Truenotch

Muffler Tech

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The velocity matters as much as flow, so a 1.5lt engine spinning at 10,000rpm will flow the same as a 3lt engine spinning at 5,000rpm. However, the velocity or the gasses will be alot higher, so the temperature will be higher, so the exhaust will have to larger than merely equivalent to a 3lt exhaust because it has to accommodate both the velocity and the increased tempature

10000rpm in a 1.5l engine is 125L/s air flow

5000rpm in a 3l engine is 125L/s air flow

Flow rate Q = Velocity x Area of pipe

There is no difference to the gas velocities under either condition as long as the pipe is the same size.

You have just failed High School Physics

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markku keep the straight through muffler as far back as possible if exhaust is coming from rear of car .trouble with side pipes no room under neath and run on the kerbs will rip it off

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is it essential to have a muffler?

 

i got a flowmaster ish style jobbie mid ships then probly a meter and half of pipe after that dumps  behind rear wheel, i put a coby in half way to try quieten it down and i think its actually louder.

 

im thinking i will get a new system made meyby keep the flowmastery thing and get a muffler on the rear (loose my pride and joy side exit)

 

want to get quiets now, kinda draw less attention because its proper noisy atm

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Epic thread.

Stuffed around with this stuff years ago when I built a 351 Clevo for track days, and what we found with hours to spend on a dyno (mates workshop so it was free) there is in fact an ideal exhaust size for any given tune.

With the 351 fitted with road cams it was around 1 3/4" duals and a balance tube with tri y headers.

With it fitted with larger cams and 4-1 headers 2" duals worked best.

As soon as we went bigger we lost power.

Only thing we could put it down to was pipe temps and inner surface area of the pipes affect flow so a larger pipe not only looses temperature but the larger surface area restricts flow due to greater friction.

I do know this sounds stupid but maintaining even exhaust temps right to the end works.

Many dyno sheets bear this out.

Example. A Harley with open mild steel pipes will produce "X" horse power.

Wrap the pipes with header tape so the temp remains as consistent as possible for the full length of the exhaust and the horse power goes up.

I've seen this myself with a VG Valiant when the complete exhaust was HP coated from engine to rear bumper, it does the same thing it keeps the exhaust temps from changing at every restriction, turn or muffler making it flow more evenly.

 

What you have to remember is as gasses change temp their density changes which can affect velocities as well in the same way area does.

Good flow is not just affected by pipe size and volume but by the changing temperature as well, start doing this 3 or 4 times with a system and the flow goes to shit.

 

Another thing we found regarding the sound was get the first reso as close to the headers as possible to remove the harsh raspy engine noise and using mufflers one size bigger than the pipe helped both for flow and sound.

We actually cut the flange of and welded the reso straight onto the collector with the header end of the reso reversed and the gap filled with a bit of pipe the same OD as the reso.

 

For N/A engines Coby made a reso with a small chamber at the entry end and that helped with the sound reduction making it possible to just run a couple of them removing the need to fit a back flow muffler in the system but I don't know if they make them any more.

 

For power the answer is as little restriction as possible, a straight pipe in a stretched megaphone shape and as short as you can achieve if sound regs at the track allow, look at a moden NASCAR for an example, they run a flattened megaphone exiting the side of the car.

For a deep rich N/A sound my solution is 1st reso short and as close to the header as you can get it, 2nd reso at mid pipe and as long as you can fit then a Magnaflow type at the rear with a short pipe added.

For a turbo 1st reso at mid point and as long as you can fit then another short reso at the rear again with a short bit if pipe.

All use mandrel bends at the pipe size and reso's / mufflers 1 size bigger than the pipe and as few turns at as large a radius as you can fit. this will maintain flow speeds and keep the temp changes to a minimum meaning the velocity will stay as constant as possible.

At least that's what nearly 40 years of spannering on all sorts of cars has taught me.

 

One thing I have read that doesn't make any sense is the guys that say exhausts have no affect in helping the engine to draw air in, this is total crap, you only need to look at a two stroke with an expansion chamber to work that out, it may not affect 4 strokes to the same degree but it does have an affect.

It's called scavenging.

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One thing I have read that doesn't make any sense is the guys that say exhausts have no affect in helping the engine to draw air in, this is total crap, you only need to look at a two stroke with an expansion chamber to work that out, it may not affect 4 strokes to the same degree but it does have an affect.

It's called scavenging.

 

Yeah, depends on how the cams are set up. Setting up the pulses to match intake and exhaust opening times.

 

If anyone is super keen on this, the book http://books.google.co.nz/books/about/The_scientific_design_of_exhaust_and_int.html?id=oIZTAAAAMAAJ&redir_esc=y has been the biz for many many years, and explains many of the principles.

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One thing I have read that doesn't make any sense is the guys that say exhausts have no affect in helping the engine to draw air in, this is total crap, you only need to look at a two stroke with an expansion chamber to work that out, it may not affect 4 strokes to the same degree but it does have an affect.

It's called scavenging.

 

Yes, and your headers should do all the scavenging if they're designed properly, making the pipe size after the collector irrelevant / normally a hinderance. 

 

I'm going to sound like Bradspencer here, but BTCC 2litre N/A cars had 3" exhausts for a reason. 

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It doesn't matter what the engine is much at all, you have to burn x amount of fuel to make y amount of power efficiency differences aside - and those only effect this a small amount. So in order to burn x amount of fuel you need z amount of air. In order to minimize pumping losses etc. the exhaust size required after the collector is going to be pretty much the same (i.e. +/- 10%)  for a certain power level on any otto cycle engine. Bigger is not going to reduce power unless there was some kind of resonance effect going on in that length of pipe and even then it would depend on what that effect was if reducing the gas velocity effected it in good way, bad way or negligible effect. Yes heat and gas velocity is important and for the same size pipe you will often see a benefit keeping it hotter - but if the pipe was a restriction, the flow area increase from going up 0.25" or 0.5" is going to far outweigh any flow efficiency decrease there might be from lower temperature exhaust gas due to surface area.

 

Going bigger will change the sound (seat of the pants dyno change increse?/decrease?), be harder to fit, heavier and be more expensive though.

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Oh man don't even want to start on that post Frosty, I don't know where you get this stuff from sometimes.

 

One thing though, what NASCAR exhausts are you looking at? the exhaust outlet on a NASCAR exhaust is dictated by rules and is actually a flow restriction at high RPM. Modern cup engines have a typical 90 degree V8 setup with tri-Y's a conventional taper collector, they run different secondary lengths for different tracks and header tuning is mostly about smooth flow and packaging. The rest of the exhaust is dictated by the rules heavily, they are only allowed a X or H section in a certain place, the exit can only be in a certain area on the side of the car and a certain size, they are oval to meet these rules and get out under the cars low ride height. I have a racecar engineering article somewhere at home all about NASCAR exhausts, they are far from ideal, lots of trade-offs for suspension and frame packaging.

 

Anyway 3" exhausts for everyone.

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Hello exhaust gurus. I plan on putting a balance tube on my Buick to reduce the bop bop under water fart sound.

I have 4 - 1 headers and flowmaster mufflers

Is there a rule to how far from the collectors the balance tube should be?

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There are lots of thoughts on where balance pipes should go. Basically do it where it works man, I doubt there is much room under your car to put it anywhere you like? Factory ones were up by the bellhousing, probably going to work best just behind the gearbox? 

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Cool. I'll just jam it where it fits. I have to raise the headers a tad too cuz they are 80mm from the ground atm. Was planning on heating with a gas torch and bending cuz that's how I roll. I have some 2.5" stainless pipe that I got for free ages ago so I plan to replace from the headers to the mufflers with that.

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Where do I get this stuff from?

From doing it and seeing the results for myself.

Since the age of 14 I have been obsessed with modified cars and bikes and to this day I just have to change things, My methods may be different from most but they work not just for the road but on the track as well.

I have been involved in building dirt, grip, drift, hill climb, drag, road, rally and with my brother and many of the lads from Taranaki they have had some very competitive cars.

I don't know everything. far from it in fact, I am still learning and will continue to do so till the day I can no longer swing a spanner.

If there's one thing I have learned is there is not a perfect car, they are all a compromise between cost, horse power, driving dynamics, practicality, this list can go on forever.

I do agree with the comments re NASCAR but you are only talking about the Headers and X / H portion not the muffler section that exits the side under the sills.

Something like this.

BoomTubesDrGasExhaustNASCARX-Pipeboomtub

People are always telling me "that won't work, your crazy" and I take great pride in the fact I can get it to work.

I can remember a session I did with Bunter (Richard Pierce) on his sprint car when we where building a set of Pulse headers, these work by timing the exhaust into the collectors in an even timing, this requires following the firing order and putting every second exhaust pulse into the collector which can be very tricky as it means taking the pipes over the engine on a V8.

It did help with power output but it was a mongrel to fit.

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Ah you mean the boom tubes as they are called? Article talks about these, they are pretty much just as big as possible to give the lowest pressure after the restrictive oval section required by the rules. On cup cars some of them are almost square not an expanding shape, they also have channels in them performing some kind of voodoo. They are hardly proof that a megaphone muffler/exhaust tip provides some kind of scavenging, if anything they are testament to larger low pressure exhausts, once the tuning of the headers and crossover is done the exhaust goes a large as packaging/rules lets them. I'm sure as its big budget racing they are CFD analysed for any gains and best flow possible.

 

Not doubting anything you have done man you, but you seem to come confusing conclusions sometimes and it comes across as touting facts. I'd love to see your dyno plots as I have never seen a healthy 350ish v8 where 2" is the optimal pipe size, I would bet a straightish twin 3" would not loose any power on anywhere on a cammed 351.

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Hiiiiiiiiiiiiiiii, now kids, if I run 3" pipe on me 86, with two straight through resonators, and a chambered muffler of some kind at the back, it should in theory be bark-less, flow better/make more power, but will it be WOF-able do you reckon? Theres a flange rite before the diff, so would have a 3rd resonator that I can slap in there for track days and so on.

 

I currently run 2 1/4 tube, with one straight through hot dog and a turbo flow at the back, its fairly quiet, but has a yuck bark. I went 2 1/4 at the time, because I read that anything over that on a 4age and you get more noise, but no more power. Sounds like I was gypped!

 

Also, with the collector on the headers, would one just cut back the taper untill its 3"?

 

Always a pleasure, VG.

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i think you need to look at how you will be using the car most of the time. people get all hung up on making max power when you might only need max power 5% of the time. a 3'' exhaust on a 1600 might be annoying the other 95% of driving you do. i get it all the time at work- rotaries that get green stickered for too much noise but they dont want to get rid of their twin 2 1/2 into single 3'' dumpy muffler cause it makes more power. no point having another 15 hp if you cant drive it because you get hassled by the law every time you drive it

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