do all motors need one. some of my friends say no. just looking for more input.

Positive crankcase ventilation had been a federal clean air requirement since the mid 60's.

NEED one? no you don't need one.

PVC systems control blowby by recycling it thru the motor. They really do no harm, unless you're blowing a ton of oil anyway. They help keep the air clean, and that includes the air under the hood of your classic, and all the surfaces that any oil in the air might stick to.

For your basic street cruiser, there is no real reason not to run one.

i might drive my car two times a month.burns to much gas lol.

i agree with jim 100 percent. if you choose to not have one.i would have a breather in the valve cover at least.for expansion and contraction. also 65 and below motors have a breather on back of motor going below tranny. you dont put a pcv valve in that motor it will have a vacumn leak. sucking air right back from the tube.

i was just worried more about back pressure.

dont worry about back pressure. a pcv valve will only suck as much as it can.put your finger on it and than let go.it will go up and down as needed for vacumn or not.

I have an old english Bradford van...long before a PVC
the block has a pipe coming out of it, then drops down beside the sump...this stops any presure building up in the crabk case
As rings etc wear in old cars, the burnt gases escaped thru this pipe, creating (the older guys here will remeber) cars going down the road spuing blue smoke from out under the engine...
Then came the Greenies
So someone got this idea...
put a vent on the tappet cover, then suck the gases back into the motor thru the inlet manifold to be re burnt ...but to do this meant a manifold leak, so they put a little valve that only opened when the crankcase pressure increased.
They did find different engine behaved a little different, so each engine needed a PVC that suited its particular quirks.

This then had another issue, they needed to get air into the crankcase, and venting to the air, then grit etc to get into the crankcase...so they connected it so the air was filtered by the air cleaner 1st..

thanks guys for all the input