pcv or evacuation system for a weber set up

[Gaz64]

Originally Posted by CompClassics

An old drag racers trick is to apply a slight vacuum to the crankcase by installing EGR valves, one way check valves, into the collector area of the header on each side and attach a hose between them and a fitting on each valve cover or a crankcase breather. The Exhaust pulses create a suction which in turn cause a slight vacuum within the crankcase, drag racers looked at it as free horsepower as the pistons were having to pressurize the crankcase when heading to BDC.

At any moment in time, any pistons heading towards BDC (4 in a V8) are cancelled by the same number of pistons towards TDC.

Blowby gases, combustion chamber pressure lost past the piston rings, is the cause of crankcase pressure.

Applying a low pressure to evacuate the crankcase, and if possible creating a negative pressure within the crankcase helps the rings seal better, since ring flutter will be minimised.

Hence the dragracer power increase.
Many guys run vacuum pumps these days, with a regulated vacuum, about 5 in Hg.

For my cars, I run a PCV plumbed to a hidden log where each runner gets a shared source from.
The fresh air side inlet which becomes the breather outlet under load goes to a Moroso air oil separator (self drains to the valve cover via 6an) 12an hose breather hose, then comes back to my airbox.
Not a skerrick of oil in the airbox/hose entry.

Gary

[Karl Bebout]

Wolf, thanks for the pictures. Nice set up.

[paul cass]

Some follow up to some of the posts. You can run lines from breathers to the headers for an evacuation system. Did this when I ran on the track in the 90's. Had blow by and it solved that problem. Later found out I had a broken piston causing the blow by. Check valves are to prevent any combustion in headers from being sucked back into engine and possibly causing an explosion. I'm trying to run a crude form of evacuation system for the engine. If it does build up some pressure, if any oil does leak out then the catch can should collect it. And I don't have to deal with a blow by with open breathers. At 71 I'm past the age of racing everything in site. A little lead foot here and there should be enough. Basically I'm looking to have an engine that can breathe with out making a mess. And I'm not looking to drill and tap. Once I get my system installed I'll report what happens. Mainly I want to have an effective system before I cruise the hill country during our Texas Cobra Club Spring Meet in April. Tried to download a video but with my limited skills it didn't work.

[paul cass]

Hey guys
The catch can came in last week just in time for me to install before our annual Cobra event in San Marcos. I got it installed and as for as I can tell it seems to be working perfectly. If one of you can tell me how to post pics I'll show you what I came up with. Came out pretty good I think.
Paul

[paul cass]

Hey guys. Here is a pic of the catch can mounted for my pcv evacuation system.

[Gaz64]

Hi Paul,

A few things that interest me here from your photo.

Your passenger side hose won't flow much with a PCV valve fitted at the valve cover, so then the bulk of blowby will try to pass through your drivers side hose.

If you study some race cars, larger breather hoses allow slower air flow, and a chance for any oil to fall out of suspension, instead of the high speed jet of air/oil associated with hoses too small.

There's no such thing as hoses too large, of course nobody runs 3 inch breather hoses , but two AN10 hoses, or a single AN12 hose are my recommendation as minimum for V8 over 300 cubes.

One day, I'll post a drawing of my closed breather system that uses a Moroso Vacuum Pump Air/oil separator that self drains to the valve cover, a pcv valve that breathes from the tank after the filter medium, and no oil in any hose after the separator. My tank is mounted high at the back of the engine bay, all hoses run "uphill" from the engine, which gives little chance for any oil to get to the separator, but if it does, it self drains via a 6AN to the valve cover.

Allowing the engine to breath freely when under load, lessens the chances of crankcase pressure related oil leaks as well.

On IDF Webers, the Emulsion tube stack (Extension/Air Corrector/Emulsion tube/Main jet) and the Idle Air inlet are exposed to outside air, as in your installation.

On IDA Webers, the Emulsion tube stack and the Idle jet Holder are covered with a screen. DCOE Webers are also covered with the "Mousehat".

They can afford to run open velocity stacks, or screen filters, one per barrel, whereas on the IDF, the engine is inhaling unfiltered air via the air corrector, albeit in small quantities.

The Idle air is particularly susceptible since the inlet is low in the airhorn.

CB Performance offers a fix called the "jet doctor".

I personally would have a K&N oval filter per carb, so all of the air inlets receive filtered air.

I hope I haven't been too critical here, and you may take some advice onboard.

Gary

[wolf k]

^ I agree with Gaz here. Also, no need to use a PCV valve if you are not returning the vapors to engine intake manifold. This appears what you are doing, just venting to atmosphere, correct?
I am also running IDF's with just stack filters, however, I am very leery of unfiltered air entering the air bleeds and also the bowl vents. I need to come up with some type of filtration for the vents and bleeds.

[paul cass]

Hi Gary and Wolf. Don't mind the suggestions. The breather cap has an internal pcv valve. So both sides have the same pressure. I know that some guys run their weber's without filters. Living on the coast with constant high winds and sand and anything else you can think off I need to run filters. I haven't seen any difference in the operation of the engine with or without these filters. I spent all day today fine tuning the weber's with the assistance of a good friend. Bill was a very successful race car driver and has much more knowledge on engine tuning than me. Found that three of the stacks were not sucking air like the other 5. And when the engine was shut down fuel bled into one of the stacks, and leaked into all the others a little. Took all of the tops off and found that the float level was set too high on all of them. Adjusted the float and fixed the problem. I played around with the idle speed, per your suggestion Gary, and eventually Bill, by ear mostly, was able to adjust all the idle mix screws to get a smooth idle and no skip or stutter. I didn't get to do a test run as we had rain in the area. Later tomorrow the weather is supposed to clear and I'll do a test drive. Still somewhat disappointed in Top End and Red Line. Absolutely no info on the weber's and why wouldn't the floats be set correctly. Noticed that three of the needles were round and the 4th had shoulders. All were set at 5 mm or less.

[Gaz64]

Hi Paul,

Both of your valve cover fittings should be freeflowing without any PCV valve.

Make the hoses as large as the smallest fitting, at least 1/2 inch ID.

Keep an eye out for any dust etc that gets near the top of the carb lids.

Gary

[paul cass]

A question Gary. I've gotten feedback both ways with the pcv system. Some say to leave them in while you and a couple of others say leave them out. What is your reasoning for leaving them out. Some guys use only breather caps but then you deal with oil contamination in the engine compartment. With the catch can I keep the vapors etc. away from the engine and engine bay.

[Gaz64]

Hello Paul,

Your PCV valve fitted to the passenger valve cover is a restriction to blowby gas when the engine produces more blowby under load.

Pull the valve out, and blow through it from the valve cover end, then compare that to a hose of the same diameter.

Your system appears to vent the engine to a catch can, which is vented to the atmosphere with a K&N filter, which is a very common way of allowing the crankcase to be vented.
Because it is vented to atmosphere, it is not classed as a PCV system, since the engine is not consuming the crankcase gases.

So, in a nutshell, there should be free flowing hoses without PCV valves fitted in your system.

Hope this helps.

Gary

[big deuce]

Originally Posted by CompClassics

An old drag racers trick is to apply a slight vacuum to the crankcase by installing EGR valves, one way check valves, into the collector area of the header on each side and attach a hose between them and a fitting on each valve cover or a crankcase breather. The Exhaust pulses create a suction which in turn cause a slight vacuum within the crankcase, drag racers looked at it as free horsepower as the pistons were having to pressurize the crankcase when heading to BDC.

Very true, and way more effective on larger cubic inch engines. Dad and I built an off topic 505 bbc for our drag car and used that exact setup; until we had it dyno'd. The machine shop said, " lets put a evac pump on the crankcase", threw a moroso evac pump on and hp went from 790 to 820hp with no other changes. Oil pressure was rock solid also, not so much without the evac.