Here is the data I have on the effects of the modifications I have done to my Miata (see my Miata page). At the time of these dyno runs, my car still had the stock cat.
My car is a '93, with the 1.6L engine. I do not have any dyno figures from my car in a stock configuration unfortunately. Dyno runs I have seen for various bone stock 1.6L Miatas suggest that the stock HP is about 95hp on average, with a range of maybe 92hp-98hp. On the same day I did my runs, a bone stock 1.6 measured 98hp on the same dyno, same weather conditions.
Based on these data points I estimate that a reasonable stock figure for my car is 95hp. I'll use this as a baseline. You can fudge the numbers up or down a few hp if you feel this is not correct. Note that in all cases I am talking of rear wheel horsepower, the only kind worth considering. Anybody other than Mazda who reports crank horsepower is probably just making up the numbers, so I ignore these.
I ran at both 14BTDC and 18BTDC. For my car, 18BTDC is clearly better. Some Miatas have reported loosing top end power at 18BTDC, but as the first graph shows, for my car it provides a nice advantage over 14BTDC up to 4.5Krpm, and is essentially identical from 4.5K to redline. Red data is 18BTDC run.
Going from 14 to 18BTDC finds 1HP, from 104 to 105hp. I regret I did not do a 10BTDC (stock) run to find out the gain between 10 and 14BTDC. According to various dyno reports, apparently there is about 2HP to be had there. Based on the subjective difference between the 10, 14 and 18 settings I am willing to believe those estimates. Thus, my car should be producing about 102hp at 10BTDC.
This means that all the power-related modifications to my car (JR catback/header/CAI) produce a net total of 7hp over a bone stock car. Not very impressive, to say the least.
I do not have any data to be able to accurately figure out how much each component contributes to this total. Based on very subjective driving impressions, I think the catback made the largest difference in power, followed closely by the header. The CAI is certainly the least effective. Based on this, if I had to guess, I would estimate that both the catback and the header give somewhere between 2 and 3hp, and the CAI gives somewhere between 1 and 2hp.
Do note that dyno numbers cannot reflect effect on hp from ram-air effect (if any). I have taken the CAI out and replaced it with the factory intake twice for back to back testing. Subjectively, I cannot tell any difference (other than intake noise) between the two until approximately 70mph. At that point the CAI begins to feel a little livelier. At higher speeds (full throttle in 4th gear approaching redline) the CAI does provide a noticeable advantage. Given the misguided speed laws, this in effect means that the CAI offers no noticeable advantage at all for street driving.
Below is a graph comparing my car at 18BTDC with a bone stock 1.6L Miata at 10BTDC. Note that power is pretty close up to 4.5Krpm, but from there to redline my car gains an advantage, as would be expected due to the mods that let it breathe better.
Note that compared against this stock car, my Miata only produced a total of 7HP more. If the timing change gives 3HP (as assumed above), then all the other mods would only be producing 4HP. However, I suspect this was a particularly strong stock car, so I prefer to use the 95hp baseline as outlined earlier.
The graph below compares my car against a '91 with the exact same equipment, including the FM cat, which is now in my car but was not installed at the time of the dyno graphs presented here. It is interesting to see that the curves are basically identical except at the very high end. Perhaps the FM cat breather a little better and might be the reason, since otherwise both cars are identical. Furthermore, this graph is interesting as a corroboration of my car's dyno numbers.
Finally, the graph below compares my car against Ony's '96 1.8L Miata which has a Borla catback exhaust, header, intake and timing at 18BTDC. No wonder those 1.8s are so annoying on the long straights! ;-)
That's it... from all these number I make up a few conclusions: