Inconel for exhaust valves

[Henry Schmidt]

We are considering having Porsche exhaust valves made. 625 is looking like a great option for cost and durability.
I would like to investigate using Inconel in depth. Is there a consensus about the best Inconel for this application?

[jac1976]

Would this be better than the X-750, or the 625 easier to work with. Full disclosure, I have no idea, but was just reading up on Iconel.

[dannobee]

Usually Inconel 751 is used for exhaust valves. I've used 625 for header tubing. Dunno how it would work when used in exhaust valves.

There's another alloy used for boosted offshore boat engines when the engine load gets really high. XH-432 IIRC. Also used in Top Fuel exhaust valves.

[draw]

I'm not an engineer nor do I have any practical experience with Inconel, but...

Inconel 625 was designed more for its corrosion resistance and 751 was designed specifically for exhaust valves. If you used 625 for exhaust valves, they might survive heat and corrosion, but could eventually suffer thermal fatigue cracks, valve face recession, or creep deformation sooner than a 751 valve.

MatWeb has datasheets for each...

Special Metals INCONEL® Alloy 625

Special Metals INCONEL® Alloy 751

[Henry Schmidt]

It looks like you guys are correct. 625 should work but 751 is better in every aspect except price.
Thanks for the help.
Now, design the valve, pick the sizing and place the order.
The next challenge is a similar process for titanium intake valves.
I hope to have valve sets available for the community.

[dannobee]

I remember Saab Turbos and Ferrari using Nimonic 80a in their factory builds. And some even more exotic engines using Nimonic 115, but I have no direct experience with 115. Just two more data points when inquiring to the valve maker(s).

[Jeff Alton]

Henry, would your valves have a smaller diameter stem on them?

Cheers

[Henry Schmidt]

Originally Posted by Jeff Alton

Henry, would your valves have a smaller diameter stem on them?

Cheers

The plan is to make them 8mm and sell them with custom guides.
Collecting bids as I write this.
We're also looking at redesigning the intake valves more in line with a LS valve design.
Open up the area under the head to improve flow and reduce weight. Of course retaining 3 groove keepers.

[Jeff Alton]

Perfect

Cheers

[Leoo]

Inconel 625 is a strong, corrosion-resistant, and cost-effective option widely used for exhaust valves, especially in OEM and performance road cars. Inconel 751, though more expensive and harder to machine, offers superior fatigue and thermal cycle resistance, making it ideal for high-performance or racing engines.

[Speedy Squirrel]

I found this info on Nimonic 80a, which is what the aircooled turbo engines used. They were also sodium filled on the exhaust side, and later on both sides.

https://www.specialmetals.com/documents/technical-bulletins/nimonic-alloy-80a.pdf

[PeteKz]

Curious: Why are you looking at Inconel exhaust valves? Are the ones you are using, or the sodium turbo valves, not holding up in some engines?

[Henry Schmidt]

Originally Posted by PeteKz

Curious: Why are you looking at Inconel exhaust valves? Are the ones you are using, or the sodium turbo valves, not holding up in some engines?

We're always looking for simple ways to improve the build process on these old motors.
Our Inconel exhaust design should improve flow and reduce weight allowing for lighter valve spring even at higher RPMs. The same with the stainless intakes. The challenge is accomplishing these goals at a cost effective price. The other benefit with the Inconel is longevity. The hope is that an Inconel will represent a 2 or 3x return on life expectancy.