3" Catback On My N/a R33

[Samon]

Hi Guys,

before you go flaming me for not using the SEARCH button... i have.

Now, here goes. My R33 GTS came with a 3" Mandrel Bend catback with twin mufflers and tips. After substantial reading, i understand that 2.5" would have been better... but the previous owner obviously didn't know this when they got the new exhaust system fitted. Now I love my exhaust... it sounds awesome (VERY loud but i love it ) but it really does lack when it comes to down-low torque. I would like to see if there is anything simple that i can do about it. It did cross my mind that i could try blocking off one of the mufflers... (with something like a potato ), but after more reading i'm not sure if that would work. Sure it would increase backpressure... but it doesn't eliminate the fact that it is a 3" system from the cat back.

Would there be any problem arising from this? i.e. I couldn't imagine any damage would be cause (apart from the fact that I'd feel like a tool driving around with a potato in my exhaust while i tested it out ). If it was much better i would just get the second muffler blocked off (probably leave it on there coz it looks nicer ).

Any feedback/opinions would be greatly appreciated!

Cheers,

Sam.

[SKiT_R31]

ok, to start with, 25de's are lacking in torque as it is

if the volume doesnt effect you, then leave it, its fine.

and the whole backpressure thing is bullshit, its not the fact that having a bit pipe doesnt have enoguh backpressure, its shit like the pipe being big makes the exaust cool down CREATING backpressure, and also some turbulence (the enemy of flow) since air shrinks as it cooles, then it goes bouncy bounce etc

going to 2.5" will barely effect anything, except sound, yeah you might gaiun SFA low end, but nothing to write home about...

you could save it for when you go turbo, or sell it to somebody who is turbo, or keep it and forget about it.

for example, if you got some extractors, and say heatwrapped them and the entire piping (if you wanted to...) then and problem with that should be solved...

i wouldnt worry about it, keep it if you like the sound, then if you want to, heat wrap the lot (paint first, rust bad) then paint over it again to protect it rotting etc, should give it a crisper note and any torque that you would have lost should almost be back... if you plan on pumping money into your na for some more power, you'd prob be better off to keep 3"...

also make sure its 3" all the way, if its 3" till the rear muffler then its say 2.5", that will cause heaps more of a problem....

[Eug]

ok, to start with, 25de's are lacking in torque as it is

in comparison to what? bar the RB30DE hybrids and Autech RB26DEs, the RB25DE is the largest capacity non turbo in the RB series

and the whole backpressure thing is bullshit, its not the fact that having a bit pipe doesnt have enoguh backpressure, its shit like the pipe being big makes the exaust cool down CREATING backpressure, and also some turbulence (the enemy of flow) since air shrinks as it cooles, then it goes bouncy bounce etc

going to 2.5" will barely effect anything, except sound, yeah you might gaiun SFA low end, but nothing to write home about...

while i don't claim to be an exhaust specialist, i do see some faults in your statement. first of all the correct term for your quoted "backpressure", is infact called volumetric vacuum. what this describes is that the gases expelled from the system "plunge" the entire area of the exhaust system and creates a vacuum. as the gases progresses through the system, this vacuum will encourage and tend to extract the newer gases being expelled. having a too large of an exhaust system will create an empty weak vacuum, this ruins the flow of the expelled gases causing the turbulence you describe.

although what you say about the gases being cooled down does hold some merit, how long do you thing the gases remain within the 2-3 meters or so of metal to have any sort of significant effect in regards to the volume it occupies in relation to molecular expansion?

i guess you could describe it as a combination of both, but what you say about changing to a 2.5" will barely have any affect is false. in high RPMs this may be the case, but i would pretty confidently bet some money most of the people on these forums drive in a range from 2-5000rpm most of the time.

also make sure its 3" all the way, if its 3" till the rear muffler then its say 2.5", that will cause heaps more of a problem....

this is true, non turbo systems (unlike turbo) are NOT under pressure, so in theory you would want to remove as much of the restrictions as possible in order to provide an effortless expulsion of gases.

many people have played around with exhaust sizing on skylines for over a decade now. 2.5" has been the ageed upon gospel for a street driven non turbo skyline.

however that said a 3" system MAY provide higher power figures on the dyno, but as most people should know a rwkw number is most definately not an accurate representation of the car's drivability.

[Samon]

Hmmm... so basically it's either whack a turbo on it (i know... everyone says it's better to just sell it and buy a turbo one, but I'd love to do it anyway as much for the learning experience as for the power gains ) or put a 2.5" catback system on it if i want more torque down low. Mine is pretty sluggish down low, even if i shift out of first just before the limiter (at about 6700), it bogs down slightly in second... and I only just beat a stock falcon or commodore at the lights. It gets up and goes alright once you really get it revving, but just has nothing down low.

Just for the hell of it, I got the largest potato that i could find, and rammed it down one of my exhaust pipes. It was not quite big enough to completely block it off, but would have been 90% covered. I went for a bit of a spin and couldn't really notice any difference (apart from it being slightly quieter)... i looked at it was still stuck firmly so i figured i'd take it out later... Anyway, I went out later tonight and totally forgot it was in there... got home and there was no potato there... must've got hot enough to slightly cook the outside and became soft and fell out... haha would have been pretty funny to be following a car and a hot potato drop out of the exhaust pipe

I have been toying with the idea of turbo-ing it for a while, but being on my P's I am subject to the 125kw/tonne limitations... dagnammit!!! (which reminds me, I've been meaning to ask... does anyone know if i were to turbo my car, would the police take the factory GTS power/weight figures? or those of a GTST? Coz technically it's a N/A GTS with some mods, not a GTST )

Edited November 17, 2006 by Samon

[Eug]

hahahah @ potato...

i wouldn't have thought that it'd work since you are technically going from:

2.5" -> 3.0" -> 2.5"

if you turbo'd your NA and didn't declare it, then i suppose you "might" get away with it, but then your "mod" would be considered illegal without a mod plate. and your royally reamed if someone cluey were to check your manifold only to find a small snail hanging off the side of the head and your back at square 1.

[rs73]

Does any of you ever felt that a 2.5" system on a non-turbo car sounds LOUDER than a 3" or 3.5" system on a turbo car? Most turbo exhaust system I've heard only felt very loud when off boost, but not when the boost is spooling up/on.

[slip]

in comparison to what? bar the RB30DE hybrids and Autech RB26DEs, the RB25DE is the largest capacity non turbo in the RB series

er, its called an RB30E eug...... feels torqueir than a 25de, but has absolutely nothing up top.

hah. your 30de obsession has melted your brain

[MBS206]

Does any of you ever felt that a 2.5" system on a non-turbo car sounds LOUDER than a 3" or 3.5" system on a turbo car? Most turbo exhaust system I've heard only felt very loud when off boost, but not when the boost is spooling up/on.

The turbo on a turbo car acts as an extra muffler. You will find this is why the turbo cars really only run a cat, and single muffler, whereas, alot of N/A cars run twin, even triple mufflers.

The exhaust pipe is WAY too big for your useable rev range.

The larger the exhaust pipe, (Once no longer a restriction) the further up the rev range your power band becomes as the pipe enters it's efficiency range.

As you've noticed, it's bogging on an upshift, as you are WELL out of your power band.

Other things that change your power band are the diameter of your primarys, and the length/diameter on your secondarys (On the headers).

Change the whole system to a 2.5" system, get a nicer note from it, and gain your low down power back.

As to how much the exhaust gasses cool down.

Go for a hard run in your car, now measure the manifold temperature, then measure the exhaust tip temperature.

The tip will be WAY colder, so a shit load of heat is being pulled out of the gasses right near the start.

Exhaust gas temps drop dramatically as you head away from the engine.

3" Exhaust system, 3 meters long, approx 1.367M^3

2.5" Exhaust system, 3 meters long, approx 0.945M^3

pV = u (moles)8.3144*(273+C)

100*2.5=U*8.3144*303

u = 0.1mole air.

Exhaust temp, measured an an EF Falcon, at cat of approx 400degrees.

100V = 0.1*8.3144*673

V = 5.59L

1 litre = 1/1000th of a cubic metre

0.00559 Metres Cubed.

Hence, at 7000RPM, 58.3 times the motor gets filled per second completely.

Hence, 0.32M^3 of gas is expelled per second.

Hence, on a 2.5" system, it takes 2.95seconds for the gas to empty,

On a 3" system it takes 4.27 seconds to evacuate.

And at 3000RPM, 25 times per second completely filled. 0.14M^3 of gas is expelled per second.

On 2.5" system: 6.5second

On a 3" system: 9.2 seconds

Now, Don't ask me what I was trying to prove when I started those calculations, except now we know how long it takes to get the exhaust gasses out (If the exhaust IS 3 metres long, and 3" all the way.

But, you can see, that's a HUGE amount of time at 3000RPM for the exhaust to take to get out (And plenty of time to loose temperature, contract in volume, and slow down even further (As all of the space is required to be filled in that exhaust *Even though there is "vacuums" in the exhaust, this is equallised by "Positive pressure" to create an equilibrium of pressure when added together*)

Also, those calcs were done at WOT. (Hence, 100KPA used for pressure)

Basically, I think that's showing, a 3" pipe is WAY Too big!

And a drop in 17% of the diameter, brings down the time the gas takes to get to be only 70% of the original (When at 3") hence, a 17% drop, created a 30% gain in time to get out.

[Eug]

er, its called an RB30E eug...... feels torqueir than a 25de, but has absolutely nothing up top.

hah. your 30de obsession has melted your brain

shut up paul... old technology no one wants a 30e lol

my bad. totally forgot abt the 30e

[rs73]

The turbo on a turbo car acts as an extra muffler. You will find this is why the turbo cars really only run a cat, and single muffler, whereas, alot of N/A cars run twin, even triple mufflers.

Firstly, great calculations... but I don't think it's entirely right with the exhaust on turbo. I have a turbo and my stock R33 standard exhaust run on a cat, then a resonator (also called the extra/middle muffler), then rear muffler just before the twin outlets. my current aftermarket one is still running on the same layout, after cat there is a middle resonator then a decent size end muffler.

Secondly with that calculations, (I also read in previous issue of HPI mag) that even a 2.5" - 2.75" inch standard exhaust system on a turbo car is still sufficient to support more than stock power level... I know this is an N/A forum, but I found that this concept is quite intriguing, considering most turbo owners will upgrade their exhaust to the biggest diameter they can afford as the first step of mods before doing anything else... and I'm sure on an M3 with +/ 246kw they still run on smaller than 3" exhaust system. Problem is probably those 3" exhaust bolt on system from japan were possibly designed for 400-600+hp application for drags or circuit, and there's just hardly any option for a good bolt on 2.75" system from well known manufacturers. but no one ever tried bolting a cat back n/a system from i.e. trust or hks intended for gts onto a gtst.

[MBS206]

For the exhaust on a turbo.

I'm running stock turbo, cat, rear muffler only. It's not over loud at all. Quite bearable, and legal (AFAIK)

As for the turbo thing, I know they're a totally different kettle of fish when it comes to choosing an exhaust pipe (As they pump a shitload more air/fuel through) and hence, I shall not comment.

As for an N/A 2.5L, it has been proven, even on a 3.8L commodore motor, that 3" is just too big and shifts the power band too far up out of the motors rev range, just as it has here, on the 2.5L motor.

[Samon]

Anybody got a ballpark figure as to what I can expect to pay for a semi-decent 2.5" system?

[SKiT_R31]

Anybody got a ballpark figure as to what I can expect to pay for a semi-decent 2.5" system?

Yeah i was reffering to the RB30e, makes more torque and lower in the rev range than the 25de...

Speaking of exausts though, drove my car without one this morning (just engine->extractors>nothing....) as extractors needed a flange welded on to be bolted to cat, and i wanted to fit them myself...

and i did notice a few things, 1. it was f**king loud, and droned like buggery lower that ~ 2500rpm under accelleration. 2. it sounded sick as and lumpy on idle...

anyway, main thing is i noticed i lost heaps of responce, the car just plodded along, hated the low revs, and it actually made the engine run a bit rough, but as soon as i hit the torque curve at around ~ 3000rpm everything smoothed out and it started to pull a fair bit harder than usual (sounding insane...)

And another thing i noticed, that if i could put up with the vibration and deafening drone, full throttle @ low revs seemed about normal power (though it is hard to judge) but under mild acceleration the car was hopeless, had no low end go, you had to really give it some pedal in the lower revs...

anyway, i'd assume that would be kind of similar to a bigger exhaust with the lack of vacuum etc, cheers aug for the info makes sense... if running a bigger exhaust causes something similar to no exhaust, i would assume you would use a fair bit more petrol also, as you need a fair bit more acceleration to move normally...

[Samon]

anyway, i'd assume that would be kind of similar to a bigger exhaust with the lack of vacuum etc, cheers aug for the info makes sense... if running a bigger exhaust causes something similar to no exhaust, i would assume you would use a fair bit more petrol also, as you need a fair bit more acceleration to move normally...

Yeah that (more fuel) and the fact that I've chewed through my clutch due to having to rev it higher and slip the clutch to take off without stalling... especially since i drive into uni and back home again often in heavy traffic, are the reasons why I would like to change the exhaust. I love the loudness of my exhaust and the power it has up around the 5000-7000 RPM area, but for practicality's sake i really need more torque down low.

[Eug]

but for practicality's sake i really need more torque down low.

you my friend have hit the nail on the head.

[342Four]

if only open exhausts were legal, and practical, ie still enough low down torque that would be sick

there is nothing better than a car belching out of a short exhaust and with a lumpy idle ie v8 territory, but from a 6

[MBS206]

There is a way to get back your low down torque.

It's called tuning.

Although your exhaust with a 3" is still aiming at a higher rev range.

Aimed at the guy who ran with no exhaust: Dial in around 4 degrees or so MORE timing down low, and a touch more fuel, and you'll have your torque back.

[Samon]

I've got a few quotes to replace the pipe and resonator/muffler between my cat and my rear mufflers with a 2.5"... varies between $180 and $300 depending on where I go and whether i get it Mandrel bent or just standard Press bent.

Pretty much every exhaust place have made comments that "it would be a bit gutless down low, wouldn't it?", so I'm pretty sure that this will help. They have pretty much all mentioned that I wouldn't have enough backpressure... some have said that if I use my existing muffler i won't have any advantage, others have said that it will be fine as long as the pipe size is reduced...

From reading what you guys have posted, i believe that it will be fine if i just replace the pipe (it splits into 2x 2.25" pipes going into the straight-through mufflers with 3" tips). My rear mufflers and tips are fine, they look nice and i would rather not replace them unless i really have to. I have a very wide cutout where the exhaust come out under the bumper (due to the two large pipes) and so i really need either twin pipes or one of those horrible looking wide oval shaped things... so using my existing setup seemed the logical way to go (it's stainless steel too)

[Samon]

Okay, so I got EPA'd the other day and now I have to get a new exhaust. If i were to get a 2.25" system made up would it be able to pass the 90dB test? Like what sort of mufflers/resonators would need to be in there to get it down to a passable level?

What do you guys have on your cars that passes EPA tests?

[mokompri]

Hence, on a 2.5" system, it takes 2.95seconds for the gas to empty,

On a 3" system it takes 4.27 seconds to evacuate.

And at 3000RPM, 25 times per second completely filled. 0.14M^3 of gas is expelled per second.

On 2.5" system: 6.5second

On a 3" system: 9.2 seconds

Now, Don't ask me what I was trying to prove when I started those calculations, except now we know how long it takes to get the exhaust gasses out (If the exhaust IS 3 metres long, and 3" all the way.

But, you can see, that's a HUGE amount of time at 3000RPM for the exhaust to take to get out (And plenty of time to loose temperature, contract in volume, and slow down even further (As all of the space is required to be filled in that exhaust *Even though there is "vacuums" in the exhaust, this is equallised by "Positive pressure" to create an equilibrium of pressure when added together*)

Also, those calcs were done at WOT. (Hence, 100KPA used for pressure)

Basically, I think that's showing, a 3" pipe is WAY Too big!

And a drop in 17% of the diameter, brings down the time the gas takes to get to be only 70% of the original (When at 3") hence, a 17% drop, created a 30% gain in time to get out.

i agree your extractor piping size and length will affect your power band, but the same principles dont apply from the cat-back onwards. least restriction is whats best, and the 3" achieves that better then the 2.5".

also i think your calculations are off. i couldnt follow your calculations, it seems like you have skipped inbetween steps, i couldnt make out all the variables and its been a while since i used moles in chemistry so it was a bit hard for me to follow. maybe a bit more step by step would be good..

but basicly what your saying is that @7000rpm, the velocity of gas out of the 3" pipe is going to be ~70cm/sec, that to me is way off, seems way to slow. rev your engine to 7K and see how fast the gas is travelling out the tip, certainly much faster then 70cm/sec to me...

putting all that aside anyways, i dont see what those calc's prove anyways. back pressure, by definition, causes a loss of power. even in the extractors, the least amount of restriction is desirable, the only reason why you dont go for the least restriction in the extractors is for scavenging purposes. the scavenging gives a net power increase, even if its not the least restrictive setup. in the catback there is no scavenging effect so the least amount of back pressure is desirable.

velocity is not desirable, its a side affect. change to a 2.5" catback and i'll guarentee you wont increase any low down power at all. also for what its worth, "sufficient" doesnt mean best