mark kibort

As i posted after our weekend with Speed GT, the ole Holbert car's timing belt skipped 3 teeth and rendered the car undrivable during the 3rd lap of qualifying. However, due to my efficient crew (freinds at the track) we were able to diagnos the issue and move the passenger side cam pulley back to the right position and low and behold, it ran and ran like it did before the near redline shift and then sputter in 4th gear when it skipped the pulley teeth.

some blamed the missing cam sprocket teeth on the cause, claiming we must have lost another, and it got stuck in the mechanism, causing a quick load on the cam belt, forcing a 3 teeth skip. Others think it could be a tensioner that lost oil and damping and just allowed some slack to happen and then it skipped.

Of course, it is possible that the cam belt had been weaken from 3 full seasons of racing (original belt from the Holbert car rebirth).

so, my question is, has anyone ever seen a cam belt slip 3 teeth???? usually, we all know what happens when the belt skips a tooth, (one tooth and then the rest!) it takes them all out and you loose a bunch of valves!!

At the track, Chris Young from CT and Bruce matesso (BMW racer) both helped to put the cam pulley back, and tensioned it slightly more than spec. it obviously worked, as it lasted for the speedGT race, the drive home and some spirited testing ever since. Im inclined to put a new belt on it, since it is time anyway, even though it looks fine.

any thoughts or guesses appreciated!

MK

sharkmeister85

Hi Mark!
Maybe I missed a thread somewhere, so this might be a stupid question, but why are you racing with a cam sprocket that is missing some teeth?
Glenn

Jim bailey - 928 International

Mark I think you have been incredibly lucky that you have not bent every valve in your engine. You really should think about replacing the cams ,the chains and the T-belt . At least look at the cams and count the missing teeth and replace the belt.

heinrich

Agreed that is insane. Dodged several bullets so far Mark On skipping teeth yes because the HTD belt and gears are smoother, I think it can happen and not cause catastrophe, but had it been my car I would be thinking hard about why this happened and when the inevitable CRASH!!!! will ensue.\

All the best, good work on-track :-)

mark kibort

I know you guys think im crazy, but if you look at the cam sprocket teeth, you can see that there are 20 teeth. Im missing one on each sprocket of each cam on the left side of the engine, and one tooth total on the right side. because the teeth have fallen out randomly, i think its more like a metalurgy issue, and the strongest have survived . if the teeth missing were an issue, that issue would be on the adjacent teeth. there is only one tooth missing on any given sprocket. the load on the adjecent teeth is 10-15% more and didnt seem to hurt the adjacent teeth over an entire race season with lots of street driving too.

without any additional teeth missing this entire season, which included a redline frenzy at Road America. (easily the longest durations at 6000rpm + that the car has ever seen), i dont think there is a great chance that the teeth could be a problem, but there certainly is a chance.

the skipping of pulley teeth on the belt, however, is something different. definitely going to change the belt but still wondering how it could have skipped. may its due to the softness of the rounded teeth at its age (not miles, but wear and heat during 3 full race seasons.)

maybe im a little bit in denial, but if the sprockets failed , im sure the one side of the engine could have some bent valves. its a big job, but as it stands now, its a big job to buy newer GT cams and put them in as well. if it "goes" then its new cams and pull the heads for some new valves too. tough bet right now in this stage of the game.

thanks for the thougths! any more??

what did you think of that picture? 928 is a pretty car isnt it!! not bad for 20 years old !!

Mk

heinrich

Mark it's one of the prettiest racecars I've ever seen.

mark kibort

still looks like this

goliver

Way to dodge a bullet man. WOW! Change the belt at least and the cam sprockets if they are missing teeth. Cool pic from the race too.

Regards,

Kaz

Wow. Mark meet fire. Fire, this is Mark. Ya'll play nice now ya hear.

Big Dave

Uh, well, I need to buy some of that good karma you have goin' on there.


Allow me to motivate you to fix it soon:

mark kibort

wow, thats a lot of shreaded belt. teeth look still attached. kind of hard to see.

heinrich

there's motivation for sure ...

Garth S

Mark,
For most who have reported a TB incident, more often it happens under relatively conservative conditions - and valves still get bent. The odds are in a track beast, things are more likely to go wrong at serious RPMs - and then, nasty things happen in a rush!
If your belt has slipped a tooth or so, I'd absolutely toss the belt, and examine every cog plus the tension roller before putting it back together and adding tensioner oil.
If you are interrested in a TB (manufacturer) that has yet to exhibit any loss of tension ( albeit in less stressful use) over two seasons, I'll PM it ( some get offended if parts do not have a 'Porsche' green stamp) .

heinrich

continental. i had a fderal mogul belt and it held tension perfectly though it was involved in my recent worn crank gear forward-walking belt unenjoyment, where i narrowly dodged a bullet too as the belt was being machined much like big dave's, from the front. So now I have a Conti on there.

mark kibort

conti is the "preferred belt"?

why was the crank gear worn, and that was the reason for the belt walking forward for shreading?

fderal mogul belt? is this bad?

thanks for the comments all!!

Mk