How to Maintain Valvetrain Stability - LS7 Exhaust Valves

Tech - A Weighty Decision

Barry Kluczyk Mar 31, 2014 0 Comment(s)
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Setup and Test Components

Katech has a Spintron valvetrain test cell that's set up specifically for such stability evaluations. It features a DC electric motor that drives a "spin buck,"which is essentially a test engine with a window cut into a cylinder. There is a dummy crankshaft (no rod journals or bob-weights) and camshaft in the engine, but no pistons or rods. And, of course, there's a complete valvetrain for just one cylinder.

The setup aims a laser at the bottom of a valve, which enables engineers to watch in real time at the control panel how it deflects under load, lofts (floats), and bounces, helping them determine the relative stability of the valvetrain. Katech's equipment measures valve lift in 0.0005-inch increments at 1 crank degree resolution up to 12,000 rpm. With such precise measurement, you can see things you might not expect. The closing event is not the only place you can see a valve or lifter bounce. Sometimes lifters can actually bounce up the camshaft lobe during valve opening.

For the record, the test used a stock LS7 camshaft and valvetrain as the baseline and featured tests with Katech's higher-lift Torquer camshaft and PSI beehive springs, as well as dual springs. When it came to the exhaust valves, the stock ones were compared with heavier single-piece, stainless valves from Racing Engine Valves (REV) and Katech's own lightweight titanium valves with molybdenum-coated stems.

The stock LS7 exhaust valve weighs 74 grams, while the REV exhaust valve weighs 98 grams—a significant 33-percent increase. The titanium valve used in the test came in at a featherweight 66 grams.

It should be noted that while lighter than the stock exhaust valves, titanium valves should not be swapped directly without additional cylinder-head preparation aimed at preventing stem and/or head wear. Some race shops insist on having them micro- polished and coated with a chromium-nitride or similar material. Katech recommends molybdenum-coated stems on titanium valves, and only when used with bronzed valve guides in the heads.

Katech Test Cell 2/16

06 Katech’s test cell is fitted with a specially equipped LS7 “spin buck,” which is used with a laser to gauge valve bounce across the rpm band. Less bounce means greater stability.

Ls7 Cylinder 3/16

07 The sacrificial LS7 engine was relieved of a cylinder wall in order to insert the laser gauge. It goes in the cylinder and is aimed up at the valve faces.

Ls7 Shop 4/16

08 Complementing the high-tech laser gauge tool is the decidedly low-tech shop vac, which is used to draw a vacuum on the test cylinder, simulating engine operation.

Ls7 Timing 5/16

09 The spin buck is equipped with a crankshaft and camshaft. They’re spun together via the standard timing chain, but there are no rods or pistons in the engine.

Ls7 White Valve 6/16

10 The combustion chamber and valve faces are painted white on the test head for more-accurate readings with the laser, which measures the movement of each valve as it opens and closes.

Stock Ls7 7/16

11 The test measured the performance of the stock LS7 valve (74 grams), a REV stainless valve (98 grams) and a titanium valve (66 grams). The heavier stainless valve doesn’t have the separation issue that has occurred with some factory sodium-filled units, but with about a 33 percent increase in weight, it is less stable at high rpm.

Katech Beehive Spring 8/16

12 Katech’s test also measured the stability of the valves with the stock beehive springs, higher-rate PSI beehive springs, and dual-coil springs. The beehive design proved to be the most stable at high rpm.

Ls7 Valve Bounce 9/16

13 The bounce of each valve on the valve seat was measured and compared with the baseline test.

Ls7 Valvetrain 10/16

14 Bottom line: When it comes to high-rpm valvetrain stability, the stock LS7 exhaust valve performs best with beehive springs. Builders worried about dropping the stock valve at high rpm can have the head fitted with bronze valve guides, which should do the trick. The titanium valve performed well, too, but it’s a costly alternative that also requires bronze guides.

Results and Recommendations

To no one's surprise at Katech, changing to solid-stem stainless valves proved to have a negative effect on the LS7's valvetrain stability. Interestingly, while the test was focused primarily on the exhaust valve, the effect on the intake valve was also noteworthy.

"Consideration for when a valve is out of control is subject to debate; however, 0.015-inch of bounce is the basis we use for our recommendations,"says Harding. "Keep in mind that we only tested for limiting speeds and did not perform durability testing. If this was a professional racing program, we would go back to the tests we want and run 24-100 hours of durability on the configuration to see what breaks, if anything.”

Here's how seven combinations performed, along with Katech's test notes:

TEST 1: Stock cam and springs, with stock exhaust valve
Intake valve: 0.006-in. max bounce at 7,000 rpm; tested to 7,500 rpm
Exhaust valve: 0.008-in. max bounce at 7,100 rpm; tested up to 7,500 rpm

TEST 2: Stock cam and springs, with REV solid exhaust valve
Intake valve: 0.006-in. max bounce at 7,000 rpm; tested to 7,500 rpm
Exhaust valve: 0.015-in. max bounce at 7,100 rpm; tested up to 7,500 rpm
Katech notes: Nearly twice the bounce by simply changing the valve. This combination is fine for a street car, if the rev limiter is set at 7,000 rpm. In road-racing conditions, we prefer to see a 500-800-rpm safety margin, which would mean a 6,300-6,600-rpm rev limit.

TEST 3: Stock cam and dual-coil springs, with REV solid exhaust valve
Intake valve: 0.014-in. max bounce at 7,300 rpm; tested to 7,500 rpm
Exhaust valve: 0.016-in. max bounce at 7,200 rpm; tested up to 7,500 rpm
Katech notes: Twice the bounce overall of the stock configuration, but within the realm of acceptability. This combination is fine for a street car with the rev limiter set at 7,000 rpm. In road-racing conditions, it would be a 6,400-6,700-rpm rev limit.

TEST 4: Katech Torquer cam and PSI beehive springs, with stock exhaust valve
Intake valve: Not tested
Exhaust valve: 0.015-in. max bounce at 7,700 rpm; tested up to 7,700 rpm
Katech notes: This is a strong combination—proven and stable. Fuel cut-off should be 7,100 rpm with it.

TEST 5: Katech Torquer cam and PSI beehive springs, with REV solid exhaust valve
Intake valve: Not tested
Exhaust valve: 0.016-in. max bounce at 7,100 rpm; tested up to 7,500 rpm
Katech notes: This combination is not recommended, because valve bounce exceed the 0.15-inch threshold, although the PSI spring did control the REV valve better than did the dual spring.

TEST 6: Katech Torquer cam and dual-coil springs, with REV solid exhaust valve
Intake valve: 0.018-in. max bounce at 7,100 rpm; tested to 7,100 rpm
Exhaust valve: 0.029-in. max bounce at 7,100 rpm; tested up to 7,100 rpm
Katech notes: Talk about instability! The severe bounce displayed at comparatively low rpm made it unwise to continue testing up to 7,500 rpm. We flat-out don't recommend this setup.

TEST 7: Katech Torquer cam and PSI beehive springs, with titanium exhaust valve
Intake valve: Not tested
Exhaust valve: 0.014-in. max bounce at 7,900 rpm; tested up to 7,900 rpm
Katech notes: Minimal bounce at 7,900 rpm! This is an excellent combination all the way to 7,800 rpm—as long as the prep work has been done to the valve stems and bronze guides are used in the head.

In summation, the factory configuration unsurprisingly delivered the most stable performance, with the heavier, solid stainless valve matched with a dual-coil spring delivering the greatest instability. There's excellent high-rpm performance with the titanium valve, but it's a decidedly pricy endeavor.

"Since we are able to get a stable valvetrain with the OEM valve and have demonstrated it to be durable when using our completely tested systems, we don't find it necessary to change the exhaust valve,"says Harding.

And what about the fear of dropping one of those OEM valves? Harding suggests that while it's certainly a concern, it shouldn't be an overriding factor when considering the valvetrain – even if it involves a "bigger"camshaft.

"We believe using the titanium exhaust valve delivers the best performance when used with bronze guides,"he says. "If cost is a factor, OEM valves are proven to be durable when combined with a valvetrain system that has been validated.”

And those bronze valve guides help, too.

Sources

Katech Inc.
Clinton Township, MI 48035
866-528-3241
www.katechengines.com

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