R500 Duratec - Valve Spring Failure (yet another...)

[Shortshift]

A cautionary tale...

My 2012 factory build, 10,000 mile car has just fallen victim to the dreaded valve spring failure issue though, almost unbelievably and thanks to great fortune, my engine has escaped from being subjected to any further damage.  In fact, on strip-down (thank you Stuart at Premier Power) we found that no fewer than four springs had failed - two inlets and two exhausts, and that on one valve, half of a valve collet had come out of its retaining groove!  And yet somehow, despite all of this, none of the valves had dropped into the cylinders...

To summarise what I think I know about valve spring history here:

• It seems that early R500s (let's say from 2008 until 2010/2011) were build using a 'straight Kent spring' and it is these items that are widely known to be prone to failure.  There's no shortage of reported failures and tales of (mostly) expensive engine rebuilds as a consequence.
   
• Because of these failures, it seems that Caterham Cars changed the valve springs around the 2010/2011 timeframe and moved to a 'conical Kent spring' but these were fitted for only a short period, possibly until 2012 or thereabouts.  Maybe these springs were also found to be failure prone which prompted their replacement in factory build engines after such a short time; it certainly appears that my car was fitted with this type of spring which adds credence to the view that these, too, should be regarded as suspect for the R500 application. For reference, I believe these are likely to be Kent VS60 springs.
   
• Finally, around 2012 or 2013, Caterham Cars changed the spec again and moved to a 'conical Cosworth spring'.  I understand that these have been fitted to all 400/420s, 500s and 620s since that time (not sure about 360s?) without any known issues and I believe they are still fitted to these engines as of today. For reference, I believe these springs are Cosworth part number: YD0093 (and Caterham Cars reference 30E365B).

If anyone can add to the potted history above, that would be great.  Though I think it's pretty accurate, I'm very happy to be corrected if anyone has better information.

Back to my car, my engine.  I had thought that, being a 2012 build, my car was likely fitted with the final Cosworth spec springs but on stripping-down it's clear that this isn't the case.  I gambled and lost, though in losing I have been very lucky indeed.  However, I hope this tale encourages all R500 owners in particular (and anyone with a 2-litre Duratec that was built before, say, 2013 and which revs beyond 8,000 rpm) to really find out what springs are fitted and, if in doubt, change them all before a costly bill is incurred.

To finish my story, I have followed Stuart's advice and the cylinder head on my car is now being rebuilt with Piper twin-springs.  This arrangement is Stuart's preference and he has built many performance Duratecs this way without issue over recent years.  Whilst I would have been equally as happy to have had the conical Cosworth springs fitted, as Caterham Cars have done for around ten years or so, I do like the idea of the second spring being there to save the day in the event of a main spring failure.   Very, very unlikely to happen now, I know, but once bitten...

Once again; not a call to check your nuts but, rather, CHECK THOSE SPRINGS!

James

[DirtBuddha]

Ford quality 

[Shortshift]

No - none of the alternatives mentioned above are standard Ford parts.  All are third-party upgraded springs, intended for use in high-lift (11.5mm with the Kent Dtec 35 cam used in the R500) and high rpm applications.  Some way away from standard Ford Duratec fitment!

James

[aerobod - near CYYC]

As mentioned in a couple of other threads, all 8 of the inlet valve springs on my 2012 R400D broke (after 10,000km of track use and 30,000km of road use), one catastrophically in two pieces, destroying a valve and requiring a complete engine rebuild.

This does not seem to be a parts quality issue, but a valve spring resonance / surge issue over stressing the coils, due to the combination of spring rate, valve weight and cam excitation. I think the fact that my R400D inlet valve springs only failed indicate there is a resonance value below 7800RPM (130Hz) for them, with the lighter exhaust valves having a resonance value between 7800 and 8500RPM (130 to 142Hz) due to both failing in the R500D. The giveaway that it is spring resonance is the coil breakage between 1 and 1.5 coil turns, with a single break at the seat end first, followed by a double break at the retainer end.

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[Shortshift]

Replying to #4:

I wonder if the R400/420 valve springs are the same as those fitted (in the corresponding period) to the R500?  I know that 400s/420s were built with an uprated (Kent?) cam and uprated (maybe also Kent?) springs, so perhaps the cam and springs are common across these two model types? 

Your springs certainly look similar to mine, James - conical, mostly open and even winding of coils, etc.  And your car and mine are both 2012 builds.  Do you have any information on the make/brand and even part number of these springs?  And, for completeness, do you know the type/model of cam fitted?

James

[aerobod - near CYYC]

Re #5, Hi James, the cams in mine are Kent KC1409 with the standard Kent springs from that period which have the conical upper couple of coils. I measured the spring rate at 40N/mm, which is on the low side compared with other Duratec performance springs which tend to be in the high 40s or low 50s. I'm not sure how close the KC1409 cams are to the DTEC10s used in the 420R.

My rebuild will be using the Raceline RLD250M cams (equivalent to the DTEC35) and custom parallel springs and retainers that Kent manufacture for Raceline.

This is a current spring from my R400 (besides the lens distortion, only the upper part is conical):

[Shortshift]

Thanks James.  So, different cams in the 400/420 compared to the 500 (as you'd expect, really) but I have a sneaky feeling that these conical Kent springs from circa 2012 might be common across both car types.

I'll see if I can get mine measured and rated when I am next round at Premier Power. Stuart works fast so it won't be long till my R500 is fully fixed and ready to go.

James

[aerobod - near CYYC]

Here is the spec on one of my springs.

Dimensions:

Top coil 14.5mm ID, 21.8mm OD
Bottom coil 18.1mm ID, 25.4mm OD
Free length 45.0mm
Wire diameter 3.65mm
6.3 coils from end-to-end

40N/mm spring rate (likely +/-5% tolerance in my measurement, with the spring compressed from about 39 to 33mm with a 25kg load).

[Shortshift]

Just measured one of my now-removed springs and, as near as I can measure with my digital calipers, all dimensions are identical to the figures you have provided, James, apart from coil count where I'd say my spring comes in at 6.5 turns, end-to-end.

If these are Kent VS60 springs (and I suspect they are) then I haven't been able to find a quoted rate for them but, working from the Kent Cams spec sheet (using known free and installed lengths with their quoted installed load) then I calculate the rate to be around 36N/mm.  Very close to your 40N/mm measured figure.

I'll get the actual rate measured when I can but these two springs (your R400/420 and my R500) look spookily similar.

James

[aerobod - near CYYC]

Hi James, it is good you have a baseline spec for the spring rate that is not too far off from the rate I measured.

My hypothesis that the issue is spring resonance / coil surge would potentially indicate the R400 rev range doesn't reach resonance for the exhaust valves, but the R500 does. I measured the weight of the retainer, collets, spring and valves. From a spring perspective 1/3 of the spring mass excites the resonance combined with the other coupled and moving masses and assuming the bucket tappet is not coupled:

Inlet valve + retainer + collets + 1/3 of spring = 0.0696kg

Exhaust valve + retainer + collets + 1/3 of spring = 0.0638kg

This gives a 1.091 difference in mass between inlet and exhaust. As the resonant frequency is proportional to the inverse of the square root of mass, that indicates a 1.0445 difference, or about 350RPM at 7,800RPM (R400 redline). If the inlet valve resonance was at 7,600RPM and the exhaust valve at 7,950RPM, then we would want the resonance for both valves to be moved above 8,500RPM, let's say to 8,600RPM, or an increase of 1,000RPM. This would correlate with a spring rate increase of 28% (frequency is proportional to square root of spring rate). If we are nominally at say 38N/m, we would need to go to 49N/m or higher.

 

[Shortshift]

Good work, James.  This is really rather fascinating. 

The R500 has a soft-cut at 8,500rpm and a hard-cut at 8,600rpm (well above the R400 rev limit) so enthusiastic track driving will likely result in regular operation in the 8,000 to 8,500rpm band.  It seems your thoughts on resonance fit well with this, with your R400 having failed inlet springs only whereas my R500, operating at higher engine speeds, has failed both inlet and exhaust springs.

I've been digging a bit too, including looking-up valve spring rate data which I table below.  In some cases, the figures are as-quoted by the manufacturer/supplier; in other instances I have had to do some rough estimation from data that I have been able to access.  However, even if not 100% accurate, I think the figures are good enough for comparative purposes.

James

Kent VS60 single                                       35 N/mm
Kent VS59 double                                     37 N/mm
Piper VSSDUR single                                42 N/mm (242 lb/inch)
Piper VDSDUR double                             52 N/mm (297 lb/inch)
Cosworth YD0093 single                         tbc
Ultimate Power single                             tbc
SBD VLSK-DUR-01K double                     44 N/mm

[aerobod - near CYYC]

A good collection of spring rates you have collected there, James. Talking with Raceline they have a parallel spring from Kent that seems to use 7 coils more tightly wound to about 18mm OD, overall giving about 25% higher spring rate than the VS60s if the wire size is the same. They say they haven't seen any problems with the many engines they have built using them with the RLD250M / DTEC35 cams.

I was trying to calculate the theoretical resonance frequency of the springs, the simple formula of a spring-mass resonance indicates about 125Hz, but the cam is rotating at half the engine speed which would point to half that value, on the other hand energy is only put into the spring when the valve is off the seat (about 3/8 of the full cycle), so that would up the resonance frequency, but overall I think there are too many factors that come in to play that constrain the spring-mass that it really needs full computer vibration analysis modelling.

[7 wonders of the world]

All good data,

The VS60 rate from Kent is 32.9 N/mm (188lb v 210llb for the VS59 doubles) so based on James calcs you would need 44.2N/mm based on singles, the mass would naturally increase on doubles do to increased spring mass and the heavier retainer, unless you opt for Kent's Ti versions or SBD's lightweight steel options.

I haven't got weights for any of these those yet, if I can find them I will post

Just for comparison, I run Piper singles with my BP300 cams, Piper advise doubles but having had 2 conversations with them both times they confirmed the doubles were well over specced, having seen several failures due to fretting on doubles I was keen to use singles, 

I'm also wondering about the metalogy of the materials used across the brands and its effect on durability - with comparative spring rates, not so easy to access this detail though.

[7 wonders of the world]

Did a little research on line last night regarding failures and asides from a few Kent ones there's a few OEM and a few bent valves which looked more like clearance issues and oe Supertech double but no images of the failed components.

Due to the popularity of Kent springs being used in engines that are regularly exploring the rev limiter the failures may be disproportionate. .

This made interesting reading though....

https://ist.org.uk/high-performance-spring-failure-a-case-study-of-a-valve-spring/

 James quite problably has a far better knowledge on this area than most and since both your engines are currently in an 'Anne Bolyn' state could we obtain a metalogy inspection on the failed springs as this could well be a contributory factor, I accept that resonance may also be a cause here, however I would have expected to see more reports of failures across other brands in the US who are running similar poundage springs, and bearing in mind Kent's experience in this arena its odd they specify a spring rate which doesn't correctly factor the working mass and hence resonance effect it will be exposed too.

The link is a specific single failure due to inclusion, my thoughts are more along the line of base material, heat treatment or surface finish.

 

[aerobod - near CYYC]

I can't see any surface defect on a couple of my broken springs with a 10x loupe, Neil (but my old eyes aren't as good as they used to be). With my failures I was thinking material defect when I took the double broken spring out, but as each of the inlet valves were taken out and every single one was broken at the same place within a few degrees with a torsional failure, it seemed material defect was less likely.

Supposedly the failure at between 1 and 1.5 turns from the spring end is a characteristic of a resonance failure as the coils in the middle are freest to move in the opposite direction and stress the coil at that position the most. The exhaust springs all seem to be pristine, too, the same length with a nice surface finish, leading to less likelihood of a material problem with all springs being the same.

It would need a lab to confirm, though, as it could always be that the material properties are out of spec in combination with a resonance problem.

[7 wonders of the world]

If it was purely resonance then its questionable as to whether the spring which Kent recommends was indeed correct for the application....

[aerobod - near CYYC]

I'm not taking any chances with using the same springs again, they won't be the VS60s. Raceline seem to be using a Kent spring that doesn't look like anything in their standard catalogue, it is parallel and smaller outside diameter than the VS60s with an aluminium instead of steel retainer.

[Shortshift]

The only comment I'd make about your choice there (above), James, is that the initial failures (from 2008 through to around 2010, when the conical VS60 spring was introduced) all occurred with straight Kent springs that are otherwise unidentified.

Just make sure you're not leaping out of the pan into the fire...  Maybe better to move away from Kent altogether, perhaps to the Cosworth, SBD or one of the Piper alternatives, unless you can be sure that your chosen Kent spring isn't the spec that caused the very-first failures? 

I know I'd not re-fit a Kent spring to my car because I'd always be worried.

James

[aerobod - near CYYC]

I'm actually buying the Raceline RLD250M cam kit with verniers, springs and retainers, James. Although Kent is the manufacturer who supplies the parts to Raceline, they fit this kit to all of the Duratec-R 250bhp engines they build, so going by their experience in it being a good match.

[Shortshift]

That sounds encouraging.  And I forget; what will your max RPM be set at?

James

[aerobod - near CYYC]

I'm going to set max RPM at 8,200RPM, as advised for longer life by Raceline. Although I've already got the 12.5:1 CR pistons (to compensate for 1000m or higher altitude), the rest of the spec in terms of cams, valve springs bolts, bearings, throttle bodies, ram pipes, fuel injectors, etc are the same as for the Duratec-R 250bhp  spec engine - about £4,000 from Raceline including all clutch and flywheel components and shipping to Canada, too. They also should be able to send me a Duratec-R fuel map to get a close enough start-up config to do the initial ring and cam break-in before putting it on the rolling road.

[Shortshift]

Sounds nice - and sensible (given what we've been learning)!

James

[7 wonders of the world]

I'm intrigued by the aluminium retainers.... whats there predicted lifespan, considering the hammering these will be subjected to I would be much happier with steel.... 

The weight reduction will be a bonus though.....

 

 

[aerobod - near CYYC]

Re #23, I talked with Chris at Raceline today about the springs and retainers they use, he says they have built hundreds of engines to over 300bhp using the aluminium retainers over a 13 year period, with many used for competition purposes, they have not had any problems.

The shear area of the aluminium retainer above the inside of the spring coil would dictate the overall durability, I would think. This area would be a cylinder with an area of about 100 sq mm (0.0001m^2), i.e. a 2.5mm thick section with a circumference of about 40mm. From a fatigue perspective, you want to keep the maximum stress of 6061-T6 aluminium below 50MPa in tension or 50% of that value in shear, i.e. 25MPa for effectively unlimited cycles (at least 10^10). With loads at full spring deflection of 1000N with the highest suitable spring rating, that would give a static shear of 10Mpa.

We also need to add the dynamic force due to the valve, retainer and 1/3 of the spring mass, or about 0.07kg for the heavier inlet valve. This is dependent on the cam profile, but maximum valve velocity is going to be over about 60 degrees of crank travel which would occur in about 1.2ms at 8500RPM and would open the valve about half of it's full travel during that period, or about 6mm. This leads to a velocity of 5m/s.

If we assume linear acceleration (a bit of an approximation) from zero to maximum velocity over that period we end up with an acceleration of 4167m/s^2, this leads to an additional dynamic force of 292N (F=ma = 0.07 x 4167), for a total maximum force on the retainer of 1292N, or a dynamic shear force up to 12.9MPa, 52% of the safe maximum.

[Shortshift]

You just beat me to all of that, James.

James