All,
Those who have been following my second season of trying my hand at "amateur" racing with the SCCA know that I had a timing chain failure at Mid-America in Omaha during my second race of the year.

I finally have my pictures back from the resulting engine rebuild.

Here are my lessons learned which may help prevent this from happening to some of you in the future.

Probable Cause: Failure of a timing chain link due to high RPM use:
- I had my PCM modified prior to last year's Runoffs to save two 3-4, 4-3 shifts at Mid-Ohio. This was probably worth a couple of tenths which was worth a few spots on the grid.
- While I won't say exactly where I had my PCM set, I will say it was north of 7000.
- During the race I was making a 3-4, 4-3 on the pit straight.
- The motor was a new 03 crate motor installed last July. It had 10 hours of break-in and was on it's sixth SCCA race weekend when it failed.
- I did not use RPMs over 7000 at the three races before the runoffs, or at Memphis, the first race of this year.
- I was using 7000+ at Mid-America in what I think is turn 4, a high speed left where the 3-4 shift point is at the apex.

I believe that at high RPMs the timing chain either oscillates back and forth, or doesn't release as it comes off the sprocket and has to curl back to make it to the other sprocket, resulting in higher than normal stress on the links.

Result:
- Timing chain breaks, valves stop, pistons don't, pistons hit valves, valves bend or break, pistons weren't damaged (somehow)

Suggested Corrections:
- I've read that the 04 LS6 comes with a heavy duty timing chain from the LS2. Part number is 88958608. The dealer's computer doesn't yet show this as an LS6 part. I can't confirm this is the LS2 timing chain. Several different sources told me this chain is on the 04 LS6 motors which makes it legal for SCCA T1, so I bought one. It fits on the motor.
- On visual inspection, this chain doesn't appear to be any stronger than the old one, but the links are thicker (not much though).
- If you are running your PCM at over 7000, beware. The chain apparently can't sustain these kinds of RPMs indefinitely.
- GM Performance Parts sells a damper that bolts to the block in the area between the timing chain and the sprockets that limits the chains ability to whip or curl. I bought one to look at it. It is a little piece of plastic shaped sort of like a W that prevents the chain from curling inwards. It bolts straight to two pre-existing holes on the block.
- It appears GM is aware of the timing chain issue and has put a stronger chain in the LS2 and made a damper for high performance applications.
- If you take off your front cover for any reason, cam work, etc, there is no reason not to put this damper and a new LS2 chain on.

Cost of engine rebuild:
$400 - Two trips to Norfolk and back
$ 75 - Timing chain
$400 - gaskets and stretch bolts
$314 - six new valves

All in all not bad. But the DNF and the two missed races likely cost me the SCCA Mid-West Division T1 title to Mike McGinley, an SCCA veteran and 2003 SCCA Mid-Am champion in CSR and ITE classes who joined the T1 ranks in my division this year.

GM PART # 88958608
CATEGORY: Belt or Chain, Engine Timing
PACK QTY: 1 CORE CHARGE: $0.00
GM LIST: $107.10
OUR PRICE: $74.97
DESCRIPTION: CHAIN

From the 2004 GM Performance Parts Catalog: page 106:
88958607 Timing Chain Damper, LS1, LS6
“This timing chain damper is intended for high engine speed applications and is used to improve timing chain life. Some blocks require drilling and tapping- an instruction sheet is included. “

From GM Parts Direct:
GM PART # 88958607
CATEGORY: Belt or Chain, Engine Timing
DESCRIPTION: DAMPENER
GM LIST: $117.30
OUR PRICE: $82.11

Timing chain showing failed link and tooth missing from sprocket:


Result of piston/valve interaction: six dead titanium valves:


Rebuilt motor with new valves, timing chain & sprockets, gaskets, stretch bolts:


Connecting all the lines, fittings, etc around the motor before raising it into final position and bolting the lower suspension cradle to the frame:


The Garagemahal where rebuild was done:


A happier but still dirty racecar:



[Modified by Subdriver, 12:46 PM 6/20/2004]

 

That's a sweet car. I hope your luck is much better with the new parts. BTW, I've read that David Farmer puts a limit of 6,900rpm on the engine. He states that the stock bottom end is good to that point but no more. I guess you are on thin ice above that. Just an FYI.

 

I just viewed all the videos on the site you put the link too. You are one Heck of a driver! Nice work!

 

Sorry to hear about your timing chain woes. I think the damper you're refering to is wused on the C5R shortblock pictured below, no?

Best of luck on your remaining races.

 

Thats simply amazing that you could do that type of damage to the valves and not damage the pistons

 

If SCCA allows it dump the chain and put a dogbone in, no more chain worries at all.

 

Excellent info. GM did install the guides on '04 LS1/LS6 engines. The stock chain is a joke. A double roller chain is a better solution. I agree, the stock LS is not engineered to run beyond 6500 RPM reliably.

 

Did that motor come out the top or bottom?

 

Great info, sorry you had to pass it along to us "the hard way". Good luck with your campaign.

 

It may not be the same exact part, but it looks like the yellowish piece of plastic sitting inside the timing chain, near the bottom. If that one is not the same, it sure looks like it serves the same purpose.

 

I think so to. I actually bought pistons which I'll use in my next engine rebuild, which may be soon. I'm trying to figure out what to do for an Runoffs motor.

 

No joy. Stock class, stock engine and drivetrain. The idea is to limit costs. Sometimes it doesn't work out that way. Some more expensive parts will save costs later.

 

Top. His garagemahal doesn't have lift. We've changed the engine out the top twice now. Isn't easy, but doable.

 

Ouch!! sorry to read of this happening

 

Thanks. I don't feel so bad about it now that it is fixed, with two subsequent races, and the car is safely back in my garage being prepped for the next (current project is new rear wheel bearings).
But it was depressing the month it was in Norfolk. Especially as I read the posts from the other T1 racers on this and other boards.

 

I had the same thing happen to my motor at Road America a few weeks ago with very little use over 6500 rpm. It was a 383 LS1 with ported and polished LS1 heads and a medium cam. A little under 400hp at the wheels (pretty good for 2001 when the motor was built). Chain broke at about 5000 rpm. Bent all of the valves and most of the push rods to various degrees. Some of the pushrods were S shaped. Cam and lifters survived. Forged JE pistons had some minor nicks but were cleaned up without even removing them from the motor. Bottom end was fine.

I looked at two chains. There is the stock chain and the GM performance parts chain. I was unaware of this new chain. The stock chain was about $25 and looked bigger than the old chain (unsure of source) and the Performance Parts chain. The performance parts chain had black links that were thinner, but hopefully of a stronger material. The performance part was also about $85. The dampner will be going on my motor soon.

Steve

 

Steve,
Good luck. Hope you don't have to go through this again. Glad to hear your pistons survived as well.

 

Better picture of the dampener:
I odered one today.




[Modified by see5, 6:00 PM 6/28/2004]

 

I dont understand why you didnt install a double roller from the beginning. Or are you limited to staying with stock parts on the engine?

VR

 

SCCA T1 rules require stock drivetrain. Only real mods allowed are PCM reprogramming and cutting out the mufflers. Other than that, the engine, clutch, tranny and diff are all 100% stock.

See5, thanks for the pic of the dampner. You can clearly see how it would limit oscillations of the chain.