I didn’t find any good threads actually comparing thermostats vs arguing about them so, through some mistake$ I made, I find myself with all 3 in my hand and thought I would share my findings. The Mishimoto pictured below is the lower temp thermostat for the S1 RX8. I wanted the lower temp tstat because my car is a race car (no street at all) and I need to keep it as cool as possible.


TL;DR - use stock or Mishimoto.


Observations follow

The following 2 pics show all 3 lined up for comparison. Mishimoto on the left, stock, middle, Pettit/motorad on the right.
- Mishimoto is the same overall height as stock
- Pettit / motorad is about 1/4" shorter. Because it is shorter, I have no idea how it would actually block off the bypass port.



Here is a pic of the Mishimoto (top), stock (middle), Pettit/motorad (bottom). Here I am trying to show that the overall diameter of the opening, shape of the opening are similar on the Mishi and stock. This is worth noting because the Pettit / motorad is smaller in diameter as will be easier to see in later pics. Smaller = less coolant flow. Also worth nothing the "bridge" thing in the middle stick up about the same amount on both the Mishi and stock thermostats. My conclusion is that the physical dimensions are very similar and should flow similar amounts of liquid.


This pic does a good job of illustrating the differences in the opening size between stock and the Pettit / motorad. The "bridge" thing on the Pettit / motorad is really low and very thick. The stock and Mishi thermostats provide much more real estate for liquid to flow compared to the Pettit / motorad.


Here's another decent pic comparing the Pettit / motorad on the left to the stock on the right. Hopefully here you can see the difference in the diameter of the holes and the larger area of the stock thermostat.


Based on this, I would not recommend the Pettit / motorad. No offense to Pettit. They are great but I don't see any benefit in this particular product from them. On the contrary, it is probably worse than stock.

 

you can just gut an OE type and use it as an orifice plate

but you have to tap the crossover and put a threaded plug in it

the issue with them otherwise is the seal at the crossover is prone to not be so reliable, and then it’s bleeding hot coolant into the pump inlet. It’s not a street car, you know. With the crossover plugged it 100% won’t leak.
.

 

I might eventually go that route for my race car but will test things out with the thermostat in place for now.

For people still using their car on the street / dual duty, they probably want to retain a thermostat.