's not a good match for your mild street engine.