In my last post, I grouped the eight primary exhaust pipes into two bundles of four. The collector's job is simple: merge the output of each set of four primaries into a tube with a single outlet. If chosen wisely the collector can also increase power. The diameter and length are two factors that impact performance, and so is the design of the area where the pipes merge.
To choose the best off-the-shelf collector, I started by researching what the best outlet diameter should be. The internet was particularly helpful since it churned up the following equation: Collector Diameter (4 -into-1 street) = Primary Pipe Diameter X 1.6 (up to 1.8) Where 1.6 would typically be used for mild street, 1.7 for street/strip, and 1.8 for all out race. I'm aiming for mild street, so I chose 1.6, therefore: Collector Diameter = 1.625" X 1.6 = 2.6" Nobody makes a 2.6" diameter tube, so I either needed to round-up to 3" or down to 2.5". The best way to decide was to work the equation backwards for a 3" diameter collector and see what that yielded for the last factor in the equation. A 3" diameter collector with my 1.625" diameter primaries gave a factor of 1.85, well into full "race" territory. So I opted instead to downsize to a 2.5" diameter collector instead. That yielded a factor of 1.54. |
Next I researched the effect of collector lengths ("L") on performance, though this was an area I figured I'd have to make some compromises given the amount of space in the engine bay. Countless examples of dyno runs on the internet show that longer collectors generate greater horsepower and torque. I couldn't find a specific example with a Northstar engine, but the example to the left used a 350 SBC running 8.5:1 compression. Adding 12" extensions to the normal collector stubs produced the red lines. That's a gain of 12 peak HP and 10 extra lb-ft of peak torque. But check out the 40 lb-ft gain at 3,600 rpm! |
I couldn't find a simple equation to calculate an optimum length for my collectors, though 18" seemed to be an average "best length" for a wide variety of engines. I set it as a target, but it seemed much longer than I'd be able to achieve. (It might be important to note that any collector can be lengthened simply by adding a longer pipe to extend it.) Finally, I learned that the longer the merge zone ("M") is, the less turbulence the gases will see within the collector, and therefore the more efficient the gas flow. I compared available collectors from a wide variety of sources that had 1.625" primaries, a 2.5" collector, long merge zones, and long overall lengths. It didn't take much looking to realize that collectors fall into two price categories: insanely expensive, and dirt cheap. The top one in the photo to the right is classified as a "converging/diverging collector" (look it up for more details) and cost $450! I opted for the simple 4-into-1 mild steel formed 10" collectors from Doug's Headers, p/n H7671, shown at the bottom of the photo at $30/pair. |
1. add the collectors to my previous drawing (no biggie);
2. find room for 8 more inches on the ends of each 10" collector;
3. compensate for the offset of the collectors biased to the right of the chassis centreline (see green lines below) so that the final exhaust outlets (blue lines) were centred on the chassis;
4. find, stuff and interconnect a cat, a muffler, and a flex joint on each system;
5. use equal length tubing for each system; and
6. have everything fit in the yellow highlighted area in the drawing below.
(I decided trying to get the outlets in the stock Ferrari location would have sent me over the edge.)
No crossover or H pipes were needed or desired since the 180 degree primary pipes took care of balancing the exhaust pulses. Keeping the systems separate would also take full sonic advantage the stereoscopic exhaust notes coming from alternating exhaust tips.
After many hours of trying unsuccessfully to stack the mufflers and converters in a conventional, horizontal position, I allowed myself to think outside the box and positioned them vertically. Why not. Then, with a stroke of luck, I was actually able to find short mufflers (in blue) and cats (in red) that would fit the design concept. Here's the layout from the rear view:
The layout is unbelievably simple and easy on the eyes compared to my first attempts.
Here's the top view:
The only compromises to my goals were that the passenger side collector is 14" vs 18", which also led to a similarly shorter overall system on the passenger side. The driver's side system downstream of the primaries measures 58.7" long and the passenger side measures 54.7".
Here's the view from the passenger side of the chassis:
To keep any part of the system from hanging lower than the cradle, I'll have to make shallow notches in the cradle's rear crossmember where the collector pipes cross under it. And, I'll have to raise the aft engine mount on the cradle which in turn means I'll have to change the angle and length of the mount's legs. It's a small price to pay.
Finally, here's the driver's side view of the system:
On this drawing I've overlaid a cross section of what will remain of the aft trunk space in purple. It's not going to be much, but then I'll have more room up front without the radiator taking up space there. Keep in mind the actual Ferrari has no aft trunk at all!