Interesting problem I've had with my '66 Barracuda since I modified the motor just a tad in 2001. When cruising with the windows down there is a strong exhaust fume odor that is quite overwhemling after I drive it steadily for around 40 minutes or more. But when my windows are up there is no odor at all getting into the interior!!! It doesn't matter if I keep the lower vent doors open or shut. Engine has a bigger cam, intake & Holley 600 double pumper and stock/original exhaust manifolds. Int. & Exh. manifold gaskets have less than 6000 miles on them. Any idea why the interior is sucking in all the fumes from the engine bay? Even working under the hood on the engine when its warmed up is bothersome. This really stops me from driving her on long cruises in the summer months.
Any insight fellas?????

MOPAR4LIFE, 273HYPO

Engine bay should have no more smell than before providing there are no exhaust leaks. If you have the exhaust exiting out the back of the car under the rear valance then it should not be any more noticable inside the car either, at least while driving. If you have an abbreviated exhaust system than ends before the rear axle then all bets are off.

pretty simple: trunk seal not sealing.

when driving, exhaust fumes get trapped in a turbulance behind the car. with open windows, you get a low pressure condition inside the car = exhaust fumes get sucked inside through the "leaking" trunk gasket.

TRUNK SEAL<<<<<<<<
when i got my old 69 charger, the guy said, in the 9yrs he had it he could never figure out the exhaust smell in the car, his wife and daughter would never go in the car with him, and he had 200 air fresheners in the car all over, bounce sheets under the seats. Shortly after getting the car i noticed that driving with the windows up was fine, then as soon as i cracked the window open it was instant exhaust fumes
Sure enough changed the trunk seal and it was totally gone. When you open the window you effectivly create a low pressure in the car, and exhaust fumes are tumbling behind the car, if there is a low pressure in the car, the fumes come into the car from the trunk seal.....

I replaced the original trunk seal about 2yrs ago and it seems to fit and close fine with the new one. Also have new front n' rear windshield gaskets. But my '66 Barracuda also has trunk "flaps" under the the rear window molding. These are original and act as a seal from the trunk to the rear passenger seats. Could these be my culprit?

Thx, 273HYPO

I'm going to vote NO.

The goal is to keep exhaust out of the trunk, not to keep exhaust gas in the trunk from getting into the passenger compartment.

edit: How are your taillight gaskets?

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Good point! My taillight lens foam gaskets are new but the rubber taillight bezel gasket is original and are not reproduced yet.

also check the body plugs, Two under the spare tire might be shot

Where does your exhaust end? Does it go all the way out back beyond the bumper or rear valance, or does it end in front of the rear axle? A buddy has the same problem with his AAR with the stock side exit exhaust. When he is stopped and at idle, the fumes come up thru the side glass when the windows are down.
OOPS I just noticed you said the fumes under the hood were really bad when you were working on it.
The change to the bigger cam may have you running a lot richer than before. Have you checked the plugs to see if it is too rich.

Exhaust comes out to rear of bumper about 1/2" past the bumper and I pulled a plug a week ago and it looked good. The post of the plug was slightly brown.
But I will check the body plugs. Anyone else??

The trunk seal is only a couple of years old and looks brand new. It also appears to be on correctly. Where did I go wrong? Is there a method to follow to ensure it seals right or is it trial and error? Also the trunk seal & trunk were removed two years ago when I did my restoration during the painting process. Trunk is very very very slightly off, maybe a 1/32". But the fumes have always been there for at least 6 yrs. I will look into this problem tomorrow.

My 68 fast back would do the same thing, worse with window up and more dangerous.. window down created enough internal cab pressure to offset the fume getting in. The exhaust comes up into a draft area and swirls around the rear bumper and enters the trunk area. ever drive a station wagon or van on a gravel road and watch the dust lay up on the rear window of a station wagon or van. same senario.

I had this problem too. The car also ran hot. Problem, not enough initial timing and too lean. is your intake mnfld a single or dual plane? what is the overlap of the cam?
A lean misfire cannot be heard or felt, but puts out more HC than runing rich.