Quote:
Originally Posted by DavidVizard-GFN
If it was a stock Ford or Chevy head prior to about 1990 the improvements can be quite substantial. In fact a pocket porting job well done can find about 2/3 of the extra air available from a basic full porting job but for about 1/3 of the expenditure in time and effort. This makes a pocket porting exercise a very effort effective deal.
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Let's put a few numbers in to reinforce what Dave says about the importance of the valve, seat and areas immediately adjacent. Back in the mid 90s I was heavily involved in a race series in the UK for Ford Fiestas using the 1600 CVH engine. The rules didn't allow any porting at all so an even more restrictive situation than the 'pocket porting' DV refers to above. Seats could be recut although the throat size couldn't be altered and for a while at least they allowed valves to be backcut at 30 degrees so that the seat width on the valve matched that in the head. The chamber being a hemispherical one imposed no shrouding so that also wasn't an issue or a place that gains could be found. In essence every extra horsepower had to come from valve and seat work and
I usually raised the bhp of my engines compared to the competition by 10% just from this area alone. That was enough to easily win the series every year I built engines for it.
Much R&D went into finding the optimum valve and seat shape profile for the engine.
The flow figures below at 25" of water show figures for 1) the standard head, 2) just the valve backcut, 3) The seat recut and blended into the chamber, 4) a fully ported head for a different race series. Inlet valve is 42mm.
LIFT.....STD....VALVE....SEAT.....FULL MOD
50.......23.8......25.0......27.5.......25.2
100.....48.3......48.9......52.8.......53.3
150.....66.2......75.5......81.3.......77.5
200.....86.9.....101.6....110.5......106.7
250....109.6....125.9....134.2......132.8
300....129.5....142.9....149.1......152.5
350....141.9....150.2....152.9......163.4
400....145.4....150.9....153.6......171.2
450....146.6....150.9....154.5......178.0
500....147.5....151.0....155.5......183.3
Power.100%..106.3%..110.5%...119.8%
The power potential figures are from an algorithm of my own devising which was found to give good correlation to dyno results on this engine.
Note how important just the valve shape is. A 30 degree backcut to reduce the seat on the valve from about 3.5mm wide down to 2mm to match the seat in the head and blend nicely into the back of the valve has raised mid lift flow by a massive 17% at 200 thou lift and strong gains all the way from 150 thou to 350 thou. The net power potential increase from just this mod is 6.3%. With the seat cut this rises to 10.5%.
Note also how most of the benefit from both valve and seat work is in the low and mid lift flow region. Peak flow (the cam in this series only lifts to 400 thou) has gone up much less than the overall power percentage increase. Another reason why trying to predict power figures from just the peak lift flow as per the Superflow manual is a waste of time.
Low and mid lift flow is crucial to power output.
The fully modified head sacrificed some low lift flow to get better high lift flow and better suit higher lifting cams. Flow doesn't start to show an improvement until 300 thou lift. This is a very common result as you push the boundaries of what the standard port shape can cope with.
It also shows that just the valve and seat profile control the flow almost totally at low to low/mid lift. Power potential has gone up to 19.8%. Flow continued to increase into the 190 CFM range at 600 thou and over although most cams don't lift anywhere near that high.
So in this case the valve and seat work alone is worth just over half of what a complete top notch porting job can achieve. This percentage will vary considerably with type of head but that's not an uncommon result, especially on older type cylinder heads.
What I must also stress is the importance of the exact seat profile. Many people take their heads into a shop and ask for "three angle valve seats please" thinking that's a homogenous commodity you can buy like a bag of sugar. What you get back can actually vary enormously in terms of seat width, top cut angle and bottom cut angle. Often there will little or no improvement over standard. It might even be worse. If you don't know exactly what to ask for then don't be surprised if the guy cutting your seats has no idea either.
Dave