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If you add a couple of microphones under the hood and in the trunk as well as have a microphone near the interior camera position as a reference for the sound, you can filter and mix the engine channels and trunk channels until it sounds similar to the near-camera mic.
If needed, put a mic in the wheel well or under the car for tire noise (put it in foam and ...
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Recording car audio can be very tough. You have a few factors to think about. For one when it comes to race cars and performance bikes you are talking about high SPLs. Road spec (at least in the US) is somewhere around 108 Db I think. Although some race classes have adopted sound regulations many have not so you could be looking at levels well above that. ...
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To answer your first quesiton, very dense and thick foam (properly attached too to prevent vibration transference - as in, not just "set and forget" flopped ont top of the car) should do the trick. That's effectively what the Rycote rain covers are in a general sense - thick and dense over-sized foam to help absorb the rain drop sounds from transferring to ...
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What microphone and what working distance are you using? The ultimate goal is maximizing the "Signal to Noise Ratio" (SNR) There are TWO ways of doing that, and typicaly you can use a combination of BOTH methods:
MINIMIZE the external NOISE. That means blocking noise from coming into your recording space. The ONLY thing that blocks noise is MASS. Not foam, ...
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Boom library did everyday cars out in the desert on a super quiet test track. They do good work.
http://www.boomlibrary.com/boomlibrary/products/cars
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