• Registered: 2009-06-25
  • Posts: 554

Topic: How much blow-by is normal?

I know all combustion engines have some, but I'm used to engines with Positive Crankcase Ventilation that actively suck in the blow-by and burn it. Type 1 engines have no such active system, but vented gases are still pulled into the air cleaner. Similar concept, just not as forceful as using vacuum from the intake manifold.

With that in mind, I had the engine in my 70 Ghia running after an oil change this week, and the oil cap was off. I was surprised at the amount of air that was coming out. Not a huge amount, but enough that an index card would not sit flat on the opening if it was level. Since I'm used to PCV engines, is this amount normal for an air cooled engine? I was a little worried that I had galled a cylinder or broken a ring despite the engine sounding good, so I did a compression check and was relieved that it was reasonably healthy.  The results on the cold engine are below:


Cylinder 3 - 115 lbs        Cylinder 1 - 118 lbs
Cylinder 4 - 125 lbs        Cylinder 2 - 121 lbs

Paul

  • From: Cincinnati, Ohio
  • Registered: 2005-09-10
  • Posts: 2,318

Re: How much blow-by is normal?

My personal guidelines are:

- above 120 psi (good to go)
- between 110-119 psi (compression getting low)
- below 110 psi (time to start thinking about a rebuild)

Of course using additional info such as:

- excessive oil consumption
- "smokey" exhaust
- excessive crankshaft end-play
- poor fuel economy
- low power (assuming familiarity with a "strong" engine)

...will help in determining engine rebuild time.

HTH,

- Nick

1979 Super Beetle Convertible
  • Registered: 2009-06-25
  • Posts: 554

Re: How much blow-by is normal?

Thanks for the info Nick. I suspect it had excessive wear under the PO because the doghouse duct through the forward tin was missing, forcing the fan re-breathe hot air and run too hot. Fortunately the engine has no unusual oil consumption, clean exhaust, no excessive end play, gets about 35mpg, and makes decent power. I like the way the engine runs, so a rebuild will keep the same setup with the exception of a counterweighted crank and better connecting rods.

I am tempted to get the seized engine from my dad's 72 super and build that so I don't have any downtime.

Paul

Last edited by Altema (2010-06-24 22:11:01)

  • From: Cincinnati, Ohio
  • Registered: 2005-09-10
  • Posts: 2,318

Re: How much blow-by is normal?

Altema wrote:

Fortunately the engine has no unusual oil consumption, clean exhaust, no excessive end play, gets about 35mpg, and makes decent power.

You know what they say..."If it isn't broke, don't fix it!!!" wink

Sounds like your engine is doing fine (especially at 35mpg & no oil consumption issues)...so work on those other 114 items on your list! wink

- Nick

1979 Super Beetle Convertible