Bigger valves, any downsides?

[mag2555]

The highest VE I have been wihtness to was over 120% on a pro stock motor.

[user-23911]

Hahahahahha.

Try looking into a ROM from a high powered EFI SD N/A factory racer........mapped up to 140% VE.

But it doesn't mean it'll get it, but it's mapped for it.
But which one?

[user-23911]

So anyway........why is everyone so focused on valve size when I made the comment about the size of the hole.

The valve has nothing to do with anything but it always comes up?????

[Warp Speed]

So anyway........why is everyone so focused on valve size when I made the comment about the size of the hole.

The valve has nothing to do with anything but it always comes up?????

Maybe because you didn't make any sense?
The valve controls how big the "hole" can be.....
The valve has EVERYTHING to do with anything! Lol

[user-23911]

The valve controls how big the "hole" can be.....

Are you sure?

The flow is determined by the size of the hole.
The valve just blocks the hole.
You can make the hole bigger so it just closes with the edge of the valve.
You can keep the hole the same size and use a 1 mm bigger valve.


Now from a wannabe machinists point of view, the whole purpose of a valve 1 mm bigger is to fit a worn seat?
If you try a bigger valve in a stock seat, too much valve lash.If the seat is worn you can cut it down to fit.Still the same throat dia. though?

So really........it's a bit like those sales and marketing things, you get what you pay for..................because you're buying bigger valves.
But you don't really because you can work the heads to use the original valves for more flow.


Well...............fitting bigger valves....it's so easy, you just swap them out at home and lap them in????
Or did I miss something?

[pcnsd]

I always thought the purpose of a larger valve was to increase the valve throat and valve curtain flow areas. One goes with the other. A 1.850"/47mm valve @ 90% has 2.1 sqin of flow area. A 1.929"/49mm valve @ 90% has 2.29 sqin of flow area at the convergence point. Valve curtain area will usually go higher. Once it exceeds the available throat area, if flow continues to improves, it does so by raising the valve area cd. Fitting a larger valve can also raise the valve on the seat which will normally improve valve area cd by moving the valve further from the SSR.

[Ron E]

The valve controls how big the "hole" can be.....

Are you sure?

The flow is determined by the size of the hole.
The valve just blocks the hole.
You can make the hole bigger so it just closes with the edge of the valve.
You can keep the hole the same size and use a 1 mm bigger valve.


Now from a wannabe machinists point of view, the whole purpose of a valve 1 mm bigger is to fit a worn seat?
If you try a bigger valve in a stock seat, too much valve lash.If the seat is worn you can cut it down to fit.Still the same throat dia. though?

So really........it's a bit like those sales and marketing things, you get what you pay for..................because you're buying bigger valves.
But you don't really because you can work the heads to use the original valves for more flow.


Well...............fitting bigger valves....it's so easy, you just swap them out at home and lap them in????
Or did I miss something?

How about... The valve controls how big the hole can be....unless you're stupid?

[plovett]

And the valve controls how big the effective hole is at any given time?

[4vpc]

So anyway........why is everyone so focused on valve size when I made the comment about the size of the hole.

The valve has nothing to do with anything but it always comes up?????

Your original comment was ignored as It would be some kind of clown that put a bigger valve in without increasing the seat diameter and throat to match and take full advantage of it. I hold the guys on this forum (which has to be one of, if not THE best of its type in the World) in much higher regard in that they would take that for granted.
It's akin to assuming someone on here would fit oversized pistons to a standard bore and wondered why it made less power before seizing.

[user-23911]

Well at least that's given the OP something to think about?

[Walter R. Malik]

So anyway........why is everyone so focused on valve size when I made the comment about the size of the hole.

The valve has nothing to do with anything but it always comes up?????

Your original comment was ignored as It would be some kind of clown that put a bigger valve in without increasing the seat diameter and throat to match and take full advantage of it. I hold the guys on this forum (which has to be one of, if not THE best of its type in the World) in much higher regard in that they would take that for granted.
It's akin to assuming someone on here would fit oversized pistons to a standard bore and wondered why it made less power before seizing.

I don't speak for anyone else but, I will presume NOTHING. Especially any modifications other that what is specifically mentioned.

People installing larger valves and making all the valve seat cuts a bigger diameter and not doing any porting of any kind in the bowl is done everywhere all the time ... NOTHING can be taken for granted.

[user-23911]

You haven't even said whether it's a 2 valve or a 4 or 5 valve head?

I could make an assumption that it's a 4 valve head and your taking advice from those who do 2 valve heads?

They're NOT the same.....are they?
A Honda to a SBC?

[4vpc]

So anyway........why is everyone so focused on valve size when I made the comment about the size of the hole.

The valve has nothing to do with anything but it always comes up?????

Your original comment was ignored as It would be some kind of clown that put a bigger valve in without increasing the seat diameter and throat to match and take full advantage of it. I hold the guys on this forum (which has to be one of, if not THE best of its type in the World) in much higher regard in that they would take that for granted.
It's akin to assuming someone on here would fit oversized pistons to a standard bore and wondered why it made less power before seizing.

I don't speak for anyone else but, I will presume NOTHING. Especially any modifications other that what is specifically mentioned.

People installing larger valves and making all the valve seat cuts a bigger diameter and not doing any porting of any kind in the bowl is done everywhere all the time ... NOTHING can be taken for granted.

Ok fair comment, but not in my world. I wouldn't go to the expense of buying new o/s valves and not doing the seat, throat and blending the bowl to match. It seems a pretty pointless exercise to me.

[4vpc]

You haven't even said whether it's a 2 valve or a 4 or 5 valve head?

I could make an assumption that it's a 4 valve head and your taking advice from those who do 2 valve heads?

They're NOT the same.....are they?
A Honda to a SBC?

Initially I was just generalising, but I did here:

Some good points - thanks.
I would say the MCSA to be part way up the port just before the SSR if it matters. Would it be increased along with the valve & throat? Good question. Initially I would think perhaps so, but maybe to a lesser degree. I'm thinking keeping velocity high will carry the air through the bigger throat.
Shrouding? Well yes it will bring the overall result down, but I don't think to a degree where it makes it not worth doing.
A CR increase in isolation will work well, but not a bigger valve with matching throat, that will need other mods to make it work. An increase in CR will work well with a bigger valve I would have thought by creating some more velocity on the incoming air.
I am thinking more of a modern two valve inlet, but would have thought the rules wouldn't change much for a single.
Thanks for making me think, maybe i'll get some port molds knocked up, some oversized dummy valves made and get some figures done with the aid of the flow bench too.

Also read the rest of the post, like this bit here; "I'm thinking keeping velocity high will carry the air through the bigger throat" and "Thanks for making me think".
It's all in there, you just have to slow down and read it.

[Walter R. Malik]

Your original comment was ignored as It would be some kind of clown that put a bigger valve in without increasing the seat diameter and throat to match and take full advantage of it. I hold the guys on this forum (which has to be one of, if not THE best of its type in the World) in much higher regard in that they would take that for granted.
It's akin to assuming someone on here would fit oversized pistons to a standard bore and wondered why it made less power before seizing.

I don't speak for anyone else but, I will presume NOTHING. Especially any modifications other that what is specifically mentioned.

People installing larger valves and making all the valve seat cuts a bigger diameter and not doing any porting of any kind in the bowl is done everywhere all the time ... NOTHING can be taken for granted.

Ok fair comment, but not in my world. I wouldn't go to the expense of buying new o/s valves and not doing the seat, throat and blending the bowl to match. It seems a pretty pointless exercise to me.

An oversize valve is usually no more expensive than the standard size valve. On used heads, if the valves are bad, (usually the stems as well as the seat area), and need to be replaced, especially if the seat area of the head is well worn, it is rather common place to do this "opening of the seat diameter" operation.

I keep forgetting that most of the people on this forum don't do this stuff for a living and have little clue about what actually goes-on as "common place" in this profession.