NemesisC5

I was reading about the LS7 in the 2014 Z28 and came across this article. While reading through it I came across a couple paragraphs relating Ti dust and premature wear of valve guides. GM weighed their options and found it was "more cost effective" to correct through development of a different material rather than use expensive bronze valve guides. I know the LS7 had issues with exhaust valves and not the Ti intake valves but I find it interesting that a compromise was used because of cost concerns with bronze valve guides. All projects have budgets but find it somewhat surprising on such a high dollar R&D and hard parts budget for the LS7 program that GM decided not to use the tried and proven bronze valve guides on a project that from what I read was "little to no compromise to achieve their goals". Hindsight being 20/20 possibly many valve issues may have been avoided...maybe not. I do not own a C6 ZO6 nor do I own an LS7 (I wish I did) but thought I would post this bit of information for those that know more than myself to read and form an opinion if this is relevant or not.

http://www.camarohomepage.com/ls7/


"We had to make a change and going to a thicker-head steel valve wasn't an option," Hicks continued, "because we wanted a 7000 RPM red line. We looked at a couple of alternatives–some really kind of radical. ultra-light steel valves. I tested a few of those and they failed miserably. Then we said, 'Alright. We gotta do what ever it takes.' We went to the titanium for mass and strength because, with the titanium head you could go to a flat, almost no cup on the combustion face–make a nice, thick section through there–and not take a big mass penalty, so you end up with a lighter valve which is also strong. (The LS7 intake) was the first production application for Del West and the largest diameter titanium valve in any production engine."

Jim Hicks, who was the lead engineer for the LS7 valvetrain explained that the cam's base circle was made smaller so the effective lobe lift would increase. That along with 1.8:1 rockers resulted in the amazing (for a production cam) valve lift. Image: Author.

(Sidebar)
Making Ti Valves

The same protective, CrN coating used on the connecting rods is on the intake valves but getting that coating just right was a tough task for Jim Hicks and the engineers working on the LS7 valvetrain. One of the issues they had to address was a "Ti dust wear" problem, similar in nature to what afflicted the connecting rod development. When the CrN coating wasn't right, it would fail and highly-abrasive Ti dust would develop between the valve stem and guide causing rapid guide wear. "Developing the stem coating which was going to work well with our production-style, PM guides was another challenge. Most racers use bronze guides or some other type of aftermarket guide which is cost-prohibitive. A moly-sprayed stem, which is typically used a lot on titanium valves (for racing), is very expensive. We wanted to come up with an alternative, so we worked on a chrome-nitride, vapor deposition coating. It took a little development, but it worked out really well for us. It was more cost-effective and just as good or better than the moly spray. There's some tricks to that process and we worked with Del West in developing them. The parts need to be very, very clean. The right processing steps need to be used. The coating thickness is very important and needs to be maintained to a tight tolerance. I think Del West is using it now for some of their aftermarket parts, too."

kenw

Very interesting article, maybe GM should have ponied up for the bronze guides. Especially after the failures started.

Thank you for posting this.

erikszr1

interesting. maybe its been posted or spoken about before, I have never seen it. thank you for posting.

erick_e

The author of that article, Hib Halverson, just measured his valve guides and all of his intake guides were out of spec. Note, only three of his exhaust guides were out of spec.

http://forums.corvetteforum.com/1586...-post2255.html

'06 Quicksilver Z06

Interesting results. He list 4 of his exhaust valves as either sticking or stuck.

At just 15k miles, as mentioned all of the intake guides are shot.

The engine was built after GM says that the issue was contained.

Rock36

Perhaps the CrN coating wasn't as successful as the CrN coating on the rods as hoped in some cases.

It also lends to the notion that the valve guide wear problem is perhaps more complex than any single root cause, but rather multiple working in concert. This is common when complex systems do fail catastrophically.

Interesting to note that Katech uses the Moly coating with the bronze guides. The article mentions these we're avoided due to cost issues. This also highlights how very smart engineers can be and are constrained by factors that prevent them from executing a design in a way they may prefer.

Thanks for sharing.

wjnjr

BZ guides are expensive? Since when? They're cheaper than pm guides.

What's expensive is a forged crank, Ti rods, Ti valves w/ special coating, cnc porting, hand assembly, dry sump system, etc.

Saying Chevrolet "cheaped out" by not using bz guides doesn't make sense.

All LS motors use pm guides btw.

vertC6

Thanks for posting Scotty, hope all is well with you.

The Ti guides are also wearing due to the chromium nitride coating, Katech uses the Moly, I plan to use the DLC costing on the intake with CHE guides.

Mark2009

You presume that the bronze guide would meet warranty requirements (5yr/100K powertrain, 8yr/80K emissions). On the EPA side the mode of failure would be increased emissions/blowby due to worn valve guide, I presume.

Of course it appears that the approach GM used is not meeting those warranty requirements either, but the exact reason is still not definitively known and does not appear to affect most other LS engines using mostly similar components (so you really can't fault them on the component selection, at least based on what we currently know).

Blackonblacksls

Tha fact that it's known to effect intake side.

That some engines have issues some don't.

And that guys like lingenfelter and katech could use stock guides and valves in their motors with no issues. After a correct vj

In my opinion points to a valve job error that leads to a lack of concentricity which can and will lead to pre mature wear and or failure.

NemesisC5

The acknowledgement by GM (per the article) of preproduction premature valve guide wear and what seemed to continue without pause as a production issue indicates to me that GM never committed 100% to resolve a critical issue in a $70-100k car (dependent on options and year model). I'll refrain from further comments as I do not own a C6z so no "dog in the fight". I merely posted this article because I was rather shocked to find out this issue goes back to 2004 in the preproduction phase and was an issue throughout the C6z's entire run. Many people did not purchase new or used C6z's because of this very "ticking time bomb issue"....although it's impossible now to calculate lost sales because of this single manufacturing. In the final analysis was it cost effective to ignore or to fix?

Mark2009

Your conclusion is not supported by the evidence.

First, the development intake wear mentioned in the article is clearly attributed to a failure (breach) of the CrN coating. On intake valves removed from service with worn guides there is rarely any evidence of any breach of the coating on the stem, and in fact most used Ti intake valves are reused in the same heads they arrived in.

Second, in most cases the excessive guide wear is on the exhaust side, which is not a Ti environment.

On the intake side the rumor is that GM cheaped out and did not have sufficient polishing done on the intake valve stem to give a smoother finish to the completed product, so the coating doesn't fail, it is just too rough (again, so the rumor goes -- none of the professional cylinder head shops with extensive experience in the management of this issue seems to have measured the finish on new GM intake valves, or at least has not published it for the masses nor their customers). Of course I'm sure that no one at GM or DW would admit to any of this in public, so rumor it will have to continue to be.

Rock36

From the article and Jim Hicks the valve train engineer:

It isn't just about the bronze guide, but the Moly coat/bronze guide vs. CrN coat/PM guide combo.

wjnjr

True, and until very recently it looked like they got it right - on the heads with excessive guide wear the intakes were usually in much better shape than the exhausts. Now we are seeing more cases like Hib's where the intake guides show more wear. It would be interesting to find out if the coating wore off the valve stems in those cases or if it was still intact and only the guides wore.

But I still don't get how using a cheaper part - BZ guide vs PM guide - is 'cost prohibitive'. Maybe someone can explain that.

erick_e

I don't think they are. I think the key word in the article is "or"

Most racers use bronze guides or some other type of aftermarket guide which is cost-prohibitive

It is the other type that is cost-prohibitive

NemesisC5

I'm not an engineer so I cannot speak as such regarding properties of various coatings or their hardness / wear characteristics. One of the stigmas of the LS7, whether real or perceived is that they have valvetrain issues. There are "stickies" at the top of sections on CF dealing with common issues, there is one at the top of this section dealing with valvetrain issues. The point of my post was to make available an article written regarding LS7 development and issues GM encountered with the valvetrain. In the article a solution was suggested (bronze guides) but was not used because it was considered "cost prohibitive". A less expensive resolution was chosen that was thought to be superior and life went on. Top aftermarket companies like WCCH, TEA and Katech use bronze valve guides on the LS7 heads. In addition on their Track Attack engines I believe I remember reading that Katech uses a molybdenum coating on both Ti intake and exhaust valves which was mentioned in the article and also considered "cost prohibitive".

erick_e

I believe this has been discussed, and the lack of availability of the PM guides is reason they choose bronze guides. Lingenfelter has always been able to source the PM guides and uses them.


Katech uses a different coating so the titanium is more compatable with the bronze guides.

tim414

It seems to me anyway, that the bean counters at GM won the day. They figure probabilities of warranty claims and which ever way where they come a dollar ahead (a head lol) is the direction their gonna take.

This is historically true with all the manufacturers.

rabrooks

Educate me. I just lost an engine, 2800 miles. GM says it was due to piston rod coating failure. I did a search and it brought me to this thread.
What is the CrN coating. Why is it used, why does it fail and cause oil pressure loss.
Thanks

2k Cobra

Ti against Ti, Connecting rod against connecting rod, doesn't work. High wear issues leading to failure. The CrN Coating is used to prevent this. If the rods wear through this coating, things will fail. Haven't seen this happen very often.

While I had my C6 Z, i got high levels of Ti in the oil. (a symptom of the con rod wear) This combined with head replacement caused me to trade the car.