PCV valves

Richard P6 · Post by **Richard P6** » Sat Sep 26, 2009 11:29 am

I am considering fitting one of these to my Rover V8.

I here they clear out moisture which as a seldom used car, would be of benefit to my engine. I know where to get one, how to pipe it up etc, but will it actually do my engine any good?

Boosted LS1 · Post by **Boosted LS1** » Sat Sep 26, 2009 2:40 pm

A pcv valve keeps the crankcase in a state of vacuum so it helps prevent oil leaks. It also draws clean air into the crankcase while purging combustible air into the inlet tract. I'd certainly use one.

crayefish · Post by **crayefish** » Sat Sep 26, 2009 3:46 pm

Boosted LS1 wrote:A pcv valve keeps the crankcase in a state of vacuum so it helps prevent oil leaks. It also draws clean air into the crankcase while purging combustible air into the inlet tract. I'd certainly use one.

Would certainly be great for a road engine. However for a performance engine... surely adding oil vapour into the induction mixture would reduce the octane which would mean detonation would be more likely... hence why no one uses those emmision control systems on performance engines.

I have seen systems that create a vacuum in the crank case by using a conector into the exhaust. All of the benefits... none of the problems! Just remember to disconnect it for MOTs!

Boosted LS1 · Post by **Boosted LS1** » Sat Sep 26, 2009 5:47 pm

Production performance engines do use emmisions circuits though. They have to. It's just people who think they have a performance engine who don't if you know what I mean. You'll always see ultimas and cobras with open circuits. Often they'll have an oil leak and a film of oil around the engine bay from two 1" open breathers. Don't forget there are plenty of high performance engines out there with active pcv circuits. How close to the ragged edge would you have to be to blow up due to an active pcv circuit. I can understand a racing engine wanting open breathers and catch tanks but most of us won't get past 7k. The octane comment you make is correct but I doubt there's much oil passing through a healthy pcv circuit. The inlet hole is usually tiny and the circuits more active under high vacuum not high rpm's. In fact the oil is usually filtered as if it did pass into the inlet we'd all have smoke coming from out tailpipes.

I've seen the exhaust extractors, I think Moroso sell them. I think it's a neat idea but I never got round to trying those.

topcatcustom · Post by **topcatcustom** » Sat Sep 26, 2009 6:24 pm

Exhaust extractors should only be used in open systems with no mufflers. There is a risk of back pressure which will actually put pressure into the crankcase, that is why they are mainly used on drag cars etc. I am putting an electric vacuum pump on mine, you can also get belt driven 4 vane pumps (like a tiny blower) but I didnt want any more belts and pulleys! A few inches of vacuum is supposedly worth a good few bhp! Probably use more oil though...

crayefish · Post by **crayefish** » Sat Sep 26, 2009 9:34 pm

topcatproduction wrote:Exhaust extractors should only be used in open systems with no mufflers. There is a risk of back pressure which will actually put pressure into the crankcase, that is why they are mainly used on drag cars etc. I am putting an electric vacuum pump on mine, you can also get belt driven 4 vane pumps (like a tiny blower) but I didnt want any more belts and pulleys! A few inches of vacuum is supposedly worth a good few bhp! Probably use more oil though...

Why would they cause back pressure? they work by a fitting conected to the exhaust which has fast air (exhaust) flow past it and hence low pressure. This creates a vacuum and pulls the oil vapour through. the amount would be so tiny compared to the mass flow rate of the exhaust that it really wouldnt increase the air pressure in the exhaust...

Sorry Topcat... this seems to be the second time I have a differance of opinion with you tonight!

mgbv8 · Post by **mgbv8** » Sat Sep 26, 2009 9:54 pm

They also have a non return valve fitted so you cant get high pressure exhaust gasses pushing back up the breather pipe into the engine.

topcatcustom · Post by **topcatcustom** » Sun Sep 27, 2009 9:29 am

I know how they work! I had just read a couple of people who tried to use them with fairly standard exhaust systems who under high revs found that there was back pressure in the exhaust (from the mufflers not flowing enough I guess) which obviously passed back through the pipe into the crankcase. Jegs do a kit 720-6002, are the rocker top boxes 1 way valves or just nice filters/breathers?

I'd use them on a set up like Perrys but not sure about a full exhaust system car.