Replying a little late to this comment, but I haz data and this is one of my favorite little simple experiments I've done. I've shared this elsewhere but since removed it for various reasons. Anyway.

The first iteration of the 949 reroute had a larger cheater hole. I think it was 3mm OD. This produced enough cheater flow through the rad to cause some significant issues with slow warm up as well as dropping below thermostat temp on long descents like we have here in WNC.
If you do some quick envelope math, the cross sectional area of a 3mm hole vs that of the ~1/2" bypass hose, that cheater hole represents about 6% of the open flow area during thermostat-closed condition.

Back in early 2024 I tested this on a cool-but-not-cold morning on my commute to work using the same route and same gentle driving both times, as one would expect from a commuter.
- Blue line is the original large cheater hole with the AC side of my radiator blocked by cardboard.
- orange line is the cheater hole sealed with RTV and no cardboard.

Conclusion - sealing the cheater hole with RTV...:

• improved warm-up time to temp
• improved the ability of the thermostat to retain minimum temperature control on long descents and cold days
• had no impact to race track performance or peak temperatures on hot days
• adds no issues when combined with the use of cardboard to cover half the rad during winter months

IMO, the yo-yo effect exists but in my mild temperatures is not concerning, as the rate of change in temperature (shock) is within reason similar to what you'd expect by turning on the radiator fan. If you live somewhere with real winter and sub-freezing temps, you may care about that cold shock more than I do. But that's what a garage and a winter beater is for.


949 has updated their cheater hole to be smaller like OP has, 1.5mm OD IIRC, and apparently it's better. I have the new part on my shelf but have not felt the need to tear into things and install it because my sealed RTV plug has been holding up fine for 2 years now.
The smaller 1.5mm cheater hole represents about 1.5% of the open flow area during a closed-thermostat condition. Big improvement there - kudos to 949 for listening to customers, understanding the functions of their product, and making the correct improvement. It sounds silly to give props for something this minor, but R Theory's recently released coolant reroute shows that aftermarket race shops *still* fail to understand the fundamental functions required in a cooling system. Arguably FM also.

I understand the purpose of the cheater hole but I wish the design had incorporated an included roll pin or something so users can choose to 95% block it if they don't plan to run a racecar setup without the bypass loop.

 

I'm glad you brought that back up, as I'd had some more thoughts on the reroute.

As both of us noted, the bypass notch got smaller at some point, despite previous posts from Emilio saying they had no intention to change the bypass notch. That said, even if you got rid of the hole/notch there is still significant bypass around the entire thermostat. Every stock thermostat housing and gasket I have used creates a solid seal around the entire thermostat diameter. The circlip used in the Qmax does not. I sealed the entire diameter of the thermostat entirely with silicone, not just the notch.

Running at steady state around town in mild 60ish degree weather, the readings off of the OBDII feed will oscillate between 185 and 195* over the course of 20-30 seconds. Previously, my car would hold steady at 195 in most any condition, from 45 degree days to 95 degree 45 minute track sessions at VIR.

I have not confirmed this, but I am choosing to blame the thermostat itself rather than my install, or the QMax design. Stant Superstats used to be the gold standard, and they have little V notches that create a more gradual change in coolant opening and flow as the thermostat opens. Stant no longer seems to make their own thermostats and just repackage Motorad now, and I'm not sure if those notches are still there on this thermostat. Every time it opens, a big slug of coolant coming off of a 36mm rad overcools the engine until the thermostat shuts again, repeating the cycle. It may be more use of the thermostat but I prefer this to overcooling on warm up.

I wish I had monitored coolant temps during the last track day, as it seemed to be running hot (205 indicated in the pits) compared to previous experience, but I only opened up the OBD reader after the misfires started. I'm not going to call that a normal operating condition if some of the combustion gasses could have been escaping into the intake manifold with my tulipped intake valves and increasing the overall operating temp.

 

If it were me, I'd buy a full set of intake valves, and pop them in myself.
A valve spring compressor is pretty cheap, and you can use sharpie as a witness ink to make sure the valve is contacting the seat in the right spot.
Likely the contact will be fine on the valve/seat as far as location goes. and a SLIGHT lapping can ensure the seat/valve are sealing well.

If the contact area is only on the outer edge of the valve, or inner edge of the seat... then it's to the machine shop again to get seats replaced.

Then you can do the bucket/shim dance on what fits where, and order new for ones that happen not to fall in where you need the lash.

note: I'm more cheap than lazy, and i'm pretty damn lazy.

 

Regarding fixing the engine, I've had some time to research and think. I haven't made a decision but I'm getting closer.

Used heads: Two different people have offered me used BP6D heads. Neither is a pristine example, and they're not local. Their pricing is fair but I want to get on track reliably, not just check boxes.

Rebuild this head: I'm going to revisit the machine shop today and explain what happened. Neither of us knew that stock valves have a very narrow margin where they can be refaced and machined, and I am blaming that for the issues I'm having. On one hand, he's not a Miata focused shop and this is pretty esoteric knowledge. On the other hand, I am paying for expertise and I've had a failure related to his work. I don't want money back, but if he would put me at the front of the line and knock out the work in two weeks or so, I can get back on track pretty quickly. Manley valves are actually less expensive than stock ($440/set), and I would get valve guides replaced as well ($120/set) just out of an abundance of caution. I called Manley and they told me their valves have a reasonably large margin for machining. I'd probably prefer to adjust with shims if possible to sidestep that entirely, which could be a pain to source though. I spoke with another shop that multiple people pointed me towards and he didn't make any promises besides 1-2 months before my head would be worked on.

Spec Miata Shops: Treasure Coast Miata charges $1600 for a head that gets the exact same work done I paid for - no replaced valves. That leads me to questions about if this could happen again. East Street Racing wants $3,000 for a fully serviced head built to the limit of the Spec Miata rules. I want all parts replaced but I don't want to pay for expensive, tedious optimizing to rule sets I'm not bound by while ignoring cheaper, lower hanging fruit that's banned by SM rules. But I would get back on track real quick.

I'm going to try to get this head off ASAP. It has to happen regardless of my next steps, and if the valve seats look good, I may end up trying to do as much of the work myself as you point out @Ironhydroxide. It would fit the overall theme of this thread... I feel the need to do everything myself.

 

The heads off and I'm not thrilled with what I've found.

First bit of concern is this oil at the throttle body. There's clear signs it's coming through the pre-throttle valve cover breather, as there's streaks in the intake pipe.



The reason I'm actually doing this - tulipped valves from cylinder 4.



Some cylinders had this yellow/bronze sheen to them. It did not wipe off.


I knew this when it went together, but for some reason the cross hatching has distinct steps in the cross hatching reflections most obvious in the back of cylinder 4 below. This is not the result of different lights in my garage reflecting funny, its down to the surface finish.

The yellow bronze coloration is still visible but less obvious.



Bronze coloration in my intake ports are the result of oil.



Every intake port had oil all around the sealing surface.



It seems to me that I've got sub optimal ring seal, giving me blowby and pushing oil into the intake. While I did open up the valve cover baffles to clean them out, I don't imagine this is just the result of a bad seal there.

Prior to the bad idle, it was making decent power, and I never noticed oil levels dropping significantly. So even if there is some blowby, it's probably best to just throw a used cylinder head on it and go forward. I don't want to spend another $1k on rebuilding a cylinder head only to find my ring seal is garbage and the whole thing has to come out again.

 

This sucks. And blows.

Honestly (and I haven't had my coffee yet, so my thinking isn't all there yet), I think the easy button to maximize track time this year would be to yank the rest of the motor and drop in a used engine.

Drop in Sixshooter's engine, even though it's not VVT. (As long as you can deal with the ECU side of things) It's just for a season (or two) and honestly, VVT isn't worth that much for a "fun" track car.

Your bottom end needs work. It's not going to get any better with another year of track time on it and it could get much, much worse and take out your head (again).