Just read this extracted bit from the Z06 article currently on the home page -

"In addition to that, the LT6 utilizes a mechanical valvetrain rather than a more modern hydraulic setup – which dates back to the 1900s, though it was largely ditched by the 1970s. The reasoning behind this is simple – a high-revving, short-stroke, twin overhead-cam powerplant like the LT6 could experience valve float if those units aren’t capable of keeping up, which can lead to catastrophic failure."

Want to make sure I understand this correctly - Does the engine have solid (mechanical) lifters, instead of hydraulic? If that's the case, is there a recommended service interval for valve adjustment, as with solid lifter engines from the 60s and 70s?

I could be off-base with those questions because I'm sure the Z06 has more than a few mechanical differences from what I'm accustomed to working with. Just looking to learn a bit more about the Z06; it's a pretty safe bet I'll never need to work on one.

TIA,
Larry

 

The valve actuation is mechanical, no hydraulics. GM says the engine will not need valve lash adjustment.

 

It's an overhead cam engine. Think of the cam shaft lobes mechanically interfacing directly with the valve stem tips. This is not a 100% accurate description, but accurate enough to understand what's going on. I think there are shims to make adjustments ot lash between the valve and cam shaft lobes. There are no valve lifters or pushrods.

 

Thanks, guys. I was off-base, entirely overlooking the OHC aspect of the engine. Appreciate the info.

 

I read that as well. What I don't understand is the comments about valve float and hydraulic valve train. F1 cars used hydraulic lifters and ran up to 15K rpm so why is mechanical better?

Not questioning the decision, just interested in the tech stuff and decisions that drove it.

 

I am unable to find any recent F1 engines with hydraulic lifters. Do you have a cite for that? What year? They have pneumatic valve springs but that is different than hydraulic lifters.

 

It has no lifters… mechanical followers actuate the valves.

 

Nope, you are right. I was reading that they are researching electromagnetic valves with no cams; read they currently used pneumatic but had hydraulic on the brain...

 

Had this in my stash of pics.

 

Color me skeptical of GM's claim of "zero adjustment" needed over the lifetime of the engine. Of course, GM tested it so perhaps they know these things are only going to hold together for 75,000 miles before they shake apart.

 

Engine lifetime is the operative term. The C8R engines are not long lived by any means, but GM has assured us that this one will be "maintainable" to some degree.

I am not sure what the design life of the LT6 has been projected to be...

 

If you look at the photo posted by Broken Halo, you can see the valve spring expands directly against the camshafts lobes.
Thus as components wear the valve spring automatically takes up slack/lash. There would be no adjustment required until the valve spring failed or got so weak it couldn't follow the camshaft lobe without floating.

No way for us to know but those springs should be good for 2 or 3 hundred thousand miles if properly designed and made of proper materials. JMHO.

 

IIRC GM engine durability testing standard is 150k miles equivalent. I don't know testing protocol or the allowed failure rate.

 

No, that's incorrect.

The top of the spring seats on the spring retainer.
The retainer is locked to the valve by two split valve keepers.
The valve tip in in contact with bottom of the cap/shim.
The top of the cap/shim is in contact with the bottom of the finger follower.
The finger follower is in contact with the camshaft.

There is no "automatic" lash adjustment at all. The finger follower is DLC coated and will not wear. The cam lobe is the wearable item. Nothing else will wear.
With this setup the lash will tighten over time like most mechanical bucket and finger follower designs.
The valve "contact" face will wear and the valve seat will wear so that tightens the lash over time. The cam lobe wear will loosen the lash but it will be negligible.
Id expect the lash to need checking at 100,000 miles and you may see it a few thousands tighter, but still within spec. But time will tell.
The good news is the cams will NOT have to come out to change the lash unlike the shim under tappet (bucket) style valve-train common in motorcycle and many automotive designs.

 

Curious do you know what the spec range is for the lash, one would think that the initial lash setting is set up on the loose side from the beginning to account for future valve face and seat wear. This could be a factor in why the LT6 seems to have varying degree's of valvetrain noise from engine to engine.

 

I think you know I didn't mean the spring itself contacted the cam lobe. Admittedly my words could have been chosen more carefully. I meant valve spring pressure keeps everything in direct contact with one another and the cam lobe.

Ok, so what keeps the valve tip in contact with the bottom of the cap/shim and then the cap in touch with the finger follower?
Let me answer. The valve spring keeps all of those components under pressure and in touch with one another while they follow the lobe.
Any wear will be compensated for by the spring pressure. As long as the spring is in good shape there will be no condition where the valve tip and follower will not be held against the cam lobe. Therefore no adjustment for slack should normally be necessary.

As you stated if anything we are concerned with the valve lash getting tighter over time as the valve seat and the valve face wear. That's if the valve tip and the valve follower have zero wear. If they wear also, then it may be a wash. If the face wears then it stands to reason the tip wears to some degree as well. We don't know the wear rates of either for sure. Hence this discussion.

We do know Its a titanium valve so face wear and seat wear should be minimal. The valves are light so spring pressure should be less (or reasonable for a high RPM engine) contributing to less valve face and seat wear. And we know modern engines which use very sophisticated valve seat materials show very little seat wear even at high mileages.

Keeping these facts in mind In my opinion it will be at least 200K miles before a valve seat and face wore enough with titanium valves to put lash tight enough to be of concern. JMHO.

 

They are not in constant contact. There is lash. So the answer is nothing.
The cap/shim and finger follower are loose and floating.
The spring does not exert and pressure on the cap/shim, finger follower and cam lobe. Unless of course the lash gets to zero or a negative number.

 

Sorry, I don't know what the lash spec is. If i had to guess .004"-.008" on the intake and .006-.010" on the exhaust.

 

No that is incorrect. F1 NEVER used a hydraulic lifter setup. F1 has used a pneumatic valve spring in lieu of steel, titanium or beryllium coil valve springs. The pneumatic spring has effectively no weight, whereas the spring has to over overcome its own inertia and can have problems over 12,000-13,000 rpm. 2.4 V8 and 3.0 V10 era were actually up around 20,000 rpm.

The actual valve "lifter" is not a lifter at all but a finger follower, which is the lightest and best way to get the camshaft to lift the valve and return it to its seat with a minimum of bounce. The valvetrains are mechanical and always have been. They use shims to adjust valve clearance because its lighter. A stud and locknut DIRECTLY above the valve adds critical weight. It is the same reason Nascar Cup engines use pushrod length to adjust valve clearance. Once they machine the rocker stands to get a good valve sweep, the actual mechanical clearance is set with custom length pushrods--getting rid of the adjuster on the valve end lets them run a lighter spring which frees up power.

 

That is not correct Sir.

When the cam lobe is at anything but absolute 0 lift it pushes the valve tip which lifts the valve off the seat. 0 lift is the base of the cam lobe profile and the valve is completely seated and closed by spring pressure at the 0 lift position.
Or said another way. Anytime the cam lobe is at any thing other than 0 lift the valve spring is exerting its pressure against all the components between the valve tip and the cam lobe.

Think about it. For example. At maximum cam lobe lift the valve is open and the valve spring is compressed. What mechanical action is compressing the spring?
The cam lobe is by pushing the valve tip down. That's its purpose is to lift the valve off the seat. So by Newtons third law the valve spring must apply an equal and opposite force against the cam lobe and any components (the follower) between the cam lobe and the valve tip.

In this design there is only one time and place there CAN be "lash/slack in the system. That is when the cam is on the base circle AND there is enough wear or if possible maladjustment between the valve tip, follower and cam lobe to create play (lash, slack, lack of contact). At any other time, anything above 0 lift when the valve is off the seat and the spring is being compressed there will by definition be 0 lash/slack in the system.