i did search but i need a pretty complex answer, hopefully someone can help me

ok.... so im getting into the head.... upgrading everything, whole nine yards, from lifters/rockers to valves and stem seals.... with a very light port and polish on the intake/exhaust runners (was gonna go all out race but then i did some research...)

right now im non turbo, but im dropping in crower stage 2's and crane cam gears (with all the other bolt on mods already)

basically, what kind of work is involved with using 1mm oversized valves? does it change the piston to valve clearance any? what kind of machining and/or calculating needs to be done for this? and would it be worth the work involved?

any info greatly appreciated

you guys are probably thinking... "a noob asking valve q's here we go"
lmfao any info is greatly appreciated

I think if you knew the kind of work required to have them put in, you might be able to do it yourself, but seeing as you don't, you'll have to have them installed for you. I believe the head has to have the holes that the valves seat into opened up by 1mm overall. Maybe like a 3 angle job, but taking extra material out of the head. Pretty sure the valves maintain the same clearances since they don't go any deeper into the chamber, they're just a bigger port opening into the combustion chamber.

:shrug

I have them on my car. The piston to valve clearance does change. The head will need to have those valves lapped and maybe the seat cut.

well i think i'll steer away from that anyhow... it would probably slow me down until i went turbo anyhow. it was just something i had in the back of my mind

but i did read an old post that said 1mm oversized valves raises compression .4:1 i did find that interesting lol

thanks guys

"i did read an old post that said 1mm oversized valves raises compression .4:1 i did find that interesting"

those would be some HUGE valves, lol. you would be safer just slopping on some jb weld to the top of the combustion chamber..or better yet, onto the top of the piston! look out for my 27:1 420a $10 piston mod in the near future.

anywho, i think oversized valves are for the most part useless on 99.999% of our cars anyway. thres much better ways to spend money on mods. :thumbsup

oversized valves are pointless unless you have the head ported to enlarge the ports to accomodate the bigger valve..

valves do not raise compression numbers..

if you can hit the valves with you stock or low comp pistons you got other problems..

std sized valves need a valve job to be installed
possible new guides depending on what shape yours are in.

Bill

Oversized valves can also hurt low lift flow capability. This would actually give you less power in the low/mid rpm range. High lift would flow more air and make power up top, but this is mainly meant for drag racing applications.

I originally wanted 1mm oversize valves and after numerous flow tests with the mitsu 4G63 head, my engine builder decided that the benefits up top did not out weigh the loss of power down low for street/autocross applications.

You may find differing opinions on the subject. Either way if you use an oversize valve, make sure the person building/porting the head knows what they are doing. I'm sure there are professional race teams that can have their cake and eat it too. I sure wish they would share their R&D results.

Originally posted by MotoFool
valves do not raise compression numbers.. if you can hit the valves with you stock or low comp pistons you got other problems.. std sized valves need a valve job to be installed possible new guides depending on what shape yours are in. Bill

Not exactly true. Rage undercut valves, back when Nemo was still in business had flat bottom valves which would take away combustion volume increasing compression. and those did give an increase of about .4:1 in compression ratio.

It's an interferance engine... all you have to do is mess up timing and you can smack any piston. I don't think dished low comp pistons are any lower than the valve reliefs cut into stock pistons, so I think you can hit those just a easily. High lift cams, bad timing or too radical an adjustment with the cams gears... never know. and high comp pistons if designed correctly should keep the shape and position of the stock valve reliefs just increasing the dome size and shape. Shouldn't be any harder or easier to hit a high comp piston than a low comp. or standard piston.

Very rarely do valve guides need to be replaced, most of the guys who have bent valves haven't replaces the guides. They are pretty study and beefy. The valve stem seals on the other hand, take wear.

You don't necessarily need a valve job when installing new valves, stock size replacing stock size if the valve seats are in good shape the new valves can seal just fine, don't install and assume they seal good though, get it checked. It's not a necessity to do a 3 angle valve job, but I do recommend getting it done, to improve flow.



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