Okay, so far we have our
tube diameter and
primary length figured out. The next step is to look at designing the collector.
The collector goes on the end of the primary pipes and brings them together. You'd like to do this as smoothly as possible to maintain exhaust speed.
Now, I think we can over complicate this part a lot, but IMO we don't really need to. Are you going to have a super fully optimized collector if you omit details? Absolutely! But, as we're finding out there really is no "perfect" design anyways. That being said, we'll at least go over the things to avoid.
So, lets do the calculations to figure out what size inlet and outlet we need. Well, inlet is pretty easy as it'll be the three 3/4" pipes coming in. But, what size outlet should we have? To do this, we're basically going back to the first step of calculating pipe diameter based on horsepower. The Metro has 55 horsepower, all 55 will be flowing through a single pipe now (vs one of the three primaries). So, we calculate pipe diameter based off of that.
Per our chart, 1.25" OD 16 ga will flow enough power for 52 horsepower. Since we're targeting a slightly lower rpm range, 1-1/8" will work. 1" will give us better power at cruising rpms, but it will most definitely kill some higher rpm power, so lets say 1-1/8" for now. Of course we could always design it for 1" and cut the collector back if we don't like it like that.
Next, we have to figure out a length for the collector.
Headers By Ed says they should be at least 4" long and 5-6" is preferable. As we're familiar with aerodynamics here on EM, this is pretty easy to see that a more gradual transition will create less turbulence and maintain more exhaust velocity.
Per this collector manufacturer, the angle of convergence should be between 12-15 degrees.
http://www.spdexhaust.com/pdfs/02-11...Collectors.pdf
