Effects of valve lift on part throttle.
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Effects of valve lift on part throttle. At low speed part throttle situations, which is more responsive, .500 or .600?
Dyno testing has proved that higher lift helps and the cam gurus say it helps everywhere but, I think this is at WOT.
I have also read that low lift is better/torquier at part throttle cruising situations.
Not sure if it really matters but, I am talking about cathedral port LS heads.
I think it is a good topic of discussion for us that use our rides more for cruising than track.
Dyno testing has proved that higher lift helps and the cam gurus say it helps everywhere but, I think this is at WOT.
I have also read that low lift is better/torquier at part throttle cruising situations.
Not sure if it really matters but, I am talking about cathedral port LS heads.
I think it is a good topic of discussion for us that use our rides more for cruising than track.
Last edited by Jake Wade; Apr 27, 2021 at 12:59 PM.
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I'm going to say for my own personal experience with Camming and 4.8L to 6.2L that The negative effect of the higher lift slowing port velocity would be something you would see in the sub 2500 RPM range and only really profoundly noticeable on the very smaller engines 4.8/5.3
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I'm going to say for my own personal experience with Camming and 4.8L to 6.2L that The negative effect of the higher lift slowing port velocity would be something you would see in the sub 2500 RPM range and only really profoundly noticeable on the very smaller engines 4.8/5.3
You mentioned the port velocity and that was something on my mind that I didn’t mention.
I am glad you did.
Interesting stuff here.
Another interesting thing to note is we picture port velocity based on rpm, but the air backs up against the valve, so although rpm is pulling air into the intake faster, it's hitting the back of the valve harder then it opens, and the piston is drawing down faster.... however I've always pictured there being vacuum there in that scenario, but over the years I've seen it more described as a low pressure area, and the atmospheric pressure actually pushes the air into the cylinder. And that's why when you add boost to an engine it has the same exact characteristics as before it was "boosted" because technically... the engine was already boosted by atmospheric pressure, you didn't "add" boost, you increased it.
So what does that mean. It means I know enough to be dangerous here and I don't know if high or low rpm necessarily dictates the answer to your question.
I will say, on the dyno tests... the higher lift cam picked up power at low rpm, at low rpm the engine is pulling in what it can, not a lot. So in that situation whether it's at WOT or not... I don't think is as important to port velocity as we are picturing it. In other words some information seems to point to the fact that the higher lift just works better. And part throttle.. versus WOT I don't know. There is more vacuum in the intake, which actually could counter act port velocity because now the cylinder isn't the only low pressure area, now the intake is as well, so therefore you don't get the cylinder filling from atmospheric pressure that would have been there at WOT, so you may not be able to have the velocity either way.
I don't know if I helped or made it worse. I'm thinking out loud about something I don't have the answer to, so it may have made things worse lol.
Another interesting thing to note is we picture port velocity based on rpm, but the air backs up against the valve, so although rpm is pulling air into the intake faster, it's hitting the back of the valve harder then it opens, and the piston is drawing down faster.... however I've always pictured there being vacuum there in that scenario, but over the years I've seen it more described as a low pressure area, and the atmospheric pressure actually pushes the air into the cylinder. And that's why when you add boost to an engine it has the same exact characteristics as before it was "boosted" because technically... the engine was already boosted by atmospheric pressure, you didn't "add" boost, you increased it.
So what does that mean. It means I know enough to be dangerous here and I don't know if high or low rpm necessarily dictates the answer to your question.
I will say, on the dyno tests... the higher lift cam picked up power at low rpm, at low rpm the engine is pulling in what it can, not a lot. So in that situation whether it's at WOT or not... I don't think is as important to port velocity as we are picturing it. In other words some information seems to point to the fact that the higher lift just works better. And part throttle.. versus WOT I don't know. There is more vacuum in the intake, which actually could counter act port velocity because now the cylinder isn't the only low pressure area, now the intake is as well, so therefore you don't get the cylinder filling from atmospheric pressure that would have been there at WOT, so you may not be able to have the velocity either way.
I don't know if I helped or made it worse. I'm thinking out loud about something I don't have the answer to, so it may have made things worse lol.
Best thing I've found for part throttle is higher octane and added timing.
Thing about part throttle is, if it isn't enough, add more throttle input and shazaam. Problem solved lol
We always want more power at WOT (well everywhere but you know what I mean). If part throttle power goes down or feels down, you can add more throttle. But if WOT power is down, you can't... So I would shoot for more WOT power. But I do get why you're thinking about it/the conversation.
Anyway, I believe part throttle testing can be done. I think a dyno operator can load the machine to hold a certain rpm and then give say 20% throttle and test for power. But I'm not positive on that. Would love to see more part throttle testing done if it can be done.
Thing about part throttle is, if it isn't enough, add more throttle input and shazaam. Problem solved lol
We always want more power at WOT (well everywhere but you know what I mean). If part throttle power goes down or feels down, you can add more throttle. But if WOT power is down, you can't... So I would shoot for more WOT power. But I do get why you're thinking about it/the conversation.
Anyway, I believe part throttle testing can be done. I think a dyno operator can load the machine to hold a certain rpm and then give say 20% throttle and test for power. But I'm not positive on that. Would love to see more part throttle testing done if it can be done.
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Best thing I've found for part throttle is higher octane and added timing.
Thing about part throttle is, if it isn't enough, add more throttle input and shazaam. Problem solved lol
We always want more power at WOT (well everywhere but you know what I mean). If part throttle power goes down or feels down, you can add more throttle. But if WOT power is down, you can't... So I would shoot for more WOT power. But I do get why you're thinking about it/the conversation.
Anyway, I believe part throttle testing can be done. I think a dyno operator can load the machine to hold a certain rpm and then give say 20% throttle and test for power. But I'm not positive on that. Would love to see more part throttle testing done if it can be done.
Thing about part throttle is, if it isn't enough, add more throttle input and shazaam. Problem solved lol
We always want more power at WOT (well everywhere but you know what I mean). If part throttle power goes down or feels down, you can add more throttle. But if WOT power is down, you can't... So I would shoot for more WOT power. But I do get why you're thinking about it/the conversation.
Anyway, I believe part throttle testing can be done. I think a dyno operator can load the machine to hold a certain rpm and then give say 20% throttle and test for power. But I'm not positive on that. Would love to see more part throttle testing done if it can be done.
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Theoretically, the extra lift should allow some extra air past the valve compared to lower lift. So in my mind, as long as the low lift cam doesn't allow total cylinder filling at a given rpm the higher lift cam will fill better. The better way to figure this out might be by CFM in and out of the cylinder.
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Negligible.
Between the fact that the intake manifold is holding vacuum, the piston speed is very slow, and you are only moving enough air that .250 lift is more than enough, there will be no measurable difference.
Your valves that normally are capable of flowing ~250+ CFM per port are only pulling about 10-11 CFM each while cruising at 70 MPH.
The opening rate of the valve will have far more effect than any lift over 0.250" or so.
Between the fact that the intake manifold is holding vacuum, the piston speed is very slow, and you are only moving enough air that .250 lift is more than enough, there will be no measurable difference.
Your valves that normally are capable of flowing ~250+ CFM per port are only pulling about 10-11 CFM each while cruising at 70 MPH.
The opening rate of the valve will have far more effect than any lift over 0.250" or so.
Last edited by AwesomeAuto; Apr 30, 2021 at 12:25 AM.
At low speed part throttle situations, which is more responsive, .500 or .600?
Dyno testing has proved that higher lift helps and the cam gurus say it helps everywhere but, I think this is at WOT.
I have also read that low lift is better/torquier at part throttle cruising situations.
Not sure if it really matters but, I am talking about cathedral port LS heads.
I think it is a good topic of discussion for us that use our rides more for cruising than track.
Dyno testing has proved that higher lift helps and the cam gurus say it helps everywhere but, I think this is at WOT.
I have also read that low lift is better/torquier at part throttle cruising situations.
Not sure if it really matters but, I am talking about cathedral port LS heads.
I think it is a good topic of discussion for us that use our rides more for cruising than track.
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I've been thinking about this some more and it's just.100 valve lift, That's all we're talking about here.
Does anyone remember the two different comp 212/218 extreme RPM grinds? One was a 114 LSA the other a 115 LSA with a touch more lift and the need for better springs?
I like to think about what kind of combinations are going to be more sensitive to this small change in valve lift than others, bigger valve and bigger port? Small valve small port? Big chamber small chamber? Displacement of course makes the most difference I would think as that is the real draw on the lift.
I had a 4.8 l with the 115 LSA, cam mentioned above it idled smooth but it was very lazy down low extremely powerful up top though
Does anyone remember the two different comp 212/218 extreme RPM grinds? One was a 114 LSA the other a 115 LSA with a touch more lift and the need for better springs?
I like to think about what kind of combinations are going to be more sensitive to this small change in valve lift than others, bigger valve and bigger port? Small valve small port? Big chamber small chamber? Displacement of course makes the most difference I would think as that is the real draw on the lift.
I had a 4.8 l with the 115 LSA, cam mentioned above it idled smooth but it was very lazy down low extremely powerful up top though
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The wider LSA plus the small engine size both made it lazier down low. A 112-113 LSA would give it a SLIGHT lope but somewhat stronger low end.
Along these lines, I noticed that while Cam Motion's truck grinds had 114-116 LSA for even the 198-204 intake durations, Tooley's Truck Torque cam (202/202) has a 111 LSA, resulting in 20# torque gain all the way from off idle to the top (about 5500RPM). It made me a believer in as tight an LSA as possible with minimal or no lope.
Along these lines, I noticed that while Cam Motion's truck grinds had 114-116 LSA for even the 198-204 intake durations, Tooley's Truck Torque cam (202/202) has a 111 LSA, resulting in 20# torque gain all the way from off idle to the top (about 5500RPM). It made me a believer in as tight an LSA as possible with minimal or no lope.
Last edited by G Atsma; Apr 30, 2021 at 09:05 PM.
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Lope is a byproduct of overlap. Overlap is total duration of intake and exhaust added together, divided by 2, minus LSA multiplied by 2. Take my camshaft in the 434 for example. 254/268 on a 114. 254+268= 522. Divide that by two= 261. 114(LSA) x2=228....261-228=33 degrees of overlap. Overlap determines lope, or driveability of a given camshaft. There are factors that affect lope or driveability such as cubic inches, compression, a given cylinder heads efficiency, and of course tuning, to name a few.
LSA in itself has NOTHING to do with lope. Stick with me here..
Or really anything. It's a number that is measured after a cam is designed. It's just a number that is... there. And should not be used to judge much of anything.
You can have drastically varying overlap with say 110 LSA from one cam to another, and over lap is where the lope comes from.
However if you are discussing several cams that have similar durations then you can somewhat gauge lope off of LSA. But a 190/200 110 will chop FAR less than a 240/250 110. Over lap between those two cams will be worlds apart.
For example, those two cams with 0 degrees advance.... the 190/200 has NEGATIVE 25 degrees over lap, the 240/250 has POSITIVE 25 degrees overlap.
Or really anything. It's a number that is measured after a cam is designed. It's just a number that is... there. And should not be used to judge much of anything.
You can have drastically varying overlap with say 110 LSA from one cam to another, and over lap is where the lope comes from.
However if you are discussing several cams that have similar durations then you can somewhat gauge lope off of LSA. But a 190/200 110 will chop FAR less than a 240/250 110. Over lap between those two cams will be worlds apart.
For example, those two cams with 0 degrees advance.... the 190/200 has NEGATIVE 25 degrees over lap, the 240/250 has POSITIVE 25 degrees overlap.