Valve and port side-view cutawayLapping Ring45 28 Back-cut 11.5 30 45 60 75 Valve Seat Val
Most modern OEM four-valve heads are already very efficient. This means it's not necessary to remove large amounts of material, but rounding and smoothing edges still shows a gain in flow. Sharp corners from machining and tight bends from the casting process were blended. The valve guides, which are normally knocked out for tool clearance in CNC porting, were left in and ground to match the profile of the port. OEMs typically use smaller profiles to increase port velocity and throttle response, so porting tends to have more beneficial effects on flow at high valve lift.
According to Cosworth's cylinder head specialist, Lyon Kaplansky, polishing has a limited effect on overall flow. Port profiles are infintely more important, but while the tools are out, you might as well get that slight increase in efficiency. It looks better for the pictures too.
Polishing helps maintain laminar (nonturbulent) flow through the port by reducing the boundary layer caused by rough casting surfaces and mold flash lines. An easy way to pick up power in turbocharged applications is to polish the exhaust ports. To a certain extent, the polished surface helps deliver more exhaust energy to the turbine. However, there's little point in getting a mirror polish because it will eventually be covered by carbon buildup.
Maximum flow, however, actually benefits from a little turbulence. That's why the intake ports were roughed with a stone-tipped grinder. The surface feels like it's been sanded with 80-grit sandpaper. The moderate turbulence generated by this less-than-perfect surface increases flow velocity near the port walls and keeps the fuel in suspension longer.
This is a stock port before any head work. Note the factory machining, which transitions t
The Flow BenchAll this head porting business could be a waste of effort without some proof that it actually makes a difference. Our engine is sitting on various shop floors, so dynoing isn't an option. We can at least measure the flow gains from the work done on the head using a flow bench. Cosworth uses a SuperFlow SF-1020 to flow test all its work. Any of its production CNC head porting designs go through repeated flow evaluation to ensure the porting has optimal flow gains throughout all ranges of valve lift.
The head is flow tested on the bench with all the valves and spark plugs installed. The valves are held closed by test springs hard enough only to maintain valve seal. Using a dial indicator, the intake valves are opened in increments of 50 thousandths of an inch all the way up to half an inch of valve lift-regardless of cam profile. Flow rate, in CFM, is measured at every valve lift increment, which gives a complete picture of where flow gains are found. The process is repeated for exhaust flow with the exhaust valves open and the flow direction reversed. Our hand-ported head was tested before and after.
The SF-1020 is set to flow test at a 50-inch pressure drop instead of the 28 inches used by most other flow benches. The higher flow required to generate the 50-inch pressure drop across the port better exposes potential weaknesses in the flow design.
Localized flow speeds around tight radii or in stagnation zones are checked with a U-tube manometer attached to a tube probe. The manometer measures pressure at the tip of the probe, which correlates to flow velocity. It's also a good way to measure how hard you can suck or blow, if you want bragging rights.
On average, flow improved 8.5 percent on the intake and 23 percent on the exhaust. At .5-inch of lift, which the stock cams won't reach, maximum flow rate over stock increased 26 CFM on the intake and 44 CFM on the exhaust.
More parts for the resurrectionOur budget is getting tight and we still have a lifeless chassis. Next up is a Quaife limited-slip diff-a must-have if we want to keep up with the Cobra.
Our Cost So Far
2003 SVT Focus: $17,480
PTE turbo kit: $3,995
GT Fabrication downpipe: $450
GT Fabrication misc. fabrication: $200
FocusSport partial installation: $375
Odyssey PC680 battery: $148
FocusSport torque mount: $99
SCT reflash: $350
Michelin Pilot Sport PS2 tires: $692
Spray paint: $15
Mounting/balancing: $50
Progress rear bar: $199
Cosworth head work: $1,800
Total: $25,853
Minus 2003 SVT Cobra: $34,750
Total left to spend: $8,897
Previous Installments
April 2003
Part 1: Starting from scratch
June 2004
Part 2: Finally, a turbo
September 2004
Part 3: T minus 0.7 seconds
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The valve cutter sits on a hemispherical head that allows the machinist to align the cutti
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Our head was flow tested on Cosworth's SuperFlow SF-1020 flow bench. The bench sucks or bl