Street Porting
Porting the standard 2.5 Alfa Romeo V6 Cylinder Head
This is a step-by-step guide to porting the standard 2.5 head as fitted to the GTV-6, Milano and 75 models utilizing the stock valve sizes.
First, some basics lifted from
the Superflow 110 Operating Manual:
“The horsepower of an engine is directly proportional to the amount of air drawn into the cylinder and retained until ignition occurs. By reducing the air flow resistance of the intake and exhaust tract, cylinder filling is improved and engine horsepower increased directly.”
“Total flow through the engine is ultimately determined by the valve diameters. Valve size is limited by the diameter of the cylinder bore. Generally, hemi head engines permit valves up to 0.57 times the bore diameter due to extra space in the combustion chamber.“
“Tests indicate there is
generally no power improvement as long as exhaust flow is greater than 60% of
intake flow. This would dictate an exhaust valve diameter of
0.77 to 0.80 times as large as the intake valve.” Click [1] for reference.
Let’s look at one of my favourites, the 2.5 V6:
88mm bore, 41 x 36.5mm valves,
Intake manifold inside
diameter ~34.67mm,
41mm intake is only 0.465 times bore diameter,
41mm x 0.85 = 34.85mm optimum venturi size,
36.5mm exhaust is 0.89 times intake valve diameter.
So our intake is too small,
but efficiently sized and our exhaust is too big.
Let’s look at what we can do to improve the efficiency of the existing valve size combination.
Besides the basics of a clean
well lit workspace and a supply of compressed air (this does give my tired
little 5hp 60 gallon compressor a workout).....
Here’s what I use:
Dividers/callipers: a standard
straight tip and an inside pair 7” long,
¼” Die Grinder: straight,
Carbide Burrs:
(a) ½”
cylinder with ball nose, single cut, coarse flutes for light alloy, standard
shank,
(b) 3/8” flame, single cut, long shank,
(c) ¾” ball, double cut, long shank,
Sanding Drums: ¾” and 1”
diameter with shanks to suit die grinder,
Additionally, I also keep an 8” long
piece of ¼” round stock with a 1”slot cut in one end to hold strips of
sandpaper,
6” Dial Calliper,
Neway Valve Seat Cutters: an
assortment of diameters with 70, 45 and 30 degree cutting angles,
Prussian blue or machinists dye,
Clover brand fine lapping compound,
Safety Goggles: I really don’t like digging bits of debris from my eye.
BTW you can use an electric die grinder.
Once the head is stripped of moving parts and cleaned you can see the valve pockets appear to have been roughed out at the factory, no doubt by some one in a hurry, prior to the installation of valve seats. Sometimes the pockets line up with the seats and sometime times they don’t and I have seen them opened up larger than the seat insert.
The intakes tend to be especially dismal as the designed port does not allow a straight shot to the valve like the old four cylinder heads. The rough shaping of the casting along with the fellow in a hurry on the assembly line conspire against a smooth transition around this bend with turbulence and separation.
As flow losses occur from changes in direction and decreases in velocity I want to smooth the directional flow and keep the velocity high. The