<VV> [fastvair] I want Compression!

Thu Dec 25 22:35:43 EST 2008

Sorry Seth , Ment to go to the group. Jack

-----Original Message-----
From: jfrost1207 at aol.com
To: sethracer at aol.com
Sent: Thu, 25 Dec 2008 5:14 pm
Subject: Re: [fastvair] I want Compression!

Three more things to change compression under your guide lines. Turbo -
Blower - Noss . Just a thaught. Jack     PS Have a Great Christmas and
New Year to All

-----Original Message-----
From: sethracer at aol.com
To: yahfastvair at oogroups.com; virtualvairs at corvair.org
Sent: Thu, 25 Dec 2008 5:03 pm
Subject: [fastvair] I want Compression!

In a message dated 12/25/2008 9:08:08 AM Pacific Standard Time,
mark.aksamit at yahoo.com writes:

I  would like options to increase compression that DO NOT involve
changing any  of the stock valve train and its stock geometry!
I am calling upon my  colleagues for input!
Thank You!
Merry Christmas!Mark R  Aksamit

A noble thought, Mark! Starting with the obvious, there are only two
ways  of
increasing compression, since it is a ratio. You can either reduce the
size
of the chamber or increase the size of the cylinder displacement. (You
can do
both.) If you want to keep the cylinder head in the same relative
position to
  the cam, that limits your options. It eliminates the most common
method,
milling  of the chamber/cylinder interface, which reduces the chamber
volume. If
you are  starting with a clean slate 140, you can increase the cylinder
displacement.  Boring and stroking are the only two options. Stroking
of the Corvair
has  not been a real option for the past 40 years, but that may change.
Someone is  now making new crankshafts and, with a bunch of attention
paid to the
rod  vs. cam interface, a longer stroke may be a real option. Bob
Coffin is
exploring  these options, I believe.

Most folks look at a bore increase, since it has been an option for
many
years. Any bit helps, but if you are starting with a fresh slate,
replacement
cylinders/pistons - some of which require boring out the cylinder holes
in the
case and the sealing surface area in the heads - can provide extra
displacement.  Doing this out at the fringe of the cylinder studs will
require both upper
and  lower sealing management and discipline. Torque values and gaskets
must
be  carefully chosen and maintained. From all visual indications, the
new
Clark's  cylinders, with their full-fin cooling might allow a
reasonable overbore
(without the head/block boring) and still provide sufficient cooling.
If  you
go this route, piston top design can also help with compression, but,
as one
person mentioned, high pop-ups can interfere with flame front travel
(from
the plug area) inside the cylinder.

Another option is to modify the chamber to reduce it's volume. As
mentioned,
the usual method is to mill the sealing surface, but that changes  your
geometry. Since the cylinder heads are cast aluminum, the chamber can
be welded-up
(in spots) to lessen the volume. You could simulate the  milled-head
effect
by welding up the area opposite the valves, extending,  perhaps into
the area
between the valves. That is a lot of welding and would  still require
final
milling for a consistent gasket surface.

Mark - before you go too far in pursuit of compression without valve
geometry changes, you need to grasp another item. What cam will you be
using? If  you
are using an original Corvair cam, or one correctly ground from a new
billet, you will be able to maintain exact original valve geometry.
That is
entirely dependent on the cam base circle being the same as original.
All  reground
cams reduce that base circle - some a little, some a lot. That moves
the
lifter down in the bore, and your geometry changes. That base reduction
is
necessary for the lift to be increased or the duration to be changed.
So, if you  are
installing a "racing" cam
in your "racing" car, most likely you will already be chasing the
correct
geometry in your valve train.

So, consider combining this reduction in cam base circle with milling
of the
gasket surface on the head. If both are "adjusted" the same amount,
that
part of the valve train equation are "as stock." The fulcrum point for
the
rocker has moved in the same amount, as has the tip of the valve. Also
of note  is
that the contact tip of the valve may be relocated outward if the valve
seats
are ground, depending on the overall length of the valve.

So, Mark, the bottom line is that, to take full advantage of all the
options
available to you building a race motor, you will have to check and
recheck
the valve train geometry. Even if you decide to use a stock cam and
rockers,
you will need to check to make sure the rocker tip to valve tip
contact patch
is correct. An adjustable pushrod will allow you to adjust that
pattern to
make optimal use of the valve tip. Once you know the correct  pushrod
lengths
for each valve - yes all twelve - you decide on what  lengths to go to
(little
pun there) to provide the right pushrods. Ray Sedman  sells pushrods of
any
length you want. Also Smith Brothers in Oregon (not the  cough drop
folks) sell
pushrods for the Corvair, any requested length.

With all this effort to get everything right, can you still screw
everything
up? Emphatically YES! I know because I just did. The motor I brought
to
Willow Springs and Ventura had valve noise that sounded like someone
had  dropped
a handful of extra nuts into the valve covers. Upon examination I found
 the
results of several shortcuts I had taken in the last few weeks before
the
races. I didn't know what cam was installed and the heads had been
milled a  bunch
(before I got them.) Despite my archeological efforts, and  measurement
after
measurement the valve geometry was severely  "compromised". It was
nobody's
fault but mine!  I would do things  differently today.

Anyway, consider increasing compression by milling the heads, all the
stuff
you would have to do to verify the correct valve train geometry - you
should
be  doing anyway!

And a Merry Christmas to everyone!

Seth  Emerson

C's the Day! - Corvair, Camaro,  Corvette,  Christmas

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