Mustang #315

I ditched the old Doug Thorley headers because they wouldn’t work with my new motor. Too bad, because they were nicely made. I bought a set of Hooker Super Comp headers, part number 6111-HKR and have set about modifying them a bit. I ground out the tubes to match my heads’ exhaust ports, had the flange re-welded on the outside instead of just the inside, and am modifying the collectors.

The primaries on the left side are all within about .5″ of each other in length – impressive! On the right side, tubes 2-4 are close to each other, but a couple inches shorter than the left. Tube 1 is almost 5 inches longer than 2-4! It’s the best I can do, I guess. Hooker says they are “tuned” equal length primaries. I guess “equal” is relative. True equal length headers would cost a lot more. Tough to make.

To make up for the difference between left and right, I’m extending the collector .7″. The left-hand collector needs to be 18.2 past the ends of the primaries and the right hand needs to be 18.9. It’s not really possible to get very exact with it, because the ends of the primaries really vary (+- 1/8 or more). But, I’m getting real close.

Making the exhaust system is fun and challenging. I’m sore from lifting pipes into place over my head, fitting, removing, cutting pipe, lifting into place, etc. The headers are a real tight fit. The driver’s side one has to go in first. I have to lift the engine and move it over towards the passenger’s side in order for the pipes to clear the steering box and shock tower on the way in. Luckily I did install some Ron Morris engine mounts that allow for side to side and front to back adjustment. So, lift, move, put the header up and a couple bolts in, then shift the motor back to center and drop it. Then install the passenger’s side. It all has to be done multiple times in the fitting process.

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I ground out the tubes to match the large exhaust ports of my AFR heads

I had my engine builder, Bob Gromm, TIG the flanges on the outside with phosphor bronze. This makes them strong, and seals up the thin areas left by my grinding

This is the stock collector that comes with the Hooker Super Comp 6111-HKR. It's too short and too big for my application.

I cut and deburred the collector and prepared it for welding

This is the header next to my new collector pipe. The new pipe has a slightly smaller ID, which should be better for my torque curve.

I ground the edges of this down a bit because it's a thicker gauge than the old collector and I thought they should match up.

tacking the new collector to the old. I cut off a piece of the reducer to use as a joining band

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new collectors with ball and cone adapters, tacked together

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I’m going to have to replace my headers, but I thought I would go ahead and measure my current Doug Thorley Tri-Y style headers so that Scott can see if my current ones will do in a pinch. Here are the current dims:

I first measured the primary tubes. I measured them from the top, so it’s longer than from the side or bottom, but it was the most accessible, and it’s at least consistent. I’ll list the primary tube length, which is from the non-gasket side of the flange, to the first weld joint where they go into the secondary tubes. I’ll also list the overall length, which includes the length from header gasket to collector gasket. Besides the primary tubes, I measured the other tubing on the side of the bend so that’s more neutral.

1: Primary: 11.75, OAL: 39.3
2: Primary: 9.5, OAL: 37
3: Primary: 11, OAL: 33
4: Primary: 11, OAL: 33
5: Primary: 14.25, OAL: 39
6: Primary: 14.25, OAL: 34
7: Primary: 9.25, OAL: 36.55
8: Primary: 9, OAL: 31.3

Collector Length: 5.25
Collector ID: 2.35-2.4
Primary OD: 1.5
Secondary OD: 1.75

Primary tube openings at header gasket: 1.15-1.25 W x 1.6 H

1,2 share a secondary. 2,4 also
5,7 share a secondary. 6,8 also

I’ve been learning a lot about headers and exhaust recently. The way headers work is pretty amazing. I’d heard references to “scavenging” before but never quite got it. For my own notes, I’ll put what I’ve learned here. As always, if I’m off-base, leave me a comment and straighten me out!

I have to start with the exhaust stroke. In my car, as in many higher performance cars, there’s a bit of  “cam overlap”. That means that the intake event and the exhaust event will overlap a bit. The 4 strokes of the internal combustion cycle are Intake, Compression, Power, and Exhaust. Somewhere around 80% through the power stroke, the cam begins to open the exhaust valve. It might seem like you lose power this way, but in practice, most of the energy from the explosion has already been used and the inertia of the crank is enough to carry it through the end of the stroke without really losing much power. The exhaust valve beginning to open a bit early allows it to be fully open for the exhaust stroke, and the gasses can escape very efficiently.

So the piston is traveling up the cylinder, pushing the exhaust gasses out the exhaust valve. Somewhere near the top of the stroke, the cam begins to opent the intake valve. The inertia of the exhaust gas flowing so quickly out the exhaust valve actually helps to draw in the new combustibles through the intake valve. There are 2 things going out the exhaust valve at this point: a pulse of gasses, and a compression wave (sound) from the explosion. The compression wave is traveling at the speed of sound (around 760mph depending on the dynamics of the gas, temp, etc.) – the gas pulse somewhat slower (I don’t know the speed). When the compression wave exits the header’s primary tube and enters the collector, a negative pressure wave heads back up the tube at, again, the speed of sound.

The length of the header primary tubes has to be calculated so that this negative pressure wave, travelling at the speed of sound, reaches the exhaust valve at just the right moment to help pull the exhaust gasses from the next combustion out of the cylinder. This plus the inertia from the gas pulse movement also helps to pull the intake gasses into the cylinder for the combustion after that. This is what they call the  “scavenging effect” of the headers. The more combustible material you can pull into the cylinder for that next power stroke, the more energy it can release, and that energy will push the piston down faster, turn the crank faster, and the smile will get bigger on your face.

The staggering thing is how fast and how often this happens. In the 4 stroke motor, it takes 2 revolutions to complete. So in one minute at 4000RPM, each valve has cycled 2000 times. That’s 33 1/3 cycles per second that this whole process has to execute. This is why the exact timing is so critical.

So the length of these tubes is critical and needs to be designed to match the flow characteristics of the intake/cam/heads. Furthermore, each one has to perform similarly in order to send the gas pulses out the exhaust pipes in the rigt order, and not on top of each other (which would create pressure and in effect, blockage). So the primary tube lengths must be calculated, and must be proper in their relation to each other. In my application, that pretty much means equal length, within, I’ve heard, .5″

Where this may be leading me is toward getting some pipe and laying out and welding my own headers. It depends on the final design of the motor, and what its characteristics will be. If I can buy something that’s close enough to what I need, then I’ll likely do that. If not, I may be building them myself, which will be a ton of work, but fun. The challenge with making my own is in trying to lay out the pipes so that they all are the same length, and end at the same spot, but start in very different spots (each exhaust port). That, and making them fit in the limited space that exists between my engine and my car’s body. Luckily, I won’t have to actually bend tubing. I can buy u-bends and j-bends, and cut them to the lengths I want. I can imagine that this will be a long process, with possibly a lot of waste involved. Fun though! And the pride of ownership would be high.