Explain to me functional difference between Mechanical vs Hydraulic Lifters

[outrun]

I dig reading about engines although I am no builder,

Can someone spell out in laymans terms the difference between these two units.

I basically know the lifter sits above the cam and as the cam actuates this unit raises the pushrod etc....

 

[Dan4510]

outrun,

The difference is that hydraulic lifters self adjust usting oil pressure. Also, they operate with no lash(space between the valve tip and rocker) due to the fact they also adjust with temperature of the engine. This is accomplished using a valving system inside the lifter.

hydralic lifters are usually not used in a high performance engine, but this is changing. The reason: if the valve train is rpmed too high the valve can float meaning the valves do not follow the cam. This creates extra lash and the lifters take it up and the valves cant seal.

Solid lifters (mechanical) must have lash in that there is no adjustment in lifter that takes up the slack. Therefore the valves must be adjusted. For example on a slant six the manufacturer recommends 12k miles of one year to adjust. On a high performance engine that sees high rpm, it can be recommeneded much more often.

Some racers at least check if not change valve lash each run.

Hope this helps.

Dan

 

[labman]

Maybe you need a description of a lifter. Don't valves go down and the springs push them back up? Lifters go back to the old flat heads where the valves were located beside the cylinder with the valve face at the top and the stem going down toward the cam shaft. To keep the camshaft from pushing the valve stem sideways, short, stubby rods were fit in bores between the cam lobes and the valve stems. The cam lobe would come around and push up the rod, lifting the valve. Originally there was a screw with a lock nut to adjust the clearance to allow for expansion, but keep slack (lash) as little as possible. A mechanical arrangement. Later the rod was split into 2 parts with its own little cylinder and piston. A hole let the oil pressure in, pushing the piston out, expanding the assembly by hydraulic pressure. Then push rods replaced the valve stems, and the valves were moved to the top of the head with the face down, and a lever (rocker arm) pushing the valves down when the cam, lifter, and push rod push the other end of the lever up. In your DOHC engines, the lifters are above the valve, and the cam is above them. The lifters now lower the valves.

My SOHC truck has mechanical non lifters. The levers (given the classier name cam followers) bear directly on the cam, and pivot on the shaft to push the valves down. The screw with its lock nut, adjusts the clearance. Mercifully the book calls for setting them cold with the engine not running. It was great fun adjusting the ones on a hot engine with it running in the good old days. The engine bay and worker both were coated with oil when the job was done.

 

[MRC01]

quote:

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Originally posted by Dan4510:
hydralic lifters are usually not used in a high performance engine, but this is changing.

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Yes indeed it's changing. Several high performance 32 valve DOHC V-8 engines are now using hydraulic lifters. Toyota (Lexus) and Ford are two I know of, though I'm sure there are others.

But threaded adjusters between the cams & valves are not all that bad. It's easy to adjust.

The absolute WORST for adjustment is shim under bucket. Here there are no lifters at all. The cam directly actuates the valve. There is a "bucket" over the valve with shims inside to set the spacing. This system is excellent for very high (> 10,000 RPM) revving engines but a real PITA to adjust.

To adjust these guys, you must measure all the clearances, remove the cams from the engine, remove the buckets from the valves, pull out the shims, calculate what shims you need to get the clearances into spec, buy new shims from the store, wait a few days with your engine torn apart for the shims to come into stock, then go pick up your shims, reinstall the shims, buckets and cams and reassemble everything.

 

[sbc350gearhead]

They even have fast-bleed lifters for traditional OHV v-8's that will rev to 7 grand without valve float.

 

[Drew99GT]

For engines over say, 8,000 rpm, I would think hydraulic lash adjusters simply wouldn't work. For one, they weigh more, and also, the valve spring pressure required to negate valve float would push the oil out of the lash adjuster every time, unless you used straight up hydraulic fluid as oil

 

[Virtuoso]

quote:

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Originally posted by MRC01:
The absolute WORST for adjustment is shim under bucket. Here there are no lifters at all. The cam directly actuates the valve. There is a "bucket" over the valve with shims inside to set the spacing. This system is excellent for very high (> 10,000 RPM) revving engines but a real PITA to adjust.

To adjust these guys, you must measure all the clearances, remove the cams from the engine, remove the buckets from the valves, pull out the shims, calculate what shims you need to get the clearances into spec, buy new shims from the store, wait a few days with your engine torn apart for the shims to come into stock, then go pick up your shims, reinstall the shims, buckets and cams and reassemble everything.

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It's not neccesarily that hard to do. My Taurus SHO uses that kind of system - but they make a few tools for the job. One pushes the shim/bucket down, one holds the bucket and another pops the shim out for removal (this one looks a LOT like what you'd see in a dentists office).
Luckily, no removal of the cam is neccesary.

 

[road_rascal]

Are you sure that's not a shim over bucket setup? My Kawasaki uses shim under bucket which requires cam removal for clearance adjustments but some other bikes use the shim over bucket which use that same 'tool' to push the bucket down to remove the shim. The older Honda Nighthawk 700's have hydraulic valve adjusters and they rev to 10,000 rpms. My '97 750 goes to 8600.

 

[Shannow]

some hydraulic tappets have the hydraulics at the fulcrum, so the movemen,t (and weight effects) are minimal.

Back in the bad old days when you could play with stuff, we only used to give our Holdens a half turn preload to stop them pumping up (too far).

Even my new Nissan turbodiesel DOHC setup needs no valve adjustments - ever.

 

[JohnBrowning]

Labman and MRC01, loved it!

Toyota is notorius for Mechanical followers, mechanical lifters and shim and bucket set up! I never minded doing a valve job. It gave me an excuse to get the valve cover off and take a look. It also allowed for more precision.

 

[TooSlick]

MCR01,

It's not necessary to remove the cams from the engine, at least on toyotas. They make a nice little, cammed, pry bar tool that separates the cam from the lifter. All you do is use a magnetic tool and remove the shim. Of course if the valve clearance are within spec, you don't have to touch those valve shims at all. The engine is easily turned over with the plugs removed, and you measure eight lobe clearances with each setting.
The flat rate time for this is about 2.5 hours, but that's padded quite a bit ....

I have the shim over bucket, type setup on my 2.4L Tacoma and it hold clearance extremely well. I'm just about to do the initial valve adjustment w/ 70k on the motor. This is the first winter my truck hasn't been garaged, due to that pesky red, Audi TT roadster taking my old space.

When I start the truck in the morning and it's 20F outside, I notice some valve noise for the first time since I got it.

I think it's an excellent design for any high rpm motor. Besides if engineers don't get to tinker with their toys, there would be no reason to live

 

[JohnBrowning]

TS, My Dads Tacoma has over 120,000 miles and he just sent it in for a valve adjustment. It was just starting to get noisey. I have to admit that I like the cam follower design much better for ease of maintence. I need to get an assortment of hims some time in the next 9 years so I can do it my self. Will most Toyota parts counters sell them individualy now? When the 3.0 V6 first came out you had to buy a complete set of shims they would not just sell you what you needed!

 

[jsharp]

I've lived with shimmed tappets in cars and bikes since I bought my first 900 Kawasaki in 1974. I prefer it to any other system although some like the old Kawasaki 650/750 had the shim under the tappet so you had to remove the cams to change the lash.

The big Kawasakis and the Toyotas have the shim on top of the tappet and you can change it out without cam removal using cheap special tools.

A few things I like about this system -

It's simple and accurate.

The wear is minimal. You have two relativly large hard surfaces sliding on one another in an oil bath. It's a low wear system compared to a small dry adjustment screw riding/sliding on the valve stem...