<VV> valve seats revisited (Engine Braking)

Tue May 8 03:09:36 EDT 2012

Randy,

Yes, there is a relative vacuum on the intake stroke which  creates most of 
the engine braking.  However, the subsequent opening of the  exhaust valve 
does not break the intake vacuum.  Keep in mind the four  stroke cycles.  
Intake, compression, power and exhaust.  The exhaust  valve does not open 
again after the intake stroke until after the  compression and power strokes.  I 
do not recall if there is any valve  overlap on a stock low HP Corvair 
engine but it is possible that the exhaust  valve is open for a very short time 
at the end of the exhaust stroke just as the  engine enters the intake 
stroke.  In effect; the exhaust valve is still  closing as the intake valve is 
beginning to open.  If so, it can be pretty  much ignored both because of its 
short overlap duration as well as the inertia  effect of the exhaust gasses. 
 In effect, gasses will want to continue  flowing out the exhaust port 
until they are physically cut of by the closed  valve.  In fact, this effect 
creates a vacuum in the chamber which  jump-starts the intake process.  I don't 
want to get too deep into it but  this effect is RPM dependant.  In any 
case, the exhaust valve is not likely  to break the vacuum on the end of the 
intake stroke.

At high speed, in gear and with your foot off the  accelerator pedal, going 
through the cycles gives us the  following.

Intake:  As you said, vacuum creates engine braking due  to the suction on 
the piston as it moves down.

Compression:  This causes braking due to the increasing  force on the top 
of the piston as it tries to move up and compresses whatever is  in the 
cylinder.

Power:  With the carb(s) closed off there is minimal  air/fuel and there is 
effectively no power added.  However, the gasses that  were compressed in 
the compression stroke will now try to force the piston down  on the down 
stroke.  The compression and power cycles pretty much offset  each other as far 
as braking power when the carbs are closed off.

Exhaust:  There will be some compression as the piston  moves up to force 
gasses out the exhaust port.  At higher RPM, this is a  fair amount of 
braking but relatively small compared to the intake  braking.

Keep in mind that there are other factors contributing to  engine braking.  
These include friction in all moving parts but the main  factor in the 
"other" category is the cooling fan.  At high RPM, the fan  draws a very 
significant amount of horsepower.  Different contributors to  Virtual Vairs have 
stated a range of HP at various RPM but the power draw is  significant and 
obviously rises as RPM goes up.  Maybe one or more of the  gurus can give us a 
HP number for the fan at a particular high RPM.

As far as the vacuum on the intake stroke; the effect of the  other two 
cylinders on the intake manifold adding to the vacuum is minimal of  anything 
at all.  Keep in mind that those three cylinders are at different  stages in 
the four stroke cycle and will not be trying to pull air/fuel in at  the 
same time.  Of course, all three cylinders contribute to the  maintenance of 
the manifold vacuum at higher RPM but probably not much more than  one 
cylinder would maintain by itself.  In other words, once you pull a  certain level 
of vacuum in the intake manifold it will maintain at that level as  long as 
the carbs are restricted and a high RPM is  maintained.

In any case, you put your finger on the major contributor to  engine 
braking.

Last, this has nothing to do with your post, Randy, but the  cooling 
(quenching) effect going from full power directly to downhill engine  braking 
being a major contributor to dropping a valve seat is questionable in my  
opinion.  I think the worst case situation for quenching would be running  full 
throttle (full load) uphill with resulting overheated heads and then  cresting 
the hill and running full throttle down the other side.  Full  engine 
braking has little to do with the dropped valve seats but going from full  load 
driving to low or normal load driving would be more dangerous because there  
is actual air flow quenching going on.  Of course, overheating the heads in  
and of itself is the root cause.

Doc
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In a message dated 5/7/2012 9:20:43 P.M. Pacific Daylight Time,  
virtualvairs-request at corvair.org writes:

Message:  3
Date: Mon, 7 May 2012 20:59:20 +0000 (UTC)
From:  judynrandy at comcast.net
Subject: <VV> valve seats revisited
To:  virtualvairs at corvair.org
Message-ID:
<1729615311.91705.1336424360910.JavaMail.root at sz0092a.westchester.pa.mail.co
mcast.net>

Content-Type: text/plain;  charset=utf-8

<snipped>

??????Anyway, I began to  think, "Why does leaving the car in gear slow you 
down anyway?"? I ran the  process over in my mind of what happens, and then 
I saw the light.? When going  down the other side, you take your foot off 
the gas pedal and the butterfly  valves in the carbs CLOSE .? So, basically, 
there is no air flow into the  cylinders.? I don't think its a complete 
vacuum, but seeing how the holes  and/or openings in and around them 
(butterflies) are very small and limited in  size? not much happens.? As the intake 
valve opens,? the piston tries to  accomplish an "intake" stroke only to be 
given the vacuum/suction from the  other 2 cylinders doing the same thing.? I 
guess when the exhaust valves open,  it breaks the "vacuum" suction and 
prevents it from being a true "vacuum"  seal.? (Please be merciful to my physics 
explanation.? Hopefully, you get the  idea of what i'm trying to describe . 
)? 

??????So, for my 2 cents  worth, I believe this theory works well as I 
haven't? lost a valve seat yet!?  Theory confirmed!? Take that, "Mythbusters" 
!!!! 

Randy (Cap'n)  Hook 

'63 ragtop 84/pg 

'65 monza 4dr. 110/pg  