Valve adjustment

[grannyknot]

Nissan's FSM and Haynes recommends Intake .010", Exhaust .012" hot for an L28.

My cam is cut to Crane mild street specs and recommends Intake .006", Exhaust .008" hot.

That seems like a big difference, is it?

Thanks,

Chris

[Wade Nelson]

Valve lash enables the valve stem to expand when it gets hot without ending up with ZERO clearance.

Ideally, you'd want just NEXT to zero clearance on valves in order to obtain maximum valve lift, and flow. Max horsepoer.

But on a car that will be neglected by consumers for 100K+ miles, the "factory" has to throw in a LOT of clearance.

Ok, so in a "racing" engine you adjust for less clearance. The assumption is you're not gonna drive 100,000 miles without ever re-adjusting them, as they, or the lifters, or rocker arms wear. In some designs the valve clearances DECREASE as it wears, in others, it INCREASES.

Noisy, clacking lifters indicate too much lash. That's far better than silent ones, which may ALREADY be too tight.

The only damage comes is when they're too tight, because of wear or mis-adjustment, and the valve doesn't close completley, and escaping exhaust gasses damage/destroy the valve and seat.

So, assuming you're not going to "set it and forget it!" tighter clearances can result in some nice performance gains --- at the cost of needing to check and/or re-adjust them more often.

Valve lash ALSO affects timing. Less lash means the valve will open SOONER than it would otherwise. So Crane adjusts their cam so the valve opens at the correct time WHEN its correctly set to .006/.008.

Maybe you knew alll this already, but I"m sure others here don't or didn't.

So to answer your question, no, it's not a BIG difference, but it's an important one.

I suggest some time you try adjusting valves on your Z or some other car to spec, and then try adjusting 'em a bit looser, say add .005, and DRIVE it under both conditions. I'd say Chances are you won't be able to discern any difference.

Valves opening/closing sooner/later WILL affect whether a given engine has more low end torque, or makes more high end power.

This is why VVT- variable valve timing --- is such a winner. On SOME systems, it not only adjusts valve TIMING, but also valve LIFT. So it can optimize for power AND economy ALL across the rpm range.

I've long wondered how much power the L28 engine in the 280Z could put out if SOMEONE retrofitted a VVT head for it. Could be a monster!

Cheers!

[grannyknot]

Thanks Wade, makes perfect sense. I'm not a "set and forget" kind of owner so I'll use the tighter setting.

Appreciate it,

Chris

[Captain Obvious]

Valve lash enables the valve stem to expand when it gets hot without ending up with ZERO clearance.

Wade, I championed the exact same theory and it was brought to my attention that the cold clearance spec for the Z is smaller than the hot clearance spec. In other words... It depends on the application. Some engines get tighter when hot, and others (like the Z) get looser. I haven't gone through the analysis, but I suspect it has to do with where the aluminum is.

Academic, but thought I would pass that on.

[Walter Moore]

Wade, I championed the exact same theory and it was brought to my attention that the cold clearance spec for the Z is smaller than the hot clearance spec. In other words... It depends on the application. Some engines get tighter when hot, and others (like the Z) get looser. I haven't gone through the analysis, but I suspect it has to do with where the aluminum is.

Academic, but thought I would pass that on.

The cam shaft rides in aluminum cam towers, which expand more than the mating steel parts when they get hot.

Oh, and when I first got the engine running I had only done the cold adjustment. I didn't check the hot setting for a week or two. The valves had huge clearances when hot. After I got the valves in "proper" adjustment the car had more power, and sounded different. It is hard to explain, but with too much clearance the exhaust note was raspier somehow...

[Zedyone_kenobi]

This is such a great post! I remember reading it last year. I have long wanted to take some of the lash out of my cam adjustment.

I run 0.008/0.010 cold and when the engine is cold it is so smooth and quiet, but when it warms up, I get a tappety tap which just drives me nuts. Even though I know these engines have a bit of a noisy valve train. Good stuff.

[Wade Nelson]

it was brought to my attention that the cold clearance spec for the Z is smaller than the hot clearance spec. ... Some engines get tighter when hot, and others (like the Z) get looser.

Ok, I stand corrected. So the "cold" valve clearance setting on the Z engine is to ensure it will actually START when it's -20 degrees out. At zero clearance the valve risks staying slightly open, you get zero compression, hence no pumping action, so no charge to ignite getting sucked into the cylinder.

Starting when cold is tricky business. A whole lot of the newer 1995 & newer Stanzas and Altimas would flood when cold started, particularly if the driver pumped the gas pedal while cranking.

The excess fuel washes the oil off the cylinder walls. Compression goes to zero, and...vehicle won't start. Solution is to hold the gas pedal to the floor, the ECM senses this, goes into "clear flood" mode, injects ZERO fuel, and hopefully at least one cylinder still ignites and gets the motor running. Most cars today have "clear flood mode" programmed into the ECM.

More than a few of these makes got flooded so bad we (at the dealership) would have to pull the plugs, use compressed air to dry 'em out, squirt of oil, then start. Along with giving the driver a lesson on proper cold weather starting.

A tad off topic, but hopefully useful info to fellow motorheads when helping others.

[rcb280z]

So running 0.006/0.008 is a bad thing on an original L28? Sorry if this is a stupid question, just bare with me. I have been wanting to do this for some time now on my 77 280. Valve train is really noisy (to me) that is. I have never done the HOT adjustment, always cold (0.008/0.010). And I do it twice a year or more just because... Have had 2 Datsun mechanics tell me they do it cold as well.

[Zed Head]

Engineering specs. are usually set after extensive factory testing and include a safety margin. Tightening up the lash will reduce the safety margin. So it really comes down to how important a quiet engine is to you. You might be able to safely quiet things down with some insulation on the valve cover, or on the bottom of the hood. Or you could get a preheater for your engine block and never run it cold. Just saying that using valve lash to make things quieter could be risky. A thicker oil might get you there with no risk.

You would think that you could shave a few thousandths off of the spec. and be okay but it's possible also that the valves will heat up faster than the rest of the parts during warm-up and you'll burn/warp a valve before the engine gets hot and loosens up the lash. So you might be okay cold, and okay hot, but too tight in-between. Just a possibility.

By the way, I will not be baring with you or any other classiczcar members (that I know of at this point in time).

[Zedyone_kenobi]

I had an idea. I am going to run the car until it is up to temp, and then let it run for 15 minutes after.

I am going to pull off the valve cover and check clearance. I should be in the 0.010/0.012 range hot, but if I am not, then I know I can probably afford to try a tighter cold lash.

The transision is the key as mentioned. There is no way to really get that clearance unless you start the car from cold each time and increase the time and temp in increments, checking the clearance as you go. That would be an interesting data point to have!!!

If only I had an entire weekend with nothing to do!

[LeonV]

The Z's valvetrain makes noise. Reducing noise by decreasing valve lash is a sure way to burn up valves over time. A 40-year-old Z will never be as quiet of a car as most people are used to these days.

[Wade Nelson]

the z's valvetrain makes noise.

[caplock on] wHAT dID yOU sAY???

Edited February 28, 2013 by Wade Nelson