confused large exhaust good?/bad?

[sophieschoice]

I've always thought that allowing your engine to flow more easily would increase horse power. A few days ago I saw street tuner challenge, and they said that young people put these large exhaust systems on their cars not realizing that they can harm performance especially if the engine is one with low compression. Is this true? What do they consider low compression? Does this apply to the ka motor?

[AZhitman]

STC is a poor source of information for all but the most basic of automotive knowledge.

With that being said, they are correct to a certani degree, and I could go into a long explanation of exhaust pulses, eddying, cooling, exhaust gas velocity, etc...

Bottom line is this: N/A KA motors seem to perform equally well with a larger exhaust system. Some will claim a small loss of bottom-end power, but I have not yet seen this to be the case.

There's no real reason to run larger than 2.5" on a N/A KA motor though - You just get into cost and clearance issues.

[SeanC]

ok i may start a huge pissing match. alot of people will try to tell you that backpressure will give you torque or "low-end" the simple answer is no it does not. backpressure always = less power everywhere. exhaust velocity does on the other hand change things, exhuast velocity is taken care of by header design, tube length, port design and size and the collector size, after that all the exhaust does is move the exhaust from under you to the back of your car so you can breath. i have yet to see the fact that 3" hinders low end power proven with a dyno graph. BTW my n/a motor with only 3" exhuast runs 15.5 in the quarter bone stock. i dont think that the 3" hindered it in any way.

[AZhitman]

^ Sean is correct.

However, the KA seems to be the exception rather than the rule.

In most other motors, excessive cooling of exhaust gasses in the system (from too large an exhaust) causes velocity to reduce, which not only increases backpressure but reduces scavenging.

3" on an N/A KA won't hurt a thing.

[BadMojo]

[QUOTE=AZhitman3" on an N/A KA won't hurt a thing.[/QUOTE]

What about your hearing and your wallet?

Someone (possibly DeviousKA) posted a dyno sheet for a 3" exhaust on a KA24DE that showed no loss of low end of any kind. Can't seem to find the post, but it's around here somewhere.

[Bigvinnie]

Most dyno charts don't incorporate a full fledged powerband from 1000RPM till 7000RPM. It isn't noted where any power is lost in the early lowend range of a 3" or 2.5" exhaust system (below 2500RPM).Lowend range of power is what contributes to a good percentage of acceleration and streetability.I have yet to see a 3" cat back exhaust that nets more overall power than a 2.5" cat back exhaust (1000RPM~7000RPM).3" is a waste of money and loss of hearing, as well as a few noise violation tickets.

[sophieschoice]

thanks for all knowledge guys. I appreciate it. Does this also apply to intercoolers as well. I just bought a 3" intercooler kit off of ebay for a turbo project. Did I waste my money? The 2.5 " was cheaper by a little, but every dollar counts. BTW I have a convertible so noise is an issue. Can you recommend the quietess exhaust.

[InsanityInc]

First question, for the exhaust:

A large exhaust system can never make you lose power over stock, except in the rare instance that your factory system is already well tuned (350Zs are like that).

To understand why a smaller pipe can produce more power for a given car than a larger one, you need to understand something about engine airflow as well as the venturi effect. The venturi effect is something based on the bernoulli principle which is a physics equation that states as the velocity of a fluid (gases and liquids are fluids) increases, the pressure across that fluid decreases. This is also why a syphon works. The venturi effect is an extension of the principle which states that a fluid forced through a smaller opening will increase in velocity, therefore the pressure will decrease. The operation of a carburetor relies on this. Therefore, a high exhaust velocity will decrease backpressure (often into negative pressures, or a vacuum, actually assisting exhaust scavenging as opposed to impeding it).

However, there is a trade off. A pipe that is too small will impede proper flow and increase backpressure. This is because any given opening can flow a given CFM of a gas at a given pressure. Therefore, if your pipe at 0 pressure cannot flow the amount of air your engine is producing, the engine will be forced to pressurize the exhaust until it can travel through the pipe.

So, you have a bit of a dichotomy. A smaller pipe can increase velocity, which can decrease backpressure into negative levels (the desired result), whereas too small a pipe will simply cause too much backpressure to possibly be negated by any velocity gains. A really huge pipe will have 0 backpressure (cooling gases can't cause backpressure, it's simply impossible) as it will be fundamentally the same as running an open header. However, 0 backpressure obviously isn't as good as negative backpressure which is why precise exhaust tuning is important.

Probably more than you ever wanted to know about that, but there you go.

As for the large vs. small intercooler, it's somewhat related to exhaust size but not all that much given that the purposes of the devices are different. However, there is a similar dichotomy. Essentially, a larger volume of air (larger hot/cold pipes and intercooler) take longer to pressurize, and also will not reach as high of a pressure (remember, pV=nRT, so if V is bigger and you're still shoving n moles through, then pressure has to drop). So, a larger intercooler and hot/cold pipe will make your overall boost pressure decrease across the pipes as well as increasing turbo lag. A smaller intercooler (or lack thereof) will do the opposite. However, the problem arises when you consider that a larger intercooler will cool better than a smaller one (generally speaking), meaning that even though your pressure might end up lower, you still have a more powerful intake charge.

So, the tradeoff here is that your turbo lag is increased, your throttle response is decreased and your power is increased as you go to larger and larger intercoolers.

[sophieschoice]

Thanks InsanityInc! That is exactly what I was looking for. You Rock Man! Not only am I no longer confused, but I feel much better about my intercooler purchase. Thanks again everyone!!

[S13FX]

Ok so let me get this straight. I can just keep my 2.5in Exhaust for my 10psi and still be ok? Cause if so this is some freakin GREAT news heh, now I can save up the 200 some dollars I was going to spend on a huge 3in exhaust and put it towards something else .

[InsanityInc]

You can run 10psi on a stock system if you really want to. However, with turbocharged engines, bigger pipes tend to be better no matter what, as the majority of the backpressure in the system is caused by the turbine in the exhaust stream, so any vacuum created in the exhaust really doesn't do anything for you, making the goal simply to not add any more backpressure to the system.

[Bigvinnie]

Ok so let me get this straight. I can just keep my 2.5in Exhaust for my 10psi and still be ok? Cause if so this is some freakin GREAT news heh, now I can save up the 200 some dollars I was going to spend on a huge 3in exhaust and put it towards something else .

Fuel dumping is the largest concern for upgrading exhaust pipe.Considering that you are running 10PSI, I would be under the assumptionm that you are using 370cc injectors. In that case 2.5" would be fine.Most backpressure on turbo systems happens within the manifold before high spooling and rpm usually at lower RPM. No need to upgrade IMO, although a 3" would eventually be nice.

[240sxRAOR]

As for the large vs. small intercooler, it's somewhat related to exhaust size but not all that much given that the purposes of the devices are different. However, there is a similar dichotomy. Essentially, a larger volume of air (larger hot/cold pipes and intercooler) take longer to pressurize, and also will not reach as high of a pressure (remember, pV=nRT, so if V is bigger and you're still shoving n moles through, then pressure has to drop). So, a larger intercooler and hot/cold pipe will make your overall boost pressure decrease across the pipes as well as increasing turbo lag. A smaller intercooler (or lack thereof) will do the opposite. However, the problem arises when you consider that a larger intercooler will cool better than a smaller one (generally speaking), meaning that even though your pressure might end up lower, you still have a more powerful intake charge.

So, the tradeoff here is that your turbo lag is increased, your throttle response is decreased and your power is increased as you go to larger and larger intercoolers.

This is if you have turbo what would intercooler size do for roots supercharger? I would asume that bigger is better since the air will compress after the intercooler. Or do superchargers have a sepcial type of intercooler?

[InsanityInc]

If you put an intercooler before the supercharger it won't do anything. Air heats when you compress it. Roots superchargers are rarely intercooled (or, as it's generally called with superchargers - aftercooled).

[mmm240]

a lot of the gain that you get from aftermarket exhaust is eliminating a lot of the bends. for example on my 93 there were two 180 degree bends just before the muffler

[knx2]

Ok but I have a question. My friend and I chopped my exhaust off of my 93 240sx and we drove it to the parts store to buy new pipe. basically a test pipe to replace the cat. My car acted like it had lost 50 horsepower. We put the pipe in and the power came back. What is the reason for that?

[mmm240]

maybe velocity (exhaust didn't flow properly)

[Burnsey]

BTW I have a convertible so noise is an issue. Can you recommend the quietess exhaust.

First off ... I absolutely hate obnoxiously loud exhaust systems and they piss me off beyond belief ... and i hate ricers too.

Moving right along, to your question ... I've done some research trying to find the right exhaust that is quiet, looks good, and you still get performance ... pretty much I think it narrows down to two items ... I will suggest BRM (AZhitman's exaust) because its a good 'ole NICO affiliated exhaust and I've been able to find 30+ ppl who say they love the exaust and that they feel the same way about exhaust as I do ... BRM is an amazing product ... another choice would be RSR Exmag Exhaust but i've only been able to find about 3-4 ppl who actually have it installed on their cars ... its supposed to be quiet as well. But BRM is ur first choice so hop on it and talk to AZ cause I'm sure he'd be more than happy to help you choose what best suits your needs.

[InsanityInc]

Ok but I have a question. My friend and I chopped my exhaust off of my 93 240sx and we drove it to the parts store to buy new pipe. basically a test pipe to replace the cat. My car acted like it had lost 50 horsepower. We put the pipe in and the power came back. What is the reason for that?

If your car had a stock system, I guarantee that an open header will do nothing but gain power.

The only thing I could think of is that the exhaust exiting there was causing something funny with the O2 sensor, or you were just driving it easier so as to not blow up your eardrums.