Turn Signal Intakes

a performance analysis by Joe Gerhardstein

Monica received a pair of Turn Signal Intakes (TSI's) last Christmas as a gift, and this article details the installation procedure and some tests we have done on them.

The TSI kit (some assembly required) is availabe from several vendors. The kit contains 2 fiberglass intakes, 2 turn signal lamp holders, 2 lenses and 2 self-tapping screws. The fiberglass intakes are unpainted when you get them. We had ours painted at Imported Dynamics in Marietta, though you can buy Miata Black spray paint from Pep Boys if you want to try yourself. Imported Dynamics did a wonderful job painting, and the color matches our 1992 Black very well. Before painting, however, I recommend inspecting the fiberglass TSI's closely. Ours had several small gouges in them which I tried to repair. The repair looked pretty good before going into the paint shop, but when the paint was baked on, the repairs shrunk and if you look closely you can see where they were. If your intakes are gouged when you receive them, I would strongly recommend taking them back and getting a different set if you can.

When painting, I recommend leaving the area inside the intake where the turn signal will go unpainted. This will improve the amount of light reflected out by the turn signals later (assuming you are painting black). Other club members have lined the inside of this area with aluminum foil to make the turn signals brighter.

Now that you have painted intakes, you can get on with the installation. The only tool you will need is a #2 phillips screwdriver.

  1. Taking the plastic lenses, drop one into each intake with the rough/faceted side out.
  2. Insert the turn signal lamp tube into the intake. Note that one end has a flange/key/"ears" on it, and the other is completely open. Put the open end into the intake, aligning the hole drilled in the side of the turn signal tube with the hole in the bottom of the intake. Put a self tapping screw into the intake and tighten until the tube is tight.
  3. Remove the two screws holding your turn signal assembly and pull the assembly out.
  4. Remove the turn signal lamp by giving a quarter twist to the base of the lamp.
  5. If you look closely at the turn signal lamp base, and the end of the turn signal tube, you will notice that there are 3 "ears", one of which is longer than the other two. Insert the turn signal lamp into the intake, aligning the longer "ears" and twist a quarter turn to secure. Ours were very snug, and you may have to trim the turn signal lamp base a bit to get it to fit.
  6. Slip your TSI into the front of the car. Put the two screws from the turn signal back in, using the longer one on the inside, and the shorter on the outside. Put the screws in far enough to keep the TSI's from wiggling around, but don't over tighten or you may strip the thin metal clips. If the screws don't seem to be going in, pull the TSI back out and make sure the screws are lining up with the metal clips.
  7. Repeat for the other side.
  8. Test!

We were quite impressed with the appearance of the TSI's after we installed them. After about 9 months of driving with them, we decided to run some tests to see if they had any performance effect. The theory here is that the TSI's are suppose to keep the engine compartment cooler. This keeps the intake air cooler. Cooler air has a higher density (more oxygen in the same volume), which in theory should allow you to burn more gas for each piston stroke. To test the TSI's, I installed a K-type thermocouple just above our K&N filtercharger, and ran the cable back into the front seats. Monica and I took the car out to a stretch of road that we were able to run at speeds between 25 and 55 MPH, taking readings after a half mile at the designated speed. The outside air was 30.5 C (87 F). This was done during the early afternoon, when temperatures were relatively stable. We did this for the following cases: stock turn signals; TSI's; stock turn signals with headlights up; TSI's with headlights up; TSI's without the hood cowl seal (45 MPH only). The graph shows the temperatures at the K&N filtercharger.

The run we did at 45 MPH without the cowl seal and with the TSI's gave a reading of 55 C. This was not appreciably different from TSI's with the cowl seal, so we did not do any further testing. Please note that K-type thermocouples have an absolute accuracy of +- 2 C (3.5 F), and a repeatability of +- 0.1 C (0.2 F).

So, what does this mean? According to Munson, Young and Okiishi's "Fundamentals of Fluid Mechanics", the following are air densities at various temperatures for standard atmospheric pressure:

 Temp, C (F)

Density (kg/m^3)

40 (104)


50 (122)


60 (140)


70 (158)


80 (176)


So in order to realize a 1% density increase, we need to decrease the intake temperature about 2.5 C (4.5 F). In some cases I saw a 5-6 C drop with the TSI's, which should yield me a 2% density increase. The headlights up, however, caused a 20 C (36 F) drop, which should yield an 8% density increase. This is offset by the increase in drag, though.

During testing, I noticed several interesting things. First, when you are driving close to the driver in front of you, the intake temperature in about 3-4 C (5-7 F) higher than when you have 1/4 mile free in front of you. Second, the intake temperature rises quickly when you stop at a light (20 C [36 F] in 10-15 seconds), but not as quickly as it cools off when you start back up (3-5 seconds). Unfortunately I wasn't able to measure the time difference between the turn signals and TSI's. Third, intake temperature varies drastically depending on whether you are driving in the shade or in the sun. I noted variations of around 10 C (18 F) easily between long stretches of shade and sun.

Our conclusion? The TSI's appear to drop the intake temperature slightly, but if you are looking for a significant performance increase, spend you money on something else. I give them 1 Miata out of 5 for performance. However, you have to admit that they look pretty awesome, especially on a black car. I give them 5 out of 5 Miatas on the good looks scale.