It's winter in Australia- your fuel consumption is up/mileage is down, right neil? air temps are cooler, making air more dense. higher density air means that more fuel is being consumed to maintain 14.7:1. further, with denser air, it's harder for the engine to push a vehicle through the thicker air, so still more fuel is getting consumed.
By all means, Neil, give it a go. reusing the waste heat radiated by the engine is an idea many others have experimented with before, but I have faith you can achieve a different result because you're tenacious.
I was trying to show you that regardless of units, your math doesn't work; disregarding pressure won't get you any closer to a correct result either:
Vacuum is negative pressure...and you're continuing to disregard the Ideal gas law PV=nRT. outside the manifold is atmospheric pressure, usually used as the reference for the amount of vacuum. a running engine develops 18 or 20" of vacuum below atmospheric pressure. your pressure times volume divided by temperature only determines the weight of the air (number of moles) times a constant. ignore pressure like you did, and you have a ratio of volume over temperature...liters per degree C for instance.
How about this experiment: blow up a balloon, neil. use your lung power or a pump - doesn't matter. tie the end, and weigh the balloon. got it? that's your molar mass, the n in PV=nRT.
now put that balloon in the fridge or a freezer. come back in an hour or 2. what's happened to the balloon? It will have decreased in size - the volume will have shrunk. Temperature and Volume will have decreased, yet there is still the same amount of air in the balloon - there is more mass per volume unit, meaning it's more dense AND because that volume of air is smaller, it's not exerting the same pressure on the envelope of the container it's in, the balloon, so it will have shrunk.
the same works in the other direction: if you're able to inflate a balloon with air at room temperature, and then heat the air inside the balloon, the balloon will increase in size, possibly to the point that the balloon can't hold it anymore (pop!). (ever seen a hot air balloon flying? it's mass and pressure and volume changes from the addition of heat energy that create the forces to lift it from the ground)
Please let us know how you get on with your experiment