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How Much Power Do I Lose If I Get A 3" Catback Exhaust.

Awesome

By Awesome,
in Engines & Forced Induction

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N-DAWG Mentor

N-DAWG

Posted
Posted

Short answer: yes

Do a search and you'll see it's been covered. a 2.5" mandrel bend exhaust is going to be the best option for an N/A Skyline. If the exhaust is too large in diameter, you lose volumetric efficiency (commonly mislabelled as backpressure). Basically the exhaust gasses aren't filling up the space efficiently. You will have more top end power, but you will lose it quite a bit where it matters day to day.

scathing Grand Master

scathing

Posted
Posted (edited)
what do u mean by top end power?

Have you noticed how, with a stock exhaust, the engine feels strong and the revs climb well at low to medium positions on the tacho but it really feels strained as the engine revs continue to go higher? A free flowing exhaust will make the engine more willing to rev up top, as a restriction is removed.

and wot u mean day-to-day..

Have you noticed how you hardly ever rev the engine out to redline every time you go for a drive (I hope)?

Edited by scathing
scathing Grand Master

scathing

Posted
Posted
I am an aggressive driver... thank you. 3inch it is =D

The problem is that all that extra revviness up top is replaced with sluggishness down the bottom, for the reasons given above. So if you can keep it high in the rev range you should be OK, but let the revs drop and it'll run worse.

In the end, a car with a wider spread of torque will probably be quicker than a car with a narrow powerband. Just look a Hondas :)

I haven't looked into RB25DEs specifically, but if everyone is saying that a 2.5" is the best exhaust to get good gains across the entire rev range then, seriously, that's what I'd be doing.

Vik Mentor

Vik

Posted
Posted

Ive got a 3inch at the moment, and its true, if your hitting it hard on the motorway, then your gonna have to keep it on high revs ( within the speed limit ).

And the noise it makes is alright, but will get very very very annoying if your driving home from the city after a good night out and you just want to sleep and all you can hear is that big ass exhaust.

Trust me, unless your planning to do a turbo conversion, then go with the 2.5 .. sounds awesome and more evenly distributed power gains i guess.

MRXTCZ

Kranker Collaborator

Kranker

Posted
Posted (edited)

in before the lock....and cant we have some kind of a max new thread rule for new members?

edit: its great you want to learn more about how cars and performance modifications work, but lots of the questions you are asking have been answered time and time agian.

if you cant find the answer by searching the forum, (sometimes it can be hard to find old topics), then ask your question.

I know that a skyline related question pops into my head 3-4 times a day, and 9 times outa 10, ill use a few different search term and find someone else who has asked the same thing.

if u still cant workout a definate answer then post up a topic, by all means

Edited by Kranker
central coast person Proficient

central coast person

Posted
Posted

How Much Power Do I Lose If I Get A 3" Catback Exhaust???

well your answer to conclude is if you are aggressive get your 3"

if not so get a 2.5"

just drive everywhere in 4000rpm burning everyones petrol :P

-FIGJAM- Grand Master

-FIGJAM-

Posted
Posted

i notice that alot of people ask this same question, dont know if this has been answered in another thread, but how much power is gained with a 3" instead of a 2.5" or 2"?

My understanding with exhaust, is limited at best, but dont they really just free up what power the engine has and is loosing due to breathing problems? I understand the compression and all of that, how a N/A will build up less emission than a turbo at a faster pace, hence the requirement for a bigger system. Am i on the ball or should i go do another wikipedia search? :)

Mitch

Eug Grand Master

Eug

Posted
Posted

the reason a lot of people ask the same question is because they don't really understand the physics behind exhaust systems.

bigger is better if you are in the high rpm applications (i.e. dyno) as the engine is expelling a lot of exhaust gases, the volumetric efficiency is high which improves the flow properties of your system. however at low rpm applications (i.e. street driving) the engine isn't expelling nearly as much exhaust gases, the volumetric efficiency in a large exhaust is low and thus the car will feel 'gutless'.

the issue i see with that is, a lot of people use the dyno as a power indicator. the reading is the highest output power at a reasonably high rpm. so of course a large exhaust is going to be seen as an improvement.

too small of an exhaust and it creates a restriction. your engine is expelling more exhaust gas faster than you can get rid of; this creates backpressure, where the exhaust gases go back towards the engine rather than out.

the perfect exhaust is a variable one, which adjusts according to the amount of gases being expelled. this would be used to create the most optimum scavenging effect which is not easy and most probably expensive.

a lot of RB25DE owners have tried 3" exhausts and most have found them to be unsuited for their regular street driven cars. However Cara has reported an improvement with a 3" system, this may be in part due to her motorsport commitments, ecu and nitrous. Not something everyone has.

I personally use a 2.5" exhaust and do a fair amount of motorsport activities which involve full use of the entire rpm range. I would not go any larger unless I started making significantly more power as I would be sacrificing driveability.

here's a simplification.

air/fuel in ----> engine ----> exhaust out

if you are putting more air/fuel in, then you will need to take more exhaust gases out. it's a balance and you would really need to do both ends to maximise the benefits.

in saying that stock exhausts are a little restrictive from factory due to emission laws. so going a little bigger will improve the efficiency, but going too big then you start to lose that efficiency again.

that's the final say. from now on i will close all 3" exhaust threads without any notification.

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