26 Head With Oversized Valves?

[RBeed]

hey guys i am looking at putting 1mm oversized valves in my 26 head and was just wondering what others opinions are? i have read a few magazines that say its not worth the money. it is on a full built motor, big cams big turbo

any input would be great

thanks guys

[RBeed]

anybody?

[proengines]

anybody?

it's worth doing if you're looking to make >450-500kw or the valve seats in the head are low. Cutting for bigger valves does give you a little more to work with on the short side plus it's easier to get the seats at a good height after you replace valve guides because they rarely end up concentric to the seat when you replace them.

[Gav]

Proengines above has likely given the best advice.

If you have the head off and you are overhauling it anyway, then it's probably not a bad option as the price of oversize valves isn't any greater than standard valves.

I used oversize valves as I was trying to extend the envelope of small turbocharges on a larger displacement engine. Installed high lift and long duration cams also for this reason.

If you are building a big HP engine with a single turbo, personally I wouldn't bother as any gains will most likely be overshadowed by the fact you have a big huffer installed.

[Swiper the Fox]

it's worth doing if you're looking to make >450-500kw or the valve seats in the head are low. Cutting for bigger valves does give you a little more to work with on the short side plus it's easier to get the seats at a good height after you replace valve guides because they rarely end up concentric to the seat when you replace them.

yes its much easier to pick the seat with bigger valves if guides are replaced...it also makes full use of the port job if its ported.

[ahh 33 s2]

will going over sized valves cause internal clearance issues. piston to valve i mean??

[discopotato03]

There are some interesting schools of thought on valve sizes and especially valve size ratios as in inlet to exhaust .

I'm not home ATM and don't have a calculator handy , basically I think the valve area of the exhaust side needs to be something like 0.73 of the inlet to do well .

I like David Vizards books on performance engine building and the one on BL's old A Series has lots of good principles outlined by someone who speaks real english .

In the third revision which I have he speaks about valve size ratios in relation to static compression ratio and why that ~ 0.73 ratio works with low compression ratios where something like the 0.68 works out better for high CR engines with big cams . To be honest he was relating to NA engines but generally turbo engines are lowish static CR before they come on boost .

I'd always try to enhance the hot side of a turbo engine because when it comes on boost it acts like a larger capacity NA engine exhaust flow wise so if it is to run well you can't really have exhaust restrictions when its running in a supercharged state .

There are advantages to running larger valves and some say that its a bit like running higher performance cams only changing valves is more involved because they are not really a bolt on .

I'd do valves before big cams because mild cams with big valves still behave nicely because the mild cam timing , wild cam splutter to me has everything to do with trapping efficiency and overlap timing .

What I think is being sort after here is increased volumetric efficiency and while there's different ways to achieve it some give better overall results than others .

I am no one in the scheme of things but given a choice with an RB25/26/30 I'd like to go one size up on the exhaust valves , I get the feeling that RB26's that have done any considerable mileage need exhaust guides anyway so cutting the seats for a slightly larger valve is no biggie . Fresh valve stems are nice in new guides to , critical heat transfer points through exhaust valve seats and guides .

If you are careful with CR cams and turbo sizing I don't think you'd lose anything significant down low with slightly larger valves all round . I also think a well ported head makes a ig difference to the engines overall breathing characteristics .

Your call , cheers A .

[RBeed]

thank you, thank you and thank you. really good imput guys and greatly appreciated. The oversize valves are a goer so ill have results as soon as its done, thank you all once again

[Roy]

But what brand of valve do you go for. The RB26, SR20 etc use soldum filled exhaust valves dont they, so help keep the exhaust valve cool. A simply stainless steel exhaust valve would almost be a downgrade wouldnt it when it comes to keepng combustion chamber temperatures down???????

[STATUS]

We got good results with our head, full ferrea kit but we also did a plenum change so its hard to quantify the valves alone but torque is broader and holds 50nm at a minimum more to redline. ill post up a comparo 2moro. Haed is off again to test quench area mods as we speak.

actually comparo is here (ignore the difference in response as the second run was on a lower ramp rate for run in which was 6 seconds instead of 10) power is the same within 2 or 3 kw between ramp rates http://www.skylinesaustralia.com/forums/Sh...96#entry4918296

Edited November 11, 2009 by URAS

[juggernaut1]

But what brand of valve do you go for. The RB26, SR20 etc use soldum filled exhaust valves dont they, so help keep the exhaust valve cool. A simply stainless steel exhaust valve would almost be a downgrade wouldnt it when it comes to keepng combustion chamber temperatures down???????

From what I have read (which of course is not gospel - but another school of thought) there are pro's and con's of sodium filled valves. Ie. that the heat is drawn up the stem which then apparantly takes its toll on the valve guides.

Edited November 11, 2009 by juggernaut1

[Swiper the Fox]

But what brand of valve do you go for. The RB26, SR20 etc use soldum filled exhaust valves dont they, so help keep the exhaust valve cool. A simply stainless steel exhaust valve would almost be a downgrade wouldnt it when it comes to keepng combustion chamber temperatures down???????

i use inconell exhaust valves

[Roy]

Yeh, i think Supertech do Inconel oversized valves for the RB20/25/26. Not cheap though

[STATUS]

But what brand of valve do you go for. The RB26, SR20 etc use soldum filled exhaust valves dont they, so help keep the exhaust valve cool. A simply stainless steel exhaust valve would almost be a downgrade wouldnt it when it comes to keepng combustion chamber temperatures down???????

not sure who does basic stainless alloy valves anymore. The fereas are all Tribological super alloys (superior to inconnel and lighter) The materials of these aftermarket valves be it super alloy or inconell are far superior than the std items sodium filled gear.

Inconell is exteremly heavy duty also but sometimes the extra weight is its downfall (valve float ect) for example its a nono in high reving big blocks or they become a consumable (ie every 20-40 passes)

I have nice site which explains all the alloys here: http://www.enginebuildermag.com/Article/25...so_special.aspx

Edited November 11, 2009 by URAS

[Roy]

Cool, but looking at their website Ferreas dont have off the sheld RB20 valves, lol so i am guessing that means more expensive custom

[STATUS]

Cool, but looking at their website Ferreas dont have off the sheld RB20 valves, lol so i am guessing that means more expensive custom

never looked for 20 vlaves TBH, i thought we were talking 26

[Roy]

yeh, yeh...carry on