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Alan L's Avatar
 
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Rocker arm bushing to rocker arm shaft; Clearance 0.016 -0.035mm. This is new tolerance - not wear tolerance.
Valve/guide tolerance; Ex 0.050-0.077mm In 0.030-0.057mm
Check the spray bar on #3. Any sign of wear on #3 lobes?
Alan

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83 SC, 82 930 (track) - Stock except for RarlyL8 race headers, RarlyL8 Zork, K27-7006, 22/28 T bars, 007 Fuel head, short 3&4 gears, NGK AFR, Greddy EBC (on the slippery slope), Wevo engine mounts, ERP rear camber adjust and mono balls, Tarret front monoball camber adjust, Elgin cams, 38mm ported heads, 964 IC. 380rwhp @ 0.8bar Apart from above, bone stock:-)
Old 02-05-2020, 01:37 PM
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I can share that if you are hitting the Rev limiter much at all, it will break rockers when on full boost, which inevitably you are at that point.

We broke many on a 3.6 Turbo engine and a 3.4 Turbo (both racecars)and it is related to the shockwaves in the cylinder that happen with the additional pressure brought on by the boost vs an NA engine. Both engines had Assco springs set up correctly, which is minimum .050" clearance after Max valve lift.

We surmised that it was breaking the exhaust first as the motor would go a little soft, then it would quickly break the intake after if you stayed in it a couple seconds, and then one could really feel the power fall off. If one caught it right away it would just be the exhaust rockers.
We "fixed" this by raising the Rev limiter where it could never get touched, say 500 RPM beyond the normal Aim, and drivers just need to pay attention.

Kevin
GAS Motorsport
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Old 02-05-2020, 02:28 PM
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My 930 only goes on the track and I haven't broken a rocker. Not doubting your experience Kevin - it is what it is. But this car is breaking them on hway driving. And same cylinder twice. So this is pointing to a misfit or misassembly or an oiling issue on #3?
Alan
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83 SC, 82 930 (track) - Stock except for RarlyL8 race headers, RarlyL8 Zork, K27-7006, 22/28 T bars, 007 Fuel head, short 3&4 gears, NGK AFR, Greddy EBC (on the slippery slope), Wevo engine mounts, ERP rear camber adjust and mono balls, Tarret front monoball camber adjust, Elgin cams, 38mm ported heads, 964 IC. 380rwhp @ 0.8bar Apart from above, bone stock:-)
Old 02-05-2020, 03:06 PM
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Moregas - that could definitely have some relevance, the 2 times it has snapped the rockers since rebuild are when a 1st to 2nd or 2nd to 3rd has been performed whilst just kissing the RPM limit. Will the forged Pauter rockers help with this or would you recommend raising the RPM limit? I'm considering fitting an MSD 6AL-2 or something similar so that I can set my own RPM limit and possibly a soft cut too. When you do touch the RPM limit and change gear there is always a huge bang and flames (I thought this was the CIS still injecting fuel with no spark and it igniting in the exhaust) but it would make complete sense that the combustion chamber is experiencing increased pressure. The only thing I can't understand is why only cylinder 3, it could be a combination of the suggestions. The head is being sent away so lets hope it shows a tight valve guide or something to suggest why only cylinder 3.
Old 02-05-2020, 11:32 PM
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What is your RPM Limit set at?

Rahl
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Old 02-06-2020, 12:40 PM
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The RPM limit happens at 6,500 RPM
Old 02-06-2020, 11:28 PM
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I would not be surprised when you tear the motor down and measure everything you may have a lot of other bent PCs and it’s just number three broke… I hope not for your sake but you are pushing this to the limit and these things do happen… Best of luck… Keep telling us what you’re finding and what you do...
We all can learn from this…
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Old 02-07-2020, 05:12 AM
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You need to measure your piston to valve clearance axially and radially (if the are reliefs machined into the piston for the valve). Axial clearance is properly checked with the cams installed and timed with checking springs in the cylinder head and a dial indicator on the tip of the valve stem. I use clay to check the radial clearance of the valve to the relief in the piston (if there is one or after one has been machined if need be.)

What ever the issue turns out to be causing your broken rocker. Without doing the proper checks you're wasting time and $$
Old 02-07-2020, 07:09 AM
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My Motec ECU has redline set at 7,000. I think it cuts ignition first and then fuel 100 rpm later. I'll still occasionally hit 7,100 to 7,200 in first gear.
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Old 02-07-2020, 07:14 AM
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Here is an example and I hope this helps.

This is the data I recorded from a recent 930 build that is running sc cams, stock p&c and heavily modified cylinder heads with a much larger intake valve.

If you notice the minimum clearance with the cams timed at 1.6 was .2mm atdc.

The piston was relief cut and this check was repeated to make sure sufficient clearance was obtained both axially and with clay to check the radial clearance between the valve and the relief in the piston.

Pelican keeps flipping the picture.....

Old 02-07-2020, 07:16 AM
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I spoke with a very experienced Porsche race mechanic this morning about your issue. He said you should borescope that cylinder and look for a hint of valve/piston interference, which could be caused by two things: either that head was decked too much, or the valve guide is pressed too far into the head. He notes that on a good Porsche engine build, the valve to piston clearance might be .4mm or less, and that a small tolerance error in head prep can lead to very bad things.
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Last edited by M5guy; 02-10-2020 at 02:26 PM..
Old 02-10-2020, 08:52 AM
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4mm is a lot. Sure it isn't .4mm?
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Old 02-10-2020, 01:51 PM
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Quote:
Originally Posted by quattrorunner View Post
4mm is a lot. Sure it isn't .4mm?
Yes, thank you. Clumsy fingers.
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Old 02-10-2020, 02:26 PM
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If piston to valve clearance is 0.4mm, then the first time the engine got revved up all the valves would be bent. At minimum you would need .7-.8mm of clearance to accommodate rod stretch and that would probably leave witness marks anyways with piston rock, etc. Tightest you can run reasonably safely would be around 1.0mm on intakes and 1.5mm on exhausts and that is tight!
Old 02-10-2020, 04:54 PM
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1.5mm Inlet, 2mm ex.
A simple way to check this - when you get the rockers installed;
Take #3 to TDC firing. Adjust the valve lash to zero on I and Ex. Then wind the I valve in another 1.5 turns on adjuster, and Ex valve 2 turns. (The adjuster pitch is 1mm).
Now rotate engine 720 deg - by spanner - carefully/slowly. Any resistance, STOP. The valve is hitting the piston. If it clears all the way thru 720 deg, you are Ok in that department.
Alan
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83 SC, 82 930 (track) - Stock except for RarlyL8 race headers, RarlyL8 Zork, K27-7006, 22/28 T bars, 007 Fuel head, short 3&4 gears, NGK AFR, Greddy EBC (on the slippery slope), Wevo engine mounts, ERP rear camber adjust and mono balls, Tarret front monoball camber adjust, Elgin cams, 38mm ported heads, 964 IC. 380rwhp @ 0.8bar Apart from above, bone stock:-)
Old 02-10-2020, 10:57 PM
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I had a client come into my shop with a motor that kept snapping rockers. The motor was built twice before by two reputable shops with extensive turbo experience and a great track record.

Turns out, the car would come on boost and it would hit so hard that the tires would light up and the motor would RPM so fast that the valve train couldn't keep up and the stock rockers would pop. This only happened when the tires lost traction in first or second gear.

Fresh rubber fixed it.

Seriously.
Old 02-12-2020, 02:28 PM
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Hi Guys, the clearance for the valves was measured and there is plenty of room - there is spacers on the cam chain housing and underneath the barrels from previous owner, I assume to lower compression ratio or create space for the valves. A borescope was also performed on every cylinder with no visible witness marks, the compression and leak down were also fine. Hopefully the heads will come off this week and reveal something that isn't right! They performed a similar test to what you have explained AlanL and there was plenty of clearance. I think the tolerence with exhaust valve guide on no.3 and shockwave at RPM limit seem most viable so far - hopefully time will tell. The Pauter rockers and shafts have now arrived and I'll ensure they are measured prior to fitment.
Old 02-13-2020, 06:20 AM
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Yeah, Stronger rockers will fix this right up!
Old 02-13-2020, 08:51 AM
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Before you buy forged rockers, make sure that there wasn't any piston-valve contact causing the breakage (for example the oiling issue discussed earlier in the thread). If contact is the reason, a forged rocker will just force the breakage to happen elsewhere in the engine. When pistons contact valves, something will break. A rocker is the cheapest, less damaging thing to break.
Old 02-14-2020, 06:32 AM
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I have bought the forged ones as all the rockers and shafts need replacing, I've read the quality of the OEM rockers has really dropped off so bought the forged ones for peace of mind.

The heads are now stripped off the barrels, absolutely no indication of valve to piston contact whatsoever. Also no sign of detonation,the piston cleaned up as new and completely smooth. The valve springs aren't snapped and look brand new, no damage to valves.

The cam restrictors have been drilled out to 6mm, end plugs removed and spray bars fully cleaned inside and out, the bent distributor stator has been replaced and the distributor fully cleaned and rebuilt, oil lines checked for debris/restrictions and nothing found.

No.3 head posted to specialist to measure everything and check all is within tolerence with no.2 head as a comparison. The cam tower will also be measured to make sure nothing is out of spec.

I'm hoping he finds something, if not there are literally no more checks to perform and still a huge question mark over why it's happening.

Thanks again all

Old 02-14-2020, 07:22 AM
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