tabs604

Hi peeps!

I have been running a custom 2.5" downpipe from my greddy turbo back for a short while now.

its making boosy quicker and is a little louder than before:



Now when i was N/A i had a custom dual exit de-cat back system made made by a custom muffler shop. It was very loud...but I liked it like that!

It is 2.1/4 inch back.



It looks pretty with its twin chrome.carbon tila pipes and sungle muffler.

Now the question is would i benefit allot from 2.5" haust all the way back? its only a 1/4 of an smaller at the moment.

if so what benefit would it make over the 2.5" d/p I already have?

I have been offered a custom exhaust to be made by a very reputable haust place in the UK and was thinking of have another dual tip exhaust but without a muffler or a CAT!

Is anyone running this set-up?

would this be unbearbaly loud with my greddy turbo and make spool even quicker or am I asking for trouble?

brgracer

iTrader: (14)

2.5" should help with spool, especially if you are removing both the cat and muffler.

I think it'll be a bit too loud, but try it and you can always add a glasspack or small bullet muffler to quiet it down if needed.

FWIW, I ran the car once to move it a few feet with just the DP on and it was crazy loud. I don't know how much just straight 2.5" piping to the back would quiet it down. Also, when I had a test pipe going into FM's turbo duals (with a muffler so just a cat delete), anytime I spent more than 30 min in the car with the top up, the drone gave me a headache.

F20turbo

I run a 2.5in dp and a 3" exhaust with no cat and its not loud. Depends on the muffler you use. Mine is a thermal muffler...they are a lot more quiet.

tabs604

with no muffler or CAt would that cause any problems?

someoen told me the trubo needs back pressure and if there is none (with no muffler or CAT) this can cause the seals to persish?

any truth in this?

bripab007

No internal combustion engine needs backpressure, especially a turbocharged engine. That's why you'll see pretty good gains from stepping up the exhaust diameter on turbo'd cars.

ColoradoSpringsMiata

correct me if im wrong, but i was under the assumption that a loss in backpressure = a loss in torque. i know when we dropped the exhuast a while back in my friends corrolla typeS (from cat back), he felt a little loss in torque on the butt dyno.
i thought you have to get it just right, not too much backpressure yet not too little. though i know turbocharged engines need more breathing room than N/A

Braineack

iTrader: (62)

The faster exhaust can exit the turbo the faster the turbine can spool. That's why eliminating backpressure is benifical to a turboed engine. N/A cars can utilize backpressure to gain low-end. That why at some autoxs you might see a civic with a 1" exhaust pipe with a baffle plate system on the muffler.

bripab007

No, naturally-aspirated cars do not utilize backpressure. They can utilize exhaust gas velocity which can be closely tied to backpressure. An idea system would strike a compromise between velocity and backpressure. For a nat. asp. engine, this would mean choosing the correct size primaries for the engine capacity and desired torque curve, then merging and opening up the pipe as quickly as possible.

It's sort of semantics, but: No internal combustion engine needs backpressure.

Braineack

iTrader: (62)

I meant to say velocity instead of backpressure.

tabs604

so to summarise if i go with a 2.5 system form the turbo back with no CAT or muffler it will be loud but will provide me with the most flow and no sideffects?

bripab007

Yes, pretty much...but, if given the choice between a completely unmuffled, but easier to route/fabricate 2.5" exhaust or a properly-muffled, but more difficult to construct 2.75-3.0" exhaust, I'd choose the muffled one.

Unless you like hella-loud exhausts

akaryrye

iTrader: (5)

I am getting a catback exhaust fabbed up tomorrow (cars at the shop now). It is just a catback with 2.5" piping and an 18" glasspack. Ill let you know how it goes. Also just fyi, I have begi 2.25" piping to cat, and a magnaflow cat that is ?" All is going through a gt2554 turbo.

white90na

NONE OF THIS IS MINE. IT IS FROM THIS THREAD http://www.aus-cartalk.com/viewtopic.php?t=21125

If you have extractors (tuned length headers) then you are attempting to 'pull' the exhausted gasses out of the combustion chamber with pulse tuning (harmonics). These match with the tuned length intake manifold to set up a resonance within the combustion chamber that has the air charges moving in well ordered packets (at least that is the theory).

Remember we are talking about gasses here. For any given diameter of pipe the gas will flow at a given speed. If the pressure is constant, yet the diameter is reduced, the speed will increase. Increase the diameter, and the speed will reduce. Unlike liquids, gas WILL locally compact (increase pressure), causing pulsation in the system. It is this pulsation that we try to use to ‘tune’ the engine by using the high and low pressure components of the pulse. The aim is to get more air into an engine. The measure of air in / air out is referred to as volumetric efficiency (VE). It is possible to increase the size of the intake charge beyond that of the empty cylinder - a very good thing. Engines with this capability have a VE exceeding 1, meaning they get the advantages of forced induction without the disadvantages. B6/BP in a NA MX5 probably works at a VE of around 0.8. Most performance modifications to an engine are about getting the biggest packet of air into and out of the engine as quickly as possible.

People often equate low backpressure in exhaust systems (due to large pipes) with loss of power. What they are actually observing is a loss of gas velocity at low RPM due to large pipes. Without the velocity, you get the pressure pulses stalling and diluting with a consequent loss of beneficial harmonics and VE. It starts to come back at higher RPM as the engine passes more gas. The optimum outcome of all of this is that the gas comes out of the exhaust port travelling really fast, moves into the exhasut system and tries to keep the speed up as it cools, contracts and meets other exhaust gases from other pistons. Your engine can only move so much gas, and the trick is to tune the exhaust system to keep gas velocity UP in the operating band you desire. You can go too big, and some will tell you ‘you must have some backpressure’.

The concept of backpressure on an NA car was originally to keep some of the exhaust gases lingering around the exhaust port to stop the incoming charge sweeping straight through the combustion chamber. This will happen with cams that have overlap. The lack of backpressure would substantially increase the fuel costs (as the fuel is not being burned in the combustion chamber, but in the exhaust system and is therefore wasted) and will raise the temperature of the exhaust system (leading to early failure). Afterburning is not applicable to piston engines.....ever.

Performance NA camshafts usually increase overlap (inlet and exhaust open at the same time) to produce increased filling and consequent scavenging of exhaust gasses. I won't go into valve angles, but this also has an effect. This all applied in the old and bold days of pushrod V8s and holley carbies. Lumpy idle (mucho overlap cam) and single plane intake manifolds developed good power high in the range, but were only saved down low by the sheer displacement of the engine generating torque. Keeping backpressure on common cars (not the performance GT types) allowed low range operation without using prodigious amounts of fuel. It impacted high range operation too by reducing flow when it really was needed (but the ol’ station wagon still went well enough).

Keep one thing in mind: Backpressure is the enemy of VE by reducing the amount of waste air that can be discarded.

white90na

The common thing that you will hear from the turbo guys is

'I put this sik dump pipe on, and now the turbo spools up quicker.'

They are kinda right.

Bigger dump pipe = lower pressure area immediately after the turbine. Lower pressure area increases the velocity of the gasses through the turbine, thereby increasing possibility for the rate of change or acceleration of the gas flow. This gives the turbo the ability to accelerate faster. There is a limit to this from a tuning point of view, and a practical size consideration as well. The rally guys also utilise both blow off valves and if they can afford the maintenance, anti-lag, to keep the turbo spooling when they are off throttle. Anti-lag is hell on turbines however (this IS afterburning for piston engines).

A major thing that the duff-duff boys forget with their ever increasing exhaust sizes, their gas velocity is dropping. A turbo does not work on the pressure differential between the combustion chamber and the exhaust system. It works because of the mass of the gas flowing through the turbine as it moves from the combustion chamber (high pressure) to the exhaust (low pressure). It is possible to get some pulse tuning happening here too. Turbo extractors are a good thing, they are the bunch of twisted pipes you sometimes see that connect the exhaust port with the turbine.

Note that the concept of scavenging has only small application AFT of the turbine. The turbine provides so much backpressure on the engine that harmonics induced by the 4 stroke process are largely cancelled out. If the gas then stalls in the exhaust system due to inadequate velocity (caused by a 4" pipe on a 2000cc engine) then you are going to lose responsiveness and power across most of the range. Where the duff-duff crowd go wrong is the ASSumption that large size is necessary for the remainder of the system. Good exhausts have a larger dump pipe straight off the turbo, but by the time the CAT comes into play, the exhaust is reducing to a smaller diameter. A CAT is also a resonator, and it slows the gasses, and by absorbing heat from them, causes them to contract.

For a race car which operates at high RPM most of the time (nods to Sabre, CT and Matty) you need a bigger exhaust to keep the backpressure down due to the increased volume of air passing through. Of course, you have fitted cams, injectors, timing systems and inlet modifications to optimise this operating range, and VE at that range.

Street cars generally don't have to operate at constant high-rpm. This is one reason the car companies are starting to build turbochargers with variable flow characteristics (by vanes directing the airflow onto the turbine). The idea is to increase responsiveness when flow volumes are low, but not inhibit them when they are high.

No accounting for style and decibel induced power however.

Factories build exhaust systems which must conform to the laws of the land, laws of physical fitment, and the laws of cost effective manufacture. Can they be improved on? Hell yes! Do you need 4” fully sik all the way back? Hell no! It is possible to have an exhaust system that is quiet, high flow and power increasing.

An ideal exhaust system would be small at low RPM, but grow to larger diameters as the RPM increases. If you watch the exhaust of a jet engine, you will see it has a variable nozzle for this very reason (they have variable intakes too). As backpressure is the enemy of VE, this would maximise VE at any point in the RPM range. Trouble is there is not any commercially available technology that does this for piston engines. Due to the wide operating range of a car engine, you will have backpressure somewhere in the higher range in order to retain gas flow in the lower range. Like any other part of the engine, you tune the exhaust to suit the operating band desired, and since we cannot change our exhaust sizes as required, you have to choose one size.

white90na

So the accepted wisdom of 2" for the 1.6 is largely (and empirically) correct. The key is to get the spent gasses out of the way quickly. 2.25" is probably good for the 1.8, but may actually be a little large. For turbo engines, you will need bigger, but 3” is probably too big, I would suggest 2.5” for most MX5 turbo engines would be a good place to be.

As an aside, the sound from an exhaust will have a lower note (do reh mi) with a larger exhaust as the gasses are leaving the tip at a lower velocity and pulse pressure. Higher speed pulses come from smaller pipes and these consequently produce higher pressure variations in the system and sharper sound. Louder comes from having less resonating chambers (which slow the gasses down and capture really intrusive audible harmonics). A catalytic converter, muffler and a resonator are resonating chambers. The pitch or 'note' of an exhaust is governed by the stroke, the extractor design and the orientation of the various resonating chambers in the exhaust system.

olderguy

iTrader: (19)

^ Like he said.




Actually some pretty good reading.

bripab007

<on naturally-aspirated Miatas> A 2.25" exhaust on a 1.6L has been dyno proven to increase torque across the entire rev range, and a 2.5" exhaust on a 1.8L has been shown to increase torque across the entire rev range. Flyin' Miata did some dyno testing on this.

The 2.5" exhaust shouldn't really lose any torque on the 1.6L, either, assuming correctly-sized (diameter and length) header primaries. Josh Cosford over on M.net has a 1.6L with a custom header and 2.5" header-back that he dyno'd before and after showing increases in torque across the board.</on naturally-aspirated Miatas>

<on turbo'd Miatas> A 2.5" exhaust should flow enough air, with acceptable backpressure, for ~300rwhp worth of air. </on turbo'd Miatas>

Braineack

iTrader: (62)

1.8 stock vs 2.5"

http://www.flyinmiata.com/tech/dyno_...ebb_082099.pdf

Kelly

iTrader: (6)

You are leaving power on the table only doing a 2.5".
I need to find the dyno sheet from Rob at Flipside's old Greddy setup at 7 psi
Full 3" Turbo back exhaust, Bipes, some super small intercooler off another car,1.8 injectors and Vortech RRFPR. It was near 180 whp if I remember right.

bripab007

If anything, a 3" exhaust on a lower-powered turbo setup at least has a greater potential to be able to unlock a couple degrees of pulled timing due to the lower backpressure.

That's not always going to be the case, but at least you know you're exhaust will not be a hindrance to timing advance.