Exhaust Geeks required
Discussion
DannyVTS said:
Mornin' all
I'm looking at getting a new exhaust for my Clio.
My simple question is, do silencers restrict air flow and create back pressure?
Dan
Ok. Exhaust design is a large and complex topic, full of misunderstandings and folklore.I'm looking at getting a new exhaust for my Clio.
My simple question is, do silencers restrict air flow and create back pressure?
Dan
If you fancy some light reading Google David Vizard. He has written some great books and articles on such things
I can only offer a few humble highlights to my level of understanding.
1. An exhaust is an entire sytem, from manifold to exhaust tip. If you want to tune it, then you need to understand it and you need to consider tuning all of it.
2. Exhaust tuning is but one tuning method, and while it can return results on its own, it has a compound affect and when matched with other appropriate modifications can be part of a significant horse power gains and changes in engine character.
3. Back pressure. You ideally do not want this, no matter what you read. What you want is the least back pressure but maximum flow rate and exhaust gas velocity.
The reason people get hung up on back pressure is that affecting it also changes the engines air/fuel ratio. If this is significantly affected then without a retune you can lose performance. But with a retune and you should see a performance increase.
4. Flow rates. David Vizard sugests the ideal flow rate for an exhaust to achieve zero resistance is 2.2cfm. Without checking the figures, I believe a 2" bore pipe would flow sufficiently for a Clio so as to produce zero resistance but maintain suitable exhaust gas flow velocity.
A silencer or muffler can affect flow rate. But many modern exhaust silencer can actually flow sufficiently not to affect flow rate or back pressure. Usually these will be straight through silencers like bullet silencers. Something with a 3" core is likely to be fine on your car.
5. But as I said, an exhaust is an entire system. So optimising the exhaust after the cat will only see small gains at best.
I see little or no point in running a de-cat on a road car. As you'll have to faff about for MoT's and if pulled over for a road side emissions test (I have) you'd fail it.
High performance cats can actually flow sufficiently for little or no exhaust flow resistance. Either a 3" ceramic core (maybe not event that big on a small displacement n/a engine) or metal matrix cats. You can buy performance cats on line off of eBay.com and get them shipped back for less than some cats in the UK.
6. Exhaust tuning isn't just about flow rates though. On n/a engines it is as much about scavenging. This is where one exhaust pulse creates a low pressure area in the next cylinder, which helps yield a higher charge density. To achieve this you will need some appropriate length tubular exhaust headers. In fact in most exhaust systems this is the single biggest area of HP gains.
7. So this gives you several choices.
-If it's just noise you want, then chopping out the stock silencers and replacing with one with less db reduction ones will be fine. A custom exhaust shop would do this for cheap, or you could buy some universal parts from somewhere like Jetex and do it yourself.
-You could buy an off the shelf exhaust. Most are probably ok, will make it sound nicer and if you are not chasing every HP it won't really matter all that much which one you choose, or why you chose it.
-Or have a custom exhaust fabbed up for you. This way you can customise how loud you want it to be and the parts used. Google Powerflow as they have lots of custom exhaust fabbers across the UK.
-If it's performance you want, start looking at tubular exhaust manifolds first, then a system to suit. Not all tubular headers will be equal either, if the primaries are too short scavenging won't occur at usable rpms.
Edited by 300bhp/ton on Tuesday 17th May 11:19
kambites said:
SSBB said:
In before someone says backpressure 'beefs up the torque'.
I have often heard it said that the K-series VVC needs a certain amount of back-pressure to operate properly - that a freer flowing exhaust can actually reduce power and/or torque. I never understood why or how. stabbed rat said:
I think it is as 300bhp/ton says, its a very complex subject, hence why so many different (aftermarket) exhaust manufacturers get such different power gain claims. Whenever I ask my dad about stuff like this it all gets a bit complex too quickly and i'm lost in seconds.
Well I like to think I have reasonable theoretical understanding of piston engines and studied fluid mechanics as part of my degree, but I certainly don't understand. 
stabbed rat said:
They do restrict air flow and I would assume that along with that restricted airflow, back pressure would arise. I should know more about these things, my Dad made after-market and OEM car and bike exhausts for 35 years! 
thanksEdited by stabbed rat on Tuesday 17th May 10:26
kambites said:
stabbed rat said:
I think it is as 300bhp/ton says, its a very complex subject, hence why so many different (aftermarket) exhaust manufacturers get such different power gain claims. Whenever I ask my dad about stuff like this it all gets a bit complex too quickly and i'm lost in seconds.
Well I like to think I have reasonable theoretical understanding of piston engines and studied fluid mechanics as part of my degree, but I certainly don't understand. With the amount of money my Dad's company spent on R&D for each and every exhaust its unsurprising that they charge so much for a full system. Some of the software they had could do and calculate things i didn't think were possible in the real world.
kambites said:
Well I like to think I have reasonable theoretical understanding of piston engines and studied fluid mechanics as part of my degree, but I certainly don't understand.
You need to bait 300bhp/ton into writing a mid-morning essay on it My understanding is that the fast flowing exhaust from one cylinder can drop the pressure at the exhaust ports of another. Think of a venturi/Bernoulli's law (having studied fluid dynamics should help). 300bhp/ton explained to me that is is referred to as 'scavenging'.
I think some people (myself included previously) assume that a larger exhaust system is more free flowing and therefore imparts a lower built up back pressure, without considering the fluid velocity effects that exist at the cylinder head.
Exhaust out means you need more fuel/air in. So a free flowing exhaust will only work effectively if you ballance it out with injector sizing and air flow. Larger throtle body, injector/s, better air filter, head flow, valve size etc. A free flowing exhaust by itself will just make the engine run lean and you'll eventually break something (number two piston usually).
The reason it doesn't work well for VVC and V-Tec engines is the ballance for when the VVC (or V-tec kicks in yo')activates is different to normal running so you can either have an engine that's almost flooding at low rpm or have a leans off at the top end of the rev' scale.
The reason it doesn't work well for VVC and V-Tec engines is the ballance for when the VVC (or V-tec kicks in yo')activates is different to normal running so you can either have an engine that's almost flooding at low rpm or have a leans off at the top end of the rev' scale.
kambites said:
SSBB said:
In before someone says backpressure 'beefs up the torque'.
I have often heard it said that the K-series VVC needs a certain amount of back-pressure to operate properly - that a freer flowing exhaust can actually reduce power and/or torque. I never understood why or how. In short it's all down to two things.
1. Is tuning. Today modern ECU's hide this and alter better for it than carbs did. But reducing back pressure will result in a different a/f ratio. Usually too rich, which reduces performance. Go and re-tune and return or even lean out the a/f a bit from stock and you should see healthy gains.
2. Power curve. The torque an engine makes is largely dictated by it's displacement. Tuning can increase torque on n/a engines, but it's normally of a far smaller degree than HP can be increased by. So what happens is the torque curve gets moved up the rev range. So at higher rpms you'll make more torque (and HP), but at lower rpms you'll now be further down the curve, so less torque and HP.
Poor graph to illustrate:
Suspect you probably know all this already though.
good post 300bhp/ton.Having had a Clio 182 - you might as well start by looking at what is available.
I found that you had :
Renault (lot of money for a steel cat back system)
Quick fit and all others just did not supply
K-Tec - great source for exhausts and other stuff
but you have to ask if you want the added noise..
the one I went for was a bit louder than standard
but some are McDonalds car part loud.
I found that you had :
Renault (lot of money for a steel cat back system)
Quick fit and all others just did not supply
K-Tec - great source for exhausts and other stuff
but you have to ask if you want the added noise..
the one I went for was a bit louder than standard
but some are McDonalds car part loud.
SSBB said:
My understanding is that the fast flowing exhaust from one cylinder can drop the pressure at the exhaust ports of another. Think of a venturi/Bernoulli's law (having studied fluid dynamics should help). 300bhp/ton explained to me that is is referred to as 'scavenging'.
Hmm, yes I can see how artificial restrictions to speed up gas-flow through the sections of the exhaust where gas-flows merge could be beneficial. However that's a world away from the statement "the engine needs back-pressure to operate", which in the case of the VVC is usually applied as an argument for not de-catting Elises. The cat is a long way after any junctions. "backpressure helps performance", the greatest "internet car myth" ever propogated ??
oh yeah, just go and look at all those F1 cars with hugely restrictive exhaust systems........
On a modern N/A road car, the compete exhaust system backpressure (static) is in the order of 30 to 40kPa, usually equally split approx 50/50 between the catalyst and noise limiting devices.
The back box usually has the least influence, as the EGT is lowest at this point.
Competely removing any "downstream" (i.e. post exhaust manifold) components typically increases ultimate (i.e. reoptimised fuel and spark) power by approx 5-10%. (un-optimised as little at 1-3%).
Generally, in the "aftermarket" world, the improvements gained by only back box substitution are less than the "noise" in the measurement (i.e., in-accurate, non repeatable rolling road data cannot actually accurately determine the change in power)
What is important in maintaining the exhaust manifold "tuning" but the losses in a large ("hydraulic diameter") smooth pipe are so small as to be virtually inconsequential, hence F1 engines with long, often stepped, multi pipe headers joined into a single(per bank) short large diameter collector before being terminated in a suitable low pressure area above the rear body work etc.
in a road car, where noise is obviously a significant concern, the dynamic pressure (inculding accoustic energy) has to be returned to as close to "ambient" entropy with as little pressure loss as possible. Here, the "frequency spectrum" of the energy becomes important, and systems that use perforated tube(s) with holes small enough to let acoustic energy escape without affecting the primary inertial exhaust energy to escape work well. (i.e. a high ratio of noise reduction vs dynamic pressure loss)
oh yeah, just go and look at all those F1 cars with hugely restrictive exhaust systems........
On a modern N/A road car, the compete exhaust system backpressure (static) is in the order of 30 to 40kPa, usually equally split approx 50/50 between the catalyst and noise limiting devices.
The back box usually has the least influence, as the EGT is lowest at this point.
Competely removing any "downstream" (i.e. post exhaust manifold) components typically increases ultimate (i.e. reoptimised fuel and spark) power by approx 5-10%. (un-optimised as little at 1-3%).
Generally, in the "aftermarket" world, the improvements gained by only back box substitution are less than the "noise" in the measurement (i.e., in-accurate, non repeatable rolling road data cannot actually accurately determine the change in power)
What is important in maintaining the exhaust manifold "tuning" but the losses in a large ("hydraulic diameter") smooth pipe are so small as to be virtually inconsequential, hence F1 engines with long, often stepped, multi pipe headers joined into a single(per bank) short large diameter collector before being terminated in a suitable low pressure area above the rear body work etc.
in a road car, where noise is obviously a significant concern, the dynamic pressure (inculding accoustic energy) has to be returned to as close to "ambient" entropy with as little pressure loss as possible. Here, the "frequency spectrum" of the energy becomes important, and systems that use perforated tube(s) with holes small enough to let acoustic energy escape without affecting the primary inertial exhaust energy to escape work well. (i.e. a high ratio of noise reduction vs dynamic pressure loss)
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