big valves? what are the main benefits

fras · 08-10-2008, 09:23

hi there people, i am going to be getting new valves for my c20let coscast head i have ben told that the +1mm valves fit in the standard seating but iff this is so then cant the real benefit in them, is the whole point of bigger valves not to get more flow through the head?
i may be totally wrong and iff i am could some1 correct me.
i will also be fitting bronze guides solid lifters and double valve springs, what sort of benefits should i see from these?
cheers for any help guys
fraser

DeXx250 · 08-10-2008, 09:34

Larger Valves = more air/fuel in and more gases out.
Double Springs = Reduce valve bounce at higher revs.

IIRC that is.

fras · 08-10-2008, 11:15

aye that was the impression i was under. But
iff the standard valve seats are being used then i cant see how they will let in more air as the bore sies etc will be the same.
iff someone could explin that would be great

cheers
fraser

Gordo · 08-10-2008, 11:40

Quote:

Originally Posted by fras

aye that was the impression i was under. But
iff the standard valve seats are being used then i cant see how they will let in more air as the bore sies etc will be the same.
iff someone could explin that would be great

cheers
fraser

Depending on the OEM valve seat, and the valve to be used, in many cases the valve seat has a large enough outside diameter that it can be machined to suit the larger valve.
you're correct when you sermise that just putting a larger valve on the stock seat will do nothing - actually that may not be true, there may be valve to piston interference as the valve'll be sitting higher out of the hear.

DeXx250 · 08-10-2008, 11:47

I'd imagine you would have to have head work as well to get the best out of larger valves as they on their own wouldn't increase flow that much. I'd say get the head worked first and only get the valves done if the standard ones become a hindrance to more power.

chip · 08-10-2008, 11:52

You recut the standard seat so its bigger, so your concerns about it shrouding the valve have no basis in reality mate

Dinoman · 08-10-2008, 14:09

Quote:

Originally Posted by DeXx250

I'd imagine you would have to have head work as well to get the best out of larger valves as they on their own wouldn't increase flow that much. I'd say get the head worked first and only get the valves done if the standard ones become a hindrance to more power.

Hi, even with std valves you can machine the Valve throat in the head at least 2mm larger and if you fit the bigger+1mm valves then you can take even more, as all the valves need to seat is a tiny contact area at the outer edge of the valve seat the narrower the better, tell your machine shop to machine the valve or the cycl head seat half a degree different and that way you will get your very narrow seat, then you can round off either side of contact area and polish and that will make a huge difference, but of course you have to enlarge the rest of the throats and right back to Manifold and Throttle Bodies, and then remember when you can get more in you must also be able to get more out, but please do not fit a large outlet back box as all that dose is suck your fuel out unburnt as if you check out anyone who dose have them they are always jet black, lots of unburnt fuel, but the pipes look great ????,so tail pipe no larger than the dia of 2 of your manifold to head pipes OK, I hope that helps,Regards

chip · 08-10-2008, 14:33

"The narrower the better" is a bit of a sweeping statement, and its one I dont agree with in some circumstances personally.

Gordo · 09-10-2008, 07:13

Quote:

Originally Posted by chip

"The narrower the better" is a bit of a sweeping statement, and its one I dont agree with in some circumstances personally.

Especially exhausts!