Valve Seat Width Considerations

[Majik]

I'm trying to take into heat considerations of different valve materials. I know the standard is 1/16th contact point on intakes and 3/32nd contact on exhaust. What I'm trying to figure is that does this change for different valve materials? Such as a stock steel valve, stellite coated stock valve, a Stainless ferrera valve and a delwest titanium valve? I would think the seat contact face would need to be different between these valves due to different heat transfer properties and if so how much? I'm cutting some seats for titanium valves and was wondering how much if any do I need to increase the seat contact area for them? del west's website provides no help. What advantage does a smaller contact area offer. a .040" is available but what does one gain by going smaller in the seat contact area other than more heat being traped into the valve? is it that big of a flow issue with having a flat surface like that?

 

[rpmag]

I am aware that Ti is a poor conductor of heat and hence the seat materials are chosen to compensate for this. However I would suggest that you spend the time to contact the specific company that has made the valves you want to use and discuss the specifics of your application. They have a vested interest in ensuring that their products work well. They will also know the specific material that their valves are made from.

 

[Rob45]

Majik:
rpmag is right - no use re-inventing the wheel here.
In general, wider valve seats are needed for hotter valves, but improved valve guides also can mean a narrower seat will work. Much of the valve heat is or can be carried of via the valve stem.

For example, I have made sets of beryllium-copper valve guides for motorcycle engines, and have noted vastly-increased valve and valve-seat life.
Beryllium-copper because it has the hardness of cast iron with twice the thermal conductivity of typical valve guide bronzes.

 

[Majik]

I guess my main question here isn't how wide to make the seat for my titanium valve. That would be impossible to answer due to chamber design, fuel used, compression ratio etc. What I am asking is this. The standard intake valve seat width is .040". What advantages are there to going to a smaller contact area? What are the disadvantages? and how do they relate against each other. I know if I shrink the valve seat valve head temperatures will go up but by what percentage? What is the linear relationship between valve seat contact area and valve head temps. Is there an advantage in flow by going with the smallest valve seat width as possible or is it a minimal gain in flow vs the gains in valve head temperature? I don't have easy access to a flow bench so it's hard to quantify things that would seem to make a large difference at low level valve lifts.

 

[patprimmer]

Greater airflow, especially at low lift where the valve tends to spend more time due to acceleration to and from the seat position, but also at higher lift as the narrower seat can widen the valve throat and increase area in that region.

Reduced seat life.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[WilliamH]

Majik,

One thing you need to consider with seat width is this. A wider seat does not necessarily directly correlate to greater heat transfer across the interface. A narrow seat (relatively) may actually transfer more heat than a wider seat. This is due to the higher unit loading (contact force) on the seat by the valve. There are obviously limits here either way.

Another aspect you may need to factor in is, Ti is a relatively soft material as compared to other valve materials. Therefore, Ti is not intended to be used in 100,000 mile engines without maintenance, i.e., frequent freshen-up valve jobs.

We have used Ti valves from every major Ti valve manufacturer in the world and some are very good indeed. Others are not so good. There are also numerous coatings used on stems, tips and faces currently.

Due to the much reduced mass of Ti valves you would be well advised to reduced the spring load appropriately. This will help reduce face wear. We never run a wider seat width with Ti valves. Use Be-Cu or Trojan seat material and you will most likely have a happy combination.

Will

 

[sledneck]

Has Pat said, you'll have a better airflow. If you decrease your valve seat area, you'll get a higher pressure when your valve will sit, resulting on better contact interface between valve and valve seat. Like Will said, this result in better heat transfer and will also result in better valve sealing.

There is something I think maybee runs in the equation, tell me if I'm wrong here. Titanium can withstand much better high heat. The total heat transfert by conduction between the seat and the valve depends on the temperature difference and thermal conductivities.

If your Ti valve operates at a higher temp, it could approach the heat transfer of a traditionnal valve maybee.

Here are some numbers I found on Matweb (very good materials site)

Titanium
Ti - 8Al - 1Mo - 1V
thermal exp 9,2 microns/meter*degrees C betw20 to 250 C
thermal cond 6 Watts/meter*degrees K

Stainless 440A
thermal exp 15 microns/meter*degrees C betw20 to 250 C
thermal cond 14 Watts/meter*degrees K

Looking at those numbers, if you had the same valve seat material, to have the same heat transfert between Ti and SS valves, your Ti valve should be working with differences of temperatures around 2,33 times the temp difference with SS. I don't quite know the heat transfert needed here but with a few other numbers you could probably figure out what the temperature of the Ti valve should be operating.

Someone tell me if I'm completely out of range here!

The heat you need to extract from the valve is to avoid pre-ignition caused by valve temp at some specific location (thinnest). Right? If so, this valve temp you have to avoid depends on the gas that enters the combustion chamber, not how the valve can withstand that temperature or heat. Check that out too.

Simon

Thanks