Can you change HP by adjusting valves?

This may be a crazy question but can you increase your HP if you adjust your valves (by changing your lifters/tappets/buckets), while staying with the specified range, to decrease the gap between the pushing surface and the cams so that the cam pushes the valve more, thus increasing the opening through which the A/F mix gets in, increasing the intake ?

What would be the pros and cons of that approach ?

 

Not worth all that work.

 

There is no gap between the "pushing surface and the cams" the lifters constantly ride on the cam lobes, and (if i understand your question) just putting in bigger lifters would cause the valves to never fully close. Your best option if you would like to change valve lift is to change the cam itself for a high lift cam. Which would require new lifters as well because they should always be replaced together as a set. But to answer your question, yes more valve lift does increase power (to a certain point). Pros= more power. Cons= uneven idle, poorer performance on the low end of the RPM range, and probably worse fuel economy.

 

Well technically there is a slight gap called valve lash.

He is talking about spacing the valve lash to the minimum within specification. The answer to him IMO is no.

Not making any significant difference.

 

True, if you do just a basic valve adjustment (a couple thousandths here and there) obviously there wont be much of a benefit.

 

That's what I thought. Thanks for the clarification. And @Styx586 , there is a clearance (trust me on this, I know it more than well by now) when the cam is not riding on the lifter, like @Jimmyh explains. It is indicative of how much the protruding end will be pushing the valve into the combustion chamber when it is riding on the lifter. The cam is not always touching the lifter.

 

We're talking about a gap of .006 to .009" (on the 3.4L). That's 6 thousandths of an inch. basically invisible to the naked eye. changing that by .002 to .003" will net you no extra horsepower. I was trying to answer your question of gaining power, which this will not do. The only way to do that via valve lift is by changing the cam.

 

If you set the lash a hair too tight, you will gain no HP and end up with a burnt valve. I learned the hard way on an old Honda dirt bike.

 

the factory specified clearances (or lashes) are 6-9" * 10^(-3) for intake, 12-15" * 10^(-3) for exhaust, each lifter measured when the cam lobe is not riding on the lifter. In metric, that is .150-.250 mm intake, .290-.390 mm exhaust.

can anybody explain why the exhaust clearances are speced bigger, which means that the exhaust valves don't open as much ?

 

Exhaust valves get much hotter than intake valves, therefore they expand more taking up that extra lash.

 

In the old days, you could replace the stock rockers on VW Beetles with what were called "ratio rockers." The pivot point was different from stock, which meant that they would increase valve lift without having to change a camshaft, which on the air-cooled Bug, meant pulling the engine, disassembling it, and breaking apart the case. But those were old OHV engines with the camshaft in the block.

 

EDIT: Nevermind! I just read up on the valve lash adjusting methods for the 4.0. Not even remotely worth the effort to test different lash settings on these motors!


General lash setting/HP info I've seen from other motors:

I have a chassis dyno, so I can test things a bit more accurately than most others. I've tested valve lash settings on both a Pontiac 455 and a 427 Ford Windsor (stroker), both with solid roller camshafts. On both, I found more horsepower than with settings suggested on the cam card.

On the Ford, the cam card suggested 0.026"/.026" (int/exh). Ultimately I found the best power with 0.021"/0.024", picking up 15hp at the wheels at the peak, but also gaining 25hp in the midrange. That motor made a bunch more power than these V-6's, and I doubt the difference between best and worst (with the factory specs) is more than maybe 5rwhp.

 

See, the thing with valve lash is, do you want to be constantly adjusting it or not. You can run tight, risk a burnt valve, have to go back in often due to tightening clearances and readjust, for what.... 5extra hp. Or you can stay on the looser side of spec, and never have to touch the valves for the life of our toyota motors...

Ill take the later.

 

I saw this post back and thought foampile was back to start more crap. Wonder If he ever got that truck going.