Headers to a single outlet, where to merge?

[2mAn]

So im installing a set of long tube headers on my eta and im also going to have a new 2.5" exhaust made. My headers are a 3-1 design so there will be dual secondaries under the car. My car is 25 years old, so I will be eliminating the cat because there is no emissions testing required in WA. The rest of the exhaust system will be a 2.5" in and out resonator and a single 2.5" in and out magnaflow muffler.

My only question is how far back should the two secondaries collect? I was thinking about having the two pipes go back about a foot or so before they merge into one. Im trying to make the most torque possible, thats the goal. if its a neglible gain moving them back a foot, then I will just merge them together immediately.

Thoughts?

[walktheboard]

I really don't think it will make much of a difference on a stock eta

[2mAn]

thanks for your comment, but im looking for educated responses. long tube headers generally make more torque, and thats what im looking for. its an eta so its not going to be a drag car. just looking for more midrange grunt and every little bit helps. its chipped too

[stillkotd]

This is a quote from an old article I found on Porsche headers, but it applies to any exhaust system.

"Each pulse of gas has mass, and as it moves down the tube it develops momentum. If you suddenly try to stop the flow of that pulse by closing the exhaust valve, it will attempt to keep moving (a body set in motion will remain in motion). The result is that something of a vacuum is created behind the pulse. If the exhaust valve is still partially open, that vacuum draws the residual exhaust gases out of the chamber, improving evacuation. This is called "scavenging" and is one of the key benefits of a header system. If the cam has a bit of overlap (intake valve and exhaust valve open at the same time) the intake charge is sucked into the cylinder, delivering a dense uncontaminated intake charge.

The diameter and length of the header tube are critical. For a given engine displacement, a smaller tube will cause the exhaust pulses to flow faster down the tube, thus increasing the momentum and the scavenging effect. Too small a tube and back pressure increases. Long header tubes provide superior low RPM performance while shorter tubes work best at high RPM. Optimal tube length and diameter depends on displacement and the desired RPM for the power band. Hence big race engines - big tubes, small street engines - small tubes."


...So it sounds like for your application you'd want the header tubes to be as long as possible before merging.

[Cinnabar325is]

Having just made my own exhaust for my car using long tubes and a 2.5" single system, I merged the collectors into one pipe just before the system made the turn to head towards the tunnel. In other words, just to the passenger side of the trans (I used the same routing that BMW did). Given the space available, that was the best fit and much easier than making the turn towards the tunnel with two parallel pipes.

[2mAn]

quality responses, thanks fellas. i will post pics of my system once its all done. someday id like to dyno the car again and see what kind of improvements the chip and full exhaust did for my eta. bone stock dyno put a whopping 90.9whp :rofl:

[MR 325]

thanks for your comment, but im looking for educated responses. long tube headers generally make more torque, and thats what im looking for. its an eta so its not going to be a drag car. just looking for more midrange grunt and every little bit helps. its chipped too

You're going to LOSE torque with headers compared to the original Eta manifolds.

You should be saving your money and just toss a nice sounding muffler on the car A full custom exhaust system isn't going to do much for you. Sorry man, the truth hurts. I've been there and done that.

[Bearmw]

I designing a similar exhaust setup but for an M30 using long tube small 1 3/8" primary headers with small 1 5/8" collectors. Should be torquey as hell. These apparently are old E12 Stahl headers (if I can make them fit, may have to modify motor mount). I will use an oval 2 1/2" resonator muffler mounted as far back as I can and will have the Y pipe probably a little behind the trans. Also using a 2 1/2" Dynomax turbo muffler in the stock location.

Why use 2 1/2" pipe for an Eta?

[TimKninja]

SimIlar setup on my m50. Ebay headers down to a magnaflow y pipe and 2.5 inch out the back.

They merged it right after the trans. So essentialy two 45 degree bends after the headers, then the y pipe.

[seatown88]

When doing research before building my headers I never found any equations for the length of secondaries. Therefore I just places my secondary collector where I thought it would fit the best (could be tucked up enough in the chassis) so I choose to put it centered in between the transmission cross brace and the chassis cross brace.

Here's what mine looks like out of the car...

[seatown88]

I designing a similar exhaust setup but for an M30 using long tube small 1 3/8" primary headers with small 1 5/8" collectors. Should be torquey as hell. These apparently are old E12 Stahl headers (if I can make them fit, may have to modify motor mount). I will use an oval 2 1/2" resonator muffler mounted as far back as I can and will have the Y pipe probably a little behind the trans. Also using a 2 1/2" Dynomax turbo muffler in the stock location.

Why use 2 1/2" pipe for an Eta?

What RPM are you planning on making peak power? Your primaries seem a little on the small side.

[dougie30]

Really sorry about the mega thread revival but there did you get that y pipe???

[MR 325]

Really sorry about the mega thread revival but there did you get that y pipe???

Looks like a Flowmaster Y pipe. Check Summit Racing.

[seatown88]

Looks like a Flowmaster Y pipe. Check Summit Racing.

exactly, I picked it up from summit