Maybe some of the following will help. If nothing else I may be able to put you asleep. My girlfriend's 96 corolla also had a crack running right down the middle of the exhaust manifold which you could barely feel air coming out of (right at startup you could get your hand close enough and around the manifold heatshield to almost touch it before it got too hot) and has had it since I met her almost two years ago. This was around 110k miles and today is at 167k. About a month after I met her I was changing her plugs and wires when I noticed the crack. I mentioned to her father and her that it could worsen fuel economy slightly and if they planned on keeping it to change it out. They didn't and the car worked fine until about a month ago when the exhaust note was considerably louder at idle one day and sounded almost ragged during acceleration. The car had noticable drag, loss of power (which will result in poor mpg ), and the smell of exhaust was easily noticed during stops or after shutting the car off. The crack had extended and expanded considerably. Went to a local junkyard and pulled one (you will need to know if your first oxygen sensor is threaded into the manifold itself or down further into the exhaust to get the right one.) for $25 bucks at a u-pull it junkyard. car-parts.com is an online database of junkyard/parts vendors and has them anywhere from 40 to 150. Shipping may be up to 15 or 20 bucks as it is not a small nor light piece)

I actually took automotive engineering a year ago in college, but I'm not a scientist nor can a class ever replace real-world experience of professionals and besides, many don't cover some common practical problems. It's more of a design engineering class, so if you're still looking for 'scientific' knowledge on exhaust issues, one of the auto schools listed above may be your best bet. A few things about the theory side just from working on my own family's cars and with some basics on exhaust. Cold air pulled through a crack near the block should normally not be pulled into the direction of the block if the valves are in good condition and seating correctly. The exhaust valve(s) shut/seat during compression stroke and power stroke of the cylinder and only are open during the exhaust stroke to allow the piston to push exhaust out of the cylinder into the exhaust manifold, and down through the exhaust pipe. Once the piston hits top dead center of its stroke, the cams release pressure, the exhaust valve closes, the intake valve opens and piston pulls back creating a vacuum in the cylinder, drawing intake air in. If your valves were already shot or damaged then it's very feasible and possible that the exhaust air along with some cold air can be drawn back into the block and by the exhaust valve. If your valves are doing this then you will have more problems than an exhaust leak to think of. Once air is pushed into the exhaust pipe, (with a properly designed exhaust system) scavenging of the air occurs. Air is matter; it has mass, density, volume, etc. First pushed by the pistons during startup, exhaust air travels down and through the exhaust pipe and actually pulls the air behind it by vacuum. The vacuum created behind a car helps to pulls air out, but most of the work done is by the pistons, a small but significant portion is done by the scavenging. The process is a cylinder's volume of air travels through the pipe, afterwards which the next firing cylinder's air follow it, and the process is repeated over and over again. It's this vacuum that pulls in the relatively cold outside air through the crack and forces exhaust air out of the crack, both of which theoretically can break the proper flow of the scavenging system process. It is possible for the cold air to rapidly cool the valves/cylinder walls, but it would have to be constantly doing this for at least several months in some very cold weather up north or in canada, if the crack is close enough and large enough to the block to constantly draw in cold enough air to penetrate the 200-400 degree areas of the block near the exhaust ports. Racers and dragsters can understandably face the ill-effects more often as they are pushing their engines harder and faster during shorter periods of time, producing the issues at a higher rate than common transport cars. A 96 corolla with a gasoline 4banger probably won't match the same lag time of an impending failure as a dragster running nitro or alcohol.

A used exhaust manifold is not a bad deal if you can get someone to check it before you buy it. Some new metal gaskets from the auto parts store, and finding an independent small shop to do it and you would probably still come out under $150, or $75 if you have a mechanically inclined friend to buy some beer for. It isn't hard, just a few hours if the bolts are stubborn, which for old exhausts they usually are. Check with muffler shops, most do welding/bending but some do oddjobs for cheap as well. Residual and induced stress on the manifold from heat cycling, proper bolt torque from the factory (or if it's been removed and why), how hard it's been driven, if the catalytic converter is clogged - dozens of factors can influence the propagation, or stagnation, of the crack. If you plan to keep the car for more than a year or 10k to 20k miles I would highly suggest replacing it with a used manifold. At that high of a mileage already, you are more than likely going to have something else more costly fail than another exhaust manifold failure. It's the nature of high mileage cars. Now that I've put half the readers in this thread to sleep, let us know if that has satisfied your curiousity or not.