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I would expect some type of spring action somewhere otherwise as the cam lobe wears, or the rocker wears, the valve is just going to bounce around and rattle more and more when closed. The intake stroke is going to pull the exhaust valve open then the compression stroke is going to push the exhaust valve closed and I expect that's going to be noisy as hell and also wear out the valve seat in short order as the valve is going to be rapping at a high frequency. That would be my guess. Interesting design but I don't see how it has any longevity.
Originally Posted by Andybump
I know. I did not mean a coil spring though. What I meant is that the lever itself - the L shaped thing, must have a tiny bit of flex to it, to create the pressure - else something would eventually fracture - I don't see how it could hold it perfectly closed. Except - you have a good point about the pressure keeping it closed during compression. I have a miniature hit and miss engine that has an intake valve with only a tiny light coil spring. On the intake stroke the downward motion of the piston creates a vacuum that causes it to open and draw the fuel/air. The spring must be weak enough to permit that to happen. On compression, it is of course forced tightly shut. On exhaust, the exhaust valve opens because there is push rod an rocker arm.
We can guess at possible issues, BUT looking at a few video's Desmodromic valves have been used since 1910! It's noted, a big advantage is no valve float, even at very high rpm. A cam closing profile on a valve spring engine can only be as fast as the spring allows closing NOT what the cam profile is capable of doing.
Ah yes recall on the early high performance, high rpm solid lifter small blocks, valve float was our Rev Limiter. As far as some exhaust getting into the intake charge, remember EGR? Quoting: "In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide (NO x) emissions reduction technique." A little doesn't hurt!
Not a fan either way. Prefer the F1 Nitrogen! But most had no spring on cam follower etc.
I love working on the good old marinized Detroit Diesel 6-71TIs 2-strokes with both a supercharger and a turbocharger. My sportfisherman had two of them. Top RPM was around 2300; however, they had mountain moving torque. Valve trains on those were beyond bullet proof.
^^^
Mentioning high hp diesels in botas, recall going to the NYC Boat Show in the late 1960's. Went fishing in the ocean most summer weekends with a colleague at work. The boat yard/dock where he kept his boat was a fast Jersey Skiff I thought would be great to own. Small block Chevy in front of the rear seat they raced as a class in the Bay.
In a booth at the Show was a ~40-foot-long Ocean Racing Boat and expected to see 2 or more BB Chevy's. It had two big diesels. I asked a fellow manning that booth who would put diesels in an ocean race boat? He said, this boat is going to Miami and if you knew what the hall, you'd understand their need for a fast long run diesel Boat!
I know nothing about the boat engine. And only what I read at the link I posted about how the valves are adjusted on the Ducatti with the Desmodic valve system. And it clearly shows that there is some clearance (a gap) on the closer (post 220) - so it does not positively close the valve. It use a little spring (visible in the image I posted) to close it the rest of the way at low RPM. I can only speculate about high RPM (it says the spring is not needed for that), but I think it must close the rest of the way (close the gap) due to momentum (the closer is pulling the valve shut at high speed) and then the compression keeps it shut. I suppose with coil springs around valves, and lack of a positive closer, it relies entirely on the valve spring to close the valve, so at high RPM the valve doesn't quite get there - hence the "valve float" - like JerryU said.
Voice coils are silent and cheap. There might be weight cost associated with the amount and size of the neodymium magnets, the size of the coils needed, and possibly a cooling system for the coils; however, I doubt it's much more than the weight of the camshaft, the drive gear/chain, the pushrods, the roller rockers, etc. The voice coil actuators would/could likely be built into the heads and act directly on the valve stems. The output stage electronics that provide the high levels of precise current control to each actuator is also very trivial and cheap. The compute that controls the valve timing and sequencing is basically free. The firmware is also quite trivial. The biggest cost would be the NRE engineering cost.
It's not the voice coils, and the one's needed for valvetrain are anything but cheap. They need to move a signifcant mass VERY rapidly, and they don't have a gradual ramp like a camshaft. They slam the valves open and closed, and the valves banging against the seats is what makes the noise. And coils with enough power to do that are not even lightly cheap. Do some research on Koenigegg's solutions and learn something.
If it was as cheap and easy as you think every ICE would be using them.
I know nothing about the boat engine. And only what I read at the link I posted about how the valves are adjusted on the Ducatti with the Desmodic valve system. And it clearly shows that there is some clearance (a gap) on the closer (post 220) - so it does not positively close the valve. It use a little spring (visible in the image I posted) to close it the rest of the way at low RPM. I can only speculate about high RPM (it says the spring is not needed for that), but I think it must close the rest of the way (close the gap) due to momentum (the closer is pulling the valve shut at high speed) and then the compression keeps it shut. I suppose with coil springs around valves, and lack of a positive closer, it relies entirely on the valve spring to close the valve, so at high RPM the valve doesn't quite get there - hence the "valve float" - like JerryU said.
If you watch some of the Video's you'll see Ducati has used Desmodromic Valves since 1956. They were not one of the 1st. In a later year they did add a light spring BUT not before and not sure all models do today. From what I see many even cars used the system and no springs.
As I said a slight amount of exhaust going into the intake stroke BUT with cam overlap and even forced with the EGR a small amount of exhaust is not an issue.
It’s what it does to the motor’s top end. Ask me, my motor lunched as a result of AFM, my neighbour across the street had her motor lunched for the same reason. Dump the AFM as soon as you can…it is expensive ($7K) to repair and it always happens after warranty ends.
Hogwash. Give us details on your failure. The car, the year, exactly what happened.
It's not the voice coils, and the one's needed for valvetrain are anything but cheap. They need to move a signifcant mass VERY rapidly, and they don't have a gradual ramp like a camshaft. They slam the valves open and closed, and the valves banging against the seats is what makes the noise. And coils with enough power to do that are not even lightly cheap. Do some research on Koenigegg's solutions and learn something.
Voice coils can be extremely powerful and can either be directly coupled to the valves, or they could be coupled to pushrod behind a rocker providing mechanical advantage. Voice coils can also open and close valves much faster than a camshaft's lobe and valve spring can. In fact, voice coils could snap open a valve far faster than any mechanical cam lobe can as well as decelerate the closing to avoid slamming into the seat. Voice coils are so fast and positionally accurate that they used them for hard disk head positioning. Intake and exhaust valve's controlled with voice coils could easily be driven with a non-linear PID controlled algorithm in software driving linear ~63V ~15A output stage BJTs.
Here is a stupidly inefficient and simple 25 cent voice coil with an equally blindly dumb 1.5v driver (on/off) that I built in less than 1 minute.
Just checked the Range Technologies website - no listing for a C8 corvette AFM disabler...unfortunately. I have one installed on my Escalade and it works great...guess they haven't put one out for the C8 yet. Bummer.
Disabling AFM is not about comfort or mileage....it's about avoiding damage to the engine, and also the transmission. Do some google searching....much information about Chevy/GM AFM damaging engines and also transmissions over time. There's even class action lawsuits related to it proving it's a real thing.
I'd like my C8 to be in V8 mode all the time. Hoping a reputable source puts out a good OBD II AFM disabler soon.
There isn't going to be an OBDII AFM disabler at any time. The Global B Architecture used on the C8 encrypts communications between modules. If you want to disable AFM, your only option (other than the provided methods like manual mode, etc.) is to get a piggyback ECU. Very expensive and voids your warranty.
I know nothing about the boat engine. And only what I read at the link I posted about how the valves are adjusted on the Ducatti with the Desmodic valve system. And it clearly shows that there is some clearance (a gap) on the closer (post 220) - so it does not positively close the valve. It use a little spring (visible in the image I posted) to close it the rest of the way at low RPM. I can only speculate about high RPM (it says the spring is not needed for that), but I think it must close the rest of the way (close the gap) due to momentum (the closer is pulling the valve shut at high speed) and then the compression keeps it shut. I suppose with coil springs around valves, and lack of a positive closer, it relies entirely on the valve spring to close the valve, so at high RPM the valve doesn't quite get there - hence the "valve float" - like JerryU said.
Found this spring used on some Ducati models. Also, this type like a DOHC where a third camshaft operates both closings!
Found this spring used on some Ducati models. Also, this type like a DOHC where a third camshaft operates both closings!
The valve guides must be removable as well as split(?) like crank journal bearings... Unless that retainment ring on the valve stem is somehow removable.
Found this spring used on some Ducati models. Also, this type like a DOHC where a third camshaft operates both closings!
Most interesting. Unlike the first generic image I found and put in post 214, which makes it look like a single arm does opening and closing, the shows clearly that there are two arms, and the spring is on the arm that does the closing opening. I cannot tell from the image which way the spring works. Is it holding the arm against the cam, or holding the arm against the valve? In the Ducati photo I put in post 220, the spring is clearly pushing the arm up, against the underside of what might be called the closing shim on the valve. Attached is better image showing that.
And, the retainment parts are removable.
Last edited by Andybump; Jul 12, 2024 at 08:57 AM.
Most interesting. Unlike the first generic image I found and put in post 214, which makes it look like a single arm does opening and closing, the shows clearly that there are two arms, and the spring is on the arm that does the opening. I cannot tell from the image which way the spring works. Is it holding the arm against the cam, or holding the arm against the valve? In the Ducati photo I put in post 220, the spring is clearly pushing the arm up, against the underside of what might be called the closing shim on the valve. Attached is better image showing that.
And, the retainment parts are removable.
The additional tappet that fits into the retainer is a nice touch. I assume you can buy oversized tappets and grind them down or slip a shim in when either the tappet or arm wears out of tolerance.
It’s what it does to the motor’s top end. Ask me, my motor lunched as a result of AFM, my neighbour across the street had her motor lunched for the same reason. Dump the AFM as soon as you can…it is expensive ($7K) to repair and it always happens after warranty ends.
so, is that 2 C8's that have grenaded motors due to cylinder deactivation?
V4 seems to mostly kick in during highway driving. I find that just keeping it in the 7th/8th gear in manual mode and holding the left paddle for lowest possible gear every time I need to maneuver is working pretty well.
I usually run z mode off highway which also prevents it. It would be nice to be able to disable it independently but it hasn't bothered me much.
Z mode does nothing to prevent cylinder deactivation...unless you have it programmed to.
Yep, need Power set to Track. Believe it's the way it comes programed. If not, why do anything else as that is MY Mode!
Z-Mode can set Power to Track, Ride wherever you want etc.
MY Mode Cannot set Power separately.
Anyone know why GM's "My Mode (Really GM's mode)" does not allow changing the Powertrain setting? What is the powertrain mode set to when My Mode is enabled?
Also, is it true that if you shut off the car in My Mode, it will remain in My Mode when you start it back up? If so, another reason WTF didn't they let you setup Powertrain in My Mode.... Is it because someone might setup Track mode in My Mode and GM doesn't want the car to be able to always start with Powertrain set to Track as that might increase the number of warranty claims? That's the only reason I can think off..
Anyone know why GM's "My Mode (Really GM's mode)" does not allow changing the Powertrain setting? What is the powertrain mode set to when My Mode is enabled?
Also, is it true that if you shut off the car in My Mode, it will remain in My Mode when you start it back up? If so, another reason WTF didn't they let you setup Powertrain in My Mode.... Is it because someone might setup Track mode in My Mode and GM doesn't want the car to be able to always start with Powertrain set to Track as that might increase the number of warranty claims? That's the only reason I can think off..
Likely because GM didn't want the EPA to be able to claim that GM permitted permanent disabling of AFM. Z Mode and Track Mode both revert back to Tour powertrain mode after an engine restart but the car will restart in My Mode.
For me it's not about the failure concerns, it's about the fact that my gorgeous C8 Corvette bogs in v4 mode until it eventually changes to V8 mode. It's quite noticable to me. Far as I know, Paragon is the only one offering the ability to re-program and disable the AFM. You need to really want it, and have some extra $ in the budget, but I think I want to start a new savings account.
Likely because GM didn't want the EPA to be able to claim that GM permitted permanent disabling of AFM. Z Mode and Track Mode both revert back to Tour powertrain mode after an engine restart but the car will restart in My Mode.
For a minute there I thought you were saying the powertrain mode setting within the Z-mode configuration automatically reverted back to tour mode (requiring you to go into Z-mode configuration and re-enable track mode every time)...
As for Z-mode enablement... I'm surprised there isn't a prevalent auto Z-mode enabler mod available.
Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
Slideshow: A Jaguar designer's personal project imagines what a modern front-engined Corvette might look like if Chevrolet revisited the golden age of the Stingray.
