Hi,
Well, since it's free, there's little reason not to go ahead and give it a try!
Most Pacesetter designs for 4-cyl. cars that I've seen use long primaries, i.e. a 4-into-1 type of header where all the primaries merge into a single collector quite a distance from the head. There's nothing inherently wrong with this design, it's just a bit biased toward top-rpm improvements and can cost a little low-rpm grunt.
The Spitfire engine is aomewhat higher revving, so this design might be more useful for street driving on it, than it is on some of the lower rpm range "big" TRs.
Assuming improved engine breathing is the result of switching to a header, you should need to go to slightly richer needles in your stock carbs too. In other words, once you help the engine "breathe" better, that leans out the fuel/air mixture and a bit more fuel needs to be delivered.
This might sound like there will be a slight drop in fuel economy, but not necessarily. Better breathing means a more efficient engine and can actually improve fuel economy. (But, more performance often means a heavier foot on the accelerator pedal, which means worse fuel economy!)
You really won't see a huge change simply installing a header on an otherwise stock engine setup. Other performance-oriented changes are needed to really make a lot of change. Folks are sometimes a little disappointed when they add one particular performance item to their car, having heard great things about it and expecting a big difference. For example, in the case of a header, eventually a freer flowing exhaust system to follow it up just makes sense.
This is true of most other performance mods too, say for example improved carbs. All the changes need to be thought of as parts within a "tuned system".
Of course, there's no reason not to do many of the changes incrementally... a little at a time. Adding a header is as good a place to start as any. Later you *might* want to improve the rest of the exhaust system, work on port matching and gas flowing the head, increase compression, put in a hotter cam, install higher performance ignition, etc., etc. You don't have to do *all* the above, but much of the real potential of an engine won't be seen until a well-matched series of modifications are done.
All that said, the main complaints I've heard about Pacesetter is their level of noise, compared to stock and other exhaust systems. But the bulk of the additional noise would be from the rest of their exhaust system, the header will be only moderately louder than the stock manifild. If you are simply installing a Pacesetter header, you could choose other exhaust system parts to use with it, if you wish. You're not limited to using a complete Pacesetter system.
The other major complaint I've heard is that Pacesetter exhaust systems don't last very long. Again, this is likely a bigger problem for everything after the header and, to be fair, might be in large part due to the way our old cars are used. A mild steel exhaust system is more likely to rust out and rot in a car that's only occasionally driven. This is because daily or near daily use blows out contaminants and dries out moisture/condensation in pipes and muffler(s). So - when mild steel is used - a car that's driven "only when the sun shines and only on Sunday" will actually probably to see a shorter exhaust lifespan than a car that's driven to work Monday through Friday.
Free is good, but if that header is used I'd check it over carefully. Any major dents can cause restriction rather than freeing up exhaust flow. Look also for cracks at welds that might need to be repaired before using, especially right around the flanges, and check for any rusty spots that are to the point they leak, which may not be repairable.
Have fun!
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