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From many head porting tests I have found the best intake runner center line angle for a "straight shot" to the valves to be 22.5 degrees. This is for the S4 cylinder head.
The stock intake port taper, roof angle 19 degrees, floor angle 26 degrees.
Åke
Does this mean you are back onto the 928 head development project???
Greg, I am always there but first I have to finish the BMW Alpina V8 engine (pictures), then fix the old Volvo (starter and starter gear), replace the cylinder head on the BMW 635CSI. Well the list goes on.
Åke
My heads are getting a bit more welding, that is for the purpose of raising the roof without getting too thin. I also may have found the custom cylinder head hardware. Its been hard to find the correct valve springs. The springs have to be of the best quality and of beehive design. I need the small end of the spring with retainer to go inside the hydraulic tappet. I also need around 0.650" max lift at least on the intake side. I'm going to spec the spring at 300 pounds per inch of compression. This is exactly the same as the Porsche 997 GT3 spring. The weights are roughly the same despite the intake valve being much larger (because its titanium) The differences are the Porsche uses a steel retainer and duel spring. The other differences are, my lift is quite different, 12.7 mm vs 16.5 mm. However if I have this right, your formula doesn't consider lift to be an issue. I can't comment on the acceleration of the cams either. Yes in the dark there. However I probably don't need the same amount of revs as the GT3. I think the red line on the GT3 was around 8,500 rpm but I am ready to be corrected.
For those interested in head work it is time for another look at this thread.
Åke Two-valve ported intake with 48mm (3mm OS) intake valve. Head put up the mill for machining. Head machined for oversize 37mm solid lifters. Lifter and guide bores spread apart to reduce shrouding between the two 42mm (5mm oversize) intake valves. Not shown but the inclined angle of the exhaust valves has been changed in order for the intake and the exhaust valves to clear each other during overlap.
How to calculate the required valve spring pressure for a modified engine.
I am not a specialist in this area but this is how I do it. The calculation is made for an 928 engine modified for higher power output. The camshafts acts directly on the lifter. Calculations for an engine having rocker arms will be more complicated.
For the calculation, the maximum deceleration at the nose of the cam lobe, a figure that every camshaft grinder can and should supply, is needed. Through careful measurement and calculation, the maximum deceleration can be found, but that requires measuring equipment which I do not have access to.
However, the maximum deceleration is usually between 0,0000045 to 0,0000075 mm per degree per degree (=per degree2) for most camshafts.
For the below calculation I assume a deceleration of 0.0000075 mm per degree2.
The total mass of the valve train is 150 grams or 0,150 kg. which includes 37 mm light-weight solid motorcycle lifters and oversize 42/36 mm valves.
Maximum safe rpm is sat to 9000 rpm which is 75 rps at the cam.
Gravity is 9.81 m/s2.
The rate of deceleration at 9000 rpm will be 0.0000075 x (75 x 360) 2 = 5468 m/s2.
Open valve spring pressure is calculated as (0.150 x 5468) / 9.81 = 83,6 kp.
Add 20% as a safety margin = 100,3 kp (open spring pressure)
Closed spring pressure is obtained by the spring rate times the valve lift. The beehive valve springs for my engine have an open pressure of 98 kp at 12,5mm of lift. The spring rate is 4,91 kp per mm.
Closed spring pressure is calculated to 98 - (12,5 x 4,91) = 36,6 kp which is very close to the measured value of 38 kp.
I make my on titanium retainers weighing 9 grams. A steel retainer would be about 15 grams. Not much of a difference but the stock steel retainer do not fit the beehive racing springs.
The 928 stock valve train including hydraulic lifters filled with oil is appr. 183 grams.
I have measured the stock S4 valve springs and found the open spring pressure to be 72 kp for both intake and exhaust. The closed spring pressure is 32 kp for intake and 36 kp for exhaust respectively.
Åke
How to calculate the required valve spring pressure for a modified engine.
I am not a specialist in this area but this is how I do it. The calculation is made for an 928 engine modified for higher power output. The camshafts acts directly on the lifter. Calculations for an engine having rocker arms will be more complicated.
For the calculation, the maximum deceleration at the nose of the cam lobe, a figure that every camshaft grinder can and should supply, is needed. Through careful measurement and calculation, the maximum deceleration can be found, but that requires measuring equipment which I do not have access to.
However, the maximum deceleration is usually between 0,0000045 to 0,0000075 mm per degree per degree (=per degree2) for most camshafts.
For the below calculation I assume a deceleration of 0.0000075 mm per degree2.
The total mass of the valve train is 150 grams or 0,150 kg. which includes 37 mm light-weight solid motorcycle lifters and oversize 42/36 mm valves.
Maximum safe rpm is sat to 9000 rpm which is 75 rps at the cam.
Gravity is 9.81 m/s2.
The rate of deceleration at 9000 rpm will be 0.0000075 x (75 x 360) 2 = 5468 m/s2.
Open valve spring pressure is calculated as (0.150 x 5468) / 9.81 = 83,6 kp.
Add 20% as a safety margin = 100,3 kp (open spring pressure)
Closed spring pressure is obtained by the spring rate times the valve lift. The beehive valve springs for my engine have an open pressure of 98 kp at 12,5mm of lift. The spring rate is 4,91 kp per mm.
Closed spring pressure is calculated to 98 - (12,5 x 4,91) = 36,6 kp which is very close to the measured value of 38 kp.
I make my on titanium retainers weighing 9 grams. A steel retainer would be about 15 grams. Not much of a difference but the stock steel retainer do not fit the beehive racing springs.
The 928 stock valve train including hydraulic lifters filled with oil is appr. 183 grams.
I have measured the stock S4 valve springs and found the open spring pressure to be 72 kp for both intake and exhaust. The closed spring pressure is 32 kp for intake and 36 kp for exhaust respectively.
Åke
Nice stats Åke.
Do you take on 16v port jobs?
Where are you located? I'm nearby Oslo.
The left hand lifter size 42mm is needed for 2-valve engines equipped with a high-lift racing camshaft. This is a hydraulic lifter that can be modified to a lower weight mechanical lifter. The picture lacks a 40mm hydraulic lifter for Mercedes that can be used for 2-valve engines, replacing the original 38mm lifter which is extremely expensive. For the 40mm lifter, the bores in the cam housings have to be altered. The same goes for the 37mm and 38mm mechanical race lifters for the 4-valve engine.
Åke
07-18-2018, 03:34 AM
