How to calculate the required valve spring pressure for a modified engine.
02-20-2022, 12:26 PM
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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
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928Myles (02-20-2022)
02-21-2022, 05:09 AM
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Thanks AKe, Thanks for providing me the very specific data. What you have described is very correct. I had an issue with the aftermarket springs. The spring open pressure deteriorates very quickly and my intended power peak at 7500rpm kept dropping and ended up changing the springs with significantly higher open pressure than the calculation. The deterioration on spring pressure(constant) depends very much on the stress level of spring wire material which can be reduced by proper design such as better material, heat treatment, stress relieving process, but it is sometimes difficult to get all these information. So I normally choose valve springs with considerably higher open pressure than the ideal value.(up to 30% more), provided that this kind of pressure is not too high for cam lobe scuffing at idling. But these days, the heat treatment process even for regrind cams are far better than before to withstand rather high PV value at idling. Can you possibly provide me(us) your solid lifter solution including the valve springs, retainer, lifters you used for yourself ? Thanks always for your comment and information with a high level of engineering background.
It is not recommended to use too much open valve spring pressure. It results in high friction and less engine performance, as well as greater wear on the camshaft and follower. Wear and pitting on top of the cam lobe has always been a problem. Use a good oil with a decent ZDDP content.
For my 928 engine build I use CHC-VT3700 oversize motorcycle WWC coated lightweight solid lifters from MTC, PAC-1214 beehive valve springs, top titanium retainers and 42/36 mm valves of own make. Ferrea provide good blanks if you like to make your own valves.
More about cylinder head porting here, start at post 218.
https://rennlist.com/forums/928-foru...ispreloading=1
You can also have a look here. https://rennlist.com/forums/924-931-...for-400hp.html
Åke
Last edited by Strosek Ultra; 02-21-2022 at 05:13 AM.