Re: Turbo cam in NA I5

["Bruce Bell" <bbell@bbell.com>]

OK folks,

This is, I suspect, old hat for many of you; however for
the benefit of those who need it and expect miracles from a
cam change.....

Time for a little camshaft tutorial.

Three issues here 1)" lift and 2) duration and 3) torque
Remember, horsepower is a linear function of torque.

Cams have three basic properties. Lift, duration and
overlap. I am deliberately ignoring lobe centers since that
is a highly specialized complication of the issues.

Given the same duration, lets call it effective duration, a
higher lift cam will provide a little more torque, if the
Intake manifold and head are up to the increased flow.
Because of the ramp shapes on the cam don't expect any
miracles here. Hey, you can only open the valves so fast. 
You will no doubt on an Audi gain more from Intake
manifold, head and exhaust work than from an increase in
lift alone.

Given the same lift, increasing the duration will move the
rpm range at which power (torque) is made higher. Torque is
NOT increased, it only happens at a higher rpm which means
horsepower is increased since hp is a function of torque
and rpm.

So, myth number one and one that is most important; "A
hotter camshaft will increase my engines power."

Truth; a "Hotter cam" will only raise the rpm at which an
engine will make its torque (and hp). This happens only
because at a higher rpm you can move more airflow. 

Regarding NA motors, they  need at least a little overlap
(both intake and exhaust valve open at same time, as the
inertia of the exhaust gas exiting is used to start the
intake flow. The more overlap (duration, given the same
lobe centers) the higher the rpm your power will come in.  

Turbo motors because of the back pressure from the turbo
(help me Scott, anyone) gain little or nothing from
increased overlap that is there is no meaningful exhaust
scavenging effect..  

Now some numbers.

ACTION                                          '87 200tq
(UK I think)   272 Schrick
Inlet Opens, BTDC, degrees	  0                             
26
Inlet closes, ABDC, degrees	41                            
66
Exhaust opens, BBDC, degrees	40                            
64
Exhaust closes, ATDC, degrees	  -1                         
  24
valve lift                                       11.2?     
         11.2 mm
overlap                                         -1         
          50

Damn, I hope I got all those btdt before top dead centers
right :)

One last note. It is true that too much cam (overlap) will
create harmonic disturbances that disrupt the flow through
the air sensor plate on CIS motors. However, the Schrick
cam above Idles smoothly and pulls strong through 6000 rpm
on a 4Kq. To cause these problems you will need a lot more
cam than that! If you happen to have carbs or a MAF then
the sky is the limit if you can handle the driveability.

OK, It was only a short tutorial and I prolly left more
unanswered than I offered. Jump in all, what is a party if
no one plays :)

Bruce