PCV, and the use theroff.

[clynos1]

I am just in the process of finalising the engine breather pipes and installation of the breather box and have just seen a thread on the use of a PCV, would I use this in place of the breather system or in addition to it? again any info gratefully received.
David

[GreenV8S]

Presumably you mean a PCV check valve? If you have a positive crankcase ventilation system (crank case breather connects downstream of the throttle) then you need a PCV check valve or some other form of restrictor to prevent it from evacuating the crank case when the throttle is closed.

[Steve_D]

If you are fitting the factory type oil catch tank then you will have a pipe going from each rocker cover to the catch tank. The factory install would then vent the tank to atmosphere via two other pipes.
Blank off one of those pipes and fit the PCV to the other. Connect the other side of the valve to the base of the carb where there is a fitting which could be used for vacuum to a servo.

When running, the vacuum at the inlet will draw gases from the crankcase via the catch tank. This will reduce crankcase pressure and so reduce oil leaks.

Some race engine builders actually run a vacuum pump to pull an even higher vacuum which, it is claimed, can make significant extra horsepower.

Steve

[clynos1]

Thanks to one and all for the explanation, time to make up the pipes etc. and fit the PCV.
David

[Graham-P]

Here's how I have done it. PCV to carb base and a crank breather to atmosphere in the mechanical fuel blanking plate.

[Steve_D]

Graham
The only problem I see with that is that under high vacuum air will be drawn into the filter on the chamber cover and from there into the base of the carb. This will make the mixture lean and will be difficult to tune out.

No doubt you will tell me it runs fine and has done for years but you see what I mean.

Steve

[60mph]

Steves' worry about a lean mixture is one of the reasons I havn't been able to work out a system to evacuate my crankcase and help to reduce my crank oil leak.
More light would help.
Frank

[Steve_D]

The method I described does not allow fresh air into the system as the connections to the rocker covers are not themselves air vents they are just a hose connection to the cover which can be pulled out of their rubber grommet in order to top up the oil.

Steve