Want to start an exhaust system theory discussion
 

I have some questions for which I seem to get conflicting answers, and I'm wondering where I can find a good book on exhaust system design theory / tuning.


I have lived under the impression that on small displacement 4 cyl N/A motors, once you get past the headers / cats / front pipe - the section of the exhaust generally refered to as the "cat-back" had little to no effect on power production or delivery. Thus headers that were properly designed to scavenge / use exhaust pulses were the most important piece of the puzzle, followed by reducing catalytic converter restrictions - the catback was just for gas routing and sound managment purposes. By the time the gasses got that far down the pipe, there was no "gain" to be had - the velocity had dropped off too far.


Are the assumptions correct? I ask because I recently installed an Ace header and catless front pipe + tune (on a track only, NOT road use BRZ), and planned to fab my own 2.5" stainless, minimal weight dump pipe with a small resonator and 6" magnaflow canister muffler to cut the noise a little.


As I poked around with some guys that are familiar with spec miata racing whom I respect, they say they have seen quite an effect (10+ hp gains / losses) on the dyno when testing different diameters and tubing lengths of the exhaust system AFTER the headers... In the low HP NA car world, 10 hp is a huge difference!


This gives me pause - I don't want to fab up a short section of exhaust after the front pipe and end up losing 20+ pounds of exhaust weight...but also losing 10 hp as well!


Thoughts? Resources? Experiences? Any good books that anyone can recommend?

Other vendors say that you have gains with a catback and others say no. Personally, I believe that there are some gains, but in low gears (2nd, 3rd gear) where your engine revs faster and there are more exhaust gases on same period of time. With a better catback you might have a bit better engine response and a bit better acceleration on low speed corner exits.

I think there is a bigger difference in the miata since the stock cat back on one of those is sub 2 inch. Stock for us is like 2.25ish.

Somewhere I have a book on the subject. I'm going to try and dig it out as much for you as my own personal use re-designing the exhaust for my car.

If you're looking for the technical side of things, this is a good starting point:

http://www.enginelabs.com/engine-tec...gn-and-theory/

I'd say, that there are gains to be had .. BUT! Only if catback is only thing upgraded.
If you had got most (and more then from catback) gains from upgrading header & ecu tune, better flowing catback will net way less on top of those two.

Flow is everything.

The "axle-back" section of these cars does have some significance but not as much as a Miata I believe.

You do need something there to help direct exhaust flow. Otherwise every racecar would just run an open header.

If your track doesn't have a sound limit requirement, just a 2.5" pipe out to the bumper is fine. We used the Nameless 2.5' resonated muffler delete for years to keep sound under 110db.

Once we got into aero stuff, the center exit was born.

We dynoed lots of variables of axle-back systems. Nothing ever showed anything more then a 2/3HP change.



Don't think about it too much. If you think your set-up is fast, it's fast!

peak numbers don't tell the whole tale

So I run HPDEs at 5 different tracks and so far, I haven't run into any DB limits. However I also don't want to be lumped in with the guy with the Ear drum shattering, shrieking straight piped ls swapped 240sx with a pickachu hanging off the bumper either...thus my plan to use a resonator and the canister magnaflow.

Doozer, if you don't mind, could you explain how you routed the center exit? Did it pass under the rear subframe in the factory location? Running the exhaust out to the bumper wasn't part of my plan - it was a turndown in front off the diff. But I'd hate to screw up aero balance royally by doing so.

Thoughts? Do you have a full flat under car pan? If so, care to share the plans :)?

Is the implication that the catback portion of the exhaust does significantly impact the area under the curve?

Don't worry about the aero stuff. A center exit exhaust in regards to the total aero package is minimal. There are much bigger gains in other areas worth your time and energy.

I would recommend just a standard 2.5" single exit muffler delete exiting driver's side. Any form of resonator in this axle back section should help your ears.

You have to think:
Japan engineers spent a lot of hours routing of the original exhaust system.
Ace also spent lots of hours engineering the header.

The only thing both Toyota and Ace engineers did the same was to measure everything with the total length of the exhaust in place, engine-to-bumper.

Hope this helps

This is the reason I did a header and just an axleback. Did the header for the power gain, and an axleback for the extra volume and weight savings. Using stock front pipe and mid pipe to keep the volume reasonable, I'm sure there is a couple HP to be freed up by changing the front pipe, but not for the added volume. Paid $224 for the axleback vs potentially up to about a grand on a good catback. Even if the entire catback can give me 2 or 3 hp, that wasn't worth it for the price.

I still believe there has to be gains coming off an Ace header with aftermarket front pipe, catback. I know there dyno results were with stock fp ect but the diameter is so damn tiny. I believed I measured around 52mm inside diameter for the FP.


Ace headers end in a 2.5/63mm outside diameter which is probably around 60mm inside. I find it hard to believe that with all the velocity and scavenging coming off the Ace headers hitting a 52mm restriction along with a cat doesn't hurt performance.


All I know is when I upgrade to Ace header I will be upgrading the rest of exhaust piping to 2.5" as well (High FLow Cat FP, 2.5") catback to keep the velocity constant.


I currently have Gruppe-S UEL 60mm, INvidia Catted FP 60mm, HKS legamax catback 60mm. All piping is consistent 60mm OD all the way through and I feel the great response this setup give.

I'd imagine you are correct. Heck Ace even offers a 3" front pipe to compliment their headers. I'm running a resonated 2.75" front/over combo from Ultimate Racing and the throttle response and drivability of the car certainly improved over oem. Less rasp too.



There is a dyno floating around somewhere that backs up my own claims. I've ran it with the Gruppe-S header, oem and now HKS. Couldn't imagine replacing it unless going for a full Ace setup.

I can tell you that my hooker catted front pipe with helmholz resonator did eliminate some drone compared to stock and no extra volume. I have JDL UEL catless headers and Q300.