It's worth mentioning again, that on a stock Y-Blk, the only crankcase ventilation intake is thru the oil filler cap screen. In service this was not a great idea, as dirt drawn into the engine compartment quickly plugs the screen, resulting in no flow thru venting. This is a major contributor to engine sludge problems..

Paul

Howdy Mate, if you are running the stock fuel pump, just check for oil leakage from the rear facing spindle shaft hole. If it's worn it will seep oil which will eventually be blown back onto the draft tube, giving the impression of an oil leak from the draft tube itself. It's one of those tests that "The Gods" sent down to check our level of evolution Bro'.

Let me know what you fi9nd.

Cheers, Louie , NZ

ArizonaMerc

I had that problem with a 292 that I bought used. When running, it was putting down about one drop of oil every 5 seconds or so. It looked from above that the oil was dropping out of the tube. However when I decided to crawl under the car while the motor was running, I noticed that the oil was running down the OUTSIDE of the backside of the tube. Made me think there might be a hole in the cannister part of the assembly. Took the entire assembly off , took off the cap and filter and cleaned thoroughly. I poured motor oil in the bowl part and tilted it to roughly  the same angle it would be when bolted to the engine block. Sure enough, oil began to run down the outside of the tube. Problem was: Not counting the cap and filter, That road draft assembly has two pieces : the bowl shaped part and the tube. There is a pinchwelded seam where the tube and bowl go together. Mine was leaking at the seam and when I cleaned mine , It appeared that the seam was also brazed with bronze brazing rod on the inside. Factory brazed seam? I don't know. These assemblies hang down low on the motor and are prone to be bent or tweaked when pulling motors out of the car. I know. I have done it to them. Mine was probably bumped at some time, maybe by me and the seam was cracked and leaked afterward. I cleaned the seam to bare metal inside and outside and re brazed the seam on both sides. I could have used a mig welder to weld the seam solid on the outside but chose not to . My problem was solved . No more oil leak. Good luck. 

Modern big inch race engines typically only have one exit from the engine for a vacuum system.  If ring seal is good, then any pressure lost at the rings is expected to be minimal.  Because my roadster uses an exhaust driven evacuation system, I have two exits, one in each valve cover simply due to using both headers to drive the system.  Exhaust driven systems do not have near the flow producing capacity as the belt driven pump models.  I’ll add that a multistage dry sump oil system can also pull a vacuum on a crankcase.  I now see dry sump equipped racing engines also using auxillary vacuum pumps to just up the ante on the amount of vacuum being pulled over and beyond what the dry sump pump can pull.

Lorena, Texas (South of Waco)

Boy, you learn something new everyday. Had no idea of the potential HP gains realized through neg. crankcase pressure. I had always assumed the exhaust-driven evac. system's purpose was akin to typical PCV benefits (drawing out blow-by gasses at idle, and relieving some pressure at high revs). If that system is supposed to be closed in order to maintain neg. pressure, then modern exhaust or pump-driven evac. motors likely don't have all the extra valve cover breathers we are used to seeing in 60s era race engines that bled off the excess pressure created at very high RPMs?

6 VOLTS/POS. GRD. NW INDIANA

A PCV valve is simply a controlled vacuum leak which requires an adjustment to the air fuel ratio in which to compensate.  PCV systems utilize the engine manifold vacuum to pull air from the crankcase whereas auxillary pumps (electric or belt driven) or exhaust driven systems do not require engine vacuum in which to operate.  But the purposes for each do vary.

 

PCV valve systems are specifically for removing moisture or excess pressure from the crankcase which would be an environmental concern if not reburned through the engine.  PCV systems are typically a flow through design that do not pull a vacuum on a crankcase but can remove enough excess pressure that any existing oil leakage can be significantly reduced.  If a PCV valve is used on a sealed system, then a reduction in crankcase pressure will be realized but its ability to actually remove any moisture or condensate from a warming up engine will be compromised.  It’s that flow through design that actually allows contaminates to be moved to the PCV valve where it can be reburned through the engine.

 

Auxillary driven pumps are for removing air itself from the crankcase and are realized more for their performance gains than for environmental purposes.  This is because these air pumps are designed to remove air on a sealed system rather than simply allowing air to flow through the engine.  Removing air from the crankcase frees up horsepower simply due to the pistons not having to work as hard in moving it around.  Air has mass and as a result it takes a given amount of power to move it around.  These same auxillary systems can also improve compression ring seal but there are some detriments to having too much negative pressure in the crankcase.  First is a reduction in oil pressure and secondary is some starving of oil in top end parts such as around the wrist pins.  For this reason, many auxillary engine vacuum systems will have an adjustable vacuum break to limit the amount of negative pressure that can be generated within a crankcase.

Lorena, Texas (South of Waco)

I guess I am confused. Can you elaborate on the differences between PCV, which uses intake vacuum to clear the crankcase, and the systems which use an even higher vacuum to clear the crankcase?

Jim - Erie Colorado, 1957 Country Sedan

Assuming the exhaust pulses are approaching the speed of sound, then an undercar road pipe would need to see ~777 mph to get an equivalent speed effect that generates the same negative air pressure as does an exhaust system driven evacuation system.  That’s the simple explanation.  Road draft tubes by design were not to make a negative pressure within the engine but instead promote air movement through the engine.

For what it’s worth, the exhaust driven system on my roadster is working well at 2500 rpms and just gets better as the rpms go up.  Many high end race cars (and especially those that use mufflers) incorporate either electric or belt driven vacuum pumps to maintain a negative pressure within the crankcase in lieu of using an exhaust driven evacuation system.  I’ve seen as much as 30HP on engines that use a belt driven vacuum pump versus no pump at all so there’s definitely some power to be found in running a negative pressure on the crankcase regardless of the method employed.

The exhaust driven evacuation systems are not tied to the intake tracts but instead to the crankcase usually by way of the valve covers.  A check valve at the header keeps any unwanted exhaust pressure from making its way back to the crankcase in the event backpressure does take place.  These types of evacuation systems are not to be confused with positive crankcase ventilation systems which serve a different purpose.

Lorena, Texas (South of Waco)

Am I missing something? Since race cars usually travel at a high rate of speed, would an exhaust rigged evacuation system (aside for brief idling periods) be any better than a T-Bird style valley cover draft tube that never leaks? Some racetracks require mufflers, and a charge mixture diluted with blow-by gasses would have to burn leaner and thus contain less energy/HP potential than pure gasoline.

6 VOLTS/POS. GRD. NW INDIANA

Welcome to the site.  You might double check the face of the draft tube where it bolts to the block and insure it’s flat.  These are easily deformed and the gasket simply doesn’t do its job in sealing against a surface that’s not quite flat.

Lorena, Texas (South of Waco)