Andy Hollis
Andy Hollis
9/22/20 9:27 a.m.

[Editor's Note: This article originally ran in the June 2013 issue of Grassroots Motorsports.]

New cars are great. They take advantage of all the latest technology to squeeze out maximum performance, all while providing the appropriate street driving experience for the intended market. 

In the case of econoboxes, that usually means great mileage, good handling and a quiet cabin—but only adequate power. To correct this compromise, you can give up the quiet cabin and gain some forward motivation through a performance exhaust. When prepping our Mazda2 for Street Touring FWD autocross, we ran into a little issue and needed to design an exhaust from scratch.

Like most modern cars, this Mazda2 uses two catalytic converters, including one directly in the manifold. This puts the cat closer to the cylinder head, which reduces cold-start emissions. The manifold runners must be quite short as a result, and that tends to hurt midrange punch. Additionally, the exhaust is at its highest velocity close to the head, so any restriction will have a more dramatic effect than one farther down the piping.

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fidelity101 (Forum Supporter)
fidelity101 (Forum Supporter) UltraDork
9/22/20 10:12 a.m.

I am working on this very same thing right now!

 

https://grassrootsmotorsports.com/forum/build-projects-and-project-cars/years-in-the-making-franken-wankel-1986-rx7/59966/page15/

 

I also like how the mazda2 is rated at 100hp but dynos at 95whp+

Indy "Nub" Guy
Indy "Nub" Guy PowerDork
11/30/20 9:13 p.m.

I'm in process of making some exhaust for my car right now, so when I read the title for this article: 

"Fabricating a Custom Exhaust Header"

I got excited, looking for some meaningful tech tip.  But I leave disappointed that the extend of tech tips begins and ends with this phrase from the article:

"Our car rested at the race shop for several weeks as the parts arrived and the crew built our header between their other projects."

 

sigh.

 

Keith Tanner
Keith Tanner MegaDork
11/30/20 9:31 p.m.

Best tech tip? Get some header lego like the stuff from Iceengineworks. It'll let you build stuff like this.

There are stl files out there so you can 3D print your own, but make sure that each segment is equivalent to 1" of tube length and has indexing marks - some of the ones I've seen miss out on these important factors.

Keith Tanner
Keith Tanner MegaDork
11/30/20 9:36 p.m.

Less extreme. This one was heavily space-constrained so equal length runners weren't going to happen.

frenchyd
frenchyd PowerDork
11/30/20 9:38 p.m.

At the typical RPM used on the street there will be almost zero impact in power output. 
 If you build it for racing you might gain 4% at near peak RPM. But you can quickly kill even that with mufflers and Catalytic converters. 
The  Biggest gains are typically reserved for V. Engines. V6-8  where there is room to put pulses out 180 from each other. 

Keith Tanner
Keith Tanner MegaDork
11/30/20 9:39 p.m.

Frenchy, no. You need to spend some time on a dyno.

Here's that first header I showed in a straight A/B test against an off-the-shelf Racing Beat part on an inline 4 cylinder with a muffler and cat.

http://www.targamiata.com/images_lrg/RB_vs_Keith_header.pdf

frenchyd
frenchyd PowerDork
11/30/20 9:56 p.m.

In reply to Keith Tanner :

How much power did it gain at 2000 rpm? 

Keith Tanner
Keith Tanner MegaDork
11/30/20 10:07 p.m.

300 hp.

Paul_VR6 (Forum Supporter)
Paul_VR6 (Forum Supporter) Dork
12/1/20 8:58 a.m.

If you can't afford to buy or borrow the ICE stuff (which is amazing!) you can use vacuum hose (like for a vacuum) and zip tie welding rod to the outside to get it to hold shape to match your mandrel bends. I have found this method to work "well enough" and you can cut the hose ahead of time to get equal length sections to then try different configurations without cutting into your pile of $ tubing.

Dusterbd13-michael (Forum Supporter)
Dusterbd13-michael (Forum Supporter) MegaDork
12/1/20 9:03 a.m.

What i did: bought a set of used headers. Cut them apart to make them mandrel bends and straight pieces. 

Took some 3/8 plate and a lot of time to make flanges for the headers based on exhaust gaskets. 

Placed mu collector in the car which was a piece of exhaust tubing. 

Bolted the flange to the head. Started connecting the two with the mose cramped tube run. I held the pieces of tubing together during mockups woth painters tape. When i got happy, tacked everything and test fit repeatedly. Adjusted as necessary,  numbered and witness marked all the peixes, welded together.

Keith Tanner
Keith Tanner MegaDork
12/1/20 11:40 a.m.
Paul_VR6 (Forum Supporter) said:

If you can't afford to buy or borrow the ICE stuff (which is amazing!) you can use vacuum hose (like for a vacuum) and zip tie welding rod to the outside to get it to hold shape to match your mandrel bends. I have found this method to work "well enough" and you can cut the hose ahead of time to get equal length sections to then try different configurations without cutting into your pile of $ tubing.

I usually start with blue pool hose to get an idea of the rough layout. I did one by beginning with pool hose then using welding rod to come up with the shape. I didn't attach the rods to the hose (good idea, that) but just left the right amount of clearance from tube to tube. I can post pictures if anyone's interested.

 

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