Paul -
I was under the impression that Larry was using a Weber set-up, where
the vent line goes to a fitting on the air cleaner, rather than the carb
itself. Since the air cleaner fitting generates much less vacuum than
the carb fitting, I would still use a non-vented cap. Otherwise, ite
seems to me that there wouldn't be enough vacuum in the crankcase to
keep the car from piddling in the driveway. <grin>
I used to run a Fiat X-1/9 that has a similar set-up from the factory
(downdraught Weber, PCV plumbed to the air cleaner) that ran a
non-vented cap.
Chris
wizardz wrote:
>
> hmmm.... I think a bit of clarification is in order here.
>
> Yes...as Chris reiterated, the whole point is to keep a slight
> vacuum on the crankcase (lower end) . This helps to keep the oil
> inside the crank and not leaking out through the rear crank journal
> AS WELL AS to recirrculate blow-by gases to be burned in the intake.
>
> Depending on model year, some breather systems use a carbon
> canister that also vents the gas tank and the valve cover
> (via a restrictive orifice).
>
> The timing chain cover breather (lower end) either went through
> a PCV valve directly into the top of the intake manifold, or was
> routed via a Y pipe directly into the side of the carbs.
>
> This carb side port is 'ported vacuum' that is to say it's on the
> 'atmospheric' side of the throttle plate. The high vacuum is only
> applied if and when the throttle is opened. Even then, the port
> (usually plumbed with something like a 1/4" diameter hose)
> still only breathes through a small diameter restrictor hole into
> the carb throat.
>
> Early models without the carbon canister, or vent pipe on the
> rocker cover, used the PCV valve to the manifold with 'vented' cap.
> Remember, you're pulling only a slight vacuum, you don't want full
> manifold vacuum in the crank. The idea was to also pull
> piston ring blow-by gases back into the intake to reduce acidic
> build up in the oiling systems.
>
> Later models using the Y pipe to the carbs, the carbon canister,
> and having the breather pipe on the rocker cover used a non-vented cap.
>
> Paul Tegler wizardz@toad.net http://www.teglerizer.com
> OBie - '73 BGT - daily driver
> http://www.teglerizer.com/mgstuff/ob_description.htm
> Punkin' - '78 Spitfire - corner ripping screamer
> http://www.teglerizer.com/triumphstuff/spit78.htm
> Lil' Greenee - '73 RWA Midget - lady killer
> http://www.teglerizer.com/midgetstuff/index.html
>
> Paul Tegler wizardz@toad.net http://www.teglerizer.com
> OBie - '73 BGT - daily driver
> http://www.teglerizer.com/mgstuff/ob_description.htm
> Punkin' - '78 Spitfire - corner ripping screamer
> http://www.teglerizer.com/triumphstuff/spit78.htm
> Lil' Greenee - '73 RWA Midget - lady killer
> http://www.teglerizer.com/midgetstuff/index.html
>
> -----Original Message-----
> From: Chris Kotting <ckotting@iwaynet.net>
> To: Larry & Sandi Miller <millerls@ado13.com>
> Cc: spridgets@autox.team.net <spridgets@autox.team.net>
> Date: Monday, January 24, 2000 8:09 AM
> Subject: Re: PCV
>
> NON-vented. Regardless of how you plumb it, a vented cap will render
> the PCV ineffective. (The function of a PCV system is to keep the
> crankcase at slightly less than atmospheric pressure, a vented cap keeps
> the crankcase at atmospheric pressure.)
>
> Chris Kotting
> ckotting@iwaynet.net
>
> Larry & Sandi Miller wrote:
> >
> > I'm installing a PCV on the 1275 this week.
> >
> > Should I use a vented or non-vented cap?
> >
> > Larry Miller
|