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just went to my stock and picked 4 random camshafts of the box - 100% stock NA engines with soft springs... so I'm sure I don't want to put more load to them
I am and appreciate your input - but don't you think thousands of 951 engines are also real life experiences? thats exactly what they did - original turbo springs are softer as NA springs... and at no point the force on turbos valves is lower as on NA springs
I took all these measurements and trials because I knew the standard springs are not working (from the first 16V engine I build 3 years ago) but the car owner wanted it that way...
The Crane Cams website used to state this;"Selecting the proper valve spring for any performance engine is important; but it is critical to proper operation of supercharged engines. Consider the fact that when the engine is in a "boosted" condition, the supercharger (or turbocharger) is trying to blow the intake valve open. The boost pressure actually reduces the intake valve spring seat pressure. This is extremely critical on engines with hydraulic lash adjustment. Proper seat pressure (working through the rocker arm and pushrod) is necessary to keep the hydraulic lifter plunger centered in the lifter body to prevent "pump-up." If an engine has 2.25" dia. intake valves, there is 4 sq. inches of backside valve area. Now add 12 (psi) of boost pressure, and you have reduced your effective seat pressure by 48 lbs. (12 lbs/sq.in. X 4 sq. in.). If you started out with 120 lbs. of seat pressure (static), you now have 72 lbs. of operational seat pressure. There is no way that 72 lbs. of pressure is going to control a 2.25" valve! For street use, the answer is to select a tall, mild-rate spring that assembles at a high seat pressure, yet keeps the open pressure reasonable over the nose of the cam to assure long cam life. (For example; a great spring on a Big Block Chevrolet with a hydraulic roller and a supercharger is our #96879 spring, which will provide 169 lbs. at 1.880" seat and 432 lbs. at 1.280" open with a .600" net lift cam.) Exhaust valve seat pressure does not need to be raised significantly on supercharged engines. Racing applications with solid lash adjustment do not experience lifter "pump-up,” but still need the high seat pressure to prevent the valve from bouncing on return to the seat against the boost pressure. Consult our easy to use valve spring pressure chart at the back of our Valve Spring and Retainer catalog."
Very good point! Supercharged engines don’t have any issues on outlet valves as they directly blow out to atmosphere, turbos create a backpressure of 1,5-2 times of the boost, means 1 bar boost loads 1,5-2 bar pressure to the outlet valves
I think everyone is getting confused by your explanation, the graph says it all. Seat pressure is higher but your full lift numbers are close to the same. By using a taller spring with a lower spring rate the additional compression bumps up the seat pressure. But because the spring has a lower rate by the time you’re at full lift the pressure hasn’t increased as dramatically as it would be if you had just replaced it with a stiffer spring. Bit of an odd way to think about it as most spring manufacturers will specify an installed height and pressure along with its rate and max travel at this setting and you just chose one to suit.
I would still be accounting for the additional boost pressure.
What cam or cams are you planning on using that would be a good start for which springs. Also if using stock springs new or 30 years old springs?
My first 3 liter 8 valve build I used 20 year old stock 951 springs in a 2.7 head and had a shop rebuild the head to specs and with only 16 psi the head rattled a lot from valve float under full boost/ load and It was bad so I pulled the head a few months later and went with titanium shims to make the springs more preloaded and the float went away!
Last edited by boost feen; 04-06-2023 at 10:49 AM.
I am about to add my build to the list, any helpful thoughts on compression ratio would be appreciated. It looks like most builds are from 8:8 to 8:1, any specific benefits to either side of the scale?
I am about to add my build to the list, any helpful thoughts on compression ratio would be appreciated. It looks like most builds are from 8:8 to 8:1, any specific benefits to either side of the scale?
Context, 968 block and crank, Darton dry sleeve, 106mm JE Forged pistons, Carrillo rods, 2.7L head Ferrea valves, LR 3Lcam. Currently planning on LR Super 65 BB with 951 header and crossover, 3" exhaust.
Context, 968 block and crank, Darton dry sleeve, 106mm JE Forged pistons, Carrillo rods, 2.7L head Ferrea valves, LR 3Lcam. Currently planning on LR Super 65 BB with 951 header and crossover, 3" exhaust.
Let me know how you make out with Darton sleeves, I talked to them about a month ago and they said they weren't making them for the 944 anymore unless I bought ten sets.
Context, 968 block and crank, Darton dry sleeve, 106mm JE Forged pistons, Carrillo rods, 2.7L head Ferrea valves, LR 3Lcam. Currently planning on LR Super 65 BB with 951 header and crossover, 3" exhaust.
If you're going to all that trouble to build a 3lt motor you could look to a more modern turbo.
04-05-2023, 06:37 PM
