Question For *ohc Motors

[Teddy]

heres food for thought...

 

Pushrod motors - grind cam = opens valves longer (in VERY basic terms)

 

*OHC motors, where cam runs onto shims = Add meat to cam lobes to open valves longer.

 

So, question is, IF you were able to work out what sizes were appropriate.. of you wanted a budget, and cheap ass way of getting more duration outa your motor, instead of getting new cams, would it be possible to put larger shims in, to increase the time the valve is open? It wouldn't change the time that it comes in, but if you think about it, because the shim has moved up point piss all of a thou, it would result in the valve opening slightly before, and close slightly after = more duration, but with standard specs!

 

If any of that makes sence...

 

Has anyone done this / or does it not work?

 

Just wondering, thats all :hammer:

[TRD ke70]

yip toyota done with the 5k, thats why they have bigger lifters.

to increase the duration and lift without changing the timing so to speak.

[Teddy]

ahhh

 

1+1 does make a 5k :hammer:

 

*keeps thinking*

[Super Jamie]

this is all essentially the same thing as adjusting the tappets in your 4K to tighter clearances, so the valve stays open those few degrees longer

 

which i remember explaining to you on msn a long time ago :y:

 

now, a question for the class. WHY can't you just keep adjusting the tappets/shims up to get more duration? why is there a specified service spec for valve clearance? and what are the advantages/disadvantages/tradeoffs of adjusting valve clearance tighter for more performance?

 

ps: nice to see another "teddy is thinking" post. haven't had one for a while :hammer:

[blown 5k]

A camshaft profile running on two dissimilar lifter diameters has exactly the same timing specifications regaurdless.

Closing up the valve clearances has a minimal effect on performance probably more detrimental than anything else.

Edited December 23, 2005 by blown 5k

[love ke70]

i don't really know the answers to your questions, but when i reset my tappets some fool had them all with smaller gaps than they should be, it gave me performance shittier than a standard 4k and i got about 250 k's to a tank. so theres my 2c, the valves are open longer so you get more fuel in, but it ruins ur economy, and doesnt increase performance...my experience, my opinion, I'm usually wrong but meh, interesting topic :hammer:

[karl]

[WHY can't you just keep adjusting the tappets/shims up to get more duration?]

 

Hi,

I'm new to the rollaclub and would like to have a go at your questions.

 

Like Nick posted, valve clearance is required for different expansion rates of the engine. Also when the valve wears deeper into the valve seat, then valve clearance/adjustment is required. If you have your valve clearance adjusted too tight, you may not close the valve completely. If you have no clearance, you are not allowing the valve spring to close the valve onto the seat, and closing force will still be applied to the valve stem. That itself can provide you with interesting problems - like a drop in compression pressure or the posibility that you will start burning valves.

 

 

[what are the advantages/disadvantages/tradeoffs of adjusting valve clearance tighter for more performance?

]

 

Adv - no tappet noise on cold mornings :hammer:

[NickZ]

you would just be pushing the valve open more and that means open longer.

 

but a bigger cam doesnt do that, the end point of the lobe will be flatter or more peaked etc and off center opening the valve longer and maybe in a diffrent part of the piston stroke for the different effects. the vlave isnt pushed down further as there is a limit.

 

People can have trouble with valves hitting pistons, thats because its opening in a diffrent part of the piston stroke (as above) or its not dialed in correctly

 

Another reason is the spring will only compress to a point where it will bindup.

Things stretch at higer revs as well as expand from heat etc thats why there are clearances.

[Teddy]

MMmmm

 

Its good to see a genuinely intresting thread like this one actually go somewhere... need more of these... !

[Mr Revhead[RL]]

push what? :hammer:

[Super Jamie]

valve clearance exists for several reasons. tappet noise, thermal expansion as the engine warms up, and yes, making sure the valve actually closes

 

possibly the most important and strange effect valve clearance has is the different rate of cooling that it provides the valves

 

if you get a valve too hot (often happens with exhaust valves) every time it closes, it will weld itself to the seat. as it's pushed open again, it takes a tiny little bit of the seat with it. do this enough times, and you get valve seat recession fairly quickly and your motor will be losing compression through the valve. setting the tappets larger/smaller gives the valve a different time to cool between each spark event

 

this is why a mentally big camshaft usually comes with a larger valve clearance spec than stock. beacuse the engine is likely under more stress due to the other crap you'll have done to it as well, you need to go a little easier on the valves and give them more time closed. some cams are also just designed like that

 

 

the tiddlywinks effect depends on the type of shim and mainly the lift of the cam. 4age engines have shim over, so you run a high-lift (10mm) cam and you need a screw-in shim kit. if you put too-big shims in a shim over engine, you'd just bind it up and bend the weakest point as the valve wipes over the bucket. which is probably the valve

 

valve/piston contact will only happen in an interference style engine, or with a higher lift cam, which makes a non-interference engine an interference engine. you'd have to run ALOT of lift in a K engine to make it interfere. i don't know about 4As

 

 

as nick said tho, essentially the best part of adjusting your tappets/shims is your ability to change how long the actual valve stays open, so you can get more air into the engine, and change how it comes out. this is tuning for optimum power on one cam profile. an engine is just an air pump, to make power, you want it to take a BIG breath, and hold it for as long as possible, without having a detrimental effect to allowing the next big breath in

 

you can also change the power characteristics of the engine with adjustable cam gears, which essentially change the TIMING of the valves, without actually changing the cam itself

 

the tradeoff to adjusting valve clearances for power is indeed reduced longterm reliability, and perhaps a bit of tappet noise if you're opening the valves up alot

 

personally, i think you can close stock valve clearances up another 4 thou on the inlets, and 2 thou on the exhausts, for a bit better breathing. any cam under about 280 degrees could just have stock clearances, no worries