Oh no a 3" vs 2.5" exhaust question on vr4.org

[trunks]

So I've been reading countless threads on tooners about this from 2006 or so.

I have a home made 3" that is pretty quiet with a high flow cat, two resonators, and a magnaflow muffler. I'm getting really old, I mean grumpy old and I want it to be quiet so I can listen to my oldies on the radio while I drive to work these days.

Anyway, I'm thinking of going 2.5 on the cat back and putting a couple of large mufflers (high flow) on it to get to Buick sound levels. Plus I have a dds alunimum drive shaft still in the wrapper to install that makes it a tight fit under there. Will I really lose that much power? I'm running e85 with a 68hta, smim, etc that is probably around 450ish whp once I get it tuned. The thing is I daily drive it and hardly get into it unless someone really pisses me off or a really hot girl is walking around but my ego won't let me sacrifice any performance. I remember reading something where it really didn't make as big a difference in actual 1/4 mile tests as you would think but can't find that thread.

All I ever read on tooners is '3" all the way' but I was hoping here to get a more educated opinion here.

Thanks!

 

My car has a full 3 inch exhaust, which I like, don't get me wrong, but my car was much more enjoyable on the daily with the factory pipe. There is alot to be said for not attracting cops because of the noise. It is also my opinion that my car spooled faster on the stock exhaust, but is happier above 5500 with the 3" pipe. Take it or leave it, it is only my 2¢.

 

[desant78]

once heard that the turbo is FAR more restrictive than the exhaust, so really 2.5 or 3 doesn't matter.

food for thought, don't have any concrete evidence for that.

 

[trunks]

Thanks guys. Yeah I really remember reading something that said like 20 hp difference between stock and a 3" on a dyno or something with a smaller turbo like mine sometime but I cannot find it to save my life. I guess it should come down to my driving preference in the end anyway.

I know what you mean about stock - sometimes I wish it was less modded and I would probably enjoy it more as a dd.

 

[turbowop]

Quoting desant78:
once heard that the turbo is FAR more restrictive than the exhaust, so really 2.5 or 3 doesn't matter.

food for thought, don't have any concrete evidence for that.




Turbos are all about pressure differential. So while they may be super restrictive before the turbine wheel, they need a nice open pipe with the least restriction possible to allow for quicker spool times and more p0wer.

 

[trunks]

So now that you've weighed in - Opinion on 3" dp to 2.5" catback?

 

[Brunoboy]

I have a full 3' with resonator and muffler. It's not as loud as I had t before but I want to go cut out with a stock catback.
-shane

 

[broxma]

Seems to me you are asking if the change in diameter is so drastic as to be a problem at your HP level.

At 300 HP a 2.5 inch exhaust is not optimal. You're looking at around 4.5 inches squared of area, each inch flowing around 120(ish) cfm. A car making 300 HP is pulling in more than 500 CFM most likely at the intake and you generally need more exhaust size than intake size for an equal system.

As for how much difference it would make going to a smaller pipe diameter, I can't say, but ti would suffer, and it would be measurable, probably on both the dyno and the track as well as SotP driving.

/brox

 

[Diego]

I rock 3"CMPE exhaust, straight turbo back into a muffler. Sounds fine to me.... others can hear me for blocks... I don't listen to music in my galant, so it doesn't bother me.

 

[trunks]

What is the actual data on exhaust cooling down stream? It cools and moves slower hence larger down the line is better? Or cooling=contraction=less need for circumference so 3" to 2.5" is okay? I haven't found any threads that scientifically explain it to my satisfaction?

 

Quoting broxma:
Seems to me you are asking if the change in diameter is so drastic as to be a problem at your HP level.

At 300 HP a 2.5 inch exhaust is not optimal. You're looking at around 4.5 inches squared of area, each inch flowing around 120(ish) cfm. A car making 300 HP is pulling in more than 500 CFM most likely at the intake and you generally need more exhaust size than intake size for an equal system.

As for how much difference it would make going to a smaller pipe diameter, I can't say, but ti would suffer, and it would be measurable, probably on both the dyno and the track as well as SotP driving.

/brox



One problem with what you are arguing here. All of these numbers are fine assuming that you are making full power @ maximum airflow 100% of the time. Generally speaking, torque is optimized with smaller exhausts, HP is optimized with larger pipe.

Quoting trunks:
The thing is I daily drive it and hardly get into it unless someone really pisses me off or a really hot girl is walking around



A 2.5 inch pipe will only hurt you where you admittedly spend very little time, at the top of the RPM band.

 

[trunks]

Yes I must say my old man told me on all our machinery (farm boy I am) to 'take it easy' and I have a hard time taking it over 6000 rpm for fear that he will be right and my car will blow up. I can almost see that look of disgust when I rev my car.

Seriously though, if we are talking 20 hp or even 40 hp I guess I'm okay with that. I actually like the idea of more torque on the low end actually.

 

[trunks]

Quoting Diego:
I rock 3"CMPE exhaust, straight turbo back into a muffler. Sounds fine to me.... others can hear me for blocks... I don't listen to music in my galant, so it doesn't bother me.



That's cool with me man. I'm just at the point in my life where I'm taking my daughter, 1st of 3 to school, and I don't want the stigma of the loud ass car when I pull up to kindergarden.

Small town. People talk you know /ubbthreads/images/graemlins/smile.gif

 

[Scott Y]

You wanted scientific evidence, and all I can give you is anecdotal:
2.5" w/cat &resonator & magnaflow. The car is pretty quiet, and has still gone 12.42@114 at 5800' alt. The exhaust is approx 8 years old and the muffler could use replacing as it's starting to get louder. 8 years is still pretty good, I think.
Turbo = BR20G.

 

[trunks]

That's cool. Anecdotal is all right with me. A 12 second trap is way more than I could probably handle anyway /ubbthreads/images/graemlins/smile.gif

 

[turbowop]

Quoting galant1517:


One problem with what you are arguing here. All of these numbers are fine assuming that you are making full power @ maximum airflow 100% of the time. Generally speaking, torque is optimized with smaller exhausts, HP is optimized with larger pipe.





This does not translate to turbo motors after the turbine. While NA motors may benefit from scavenging properties of a smaller exhaust, turbo engines benefit from the largest exhaust possible...or tolerable. Larger exhaust = m0ar torque, quicker spool, m0ar horsepowerz.

 

[SmoothCustomer]

This is completely unfounded, but I hear that if you put a glass pack in before the muffler it quiets things up a ton and doesn't restrict flow at all. Again, I have no personal experience but I hear good things.

 

[VR_IV_MR]

what kind/ size of resonators do you have? I was told by Keith from turbotrix that the only resonator that is worthwhile is the Vibrant Ultra Quiet Res...

Im kind of upset reading this because Im looking to do an exhaust that is exactly like yours (3", 2 resonators, muffler) and I was hoping it would be quiet enough to drop the kids off at school... /ubbthreads/images/graemlins/frown.gif

 

[Brunoboy]

I'm running the vibrant ultra quiet with a vibrant single tip muffler. I can tell you for sure the resonator quiets the exhaust down. I used to drone and couldn't have conversations in the car but now I'm able to talk to everyone clearly without yelling.
-Shane

 

[VR_IV_MR]

and you said you had two of those resonators? damn... do you have poly motor or suspension mounts?


well if you aren't concerned about losing a little power (or adding weight) have you thought of doing some sound deadening material? this would help with the oldies station but not so much with the neighbors...

edit... thought you were the op for a second, sorry...