Impact Of Valve Lash On Idle Quality

[Diogenes]

As the subject states, what is the impact of increased and decreased lash on idle quality? Specifically, carb adjustment was never touched, the car runs great, but it idles at a somewhat lower RPM than before, using the same 1/2 turn past zero lash. I realize zero lash using the "twist the push rod until you feel resistance" is a rather imprecise manner for determining zero lash, so I expect some variation, yet it is the same proceedure I've used in the past.

The sound out the pipes is also "different", so it seems I may need some further adjustment. I replaced the adjusters on the factory rockers (from DSC), as some of the OEM adjusters were worn and not staying tight.

Again, the car runs very well, but seems slightly off given the past 11 years of experience with this engine (bored 390, hydraulic Bullet Cam).
 

[Stangman]

I’m assuming this a hydraulic flat tappet. I’ve always found a half turn to be about right after zero lash. 3/4 always seemed like it was too much, I thought that’s when they sounded different. Pushing the lifter down that much seemed like it was always like a laboring sound in the motor. Idle never sounded right to me. I always loosened till slight tap then tighten till it just went away and 1/2 turn. And always do it on a warmed up motor.

[Rory428]

This post is somewhat confusing. Valve lash is clearance between the rocker arm and the valve, as used on solid lifter cams, hydraulic lifters use lifter preload, which is NO lash, but rather is the lifter plunger being pushed down from it`s "topped up position.  On a solid lifter cam and lifters, valve lash is used to provide some clearance , to allow for the expansion of engine components as the engine heats up, which reduce that clearance and can prevent the valve from completely closing, to seal the cylinder to allow full compression, as well as giving some time for the valves to transfer heat to the head casting. On a solid lifter application, increased (more) valve lash, typically leads to a smoother idle, with more noise. With hydraulic lifter PRELOAD, as long as the preload puts the plunger in the correct position range, idle should not really be effected, but if the plunger is down to far, it CAN effect idle quality , personally, with normal hydraulic lifters, with mildish cams, I have found 1/2 turn of preload is usually good, of course in the case of aluminum heads and blocks, heat expansion preload can be effected more than with cast iron components. Lifter preload, can limit engine RPM capability, so in some cases, like NHRA Stock Eliminator, where hydraulic lifters are mandated by rules, it is common to run basically zero preload, and with some special "low travel" hydraulic lifters, a slight amount of actual lash is used.

[Stangman]

I picked up on that Rory. But realized he was talking about preload. That’s why I said 1/2 turn. I think he was going to far and effecting the idle. When he gets back maybe he with explain better

[mike7570]

like NHRA Stock Eliminator, where hydraulic lifters are mandated by rules,

Rory, you do know that rule is superseded and solid lifters can now be run in any combination. 😁

[Diogenes]

Yes, I did mean pre-load, not lash. Sorry about that.

1/2 a turn is what I've always used. Finding that spot on the lifter, where you go an additional 1/2 turn, is not an exact science with the feel of the push rod. I will likely go through them again.

[blykins]

If it helps, don't twist.  Twisting will give you a false sense.

Loosen the adjuster up so that you do have lash, then you can move the pushrod axially back and forth while you're tightening the adjuster and you can feel when the lash is taken out.  When you have zero lash, then set your preload.

[MeanGene]

JMHO but I believe if you stop sticking with the whole "twist" thing, you will probably stop having problems. One can still rotate a pushrod for a ways past zero lash, which introduces a variable that can throw off your adjustment

[Diogenes]

I'll do the axial method on this, readjusting them all. From everything stated here, I have probably over-tightened some of them. Thanks for all the feedback.

[Rory428]

like NHRA Stock Eliminator, where hydraulic lifters are mandated by rules,

Rory, you do know that rule is superseded and solid lifters can now be run in any combination. 😁

Mike, yes, I understand that NHRA does allow solid lifters in Stock Eliminator now, but when I was running my 85 Mustang in M/Stock some time back, we still had to use a hydraulic lifter (I used Sherman short travel lifters), and factory rocker arms, which I used with adjustable pushrods. Between wheelie bars, fuel cells, lightweight race seats, "approved" aftermarket aluminum cylinder heads, Quick Fuel carbs etc, not to mention all the limited production late model  "factory drag cars", like Drag Pak Challengers, COPO Camaros, and Cobra Jet Mustangs, that are allowed to use engines, transmissions, and rearends that are not even similar to what the actual production street cars , have certainly changed the Stock landscape.