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Don't overrev your motors!
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924RACR  



Joined: 29 Jul 2001
Posts: 8094
Location: Royal Oak, MI, USA

PostPosted: Tue Aug 19, 2008 11:26 pm    Post subject: Don't overrev your motors! Reply with quote

Just pulled the head apart from the racecar after having a valve stuck open; thought in fact that we had a bent valve... but after removing the head, the valve didn't look bent, and a light tap with a mallet popped it right back up in place. Then we removed the cam, and found that the lifter was split right down the side (by the adjuster hole) - clearly allowing it to expand and stick in the bore!

On removing all lifters, we found that they were all split at the adjuster holes, and are all junk/trophies. We also found a couple of cam lobes damaged, one with a bit of material flaked off, one with significant rounding.

So, running hard past redline and overrevs on downshifts are NOT good things for the 924 motor, and especially hitting 7300rpm!!!
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Vaughan Scott
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924turbo_sout_africa  



Joined: 29 Oct 2007
Posts: 215
Location: Sout africa

PostPosted: Tue Aug 19, 2008 11:42 pm    Post subject: Reply with quote

i did not even think you can rev these baby's that high...damn and she didn't blow ...good builder
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924RACR  



Joined: 29 Jul 2001
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Location: Royal Oak, MI, USA

PostPosted: Wed Aug 20, 2008 12:11 am    Post subject: Reply with quote

Yep!

All new valves and springs doesn't hurt...
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Vaughan Scott
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Rasta Monsta  



Joined: 12 Jul 2006
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Location: PacNW

PostPosted: Wed Aug 20, 2008 1:52 am    Post subject: Reply with quote

There is evidence of that on a 931 head I was just tearing down. . .cracks around the adjusting screw holes. Another disturbing thing was the fact that the screws "rattle around" in the holes, doubtlessly hammering the holes and contributing to the cracks. And as a final bonus, the flat parts of the screws that ride on the valve stems were also hammered out of shape.

Crappy design. . .Dan's Piper approach is looking better and better all the time.

Vaughan, wanna buy a NIB set of stock lifters?
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924RACR  



Joined: 29 Jul 2001
Posts: 8094
Location: Royal Oak, MI, USA

PostPosted: Wed Aug 20, 2008 2:12 am    Post subject: Reply with quote

Yours are 931 though, right?

Would love to buy them, but the funds pool is cached right now after having my minivan worked over at a shop...
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RC  



Joined: 25 Mar 2007
Posts: 2624
Location: Australia

PostPosted: Wed Aug 20, 2008 3:25 am    Post subject: Reply with quote

Hmmm not good

Interested in how you determined the cause to be occasional revving >7K?
Do you have any idea as to how much seated spring pressure you`re running?
Some pics would be good, if only as a warning to other rev heads.

Had this problem only on one lifter previously and was an expensive one to repair as it chewed out the bore in the head. This is what came out, along with a severely deformed screw:




Assume you figure its valve float/bounce. Did the valve jam open because the lifter was holding it down or was it scored and caught in the guide? Sure it wasn`t lubrication, overheating, foreign material or just old used springs and reground valves/seats, reducing the effective spring pressure?

Sure others would like to get to the bottom of this too since these engines DO rev to 7300.
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ideola  



Joined: 01 Oct 2004
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Location: Woodstock IL

PostPosted: Wed Aug 20, 2008 4:10 am    Post subject: Reply with quote

I was under the impression that the lifters and cam were identical on NA and 931 heads...

...if that turns out to be the case, Vaughan, I have a spare cam and set of lifters if you need them for this weekend. It's the set that came out of my UWB project that will be replaced by the Integral cam and the Piper lifters.
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924RACR  



Joined: 29 Jul 2001
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Location: Royal Oak, MI, USA

PostPosted: Wed Aug 20, 2008 5:53 am    Post subject: Reply with quote

Thanks for the offer, Dan, but we do have enough spares amongst the many torn-down heads that I'm all set now.

Did not measure seated spring pressure. Springs were new 4 seasons ago, not old. Normally revs are 6000, maybe 6500 tops. My failure was similar to the one shown above, only we caught it before it came to pieces. But the crack is very clearly dead-nuts aligned with the screw holes. Shims, while more of a PITA by far to set up, would be more robust it would seem!

The valve only jammed because of the lifter; once the lifter was removed, it operated normally, without issue. Lubrication and heating are fine on this engine, beyond reproach, no contamination, etc. Seats and guides were new too, BTW, when the head was built a few seasons ago. Still seals perfectly, great compression when the valves are shut.

Yes, I would have no fear revving the engine higher; these lifters and the retainers are the only remaining old/original items in the drivetrain, 'cept the cam of course. That's why I posted, to share the knowledge.
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ideola  



Joined: 01 Oct 2004
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PostPosted: Wed Aug 20, 2008 6:11 am    Post subject: Reply with quote

So all things considered, I guess the outcome wasn't as bad as it could've been, eh? Did the head itself survive OK?
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RC  



Joined: 25 Mar 2007
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Location: Australia

PostPosted: Wed Aug 20, 2008 12:16 pm    Post subject: Reply with quote

Thanks for sharing this with us Vaughan.
Luckily you`re on the ball and caught it before more damage was done.

In my case it started as an intermittent lifter noise and developed into a persistent tap,tap,tap that was getting more severe. Checked the valve clearances that were all reasonable. Realize now that at this stage was probably only the crack progressively getting worse. A few hundred miles later the engine stopped abruptly when the lifter crapped itself completely and stripped all the teeth off the belt.

An expensive lesson learnt. Should have pulled the cam and checked it thoroughly.

IIRC seat spring pressure was around 65 Lbs with new springs shimmed to spec installed length while old springs averaged 58 or so. The pair that ran on the dead lifter was only 37 I think. Definitely revved higher and faster with new springs.

Best of luck getting it back together and racing.
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924RACR  



Joined: 29 Jul 2001
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Location: Royal Oak, MI, USA

PostPosted: Wed Aug 20, 2008 11:37 pm    Post subject: Reply with quote

Yep, head was fine, set up a fresh good set of lifters last night, head back on, torqued, intake in place, just the remaining hookups and installing the new header, and we're ready to rock! She'll be out Friday, no worries...
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Raceboy  



Joined: 01 Mar 2004
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Location: Estonia, Europe

PostPosted: Thu Aug 21, 2008 12:47 am    Post subject: Reply with quote

We tried to rev 924 motor back in 1998 and we wanted to see how high it goes. When hittin 8300, goes and and lifter is in two halves. It's clearly valve float, stiffer springs and lighter lifters (Piper) will cure that.

But, 7000 rpm should cause no problems if valvetrain is in decent stock condition.
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PORSCHEV  



Joined: 02 Nov 2002
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PostPosted: Thu Aug 21, 2008 10:57 am    Post subject: Reply with quote

I go 6500 on every lap...but onoy for a few seconds. That is a good safe limit for me.
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924RACR  



Joined: 29 Jul 2001
Posts: 8094
Location: Royal Oak, MI, USA

PostPosted: Thu Aug 21, 2008 11:04 pm    Post subject: Reply with quote

Thanks for the info, Raceboy. Yes, it is known that the lifters were old, none perfect, and the cam did have 2-3 weak lobes (with wear on the tip of the lobe).

6500 should definitely be no concern, with fresh valves and springs and good lifters etc. My problem wasn't so much hitting that on accel (had it set as my shift point, but was probably hitting 6600 - next time I'll set the shift point at 6400 to offset the delay in the driver). Rather, it was overrevving on downshift - 4->3 downshift in a braking zone, heel-and-toeing, from 110mph. I was dropping it into 3rd at too high a speed, like starting the shift at up to 100mph, that was causing a mechanical overrev. An overrev like this will not be helped by a rev limiter, BTW.

Note that I have a data acquisition system, which tells me in exact detail all of these facts... very nice to have!

There were other spots on the track where I spent a bit of time at 6500-6700, but I don't think that was as bad...

EDIT: Almost forgot. Car's back together now, running great. All ready to go racing tomorrow!
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ideola  



Joined: 01 Oct 2004
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Location: Woodstock IL

PostPosted: Thu Aug 21, 2008 11:58 pm    Post subject: Reply with quote

924RACR wrote:
Car's back together now, running great. All ready to go racing tomorrow!

Fantastic! Good luck this weekend! I'd come out for sure this weekend, but I have some performances at the State Fair over the next four days, so once again, won't be able to come out Just too darn much stuff going on!!!

Anyway, good luck, bring home that championship!
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