[Supras] Head Gasket Choice For N/A

[Jim Jobe]

The term non-interference suggests that at peak lift and TDC
the pistons and valves do not intersect.  So if the belt
breaks and there are valves open at peak lift, the pistons will
not hit them at any point.  Thus cam timing is irrelevant.

Seems to me the stock valve springs coil bind fairly quickly
too, so I'm not sure that floating the valves fully open would
cause them to hit.

Not many have degreed the cams.  My local group has and we
experiment with valve timing quite a bit on our big turbo cars.
However the rest of the group tends not to worry about it because
most of the cars in this group are turbocharged and there are far
easier and simpler ways for the average owner to increase hp than
learning how to properly degree cams.

On Fri, Mar 02, 2007 at 11:50:52AM -0500, Christian, Skip wrote:
> Jim,
> 
> 	When the lobe centers are decreased, either by design or by
> adjustable sprockets, the minimum piston to valve distance is reduced.
> This combined with my tight clearances is what, I think, caused my
> problem.
> 	Lobe center is the halfway point between valve opening and
> closing.  I have not been able to find anyone, and site, or any book
> that lists the cam timing specs for the 7M Supras.  No one seems to want
> to understand these days......  For the Alfa, optimum lobe center for
> street (broadest power curve) was about 102 degrees for both intake and
> exhaust valves.  When the intake was retarded 12 degrees in '72 (sorry,
> I was wrong before - it was 12 degrees, not 14), this changed the lobe
> center from 102 to 114.  When the exhaust was retarded, the new exhaust
> lobe center became 114 degrees.  Less overlap, less chance of unburnt
> gas in the exhaust.  Alfa went to variable valve timing in 1980.  On my
> '73, I re-set the intake to 102 and, on a 2 lane highway, no longer had
> to downshift to fourth to pass :-)
> 	The cam setting for best peak HP (in the Alfa) is 108 degrees,
> but the gain in top end comes with a big loss in low RPM power.  About
> 108 is the narrowest new car can do and still pass emissions without a
> big exhaust restriction.  That is why new cars (without variable valve
> timing) all seem to have good power but no bottom end.  Manufacturer's
> have figured out (FINALLY) that with the cams set for best peak HP, the
> exhaust flow restriction can be reduced & still pass emissions.
> 	I use the Alfa because it is a true hemi and similar to the 7M.
> 
> Sorry For The Rambling,  Skip
> 
> 
> -----Original Message-----
> From: supraman88 [mailto:supraman88 at verizon.net] 
> Sent: Friday, March 02, 2007 10:59
> To: Christian, Skip; supras at supras.com
> Subject: Re: [Supras] Head Gasket Choice For N/A
> 
> Ooops...missed this part. Thought it was awful hard to damage the valves
> on one of these engines. Yes, please tell us more!
> 
> 
> Christian, Skip wrote:
> > 	Unfortunately, I used a "Gates" timing belt....... Because I was
> in a 
> > hurry...  It shredded itself at 82K miles and I ended up with bent 
> > valves.  Intakes were set at .0004 because dealer had no shims and cam
> 
> > timing was changed to increase overlap (big power difference - more 
> > than all the aftermarket parts combined & still passed emissions).
> > 	So, here I am.  Planning to buy a "rebuilt head" & install.
> > Maybe, I just don't learn :-)
> >
> >   
> 
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