Thread: Exhaust sizing...

I did a few random searches thru threads on both sites today, and didn't really get a firm answer to my question.

I do remember seeing a standard to go by, that BenFenner posted (I think?), regarding exhaust sizing vs. power output.


What I'm being told now, by a few good friends (Albiet Honda heads), is that I should just forget about the 3" exhaust and go to a 4" cat-back exhaust (without a cat). I was actually pretty impressed with one of the cars, being a civic hatch with a cheap slapped-on turbo kit that was laying around the shop. They ran nothing but 11.8 and 11.9 at the track consistently, and decided to run open downpipe. That same night, they shaved down their 1/4mi. times by .5 seconds! The following weekend he put an in house 4" catback exhaust on that he welded pretty quick, and his times stayed down in the 11.4 neighborhood. Car sounds really good, quiet, and feels really strong.

Is it worth it to part my 3" magnaflow exhaust for a larger 4" exhaust?

I would do it. Also do it in AL because it will weigh next to nothing. I am doing 4in turbo back on my 35r. The AL exhaust will start just after the oil pan.

Matt

ive wanted to go with a 4'' exhaust but not sure on my setup if the 4'' would be overkill for me.

but if you were going to go with 4'' i wouldnt do it unless you go with a 4''downpipe to 4''catback otherwise i think its a waste, reason being is your turbo is still going to be flowing the volume of 3'' out of the downpipe then yes it will open up, but technically the 3''downpipe will be a bottle neck in power...4'' to 4'' will be optimum

i feel if you dont go with the downpipe also, you will not be completely utilizing the power you could be gettingr

your whole exhaust starts at the downpipe, if you have 3'' diameter of volume verse 4'' diameter in volume, you are going ot be runnig more volume off the bat with 4'' obviously over the 3''

The general consensus seems to be bigger is better for turbo applications. What type of horsepower goals you have, what kind of driveability you want, and what size turbo you are going to run must be considered, as there is certainly a point of diminishing returns.

With a turbo, the greatest determining factor of how much power any given impeller combination is capable of is going to be determined by flow rate and pressure differential. Let us disregard flow rate (as this will be determined by impeller design). The way that pressure differential can be promoted on the exhaust side is by having big boost relieving into a vastly lower pressure (read larger volumetrically) downpipe.

Although you are pressurizing the air/fuel charge via the induction impeller, the scavenging effect of the exhaust flow exiting the cylinder, and direct flow created by the intake and exhaust valves being overlapped (open) at the same time should not be disregarded.

In choosing a size for your exhaust, you need to consider where you want your powerband. Do you have a smaller turbo, and want to have a little torque-monster, that makes power down low? If so, think 3" rather than 4". Do you have a big turbo and want flow and higher horsepower at high RPMs? You might consider 4" for your piping.

Another consideration is fittment; are you going side exit, or conventional routing out the back? A four-inch system would require some careful execution around the rear suspension to avoid clearance issues.

As far as materials, I don't know that I would consider aluminum, as the expansion rate of it may create it's own fitment issues as it heats up.

As to whether it is worth it or not... well I guess that will depend on your need for speed, and if you are tracking/dragging the car; in that case, every pony counts, but are you willing to part with the bucks to extract every last bit of power from the engine? Looking beyond the potential exhaust upgrade, are you looking at bolting a big turbo on? As I alluded to before, what are you willing to live with ultimately? Is this a daily driver, or something more mission specific?

More questions than answers, I know... but I hope it helps!

Darrin

Damn that was some good information In a nutshell, I should probably keep what I have. I'm running a 3" turbo-back exhaust with no resonator (albeit still running stock cat), on a gt3076r .82 A/R.

Going larger would possibly kill a little more low end, and give me high end. I'm not concerned with topend, tbh.

Thanks for the replies guys

I would just go with a cutout, it would be a REALLY tight fit with 4" piping. Unless you weld up a few more stiff hangers throughout, it will be banging around under there.

It already bangs around

It's not hanging on the hooks where the cat is. I couldn't get it to hang with the stock hangers, so I need to have the shop weld up something nice to isolate the exhaust.

If you still have the bracket that is normally attached to the stock location where the cat goes, maybe the shop could use that as a template to weld in a U-shaped piece of rod to be bent into the rubber hangers there, or else just do it free-hand. That could go a long way to stabilizing your exhaust, not to mention prolonging the life of your flex-pipe.

Go as big as you can. The only reason to go with a mid-sized exhaust is to take advantage of pulse tuning for NA cars. Since the turbo absorbs the pulse, it is useless. Go as big as possible. Here is a dyno sheet for comparison...



The squigly line is the same boost level, but with open downpipe and 4* LESS timing. I cannot even describe how much stronger the car feels on new plugs and that 4* more timing. There is no comparison, taking the exhaust off made more power everywhere. But this gives you an idea. I made over 50whp in the midrange, even with less timing and plugs with very small gaps, around .020".

I would like to dyno again with the same setup but with the timing back in and some good plugs. I put in some iridiums and they are doing great. Hope this helps.

^ did you have no exhaust at all or?

exhausts are all about volumetic flow, larger diameter larger ammount of volume, smaller diameter, smaller volume. if you stayed 3'' downpipe you will get 3'' diameter of volume flow.