You have to
1. take the chamber out another 7 cc from 50 cc to 57 cc,
2. use 1.5" compression height 250 US pistons with a 12 cc dish in each slug.
CR then = 9.2:1
HERE'S HOW COME:
The after market US pistons are shorter, but the factory Aussie X-flow 250 piston is approximately the same as the factory 200 piston in deck height but has a deaper dish.
The bigger piston, 40 thou, and the shallow dish (I've looked at 6 cc) of a 1.53" tall 200 piston, makes 11.5:1 compression ratio on your 4.1 X-flow!
The difference with the Crossflow is all in the dome. An Aussie/Argie and US Log Head 188/200/221 Falcon runs a 5.5 cc dish.
A 200 X-flow runs a 15.3 cc dish.
A 250 X-flow runs a 22.0 (leaded 96 RON) or 27.9 cc (91 RON ULP) dome.
The US 2.3 HSC runs a flat top, 1.500" in +40.
The US 2.5 HSC runs a dished top, 1.583" deck height.
The US 250 piston is about 1.50", and has an ~ 12 cc dish
The 255 piston is a mild dish, 1.583" deck height.
The aftermarket +40's in any US car will be lower deck.
If it were me, I'd never go for an HSC 2.5 or 255 dish top piston unless I could afford to cut it down 50 thou.
Best option is a strong set of US 250 pistons, and hog out an extra 6 cc from the chambers. Beats getting a thicker gasket or importing a set of pistons. Guys who do small cahmber 'quench' Clevelands will know where the best areas to remove the metal is. They are just duplicates of the closed chamber 57 cc Boss 302 heads, but with much smaller valves.
You must aim for a clearance volume equal to the stock head. CR for 1981 was 9.35:1, and that is, with a 3.68 bore and 3.91" stroke, a clearance volume of 81.60 cc's. Each cylinder has 681.53 cc's stock.
CR=
(Dish/Dome+Piston volume+Gasket+Chamber)+Volume of cylinder
____________Dish/Dome+
Piston+
Gasket+
Chamber
(2.6+22.0+7.2+49.8)+681.53
___(2.6+22.0+7.2+49.8)
Dish/Dome+
Piston volume+
Gasket+
Chamber = Clearance Volume 81.6
The gasket is ~ 41 thou compressed.
The piston sits ~ 15 thou below the block deck.
The factory cc is a blueprint 53.1 TO 56 cc for the cast iron combustion chamber,
All Alloy Head and Alloy Head II's were still quite big chambers. Jack said a while back the HF-6 and HF-4 castings have 56cc "bathtub shaped" chambers but no swirl vane.
The last head has a smaller 50cc heart shaped chamber and no casting number that I can find except for the C2 on the outside indicating it was carbed.
The post 85 and later Alloy Head XF engines got kidney shape high swirl chambers of less than 49 cc. Other people call it the heart shaped chamber and no casting number except for the C2 on the outside indicating what it is.
I go to the
http://falconperformance.sundog.net/compcalculator.asp compression calculator, and input the data.
In practice, the Stock 4.1 Alloy Head Falcon GL Input Values are:
Number Of Cylinders 6
Bore Diameter [inches] 3.68
Stroke Length [inches] 3.91
Combustion Chamber Volume [cubic centimeters] 50
Head Gasket Compressed Thickness [inches] 41
Head Gasket Bore Diameter [inches] 3.81
Piston To Deck Clearance [inches] 15
Select Piston Type Flat Top Dished Domed
Dished
Dish/Valve Relief/Dome Volume [positive cubic centimeters] 22.0
Volumetric Efficiency: 80%
RPM: 3500
Results:
Displacement, [cubic inches] 250
Displacement, [liters] 4.1
Static compression ratio 9.3:1
Cubic Feet per Minute required @ 3500 rpm, [cfm] 202
Estimated Horsepower @ 3500 rpm 120 hp
*(Assumes altitude of sealevel, barometric pressure of 14.696 and 60° air supply to carburator)
