Bigger valves, any downsides?

[4vpc]

As mods go there is a trade off to most of them, few are win - win. Bumping up the static CR is generally win-win apart from having to use a higher octane fuel, from the engines point of view there is no downside I can think of.
What about putting bigger valves in? I can't think of a negative to that apart from maybe some weakness to the head.
Or is there?

[pdq67]

Probably install bigger valves up to the point that shrouding becomes an issue, then you have to contend with chamber size increase and a drop in compression.

Or plane them to get the compression back up. ------ Or changing pistons.....

Heck, maybe a head gasket change to thinner ones might help here?? Say you are running .060" thick HG's and go down to .015" or .020".

pdq67

[Circlotron]

Larger valves might be heavier so need stronger springs therefore greater valve train friction. Or same springs but with lower rpm limit.

[mag2555]

Dependant on your motors valve lift at overlap and its piston to deck clearance a bigger valve that makes for a tighter valve notch clearance can limit the amount of cross flow at over lap which cuts into the VE percentage and hence power output!

[mag2555]

Dependant on your motors valve lift at overlap and its piston to deck clearance a bigger valve ( Intake or Exh ) that makes for a tighter valve notch clearance can limit the amount of cross flow at over lap which cuts into the VE percentage and hence power output!

I bigger Intake valve in a NA motor may also pick up enough added low lift flow that the total amount of air mass wanting to move thru the port now its hits the sonic limit and you tare off a bunch of top end power and narrow up in the motors power band to Boot!

[Carnut1]

Don't waste your time unless you are going to work the bowl, ssr and chamber. Many heads just don't have enough material to work these areas to really make the bigger valves work. Thanks, Charlie

[user-23911]

Maybe you should be asking about bigger seats?

The valves don't do a lot, they just block the hole. It's the size of the hole that counts.

[steve cowan]

the best suggestion i think would be to do a google search here and look at valve size percentage to bore size,what is optimum ?? 52-53 % of the bore.
discharge coefficient - max cfm/sq in of valve area based on theoretical max of 137 or 146 cfm/sq in.
i am just suggesting google search as i have spent weeks and months reading all i can,and go back through old posts and threads,there is more info there than you will know what to do with,
enjoy

[randy331]

Dependant on your motors valve lift at overlap and its piston to deck clearance a bigger valve that makes for a tighter valve notch clearance can limit the amount of cross flow at over lap which cuts into the VE percentage and hence power output!

What's the most ve you have seen with the piston at TDC and both valves open ?

Randy

[pcnsd]

In the EnginePro software I note a HP loss when modeling with a bigger valve unless a reasonably significant flow increase at peak valve lift was included. For my model, I needed 16 cfm above base to break even in projected output. Base valve 47mm, bigger valve 49mm.

[Walter R. Malik]

As mods go there is a trade off to most of them, few are win - win. Bumping up the static CR is generally win-win apart from having to use a higher octane fuel, from the engines point of view there is no downside I can think of.
What about putting bigger valves in? I can't think of a negative to that apart from maybe some weakness to the head.
Or is there?

It all depends upon the kind of engine and the way it is to be used.
Bigger valves, (seats), slow down air speed across the seat area at a given RPM so, it is not always the best thing to do.

[Ron E]

Is the MCSA in the throat? I.E. will it grow with the valve? They seem to work either way. But, I'd rather keep the same size valve if the choke is upstream and not increased with the valve.

[NewbVetteGuy]

As mods go there is a trade off to most of them, few are win - win. Bumping up the static CR is generally win-win apart from having to use a higher octane fuel, from the engines point of view there is no downside I can think of.
What about putting bigger valves in? I can't think of a negative to that apart from maybe some weakness to the head.
Or is there?

It all depends upon the kind of engine and the way it is to be used.
Bigger valves, (seats), slow down air speed across the seat area at a given RPM so, it is not always the best thing to do.

Darin Morgan: "Make no mistake, velocity is the primary variable in the design of the entire induction system. I often say that my job title should be Velocity Manager instead of Cylinder Head Designer. Air speed is 10 times more important than raw flow numbers. If you kill the velocity by 10 percent, you will kill almost 40 percent of the wave and ram energy that dynamically fills the cylinder! Raw airflow cfm is an important variable as well; it's just not as important."


Just guessing and talking out loud because it's how I learn, but a bigger valve increases the curtain area at each lift, if flow increases at the same rate as the curtain area increase then airspeed stays the same; if your valve curtain area increases faster than your CFM increase your velocity at low to mid lift is decreasing.

-If you calculate the valve curtain area and compare it to the MCSA upstream in the intake tract, you'll know the lift at which the choke point transitions from the valve curtain to the throat. A bigger valve only will mean that the min CSA moves from the valve curtain to the throat sooner (At a lower lift). Made up #'s: If you're starting with a head+valve combo where the valve curtain area exceeds the port's min CSA choke point @ 0.400" and your cam has a max lift of 0.600", I'd expect that a bigger valve would give you improvement (in flow) between 0.001" and somewhere below 0.400". -The bigger valve helps reduce the bottleneck in the flow from 0.001-whatever lift the choke point transitions from the valve back to the throat; it also reduces the lift at which you hit max flow, meaning that you spend more time at max flow.


I'd guess that the perf downsides center around:
1. that you have less velocity as the intake valve closes (and really from 0.001" - the point at which the throat becomes the choke) because of the larger curtain area (with higher velocity you can delay the intake valve close and continue to fill the cylinder as the piston moves up the bore)
2. Low lift flow moves backwards just as easily as it moves forwards; reversion now steals some more power (if a larger valve is increasing low and mid-lift flowz only, and low lift flow increases increase reversion, then I'd expect that a larger valve means an accompanying increase in reversion if all else is equal)

(Others have already covered shrouding via chamber/ cylinder wall)


Adam

[4vpc]

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.

[Walter R. Malik]

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.

Some more to think about intake air speed.
Wide Open Throttle air density and high intake vacuum density at part throttles create totally different scenarios at the valve seating area.