First without the ashcatcher, you will be inhale alot dirt in your mouth, and you will be cough alot, So how do you prevent  smoking and not cough ?

Take a joint for instance. If the joint is rolled too loosely and there is alot of space between the paper and the marijuana, this can cause you to cough very hard, if the joint is rolled too tightly there will be no airflow and it feels almost like trying to such a piece of fruit through a straw. The best way is to roll so the paper is tightly and evenly wrapped around the weed, and have a tiny bit of room for airflow through the joint. This will make for much less harsh smoke and you will cough less. Paper itself is noxious to the lungs when burning, so even if you do not cough while you smoke a joint you will likely have some coughing going on later.

Once you get a feel for how much oxygen you need for how much smoke, you can take small air inhalations right before/after you inhale the joint, and get the right balance in your lungs to avoid coughing, I figured this out around 7 years ago, and I never have an unpleasant reaction when smoking.

If you are smoking a bong make sure it has good airflow, no restrictive drag where your lungs feel they are over working for the amount you inhale, I find an 18 inch beaker bong with a 1.5–2 inch mouth piece is perfect for getting a smooth toke. If there is drag, what I do is mid toke, break the seal of my mouth to the bong and get a little extra oxygen with it.

If the piece has no drag, you can use a diffuser downstem (which splits the stream of smoke into many streams in the water making it less harsh on the throat) and put some ice into the shaft of the bong. The diffuser will make the smoke easier to take and the ice cools it so your chest is not inflamed by it. Make sure the weed is packed into the bowl well but not so much it is a strain to inhale.

anyway ashcatchers and Pre-Coolers are optional bong attachments that provide an extra stage of filtration before the smoke even enters the main chamber of your Bong