Re: 1/4 vs 1 turn on hyd valve adj = performance?
This is a very easy read:
http://www.corvettesdownunder.com.au/docs/adjusting_chevy_valve_lash.pdf
How to Adjust Your Chevy Valve Lash
by Lars Grimsrud
Colorado Corvette Crazies (CCC)
The Ultimate Corvette Tuning & Beer Drinking Fraternity
Lafayette, CO Rev. A 2-7-01
This tech paper will discuss the adjustment of Chevrolet hydraulic lifters (“valve lash”).
The procedure outlined here differs slightly from the Service Manual, and is based on my years of experience doing this work in the quickest, least painful, most economical way while keeping the level of quality high. It is recognized that other people will have different methods of doing things, and may disagree with specific methods and procedures that I use.
Overview, Theory and my Thoughts on Lash Settings:
Hydraulic lifters are wonderful little innovations which reduce valve train wear and virtually eliminate required valve train maintenance.
Without the use of hydraulic lifters (mechanical lifters), the valve train must be adjusted with a certain amount of “slop” in it (“lash”). This lash is necessary, since the various components in the valve train tend to “grow” and expand as they heat up from normal engine operation. As the components “grow,” they take up a large portion of the lash, but some lash must still be retained as a safety margin. If there were no lash, there would be a risk of the valves not closing fully, resulting in poor engine performance and burnt valves. This lash, however, results in a bit of valve train noise as parts “clank” together, and this clanking induces wear of the valvetrain components. This wear, in turn, requires that the lash be re-adjusted at regular intervals. If only there were a way to eliminate the lash…. hmmmmm….
Enter the hydraulic lifter. Believe it or not, but the internal components of a hydraulic lifter are the most precise, close-tolerance parts on a vehicle. The basic operation and principle of the hydraulic lifter is as follows:
When the hydraulic lifter is at the “low” point in its bore (the valve is closed), the body of the lifter is exposed to pressurized oil in the lifter oil galley. The lifter body has a little hole in it, and this hole allows oil to enter and/or exit the lifter body. The pressurized oil in the galley thus enters the body of the lifter, and pushes lightly on a plunger in the roof of the lifter body. This plunger is about a half inch in diameter, giving it a total area of approximately 0.12 square inches. If you’re running 60 pounds of oil pressure, that means that the oil is pushing upwards on the plunger with a force of about 11 pounds max. This 11 pound force is not enough to open the valve, but it will remove all slack out of the valve train.
As soon as the lifter starts moving upwards in its bore (the cam is opening the valve), the oil hole in the lifter body moves out of alignment with the oil galley. The lifter body is sealed off, and oil can’t get in or out of the body. The lifter, thus, goes into “hydraulic lock,” and suddenly acts like a solid lifter. The oil under the plunger is not compressible, so the lifter now opens the valve.
As the lifter comes down the bore after completing its valve opening chore, it is once again exposed to the oil pressure in the lifter galley, and the pressurized oil once again assures that all lash is taken out of the valvetrain before repeating the opening cycle. As the valvetrain wears, the oil pressure simply constantly pushes the plunger upwards to remove any slack caused by the wear. The plunger can be pushed upwards in the lifter bore within the design limitations of the lifter, and will eventually be stopped by a snap ring retainer in the top of the lifter body. Once the plunger reaches the retainer, it can no longer provide effective valve train adjustment, and the valvetrain will start making noise.
The distance the plunger is compressed into the lifter body when the lifter is at the low point in its bore is referred to as “lifter preload.” This is the “valve lash” or “valve adjustment” on a hydraulic lifter. The further the plunger is depressed, the more wear the lifter can “absorb” before reaching the snap ring retainer. However, the more the plunger is depressed, the more prone the engine becomes to “lifter float” or “valve float.”
As we noted earlier, the oil in the lifter is not compressible. If, somehow, the lifter body were filled with just a few drops of oil too many, and the lifter were moving so fast in its bore that the oil did not have a chance to bleed out and re-stabilize the valvetrain lash at the bottom of the lifter travel, the lifter would keep the valve open when the valve should be closed. Further, if aggravated, this condition could cause the lifter to open the valve beyond its design limitations, out of time with the intended valve cycle. This is what is known as “lifter float” or “valve float.” It can have disastrous consequences if the valve were to hit the piston. We, therefore, adjust hydraulic lifters with some pre-load, but not too much. So what’s the right spec…?
Obviously, we can eliminate valve float completely by simply adjusting our lifter pre-load such that the plunger is right at the top of the lifter body; right up against the snap ring retainer. The problem with this approach is that there is the possibility of the same hydraulic lock conditions exerting so much force on the snap ring that the snap ring is forced out. With nothing retaining the plunger, we would have the same disastrous ending to our engine… Also, with no plunger travel available, the non-maintenance feature of our hydraulic valvetrain is defeated, and we must now constantly adjust the valves as if they were mechanical.
The factory setting on a Chevy lifter pre-load is ? to 1 turn lifter preload with the lifter on the low side of the cam (valve closed). This eliminates valvetrain maintenance for at least 100,000 miles, and is a good compromise setting. However, it can allow the valves to float at rpms as low as 5700. This, effectively, becomes a factory-installed rev limiter: if they can make the valves float lightly around 6000 rpm, GM can reduce warranty claims from customers over-revving their engines. Hey… these boys and girls designing this stuff in Detroit aren’t dummies, are they?
So for a performance application, we split the difference. A ? turn lifter pre-load will raise the rpm limit of the engine, yet it will still provide quite a bit of plunger travel so the lifter can do its valvetrain wear adjustment thing.. It will also keep the plunger away from the snap ring retainer, and it will keep our operation safe. Safe, reliable, improved performance and good durability/life: what more could you ask for?
