Its been proven and tested out that removing the cover is worse for the engine building heat up, big write up on a USA site,
In May 2019, there was some debate on the Facebook group if this removal was a good idea or not. The counterargument was that the small opening would create a Venturi, which would actually help draw air out of the engine bay. I decided to run a test with a remote temperature sensor to get some data on this question.
I did five runs on a sunny spring morning on the East Coast US with the hood temperature sensor mounted just aft of the scoop opening (see image above). The readout was then placed at the base of the driver's side A-pillar so I could observe the temperature as I was running. In addition to this hood sensor, I also used the built-in ambient air sensor of the car, and a separate hand held temperature/humidity monitor.
After letting all three sensors settle overnight in my garage, I first took baseline readings on all three. I then drove for half hour to the test road to get all coolant and oil temps to the normal range. I then did a run with my standard configuration (no cover), then with the hood scoop taped shut (so non-functional scoop), and then next the stock configuration. I finally finished with two runs of the no cover configuration to see if conditions were consistent. All runs were done in S+ mode (so no Atkinson cycle), and at a sustained speed of 70 mph. The data is summarized below.
Ambient (car sensor)
Under Hood (wired sensor)
Ambient (hand held sensor)
Morning baseline reading
Scoop taped shut
No cover (60 mph)
All readings in F
As you can see above, and to my surprise, the best reading was with the scoop taped shut. Now in full disclosure, there was a small opening as the tape pulled away from my paint (ceramic coated and waxed, so very slick). While driving, I could see the tape bulging out and up, indicating either lift from the air flow above, or positive pressure below. By the time I had stopped, there was a small opening pealed up.
But the second surprise is how much worse the no-cover configuration was. It was a consistent 53F rise with the 70 mph runs and slightly worse at the lower 60mph speed run. Presumably , in the case of the latter, the lower speed meant lower air flow.
In the light of this, I have put back my rain cover. It is possible that at even higher speeds, the stock configuration would win out over the no-scoop configuration. This result reminds me of the RR-Racing find that the stock intake is already very good. The only improvement they could justify was the intake diverter. Lexus must have done testing on the design already. As a final note, I have always liked seeing the blue intake runners when the hood is closed. I guess I will now have to unfortunately do without that.