<VV> blow-by?

[Chuck Kubin]

Hey Ryan,
   
  "Blowby" is whatever blows past the rings and valves on the compression stroke and while combustion takes place when parts don't completely seal up.  
  For the cylinders, worn, stuck or cracked rings; worn, scored or out-of round cylinder walls; pistons holed from knock (worst case scenario)  or with cracked or broken ring lands can all contribute. Combustion gasses blow past the rings and pressurize the crancase, and you get smoke puffing out of the vent tube and possibly out of the oil filler.
  For the valves, it happens when the valve face and seats aren't perfectly seated, loose stems-to-guides allow some movement, or the valve and/or seat surfaces are burned or pitted. Burnt valves are pretty obvious, but the others less so because nearly all of the blowby goes out the exhaust. If the stems are loose enough, and enough in this case means a lot, some pressure in the exhaust can blow up the guide and pressurize the air in the valve galley, pushrod tubes and crankcase.
  A leakdown compression check setup includes a hollow fitting hooked up to a compressed air source and two gauges. With the piston at TDC, one gauge tells you the pressure going in. The other tells you how much the cylinder retains and gives you an idea of how fast it leaks down. You get a more accurate picture of how healthy the cylinder really is than you would from just a compression test.
  It won't identify the problem, but it will tell you how bad it is and where to look. You listen for the leaks. If you hear it at the carb, it leaks at the intake valve. At the exhaust, exhaust valve. Crankcase vent, the rings, piston or cylinder walls. Firewall? Someone is snoring in the back seat.
  This takes more time and equipment than a regular compression test, but it gives up a lot of information and the basis for intelligent decisions. Let's say your 110 isn't exceptionally strong but it runs well enough. The compression test indicates and the leakdown confirms there's even wear across all six cylinders (let's say 100 pounds of compression).  The leakdown says the rings are worn, but no huge problems. Why spend $1k rebuilding an engine that get you around plenty well enough?
  Or, the same engine runs fairly well but is puffing out the vent tube. The leakdown shows bad compression on one cylinder, weak on two more and a lot of rushing air noise from the vent. Bad rigs, scored cylinders, a cracked piston...a matter of how long you can drive it that way, or should you dump the car, spring for a rebuild...you see where I'm going.
  The true value is is isolating the problem areas and determining where to go next.
   
  Chuck Kubin

Ryan Verthein <daretocorvair at yahoo.com> wrote:
  
>>>I have noticeable blowby and am awaiting time to do
a leakdown 
compression test.<<<<

What does blow-by have to do with leakdown? what
happens that would cause that?

I DID just start my car for the first time in many
years, but I have a LOT of blow-by.-and yes, I can
attest to the fan sucking it all in. in fact now that
I thin about it. most of the exhaust and blowby that I
see coming out the bottom probably isn't from the
exhaust istelf...could just be whats being pushed
through the heads and just LOOKS like it's coming from
the exhaust. 

I do not know this, but could be.

Anyway, I was just curious as to why they had to do
with each-other, cause it might help me if I have that
problem too.

Lets hope not.

Ryan V.

1965 Corsa/110/4
http://www.edselmotors.com/corsa.html
and http://free4allband.tripod.com/corsacorvair.html

__________________________________________________
Do You Yahoo!?
Tired of spam? Yahoo! Mail has the best spam protection around 
http://mail.yahoo.com 
  


		
---------------------------------
Yahoo! Mail
Bring photos to life! New PhotoMail  makes sharing a breeze.