The small journal blocks have the hole behind the intake/distributor for a road draft tube. The later large journal blocks don't. Or you can check the casting numbers. But I think the large journal 327 and 350s used the same block, so you would need to look at the crank flange to tell if it was a 327 or 350.
A road draft system works this way, the tube sticks down under the car into the airflow going under the car. The air flowing past the end of the tube creates a vacuum that sucks the fumes out of the engine. It only works at speed so if you are in bumper to bumper traffic you will notice the fumes and blowby from the engine compartment. The PVC system works off of engine vacuum so it is always pulling the fumes out and burning them through the intake system.
Basically yes, mainly a messy and smelly way to get some of the vapors out of the engine but only if the vehicle was in motion at a decent speed.
The hole for the road draft tube was used for PCV starting around 1961-62, depending....but it's the same hole. The PCV setup used a little cap that had a hose fitting on it, instead of the road draft tube. The hole in the block is how you tell a small from large journal main bearing size, though.
I believe there is a difference in the front of the block as well, isn't there? The small journal block has a small well cast into the block along with a drilled and tapped port into an oil galley, the large journal block doesn't. Look at these pictures, in the area just above the water pump just off center (to the passenger side), right below the intake manifold. In this picture there is a feed line coming off the oil port going to the oil filter. And just to the left of that port is a webbing cast into the block, which you can clearly see in this pic: The large journal block doesn't have that:
The term "road draft tube" has been used universally around here it seems. It is such an antiquated design that I can't honestly see why anyone would even consider using one. Chevrolet had this in 66, it is called a fresh air tube, connects to bottom of air cleaner housing, it is designed to work in conjunction with pcv hookup on oil fill tube. There is a version from 65 that is straight, from Corvettes and Chevelles. There have been numerous pcv related threads here and as I have mentioned before; depending on carb(s) and air cleaner combos, there are many different systems that can be used.
That rib is only used on 265 and 283 engines. the 67 283 didn't have that rib. and the 327 didn't also.
The large journal blocks were '68 and '69. They were actually just 2 bolt 350 blocks since the 350 started production in '67. The large journal blocks had a main/rod journal dia. of 2.45/2.10.
Paragon reproductions sells that little cap..they call it a vent tube. https://www.paragoncorvette.com/c-23256-crankcase-vent-tube.aspx
Thanks for the clarification Old wolf, but I believe the 327 still had a well in the block with that oil galley plug you don't see on large journal blocks, like this: Or this: Please correct me again if I'm wrong, but it seems to me it's the easiest and quickest way to ID an engine in a car, often the rear of the block where the road draft hole is located is blocked by the firewall or other items, the front is usually easier to see. It doesn't get you the specifics that you can get from the numbers, but it's a pretty quick and easy estimate of what your'e looking at. ID the block, look at the heads, in an instant you can know the basics of what your looking at.
I'd say just look at the block casting number, and then go to any of the casting number sites, such as Mortec.com, to find out exactly what you have. I am Butch/56sedandelivery.
Yes the 327 has a rib but not nearly as prominent as the 283 rib in the picture, that I commented on. The 265 & 283 rib you can hang your finger in it. The small journal 327 and 67 283 you cant catch your finger in it.
The Jalopy Journal
