I'm not the expert here by a long margin, but the powerband an exhaust creates is a function of the primary pipe size, and collector size (primary pipe length plays a role, but much less than primary diamter) and of course if there are secondary pipes before the collector, beynd that the rest of the exhaust system is either going to offer little/no backpressure and 'work' or it will be restrictive and cost power, and obviously it costs more as flow (which more or less = rpm) goes up.

As dodgy as it sounds, what you really might want to try it to make up a new outlet to hook up to the collector, that is perhaps 12 inches long and opens up to atmosphere with about a 7 degree taper. Admittedly this is 'old school' and there are better exhaust exits around that make more power. All of them would be too loud of course, but fitting this small length of pipe _temporarily_ will help things in one way. As mentioned there are more efficient options, but what this tapered 'megaphone' exit will do is give you an indication of whether or not, and by how much the exhaust was holding things back. If it doesn't pick up the power/top few thousand rpm of the powerband after fitting this, then it's not just the system after the headers that needs to be bigger, it's the primary pipes and possibly the collector too..

As a basic rule of thumb you'd want 1 1/2 OD primary pipes and under 30 inches.. Ironically enough - the JTS are pretty much spot on in that regard.

EDIT

- sorry - brain fart - I got them mixed up - I was thinking of the hurricane headrs 4 into 1 exhaust when I mentioned primary pipe diameter and length and collector size as being spot on.

I'm not sure the exact dimensions of the JTS ones, but for some reason think they are 4-2-1. You could probably make a call on that that they are (if the pipe sizes were similar overall) probably a little bit better for mid-range than the hurricane ones, but the difference is likely not dramatic enough to justify the cost of trying the other type if you already have the first type. Same general thing though - you'd look for 1 1/2" od primaries, and theoretically the same size secondaries (since there is technically no overlap of exhaust pulses in the secondary pipes when 1/4 and 2/3 are 'paired'), probably looking at around 12-14" primary pipe length before merging to the secondary pipes, and about the same secondary length..

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About the only place they might have a restriction, is where the collector/outlet pipe flares over the 4 into 1 section - maybe the flange inside that is under the diameter of the exit pipe making a slight restriction.

Assuming it (the open pipe megaphone deal - and you'll need at least 12 inches of length to it for it to work properly, shorter and it can throw off the result) DOES sort out the power tapering off, then you should look at (imo) a 2 1/4" collector (maybe 2 1/2 but it'd want to be full race, no holds barred, I'd bet the farm on 2 1/4) and run it about 12 inches long, then either fit a pressure wave termination box, or taper the pipe up to 2 1/2 inch and run a single straight through muffler, with the muffler as far back as you can. This might not work great for sound, but will do good for power.

In very general terms - a 2 inch system from the collector back is good for about 100-130/140 bhp, above which 2 1/4 becomes the go. 2.5 would typically from about 200bhp to 240 or so, above which you look at a 3 inch setup, and once you get above around 300-310bhp then there's a little bit of power usually to be had with 3 1/4 (not that they readily exist necessarily) or 3 1/2.

I'm basing those power levels and main system sizes on experience with engines ranging from 998cc mini engines, to some datsun/toyota larger 4 cylinders, holden 6s, hemi 6s and even some of the ford gear.

Another quick test you can do - weld a pipe onto the exhaust before the muffler, and route it (use a length of copper piping or something, so it doesn't immediately make contact with rubber hose) to a pressure guage. You're going to have to live with 'some' exhaust back pressure (zero is always the goal, but it's often hard to get noise down without costing just a little bit of flow/) - but if it is above 5psi, then you are losing considerable power from the exhaust alone.

Another old school trick you can do, instead of the megaphone, run a straight pipe 2 1/4" diameter from the collector, about 18-20 inches long, side exit or wherever. Get some el-cheapo paint (no por stuff or high temp exhaust paint or epoxy enamel, needs to be cheap stuff) Paint a stripe along this added pipe, and find somewhere you can do it, and essentially drive it like you stole it for about 10 minutes or so. Then look underneath, and mark where the paint stopped burning off that stripe. Record the distance from the collector/flange. Then on the real exhaust run the collector/exit pipe for this distance in the same diameter as the collctor, but once you get to the distance the paint stopped burning off (or a touch before it) and at that point, taper it from the 2 1/4" diameter to the 2 1/2 and route that back to the muffler (and again, mount the muffler as far rearward as you can)

I'd still prefer a pressure wave termination box in there instead, but this old school method actually has worked in the past . (one of the more common applications was early drag bikes and dragsters (not top fuel, I mean some of the more 'fun' than serious stuff people build and run) where they had engine pipes leaving the exhaust manifold, and just cut them to vent to open air at the point they stopped burning the paint off.

There was _yet_ another trick to selecting the rear muffler, and that is/was simply to put it on a flowbench. I forget the figure (and it is a guide not a 100% absolute) but it was something along the lines of needing 2.2cfm of flow (28 inches water pressure drop) per bhp at the flywheel. that would give about the best you could get as far as not causing exhaust related power losses.

Obviously not all mufflers are created equal, and there'd be considerable differences between various brands of 2 1/4 and 2 1/2 mufflers. For a streeter I'd probably just measure exhaust back pressure and if it was around 2-3psi, I'd be ok with it. If it was more race oriented, then I'd probably go for the idea of flow testing the muffler (some manufacturers might even have tested and listed their own. I know some truck mufflers are rated by size and the nominal hp engine they'll suit.