As I understand, the resonator on twincam engines helps to maintain maximum torque after peak parametr (so on 20VT it will be after 2800 rpm).

So moving it closer to the engine will make some effect. And more important then from backbox... And the volume of resonator also matters a lot.

Could anybobe provide texts or other instructions on how to calculate correct position and volume of resonator?

With my 2,4l block and big valve head there are some changes to be made, I think.

Blueflame doesn't use resonator in their products, and this is odd for me. So it's another challenge to educate myself )

Doesn't the turbo have a big effect on exhaust 'pulsing'?

there is a great post about exhausts and the different requirements they both need (turbo vs n/a) i cant for the life of me find it. It spoke in depth about back pressure and was done by some one who looked to know there stuff

ill keep looking...

this one 5 threads down?

“There is one major goal for the design of the exhaust system downstream of the turbocharger and that is low back pressure. The higher the back pressure the slower the wheel speed the lower the horsepower. There is no getting around this fact. Unfortunately, this message is lost with many “experts” who suggest bad designs. You should ignore advice that minor choking is required to cause turbine spool. This is an old wives’ tale that should but won’t die. There is also no such thing as cylinder scavenging downstream of the turbine. In other words, there is no reason to try to design for high exhaust velocities downstream by necking down any part of the exhaust or by any other means. You will only introduce backpressure which is bad for any turbocharged vehicle. The ideal exhaust would be a very short “megaphone” but for street application there is a minimum acceptable pipe diameter required to support a given gas flow rate. Air flow mass into the engine is directly proportional to exhaust gas outflow, horsepower is related to that inflow, so it is possible to establish a relationship between horsepower and exhaust pipe diameter.”

You may have 3” turbine outlet and 3” downpipe but if the system necks down to 2.75” anywhere then this is your effective diameter. If you have to neck down for any reason do it as far away from the turbo as possible.

2” for anything upto 200BHP
2.5” for anything upto 300BHP
3” for anything upto 400BHP
3.5” for anything upto 500BHP

I hope this clears thing up.


There we go, that will tech me to use the search function and not look with my eyes

rules of thumb from my experience:

1. NA exhaust tuning is very different from turbo exhaust tuning. Resonators work more for NA cars than turbo cars.

2. Bigger is better for turbos.

3. 1" pipe diameter per 100HP rule of thumb applies. especially so for NA cars. for turbo performance, its better to be slightly too big than slightly too small.

4. its better to have bigger piping closer to the engine as exhaust gases are hotter and more voluminous closer to the engine.

5. the bigger it is, the noisier it will be.

6. the bigger it is, the more low-end torque u will lose. for turbo cars, this is most felt when u are off-boost.

7. the bigger it is, the quicker the turbo will spool.

purely my 2 cents. some will disagree im sure.

redd

Resonators "filter" higher pitch exhaust noise. They are not necessarly needed when used with a turbo. position is irrelevent.

A question for all you exhaust experts -

I have a full 3" Torque Exhaust system - www.torque-exhausts.com . However, it is VERY loud and resonates very loudly between 3-3.5k revs.

Is there anything that I could do to cut the noise down? I was wondering if heat wrap would help. If not, is my only option to stuff sound insulation unter the carpet, or can I fit it under the car?

You dont need a resonator on any system unless you are looking to address a noise issue. If your car is too lound you need a silencer that does it's job and old or sheap silencer will be in-efficient in its deadening. Re-packable silencers exist to keep a high level of noise reduction efficiency without re-fabrication.

My race car was too noisy - I added a silencer, it will do nothing for power, but being black flagged off a circuit is a greater loss.

For the turbo then the least restriction is the best for performance, but as has been noted too little back pressure cause the OE turbo oil seal problems, and also noise.

The NA situation is incredibly complex and not applicable to the turbo car on boost.

As Rich said, a 3" exhaust can be quiet, mine is, but this could be done to the mammoth sized back box i have

Can any tell me which ehaust system is considered to be the best for a 20vt??

Mine's was too noisy withh just one 600mm silencer so i added in another one at the back end and i am now at acceptable levels. Everyone needs to look at their own specific requirements really i think. My car still sounds pretty good i think.

sounding like a grumpy old man I don't really agree with a lot of the stuff written here.

I would worry about overall flow more than volume of the exhaust. They are not the same thing.
If you have a 5" exhaust you will see backpressure because the gasses will lose so much energy in the exhaust that they will back up.
The inch per 100BHP is not really true on turbo cars, otherwise my exhaust would have to be much larger than it is and my exhaust will happily run over 600BHP on pump fuel.
The sizing of 2" for 200BHP and 3" for 400BHP is not correct either because the volume is not going up proportionately but the power is. A 3" exhaust has 2.25x more volume than a 2" exhaust all other things being equal

There are compromises for going large - in my 5" exhaust example - even on a 1000BHP car it is not always running at full pelt and therefore it does not always benefit from the size of the exhaust and that size is actually impeding the flow of the gasses out.

In an ideal world you would have a variable flow capable exhaust. The best real world compromises are the taper exhausts and the exhausts with pressure related valves that increase the effective volume and flow (like most modern ferraris have)

Really, Can you elaborate?



It’s more like 1/2" per 100HP on a single exhaust system
For your 600BHP setup on a single exhaust system you "should" be running 4" . Air mass flow into the engine is directly proportional to exhaust out flow, which is related to horsepower. I bet upgrading your system to a full 4" you will see power gains. Have you ever measured your exhaust back pressure?

Yes as gases lose energy they slow down in travel and therefore if you consider the air from the turbo out to be one stack the slower gases cause the gases earlier up the stack require more energy to travel down the exhaust and this they slow down causing the whole stack to slow down. Gas will always try to fill the volume of the container in which it is in so energy is wasted in not expelling the exhaust gases but filling the area that would not otherwise exist on a lower volume exhaust

The reason why a taper exhaust is useful is that if you reduce the volume over the course of the exhaust then the gases make better use of their limited energy as the tube takes less to fill. As gases slow down they get more dense as well so the requirement for the volume of the exhaust reduces also.



Since you are asserting the proportionality which is correct you should realise that the volume of a tube increases in a non-proportionate way compared to the diameter of the tube

The increase from 2" to 3" = 2.25x increase in volume - i.e. I need more than two 2" exhausts to get the same flow as one 3" exhaust or 125% more in volume terms

going 3" to 4" I get 77% increase in volume
going 4" to 5" I get 56% increase in volume

So you see it isn't proportional, and that is why I don't need a 4" exhaust. By your assertion of 1/2" per 100BHP and saying I need 4" for 600BHP you mean that I need a 3" exhaust for 400BHP - but then you are saying for 50% more power I need 77% more volume in the exhaust.

In terms of my exhaust I am already producing as much as my turbo is rated for and peak power is at 7700rpm so I might at best get another 30BHP out of it (being optimistic). So yes there is a compromise there but I think it is pretty close to optimum
In my opinion if I put a 4" exhaust on my car it would make it gutless at low rpm because of the problem with the exhaust being too large and therefore causing more resistance to flow.
This is all aside from the practical problem that it would be very difficult to make a 4" downpipe fit in any form on a coupe

Makes a good theory but I don't think this actually happens in reality. To make the most use of the high gas pressure upstream of the turbo you need to ensure that there is the lowest possible pressure downstream of the turbo. The only thing that will cause a pressure build up is a smaller pipe. Bigger is always better on a turbo car but the compromise is always space and noise.




That’s a big no no, you do not want to taper down any part of a turbochargers exhaust, no matter how far away from the turbo. You are not designing for high exhaust velocities, this will introduce backpressure.




The fact is this:
Air mass flow into the engine is directly proportional to exhaust out flow, which is related to horsepower.






I'm sorry John but I couldn’t disagree more. The bigger exhaust the less back pressure the faster the turbine wheel will spin. If it becomes gutless then there is an issue elsewhere that the bigger exhaust has just revealed.

I love these threads on here, reminds me of the ones on the 205GTi forum when Dave Baker (of Pumaracing fame) used to post!

Anyway, it seems a Garret turbo development engineer has a similar opinion to TurboJ

http://www.tercelreference.com/tercel_info/turbo_exhaust_theory/turbo_exhaust_theory.html

Whats going on?? who come up with the theory that choking an exhaust in a turbo application improves performance??

Its really quite simple

Get the exhaust gasses in and out of the turbo with the least amoumt of restriction possible, job done

There are some theories that suggest that some kind of pulse tuning between the exhaust valves and the turbo may improve cylinder scavenging but many great engine tuners have come to the conclusion that its near on impossible to make work

Back pressure and gas speed have very little to do with pulse tuning

In a N/A engine its common practice to "tune" the exhaust to gain performance, it cant work with a turbo in the middle of the exhaust system

Ricky

You don't understand the principles behind the column of gas you are creating in an exhaust. The bigger the exhaust the more the gas will fill it and therefore less energy is available for the gases to leave the exhaust. The gas will naturally fill the area wasting some of the energy it could've utilised exiting the exhaust. The gas will cool more quickly causing loss of energy and increasing density

As gases cool they take up a lower volume and therefore leaving them with more volume available will unecessarily use up more energy. This is all fact not theory. If you really want to read up on it then it is called Charles's law if I recall it was devised in the 19th century so not exactly rocket science

The whole way through an exhaust the gases are losing energy. That is a fact because they do not have infinite energy and the gases are much cooler coming out the back of the exhaust than the back of the turbo
As gases get cooler they get denser. This is a fact and you will know it from how a good intercooler works if nothing else.
So what I am talking about is common sense you would get from GCSE/O-level physics let alone an engineering degree.
What you are saying is an exhaust is mysteriously breaking the forces of nature


You also do not understand the principle behind taper exhausts. The taper is not to increase the velocity of the gases as I agree that design would create backpressure. The taper is to reduce the volume in line with the reduction in volume of the gases and vaused by increasing density of the gases as they lose energy through the exhaust (partly exhibited by the cooling of the gases). The goal of the taper is to maximise the flow as this occurs and it should cause the velocity to be equal throughout the exhaust thus substantially reducing the backpressure.


I don't dispute the fact! The diameter of the exhaust increase is not linear with the increase in volume of the exhaust. That is why your 0.5" for 100BHP is clearly incorrect. you need to look at the volume not the diameter as the flow of an exhaust is related to volume not diameter

John, your theory is correct as far as i can tell but doing what your saying cures one problem by creating another

By doing what your saying you will be creating a restriction down stream of the turbo and as such back pressure

This back pressure saps more energy than your saving

I'm not saying it is breaking the forces of nature. Charles's law has its limitations, go and check. I’m relating to fluid dynamics & thermo dynamics. You are relating to static gases in a sealed unit. The exhaust is not sealed as it is opened to atmosphere. You need to look at Bernoulli's principle for exhaust gas flow.

Dense air is heavier than less dense air. Allowing the air to cool excessively means you have a large volume of heavy gas to push out.
What John is saying is about finding a happy medium.
You should see a logjam of air as a restriction as mush as the pipe.

You need to stop looking at basic Physics. This is flow. Take this quote:

"A fluid flowing horizontally, the highest speed occurs where the pressure is lowest, and the lowest speed occurs where the pressure is highest."

Air is considerd a fliud.

Any how the bottom line is regardless of all this science. Is a bigger exhaust is better.

I know it has its limitations, I assumed no knowledge given you implied 'it just is'. In actual fact Bernoulli cannot be applied because energy is lost because there is a transfer of energy which is obvious by the heat exchange along the exhaust. If you could make the exhaust a regulated temperature at the same as the exhaust gases then it would apply, but the exhaust is acting as a heat sink. Therefore the gases get denser and take up less volume which is my point.

I'm not at all disputing the minimisation of backpressure is ideal for a turbo engine. When you put a 3 metre pipe on the back of a turbo though how it is designed makes a big difference to how much back pressure it gets. It will get some no matter how big the exhaust is up to the point where it is effectively direct to atmosphere. It is not a fully open system though in reality there is one hole at the end which is why it has to be considered a column of gas and the goal is to maximise the end-to-end flow of that gas out, not just being maximised for the initial flow out the turbo

Hi John,

There might well be mileage in the theory of reducing the diameter in the length but I'll bet it's a hellish calculation to determine the rate of taper for a given length, variable hugely by the fluctuation in temperature of the exhaust gases for a given engine. There is no definitive formula for any engine simulation only the ability to get one in the right region. Say for instance an inlet length calculation, close from theory and then (if you can be bothered) fabrication of spacers or even new manifold to "see" if they make any odds. This is a relatively stable environment when compared to a turbo charger and it's exhaust system so I'd be prepared to believe the calculation if such a thing existed would be unreliable at best and liable to be restrictive at worst. The final little % might be possible I'm prepared to believe but it's not a practical task to undertake (unless of course you have a mathamatical method thats plainly obvious) unless you have funds pouring out of your ears or fabrication skills to be proud of and don't mind making loads of different configurations.

Now, you refer to volume a few times which is where I would be coming from if I attempt an explanation, I'm wondering where you've got the figures for this "volume" To me the increase in temperature is the all important influence on establishing volume of exhaust gases. My understanding of the much higher temperature of exhaust gases when compared to inlet charge says it all at just a moments thought. The much cooler inlet charge has a density thats worlds apart from the exhaust gases, the mass is similar but the volume isn't, it's much bigger. Where do you source the volume figures you are refering to. I'm looking at the exhaust gas volume being quite easily 4 times more than the inlet volume due to the big difference in temperature but would like to hear your version of exhaust volume.

There are a few turbo books out there which don't all agree on exhaust sizing but they range from big to much bigger, Mr Bell for instance isn't taking any chances with back pressure and quotes 3.0" as being suitable for 250-320 bhp, 4" apparently suits 400-500bhp. This is in the context I think of not being at all detrimental to the engine and the power it might be capable of given the specification as a whole. I'd imagine he's seen an increase in back pressure with smaller pipes in his extensive experience

Nik

I have no experience in this area so excuse my ignorance ....... i'm assuming that somebody has actually measured the back pressure with different sized exhausts on a turbo car or atleast the rate of flow over the entire rev range?

From what I can work out John is saying that if the exhaust is too big towards the end of the system then the gasses will slow down due to them being more dense as they cool and trying to fill the area around them rather then using all their energy to be carried straight out of the exhaust in which case a tapered exhaust would help?
It seems to make sense but then I don't know how fast the exhaust gasses are travelling in the first place on a modded or 2.4 coupe and if it is fast enuff to overcome any reasonable exhaust size i.e up to 4", surely its all relative to the car and the rate at which the gasses are travelling, I would imagine that off boost a huge exhuast perhaps would not help flow of the gasses?

I feel like a midget amongst giants, I only comment as I have always gone for an exhaust just shy of 3" in the hope of better torque low down on mildly tuned coupes which is what I prefer as I live in London and the most I can hope for during the day is a few seconds of hard boost before I hit traffic!

Cheers
John (Not JohnS but to confuse things my surname starts with an S also! )

OK thought i'd chime in after reading the Garrett Turbo Engineer's post. John, your argument seems to revolve around the temperature of the gasses in the exhaust, and that as they travel along the exhaust they cool (as proved by the transfer of heat into the exhaust itself) and therefore slow down. However, and i quote:

"The temperature of the exhaust coming out of a cat is higher than the inlet temperature, due to the exothermic oxidation of unburned hydrocarbons in the cat. So the total heat loss (and density increase) of the gases as it travels down the exhaust is not as prominent as it seems."

Jay Kavanaugh, a turbosystems engineer at Garret.

So please continue the discussion. I know nothing of the science behind all this it was just this one point i wanted to make as it stood out after reading that article.

Ah, but Truffle, quite a few people aren't running with CAT's on the car, so kind removes that point from this discussion, and would imagine this is based on no CAT being present in the exhaust

In reality though, how many people actually run a cat in their exhaust system?

Aha, of course. Stop!.....Carry On!