well thanks for that tickford_6 you know your stuff.
sent ya a pm

Tickford_6 - do you have any flow charts or dyno's showing the gains with bigger valves?

Does anybody have any results from real heads to indicate that 41/47 valves are holding these engines back?

No doubt that bigger valves increase the maximum potential of a head, but I am skeptical that this will result in a better engine. Also, what is going to happen with the torque curve at low rpm with bigger valves?

stockstandard- i have no flor test for real heads modified in this way.

and i'll go on record as stating i don't believe the 39-47 for 41-47 valve are going to hold an engine back that is intended to be a road going engine.

it's based and the theory of a company in the UK that build race engines.

it does seem to hold true when aplied to some known engines that have been wel devoloped. and as far as i've seen given the 47mm intake valve the I6 has still holds true with people struggling to make 400hp

the only time it doesn't is when there is a major design flaw in the engine.

i agree there would most likely be some low end losses due posibly needed a larger port volume to to make use of the valve size, but i'm sure you'll agree by the time you start needing the bigger valve and posibly port, the Cam being used would have already dictated a struggling low rev performance.

if you like i'll pm you the website, although not dealing directly with the I6, still a very interesting read

this looks to be a good subject..


thats something to consider aswell huh, the power down low cos its not much point if ya got a s**t load of torque/power up high if the engine struggles to get above 3000rpm (i may have exagerated a bit, but you all get what i mean)

i was just goin to go for some shorter diff gears if it was too slugish down low, but yea

this valve stuff is difficult s**t to work out

wanna send me the site aswell, sounds to be interesting

Well, to get around the "use a bigger exhaust valve", I'm going to try a cam that has a longer duration ( about 10dgs) for the exhaust. Hope this works.
I used this idea with 2V Clevos, and it worked.

IMO you need to have a compelling reason to go bigger with the valves because bigger is not always better. 4.0's that are used on the street dont have a good record of turning headwork into power. There was a member here with worked AU engine, in the search for more power he pulled the head and had a work done to it by a reputable workshop (head specialist). It flowed close to 390hp which is a substantial improvement over stock (just nudged past 250cfm@.600"). Head went back on and he lost power, about 5rwkw from memory.

If you cant use it you dont want it.

the problem with using more duration to cover up head problems is the loss of tractability down low

Thats where diff gears and cam timing come into the picture.

ahh the domino approach to car modification

Well thats how it is, you modify to a package that works, everything in unisome. A big cam doesn't work without compression being raised, and to benifit
from that, then head work, exhaust, etc.... the list goes on.

Sure is, what you were describing was a little reversed though.

will do X instead if Y, then Z to fix problems caused by X...

what sort retunning was done and was a new cam that matched the head installed?

as has been seen from head flow figures posted on this site even some specialist don't get the best from these heads. some make better numbers in high lift but suffer in low and mid lift. and currently i know of only two comonly available cams that have .600" lift.

A ported head should out flow a stock head from almost .050" and most certainly from .100". and if that head is used and the managment system is properly retunned to suit there will always be good power gain across the entire operating RPM.

losses only come about by miss matching parts and specs.
things like poor state of tune
a low lift cam with a head that has good flow at high lift
long duration cams with out using enough compression
the list goes on.

but back the my first post. the point was to front the information so people can make up there own mind
the web site it cam from is

http://www.pumaracing.co.uk/


you asked me if i had proof of the larger valves working.

so i ask you, do you have proof that it doesn't work?
lets not damn the idea untill it's proven right or wrong

No proof at all, thats why I have been asking for flow charts etc so I could work things out. There is really very little out there on modified 4.0 heads (I have actively been seeking every bit of info of forum members for the last couple of years) so it is really hard to prove or disprove anything.

My skepticism comes from seeing many cars (including mine) not even coming close to pushing the heads flow limits with std valves. I cant see the need for anything bigger, and although I would be very interested in seeing what the results are I dont think it will give he what he wants.

Please dont think I am damming this idea, the discussion has been very interesting so far.

oh, and the car I spoke of above - It was unichipped and I believe it was retuned for the head (not 100% sure though).

The key is Air speed, big lift or not, you gotta have air speed, thats where a decent inlet manifold comes in. Having all the head work in the world is useless without a matching In. & Ex manifolds to suit. And the right compression for the cam too.