<VV> I want Compression!

[Sethracer at aol.com]

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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