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Old 04-11-2005, 04:03 AM   #1
windsonian
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Exhaust Efficiency

Note: I've posted this elsewhere, but thought this might have been a more appropriate place for it ... might give me more chance of a response

Ok, here's a question I should know the answer to, and should be able to look up, but I thought I'd probably get better and more concise responses from people here.

I understand how getting a more open, less restrictive exhaust can improve the power of an engine by increasing flow, but what I want to know is whether or not this will increase fuel efficiency?

These are my thoughts and they may be way off:
The exhaust will increase the efficiency of the engine in terms of power produced per amount of fuel burned. But if I'm just driving around the city normally (90% of my driving is to and from work in traffic), would the more open exhaust cause more fuel to be burned each combustion stroke of each piston? Because unless I start driving in a higher gear at lower revs (which isn't going to happen realistically), I'm going to be doing exactly the same number of rpm as with the old exhaust. Therefore the exhaust which causes the least fuel to enter the chamber each stroke would be more efficient.

However, then I thought, well if the engine is getting more power for the amount of fuel burned, then maybe exactly the same power is generated by just using less fuel. This makes sense from the point of view that to keep the car at 60kph in a certain gear (ie constant revs), the amount of torque/power needing to be supplied to the back wheels is just enough to overcome drag, friction and other losses. Any more power and the car will accelerate. And no matter what exhaust is used, a certain gear will have a certain rpm for a certain speed (eg 4th gear 2100rpm at 60km/h - guess). Therefore, to get the same power out of the more open exhaust, the throttle would be more closed, letting LESS fuel into the chamber to achieve the required rpm.

But then, doesn't an engine give a certain power at a certain rpm? If this is so, then you can't just say "I need 2100rpm and 120kW", because the engine might now be giving 135kW at 2100rpm. So closing the throttle would then slow the revs down, even if the power being applied was still sufficient to keep the car at 60km/h.

As you can see, I'm going round in circles. I'm sure this sounds very confusing to someone reading it, but I'd love to understand it more clearly. I'd be more than willing to clarify anything I've said (tried to say) if it helps you to give me the answers I want.

Much obliged ... thanks for reading all the way to here.
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Old 04-11-2005, 04:04 AM   #2
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Quote:
Originally Posted by windsonian
Note: I've posted this elsewhere, but thought this might have been a more appropriate place for it ... might give me more chance of a response

Ok, here's a question I should know the answer to, and should be able to look up, but I thought I'd probably get better and more concise responses from people here.

I understand how getting a more open, less restrictive exhaust can improve the power of an engine by increasing flow, but what I want to know is whether or not this will increase fuel efficiency?

These are my thoughts and they may be way off:
The exhaust will increase the efficiency of the engine in terms of power produced per amount of fuel burned. But if I'm just driving around the city normally (90% of my driving is to and from work in traffic), would the more open exhaust cause more fuel to be burned each combustion stroke of each piston? Because unless I start driving in a higher gear at lower revs (which isn't going to happen realistically), I'm going to be doing exactly the same number of rpm as with the old exhaust. Therefore the exhaust which causes the least fuel to enter the chamber each stroke would be more efficient.

However, then I thought, well if the engine is getting more power for the amount of fuel burned, then maybe exactly the same power is generated by just using less fuel. This makes sense from the point of view that to keep the car at 60kph in a certain gear (ie constant revs), the amount of torque/power needing to be supplied to the back wheels is just enough to overcome drag, friction and other losses. Any more power and the car will accelerate. And no matter what exhaust is used, a certain gear will have a certain rpm for a certain speed (eg 4th gear 2100rpm at 60km/h - guess). Therefore, to get the same power out of the more open exhaust, the throttle would be more closed, letting LESS fuel into the chamber to achieve the required rpm.

But then, doesn't an engine give a certain power at a certain rpm? If this is so, then you can't just say "I need 2100rpm and 120kW", because the engine might now be giving 135kW at 2100rpm. So closing the throttle would then slow the revs down, even if the power being applied was still sufficient to keep the car at 60km/h.

As you can see, I'm going round in circles. I'm sure this sounds very confusing to someone reading it, but I'd love to understand it more clearly. I'd be more than willing to clarify anything I've said (tried to say) if it helps you to give me the answers I want.

Much obliged ... thanks for reading all the way to here.

You lose bottom-end when you go with a bigger exhaust system, but gain more top. Fuel economy also drops, as most driving is done in the bottom-end.
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Old 04-11-2005, 04:08 AM   #3
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oh

care to explain this at all, rockstar?
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Old 04-11-2005, 04:22 AM   #4
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Quote:
Originally Posted by carls47807
care to explain this at all, rockstar?
I see 3 options here, please select the one you intended:

1. You (like me) believe he probably knows what he's talking about, but further, more intricate details would be appreciated. This one means you accept the reply.

2. You accept his reply, but you believe that the information in it is incorrect ... in which case, would YOU care to explain?

3. You are just being annoying by telling him to explain for the simple reason that you think he's right but doesn't know why he's right. If this is the case, you've just flamed someone for poorly replying to a thread that included a question asked by ME. Surely this means that his response needs to answer my question, not yours. This option means you don't accept his reply.... and let's face it, you don't have to, because it's not your thread.
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Old 04-11-2005, 04:23 AM   #5
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Quote:
Originally Posted by 88GrandPrixSE
You lose bottom-end when you go with a bigger exhaust system, but gain more top. Fuel economy also drops, as most driving is done in the bottom-end.
Is it sufficient power/fuel efficiency change over the rev range for anyone to have considered a variable restriction exhaust system?
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Old 04-11-2005, 04:24 AM   #6
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well then i'll take his approach...let me know how this works for you

"well having a bigger exhaust would cause the engine to warm up a lot slower, causing less effeciency in single mile trips"
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Old 04-11-2005, 04:28 AM   #7
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Quote:
Originally Posted by carls47807
well then i'll take his approach...let me know how this works for you

"well having a bigger exhaust would cause the engine to warm up a lot slower, causing less effeciency in single mile trips"
Believe it or not, I know something of what I'm talking about, so you don't need to spell everything out in baby words.

Correct me if I'm wrong, but your reply means the following: A bigger exhaust would increase the airflow through the engine and out the tailpipe. This would increase convective heat transfer, thus maintaining the engine at cooler temperatures for longer.
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Old 04-11-2005, 04:37 AM   #8
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I think this may or may not help but I have always been of the opinion that for maximum exhaust flow you have to calculate the diameter of the individual exhaust ports (lets say for argument's sake each is one inch) add them together and use that number to determine the size of the header collector and exhaust pipes (if you have a v8 then the size of the header collector should be four inches each using the above example). There are other variables such as mufflers and wheather or not you need catalytic converters. Not enough backpressure could result in low torque, but I think that high horse power kind of off-sets that, as in the case of top fuel dragsters that use pipes that turn up right off of the heads (sorry, I can't remember what they are called ).
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Old 04-11-2005, 04:37 AM   #9
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whether that's true or completely bullshit, i don't know. that's what experiments are for. i am just stating you should take grandprix's advice too scientifically. he is by no means a rocket scientist.

http://www.findarticles.com/p/articl...11/ai_n8954917

here's an article i find to have pretty solid, factual, and informative information.

again, the only reason i asked for more info from grand prix is because this is merely a forum. and usually when people make posts you expect them to back up what they say with some sort of description and detail. there is a lot of trash on this forum, as you must know.
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Old 04-11-2005, 04:40 AM   #10
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Quote:
Originally Posted by carls47807
care to explain this at all, rockstar?

Sure.

As you may know, daily driver vehicles depend on back-pressure to perform properly. Let's start with the exhaust manifolds/headers. You have to understand this to why it affects your exhaust the way it does. This is why i didn't want to explain..

As you probably can easily understand, when exhaust is pushed through the headers, it eventually meets into one pipe, or manifolds, it just goes in a straight line, which is not quite as effective as headers, so let's just talk headers. Okay, As the pistons push the exhaust through the headers, it pulses, right? (First cyl fires (pulse #1) second fires (pulse #2) etc) So to work correctly, the pressures of the pulse have to suck eachother along the exhuast system. So the high pressure will attract the low pressures. The initial pulse has the high, the rest has a low, so they in a sort suck eachother along.. sorry, exhaust isn't really my proffession, not the best at explaining it.

Okay, I'll try and make it easier... For the headers, as they turn into one later, they're designed to make the exhaust meet up so the exaust of its predesessor sucks the next one along. So as pulse #1 goes, #2 follows soon after, the high pressure "front" of #2 trailing to low pressure tail of #1. Hope that's easier.

Now as I hope you understand the pressures, you can assume that making a bigger pipe would disrupt these pressures? Right. Right, it does. It makes the exhaust flow more freely, making LESS high pressure, and LESS low pressure, as it's allowed to cover more area, you see?

You can EASILY hurt an engines power by putting a bigger exaust system on it, though you may be able to gain some power if you put headers on it.

Understandeable..?
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Old 04-11-2005, 04:47 AM   #11
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whoa

As you probably can easily understand, when exhaust is pushed through the headers, it eventually meets into one pipe, or manifolds, it just goes in a straight line, which is not quite as effective as headers.

so you are saying headers aren't as effective as headers. perhaps you meant "Exhaust Manifolds".

the rest i suppose is an alright explanation.

This is why a lot of people stick to the "cat back" exhaust customizations. You save the ingenious engineering of exhaust manifolds and partial pressures, but you have more control over back pressure and sound.

like i said that article i posted give a pretty good background.

and lets face it, headers are garbage.
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Old 04-11-2005, 04:50 AM   #12
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Quote:
Originally Posted by carls47807
As you probably can easily understand, when exhaust is pushed through the headers, it eventually meets into one pipe, or manifolds, it just goes in a straight line, which is not quite as effective as headers.

so you are saying headers aren't as effective as headers. perhaps you meant "Exhaust Manifolds".

the rest i suppose is an alright explanation.

This is why a lot of people stick to the "cat back" exhaust customizations. You save the ingenious engineering of exhaust manifolds and partial pressures, but you have more control over back pressure and sound.

like i said that article i posted give a pretty good background.

and lets face it, headers are garbage.

Re-read it, it says manifolds aren't quite as effective as headers.

What do you mean headers are garbage? That's probably the best mod you can do to your exhaust for power.
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Old 04-11-2005, 04:51 AM   #13
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Quote:
Originally Posted by carls47807
and lets face it, headers are garbage.

After reading the article (which was good apart from being for LPG RV's), I thought you'd redeemed yourself in this thread, but now you've just made an outrageous statement without backing it up .... remember, it's a forum, so you need to back up your statements.
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Old 04-11-2005, 04:52 AM   #14
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it was clearly an ironic comical retort. i was making a ridiculous claim without backing it up. like the rockstar.
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Old 04-11-2005, 04:54 AM   #15
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Quote:
Originally Posted by carls47807
it was clearly an ironic comical retort. i was making a ridiculous claim without backing it up. like the rockstar.

And what havent I backed up?
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