Coach62

Please - let's keep this civilized and mature, I think it's a discussion that needs to be had.

I think Jason at Katech raised a legit concern with the SS valves having .015" bounce at higher RPM. This brings about a few questions that were never answered, unless I missed it somewhere.

Obviously GM considered it a concern since they went through the extra expense of the hollow stem valves.

My 2 questions are:

1. If the guide is really the problem, then why are we (that are going the WCCH route) switching to SS valves? Yes, I know they are stronger, but I guess the real question is, is it really necessary? Couldn't we just replace the guides and install new factory GM exhaust valves???

2. Just how serious is the valve bounce issue? Jason at Katech considered it a potentially significant issue, and I think we have to respect that opinion. What is the potential (serious) problem here? I doubt a piston to valve contact event is very likely. I know cylinder pressures would drop slightly due to the bounce, but how much would it really drop?

Again, let's please keep this discussion professional.

Mark2009

Obviously bounce can be a damaging issue to the valve and other components in direct contact with it (seat, rocker, pushrod, lifter, camshaft lobe). I suspect that damage to cam lobes and lifter rollers that are typically blamed on poor quality parts are instead sometimes caused by lifter loft and valve bounce.

Coach62

Here is the full quote:

"valves is evident in our comprehensive
fatigue tests. In normal operation where valve float is
non-existent, a valve will experience 20,000 to 25,000
lbs of applied stress. However, when valves bounce
on the seat the stress immediately soars to 40,000 to
60,000 lbs. Manley exhaust valves ( at 40,000 psi and
1400°F ) ran to 100,000,000 ( one hundred million )
cycles while competitors’ offerings failed at 100,000
cycles. That’s 1000 times better fatigue life"

Interesting. So, what are we gonna do with that info?

Also interesting that they don't say HOW MUCH bounce they are referring to.

Mark2009

We're going to try to avoid bounce since, in response to your question #2, it appears to be a serious issue.



.

'06 Quicksilver Z06

Asked, and for the most part answered in your part in bold.

Beyond that, the finding of inconsistent wall thickness in at least one random example of a stock LS7 exhaust valve, the near 8 year history of breakage reports, it would simply be better to contact one of the forum vendors with experience in their heads and cam, and cam only packages using both valves.

The practice of using solid stainless exhaust valves in the LS7 engine, goes back to late 2005. This would correspond to shortly after the C6 Z06 came out.

I can provide a link verifying this, if you wish.

There has been no report that I can find, on this forum, of a solid stemmed stainless valve, Inconel valve, or Ferrea Superalloy valve, all of which are heavier than the stock exhaust valve, having failed in here, or having lead to engine failure, due, to "valve bounce". Nearly 300 members on this forum, many of them quite vocal, and have no qualms about reporting mishaps, are among those who have selected a heavier non stock exhaust valve.

In a 2008 article in GM High Tech Performance magazine, the subject of valve bounce was discussed, and the prediction made:

http://www.gmhightechperformance.com...e/viewall.html
"It is apparent the valve is out of control at this point-so much so at 0.030-inch of bounce off the seat that failures will result if you operate in this range periodically."

Five years later, and I can find no descriptions of these type failures attributed to "valve bounce", on this forum, with as many cars in here outfitted with such valves, and which see both street and track duty.

Talk to the shop professionals, and many of them will tell you that they have not had cars coming back having failed due to "valve bounce" in their offerings and customer cars, using these valves.

Furthermore, and more importantly, ask yourself, if these professionals were in fact, having cars come back having failed from "valve bounce", then would they continue to do this procedure, when they could just as easily, and under no restriction, use the stock valves?

If valve bounce in LS7 cars, outfitted with these valves, were an issue of practical importance to the point to where cars were coming back with broken motors from it, after having these valves installed, well then why would those shops continue with the practice of using them?

Knowing all of that, then you will have to decide for yourself, just how big of a concern you think that testing indicating the presence of "valve bounce" is, from a practical standpoint.

Good luck.

Coach62

Thanks for the input Quicksilver.

I wonder how much stronger springs would help the bounce? Any thoughts on this?

Coach62

And how do you propose we do that?

propain

Avoid bounce and float, reduce mass in the valve train. Its a no brainer in the race engine world.

Simply throwing in a valve and adding a lot more mass because it stronger is short sighted at best. We are already seeing how short sighted it was as the dual spring configuration once believed to be the best has shown with actual data to not be the case. Beehive springs are now being used and people who have been using dual springs are switching to them. You won't see this vocalized around here much though.

MTPZ06

Good luck...all it takes is one to cause a

MTPZ06

There's a careful balance there, because springs too stiff can wipe out a cam lobe.

Mark2009

True, but there have been numbers bandied about by various vendors... ranging from about .015 to .025 or thereabouts, IIRC. I would think it would be safe to say lower is better, while .000 is probably unattainable throughout the entire RPM range (in which case minimizing the range in which it does occur would be the secondary goal).

Mark2009

As propain said, and use of a spring that tends to not have a resonant frequency in the desired RPM range of the engine (this generally favors beehives over dual springs, but beehives have a limited amount of available pressure vs. duals, which limits their use unless the valvetrain is fairly light).

Coach62

I get the impression that propain is a proponent of the stock valve???

propain

There is nothing wrong with the stock valve. The enemy here is the guides. The valve is simply paranoia because it breaks at the head and is viewed as weak. We have seen the full gamut of accusations. Weld points from a two piece design, sodium failing and inconsistent wall thickness. None with any evidence only guess. Solve the guide problem and the OEM valve is your best bet.

pkincy

I have no idea if it is necessary in the LS7 but when I built my LS2 402, I set the rpm limiter in the pcm to 6600 rpm although there was no question that the valve train was easily good to 7000-7200. Given the dyno curve with hp dropping off above 6600, I simply felt no need to tempt fate and go there.

Quite obviously you can build a race SBC to run 9000 if you have the right parts put together in the right way, but it appears that the LS7 as built is having trouble reliably running at the limits that GM has set. Whether it is due to side loading on the valve from the valve train or bad guides, I would think limiting rpm slightly would save a lot of peak stress on the motor.

Given that (having a GMPP est warranty) I can't reset the rev limiter in gears I do artificially limit it by short shifting somewhere slightly above 6000.

In the mid term we will have a lot more experience both with the SS valve fix as well as the Ti exhaust valves in street use. As I don't think we will ever see a 427 again, I plan on keeping mine long enough so that I can react when we do get that long term data.

Although I wouldn't hesitate to use SS valves if my warranty wasn't due to run 4 more years.

Coach62

I guess worst case scenario if that proves to be wrong you're only looking at replacing the valves again in 5 or so years. I'm thinking about going that route and just doing the wiggle test every year or so.

I assume that would be your plan??

Coach62

Anyone get a price on new stock exhaust valves from GM Parts warehouse or other place to buy them??

propain

My plan is to consistently inspect my guides for sure. I will be changing my guides (undecided at this point) and using the OEM valves with a full head job. I will also be going with a custom grind cam.

We have seen inspection with heads running bronze guides and SS valves that show guide wear already on a daily driver with 20K miles. Not a good sign. This driver is now changing his plan on his guides and I think I might follow his plan as well.


Yes if I am wrong worst case scenario I inspect and see major guide wear forcing me to refresh my heads again every 20K miles. At the rate I drive and race the car that would be about every 5 years so yes that is my plan.

Mark2009

No, it does not.

With respect to the guide issue, we are seeing garage queens with shot guides, so running at the limit is simply not a factor in the equation.

With proper machine work in the cylinder head I have no doubt that a stock LS7 would run at 7000 RPM pretty much all day long as long as temps were kept under control. I would not be trying that with a modified valvetrain, however (unless there was some testing to verify stable operation).

Mark2009

There is no need to replace the valves unless they have been damaged by worn guides. The one possible exception is that newer (post 2/2008 IIRC) OEM valves have thicker-walled stems than the earlier ones... for reasons unknown. As best anyone can determine (vs. guess), there is nothing inherently wrong with the valves or the guides.

If you monitor guide wear periodically, as soon as you see them going (say, .0035 clearance) then you know you'll need to replace them sooner or later. If it is sooner, it is quite likely no damage will have been done to the valve stems (but of course magnified inspection is only way to tell with any degree of certainty).

OEM exhaust valves are about $35 discounted, give or take. Ferrea makes what appears to be a nice semi-hollow stem version that is not terribly heavier (about 15%) but you're not going to get the sodium heat transfer ability that the GM engineers apparently thought useful.

This topic is a tough cookie to crumble since so many people in the repair industry have directly conflicting positions. There is no consensus, other than general valvetrain theory that stiff and light is good, of course as long as it holds up under the intended use.