Hi folks, first post here. I am in the process of building up an LC9 (Al block, VVT, AFM disabled) 5.3L for my C3 Corvette. The final build intent is autocross, 11.999 in 1/4 mile, and continue to drive on the street as much as feasible. I am grinding a custom cam (0.587/0.571 lift, 204/228 duration, 114 LSA, 106 parked ICCL) to take full advantage of the VVT system and my specific engine combination, which has given me some significant advantages over a fixed cam. Keeping NA to reduce nose weight and this setup should be able to achieve my brake output goals without forced induction. I am fortunate enough to have a library of simulations I've built and tested over the years that can get within about 3% accuracy regarding most major engine parameters.

To support the higher lift cam, I am changing valve springs to something that gives me reasonable float margin with the new lobe design (Comp 26918CS-KIT). I wanted to hear from some folks who've kept their VVT system with stiffer springs and a more aggressive cam to see if there are any concerns with heavier valve springs starting to overpower the phaser advance spring or just simply making the system difficult for the oil pressure to control (increased oscillation tendency or something).

Second, I was originally planning on keeping the hydraulic rollers, but had some concerns with bleed down. Is this just internet fear mongering or is it a real concern for this generation of GM lifters? Solids are about $500 more, so I'd rather not spend that if there's no reason to.

I look forward to hearing your 2 cents, thank you!

 

Welcome aboard. I'm doing an LC9 build myself, but I decided to NOT use VVT/ AFM.

Anyway, there's a few things we need to know about your plans.

1) What transmission are you planning to use? Because VVT/ AFM is incompatible with a manual transmission. The rapid changes in engine speed resulted in piston-to-valve contact in GM's testing. That's why GM never offered that in any manual transmission cars. I suppose you might get it to work with generous valve reliefs, conservative cam, and/ or some really stiff springs, but that's still not a risk I want to take.

Speaking of springs...

2) Since the VVT system is powered by oil pressure, and it's basically fighting against the valve springs, sufficient valve spring pressure could nullify any VVT activity. I do not know exactly what that pressure threshold is (I'm sure that it depends on a lot of variables). I, and plenty of guys I know have done AFM deletes on trucks and left the VVT parts in place, but many of them tuned out the functionality.

I can't say if your springs will be too much. Someone with first-hand experience will have to speak up.

3) If your lifter preload is good, I wouldn't worry about bleed-down.

 

Thanks for the reply! Cool to hear there are other LC9 people out there, they are wonderful little engines.

1) I will be first using my M20 4 speed manual I swapped into the car some years ago. In the future I plan to move to a wide ratio T56 6-speed manual, but I don't have a timeline there. I don't quite understand the part about GM not offering VVT in a manual transmission. My understanding was that VVT and AFM were usually implemented together and a manual transmission does not damp the NVH concerns of AFM nearly as well as the auto. Because of that, manual cars usually got a fixed cam as a cost cutting measure. I am not particularly worried about PTV clearance. In the powerband, I will have more than sufficient PTV with a minimum of 0.100" at parked cam position (fully advanced) and 0.160" at peak power. My math is showing maximum con rod elongation of 0.030" and a conservative valve float margin. There should be a decent factor of safety here to take up any errors. The only way I can see contact happening would be to skip a tooth or shear something nasty.

2) Interesting to hear that folks electronically tuned out the VVT when deleting AFM. My thought is that the springs will also be pushing back down on the cam as it gets past the hump of the lobe, releasing their stored energy. But yeah since that's not 100% efficient, I suspect there might be a certain stiffness that is just too much for the system. I could measure spin torque of the cam alone in OEM form and then again after the new springs are installed and one more time after installing the custom cam, but that's a bit late to return parts if it doesn't work I suppose... Either way, I think I will do the static spin test, sounds like a fun way to quantify. Would definitely be helpful to hear what people have/haven't experienced before I pull the trigger.

3) Thank you for this! Preload is on my blueprinting checklist, I'll just make sure to hit it and will plan to use a good quality set of repro hydraulics then. I appreciate the insight.