Are you suffering from oil covering your engine bay? Have you tried breather towers and rocker vents but still find you are loosing oil?
Unless you are planning to race and need a catch tank style breather, then I’ll let you in to a little secret…..breathers and towers do nothing for an aircooled engine. In fact the breather towers are a marketing gimmick. Shock horror. I have tested almost every kind of setup, empi, bugpack, cb, csp, speedshop, mocal, breathers, towers, catch tanks, vented rockers, non vented rockers, vented fuel pump take off on 1200- 2110cc engines.
After many years of trying to overcome the symptoms of blow-by oil getting out from non sealed areas of the engine I got fed up. I have ditched breathers and towers and rocker venting and guess what… I’ve had no issues since! Read on if you want to know how….
They knew what they were doing
Essentially the non secret is you have to match the air intake bore with the output bore. This means that as the the engine sucks air in (from behind the pulley) and pushes it around in the case it needs to vent the same amount of air out of the case, whilst somehow being at a lower pressure and separating any oil that has become vaporised in the air flow along the way. Simple right? Well unfortunately when we started modifying our engines the waters got muddy…very muddy.
Vw of course knew what they were doing, on aircooled engines the output is the large outlet from under your alternator stand. This has a baffle plate that sits on top of the case to catch any thrown oil inside the case and feeds up to a baffled section on the oil filler and exits via a breather pipe. This breather pipe on early setups was 1/2″ but also included a down pipe called a road draft tube. The theory behind a road draft tube was that when the vehicle was in motion a slight vacuum would be hopefully obtained, helping to extract excess combustion gases as they collected in the crankcase. This of course only worked when the vehicle was moving and had downsides like not working at all or actually sucking up water vapour.
It was abolished by vw as they increased engine capacity. Vw changed their setup by venting to the carburettor as the source of suction provided good enough pressure differential through the crankcase. At this point they increased the bore to 3/4 inch which went to the air filter.
Sand Seal..say what?
Then came the sand seal boys, less said about this the better. If you run a sand seal then you have blocked off the main air inlet on your engine. You will not be able to stop oil blow by unless you 1. remove the sand seal or 2. Use a closed system with non return / egr valves on every outlet like a modern vehicle.
The aftermarket manufactures (when us vw owners stated fitting sand seals to stop leaks from dodgy aftermarket pulleys and crankcase over pressurisation) jumped on this and produced alternative inlets via the rocker covers and an airfilter/ baffle/breather box. This function of this setup has long been misunderstood with most people and parts companies jumping on the bandwagon and selling them as a new kind of outlet instead of an inlet. Then with removing the stock breather system or blocking completely the oil blow-by was actually made worse. For drag race applications or revving well over 6000rpm where you are also running a big catch tank and don’t go round corners they are a compromised necessity. For road use you’ll end up totally filling up the 3/4 valve cover and sloshing oil back up those tiny 1/2 pipes in to your breather, and out through your airfilters where it will be burnt off, reducing horsepower and your oil level. That or spread all over your engine bay. You’ll also have more connections that could potentially leak and have hassle each time you need to tune your valves.
Along with modifications such as twin carbs and prettying up the engine bay many people remove the stock oil filler and breather and fit a shiny oil filler with 1/2 hose outlet. This is sometimes left just venting in to the engine bay spraying oil all over the place. The better way is to attach that 1/2 outlet to one of your air-filters. This provides at least some negative pressure which the engine needs to not spew oil everywhere. In addition many attach that 1/2 outlet to a breather box or even fit a breather tower. Unfortunately this doesn’t work. The 1/2″ outlet is smaller than that of a stock 1600 engine which ran a 3/4″ outlet. Add to this any kinks, 90 degree elbows or very long pipes and the outlet the engine sees is reduced and often equivalent to almost nothing at all. The engine will over pressurise and you’ll be left wondering why after spending all that dosh on a fancy breather or tower that you are still loosing oil.
What the heck are we missing? Why do you still have oil everywhere?
The issue is the ratio of air in and air out. It needs to be appropriately matched. The breathers and towers are by design ‘oil vapour separators’. They attempt to solve the symptom not the cause. They are attempting to solve the issue of separating the oil vapour from the air by using various baffles, drilled plates and sponges. The surface area, number of baffles, temperature and pressure difference for all the attachments effect it greatly. The limited space in aircooled engine bays as well as misunderstanding how the system works essentially causes these to be massively inefficient, they become full with oil and yes you guessed it…start slinging oil everywhere.
Leave the oil separating headache behind, its a fools errand. What you should be looking at is proper crankcase in / out venting. The ratio of inlet to outlet is very important, VW had it spot on with their open pulley inlet and 3/4″ outlet. So when we come along and add bigger engines and start modifying this stock system you must keep the correct pressure differential across whatever you install. Its not easy with breathers or towers with tiny inlets and outlets and non changeable baffles. All you need to do is to keep the outlet larger and at a lower pressure than the inlet. Keep in mind that temperature, the length of your pipework, 90 degree bends and kinks all effect efficiency.
There is no one size fits all i’m afraid guys, so you will have to find what works best for you.
On my 2110cc I rev up to 6k and have found the best venting system to be a stock oil filler that has baffles (available new) with a 3/4″ outlet, a thick non kinking big bore rubber pipe up to my RH carb with a sweeping bend is sufficient. Its stock baffle and 3/4 bore means large surface area so plenty of opportunity for the oil to condensate and run downwards back to the sump. On other engines I have fitted the same with great success, on occasions I also vent the fuel pump block off either directly to the LH carb or via a small baffle. Trust me, its all you need.
