More power with .600 lift cam?
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Joined: Nov 2001
Posts: 2,985
From: In a house by the bay
Rich,
Good question. I'm gonna e-mail Chuck because I want him to explain this the same way he explained it to me. I will say this much as I believe I have a firm grasp of the concept.... it boils down to heat in the valvespring. More lift per degree and you have more heat in the valvespring.... more heat and you have a greater loss of "running" spring pressure. In other words, the spring may lose as much as 20-30 psi at high revs due to heat and greater valvetrain acceleration. Chuck uses a method to measure the spring pressure of a running engine on the dyno. According to him, you need to get the heat out of the spring to maintain the spring pressure. Two ways to it.... use a less aggressive lobe or run valvespring oilers, coatings, other tricks I don't know about.
So, stands to reason that the valvetrain has a greater propensity to lose "control" with a more aggressive lobe profile than it would with a milder lobe. It's interesting and I hope Chuck will share some of his research in that area with us. It really opened my eyes..... now I see little pinholes in the black sheet.
My personal experience is that a milder lift is much easier on the valvetrain. The part most likely to break is gonna be in the valvetrain too, so I would rather err on the side of "mild" lift per degree than on the other. Especially if we're building street engines, using stainless valves and stuff that's not exactly the lightest.... but is the most durable. Just one opinion though.
-Mindgame
Good question. I'm gonna e-mail Chuck because I want him to explain this the same way he explained it to me. I will say this much as I believe I have a firm grasp of the concept.... it boils down to heat in the valvespring. More lift per degree and you have more heat in the valvespring.... more heat and you have a greater loss of "running" spring pressure. In other words, the spring may lose as much as 20-30 psi at high revs due to heat and greater valvetrain acceleration. Chuck uses a method to measure the spring pressure of a running engine on the dyno. According to him, you need to get the heat out of the spring to maintain the spring pressure. Two ways to it.... use a less aggressive lobe or run valvespring oilers, coatings, other tricks I don't know about.
So, stands to reason that the valvetrain has a greater propensity to lose "control" with a more aggressive lobe profile than it would with a milder lobe. It's interesting and I hope Chuck will share some of his research in that area with us. It really opened my eyes..... now I see little pinholes in the black sheet.
My personal experience is that a milder lift is much easier on the valvetrain. The part most likely to break is gonna be in the valvetrain too, so I would rather err on the side of "mild" lift per degree than on the other. Especially if we're building street engines, using stainless valves and stuff that's not exactly the lightest.... but is the most durable. Just one opinion though.
-Mindgame
Registered User
Joined: Oct 2002
Posts: 2,931
From: Upstate NY
Originally posted by rskrause
Mindgame: your post reminds me of something I have wondered about for a time. Would the "ideal" valve lift graph out to a "square wave" kind of shape? Just in terms of engine operating characteristics, ignoring the mechanical effects on the rest of the valve train, is more valve acceleration always better? What do you think? I do think that within the limits of imposed by practical spring pressure, etc. that steeper lobes are better, but is there a limit to this concept other than mechanical?
Over the years, the trend in street performance (my area of primary interest) is certainly toward steeper and steeper lobes.
Rich Krause
Mindgame: your post reminds me of something I have wondered about for a time. Would the "ideal" valve lift graph out to a "square wave" kind of shape? Just in terms of engine operating characteristics, ignoring the mechanical effects on the rest of the valve train, is more valve acceleration always better? What do you think? I do think that within the limits of imposed by practical spring pressure, etc. that steeper lobes are better, but is there a limit to this concept other than mechanical?
Over the years, the trend in street performance (my area of primary interest) is certainly toward steeper and steeper lobes.
Rich Krause
The charge of air flowing thru the valve has mass so it starts and stops gradually. If you could instantly fully open an intake valve, would the flow instantly become maximum, even it there was a +5psi pressure wave arriving at the same time? I don't think so.
Like a lot of things in nature, instanteous poppet valve action doesn't happen and air doesn't instantaneously accelerate, so these two things are complimentary. By opening the valve more rapidly we can maybe get closer to the max acceleration of the airmass. IOW, If Ma Nature had intended square wave valve motion, She would have made air massless.
On the other hand, maybe instant closing would be beneficial. I'd guess Cup engines come closer to that than most pushrod engines.
With pneumatic valve springs, like in F1, the disadvantages of metal springs isn't a probem. IMO,the limiting factor is mechanical valve velocity (mm lift per degree of cam rotation). This can be very high with the large diameter buckets on an OHC.
Maybe they are dropping the valve off a cliff to close it, and controlling the bounce with the pneumatic springs. These can be constant force throughout the lift and varied as needed, like with rpm. Wouldn't that be cool? Shoot, these guys have 40K/sec telemetry. Sensing valve toss and compensating with spring load should be a cake walk for them, even at 158 valve events per second!
My $.02.
Banned
Joined: Aug 2001
Posts: 3,505
From: looking for a flow bench so Brook and I can race
Originally posted by OldSStroker
My thoughts on the "square wave" valve motion:
The charge of air flowing thru the valve has mass so it starts and stops gradually. If you could instantly fully open an intake valve, would the flow instantly become maximum, even it there was a +5psi pressure wave arriving at the same time? I don't think so.
Like a lot of things in nature, instanteous poppet valve action doesn't happen and air doesn't instantaneously accelerate, so these two things are complimentary. By opening the valve more rapidly we can maybe get closer to the max acceleration of the airmass. IOW, If Ma Nature had intended square wave valve motion, She would have made air massless.
On the other hand, maybe instant closing would be beneficial. I'd guess Cup engines come closer to that than most pushrod engines.
With pneumatic valve springs, like in F1, the disadvantages of metal springs isn't a probem. IMO,the limiting factor is mechanical valve velocity (mm lift per degree of cam rotation). This can be very high with the large diameter buckets on an OHC.
Maybe they are dropping the valve off a cliff to close it, and controlling the bounce with the pneumatic springs. These can be constant force throughout the lift and varied as needed, like with rpm. Wouldn't that be cool? Shoot, these guys have 40K/sec telemetry. Sensing valve toss and compensating with spring load should be a cake walk for them, even at 158 valve events per second!
My $.02.
My thoughts on the "square wave" valve motion:
The charge of air flowing thru the valve has mass so it starts and stops gradually. If you could instantly fully open an intake valve, would the flow instantly become maximum, even it there was a +5psi pressure wave arriving at the same time? I don't think so.
Like a lot of things in nature, instanteous poppet valve action doesn't happen and air doesn't instantaneously accelerate, so these two things are complimentary. By opening the valve more rapidly we can maybe get closer to the max acceleration of the airmass. IOW, If Ma Nature had intended square wave valve motion, She would have made air massless.
On the other hand, maybe instant closing would be beneficial. I'd guess Cup engines come closer to that than most pushrod engines.
With pneumatic valve springs, like in F1, the disadvantages of metal springs isn't a probem. IMO,the limiting factor is mechanical valve velocity (mm lift per degree of cam rotation). This can be very high with the large diameter buckets on an OHC.
Maybe they are dropping the valve off a cliff to close it, and controlling the bounce with the pneumatic springs. These can be constant force throughout the lift and varied as needed, like with rpm. Wouldn't that be cool? Shoot, these guys have 40K/sec telemetry. Sensing valve toss and compensating with spring load should be a cake walk for them, even at 158 valve events per second!
My $.02.
)but did you get my email? i would really appreciate your 2 coins
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