Excessive Engine Heat In Cabin of 2012 V6 Coupe

The greater heat could be created by a greater greenhouse effect due to the larger rear window. The sedan’s windows let in less sunlight. And previous generation sedans may have let in even less light.

I found your observation about VCM interesting. It makes sense the engine must be under light load for VCM to engage. I’m surprised to hear that the added load of the A/C compressor can be enough to partially disengage VCM. I would not expect it to be that sensitive. And what I find more interesting is your comment that at below 65 mph VCM will not fully engage. I wonder if that is because the rpm are so low the engine has insufficient torque to operate on fewer cylinders.

Is a 20% fuel loss because of A/C correct? If your car gets 30 mph highway, a 20% loss would be 6 mpg. Running the A/C usually decreases fuel mileage by 2 to maybe 3 mpg. That would be more like a 6-10% loss. Or are you basing your 20% decrease in mpg on the combined loss created by a partially disengaged VCM and A/C drag when the A/C is on - a loss of 3 mpg for the A/C and another loss of 3 mpg for no VCM?

That raises another question, how much does VCM improve highway fuel mileage? I ask because according to my computer, on my last highway trip I got about 34 mpg at a steady 70 mph in my 6-6 which does not have VCM.

Circling back to your cabin heat issue, have you considered window tinting? If that contributes to less A/C use, your fuel mileage may increase. But do you really expect to get 34-36 mpg combined from a V6 Accord? That seems very optimistic to me.

OK. I did not graduate from MIT with a PHD in Mathematics, so the MPG math may not be exact. Here are some facts: The 2012 EX-L/Navi V6 Coupe [Car in question] achieved 26.4 mpg avg over 16,581 miles. On a recent trip from Ontario Ca up the I15 to Salt Lake City Utah, 1428 miles round trip, 32 mpg, avg, speed 75 mph. 89/91 octane, heater on, a/c off. [car burned an average of 2.5 gallons of fuel per hour, during an 9 hour trip, 22.5 gallons of gas one way]. I can't speak to another V6 Coupe, but, the VCM in my 2012 V6 Coupe operates fully above 70 mph, and between 60-65 in my Wife's 2012 EX-L/Navi V6 Sedan.

In a work week, I travel 400+ miles and I fill up with 16.8 gallons on average. By my math, that's inline with the average mpg over the life of my coupe, especially in Southern California, Ontario Area, using the A/C.

I will measure the heat difference in the coming weeks. My suspicion - smaller interior volume. BTW, Side, quarters and back windows tinted 20%. I also think this heat issue is just an oddity like stalled starts and acceleration spurts with minimal pedal input I experience every blue moon. Was just wondering if any other coupe owner felt more than normal heat in the cabin. And the VCM discussion is just from my experience, and has no bearing on the design of the system, it's effectiveness to achieve better fuel economy or how it affects other drivers - just how it has been working in my 2 2012 EX-L/Navi Accords.