[Supras] Head Gasket Choice For N/A

Fri Mar 2 11:27:49 CST 2007

 Jim,

	I correct the wrong numbers in the E-Mail below.  Sorry, I
sending these from work while working at the same time.
	For the Alfa, 102 degrees gave the broadest power curve.
Emissions caused the overlap to be decreased.  Initially by 12 cam
degrees and later by 24 cam degrees.  Decreasing the overlap prevented
and unburnt charge from entering the exhaust.  Overlap is big in racing;
it is when the intake valve and exhaust valve are open at the same time.
During overlap, the exhaust can pull in charge that, without (or with
reduced) overlap, would not be in the combustion chamber.  Picture the
exhaust rushing down the pipe and before the valve can shut, the intake
valve opens sucking in more charge.  Its one of the ways N/A engines can
use to make greater power.  Not nearly as important for a turbo engine,
but I'll bet there is still a sweet spot where the power curve is
maximized.
	So for my Supra, I installed the adjustable sprockets and
gradually started increasing the overlap.  When I went over eight cam
degrees, power dropped off.  Just think about what advancing the intake
cam does.  It opens the valve earlier.  So its open (or farther open)
when the exhaust valve closes.  Retarding the exhaust cam closes the
exhaust valve later.
	Its fairly common knowledge that advancing a cam (intake and
exhaust on one cam) increases bottom end power.  So, for my automatic, I
tried advancing only the intake cam and it worked great.  It isn't world
beater power, but very noticeable.  I can keep up with (or get ahead of)
traffic and never go above 300RPM.
	Hope this makes more sense.  If not, please let me know what's
confusing (where to start).

Skip

-----Original Message-----
From: supraman88 [mailto:supraman88 at verizon.net] 
Sent: Friday, March 02, 2007 12:04
To: Christian, Skip; supras at supras.com
Subject: Re: [Supras] Head Gasket Choice For N/A

Now, you want to put this in terms an idiot like me can understand?  ;)

I used to be an Engineering Technician, but that was Electronics!  I can
usually figure out mechanical things, but this dissertation you wrote
did a 747 Syndrome on me!

Christian, Skip wrote: 

	Jim,

		I have adjustable sprockets.  Alfa Romeo used the same
engine w/
	same cams from 1972 until 1995. All Alfa cam specs are common
knowledge
	(no silly secrets).  As emissions rules got tighter, '72-'74
cars had
	intake cam retarded 12 degrees, then from '75-'77 the exhaust
was
	advanced 12 degrees.  Cat was added in '78 and cam timing was
restored
	to pre-emissions settings.  However, exhaust restriction kept
power
	down.  In the Alfas, when overlap was reduced, bottom end power
went
	down.  Longer durations disappeared in 1969 - possible as a
result of
	the mechanical fuel injection.
		So, for the Supra, I started by advancing the intake cam
2
	degrees and retarding the exhaust 2 degrees.  Drove & check
power &
	response.   Bottom line is if cams are set at intake advanced 4
degrees
	and exhaust retarded 4 degrees whole power band is improved.
Since my
	car is an automatic, I set all eight in the intake to produce
the best
	bottom end power.  Only problem is power now runs out at around
5200 RPM
	and the blasted automatic cannot be re-programmed to shift when
I want
	it to under full throttle :-)  Anyway, (when the car was
running) if the
	throttle is punched (about 1/4 throttle) at 2000-2500  RPM, the
nose
	comes up.  Just what an N/A automatic needed.  FYI - I tried 6
retarded
	for the exhaust and 2 advanced for the intake and the engine
power was
	still increasing when it hit the 6500 RPM redline.  The magic
number
	seems to be eight degrees (if the valve clearances are within
spec).  If
	I went higher than eight, power dropped off due to a flow
restriction.
	For all this, car had no CAT, HKS sport exhaust, K&N open
filter, and 3"
	intake pipe (in place of the plastic noise canceller).
		Car always got 20-22 MPG before adjustments.  After, its
20-21.
	Not much change.  Also, easily passed emissions (with CAT
installed :-)
		If you but the adjustable sprockets, try to find the
ones with
	the degree markings (if the government still allows them).
Finally, if
	I remember correctly, each tooth slot on the cam sprockets
corresponds
	to eight cam degrees (but please check this first).  All the
above is in
	CAM degrees.
		I tried to keep this short, but, sorry, I'm an
engineer.......

	Have Fun, Skip

	-----Original Message-----
	From: Jim Wooden [mailto:Jim at WoodenU.com] 
	Sent: Friday, March 02, 2007 10:52
	To: Supras at supras.com; Christian, Skip
	Subject: Re: [Supras] Head Gasket Choice For N/A

	Skip:

	If you have a second, tell me about that increased overlap on
the cam?
	;-)
	Did you use custom gears on the cams or move the dowel pins?

	I have MkIII 86.5 NA and am due for a new timing belt soon....

	Along with the increase in performance, did you notice any
dramatic
	decline in gas mileage?

	Thanks,
	Jim
	Original owner 86.5 MkIII NA

		Bob,

		        Thanks. I've done the head gasket once already.
Stuck on a 
		stock gasket, torqued to 54/58 (?) ft-lbs (from TSRM).
Lasted two 
		weeks & I started hearing the tell tale bubbles in the
heater.  I 
		figured the engine was trash, so I got out the torque
wrench and 
		torqued all of them to 85 ft-lbs.  FYI - Block deck is
trashed from 
		acid damage.  After the
		85 ft-lbs, as long as the car was COMPLETELY warmed up
before touching 
		the accelerator pedal, the car used NO coolant no matter
how hard it 
		was driven.  But, touch the gas before and gurgling from
the heater.  
		Gasket would seal with head expansion at 85 ft-lbs !
		        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 :-)

		Thanks, Skip

		P.S.  Wonder if those 85 ft-lbs stock bolts are going to
come off.  My
		'88 had one of the Allen heads crack...

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