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Are harmonics killing my engine?

Started by _jklumpp_ , May 14 2013 11:10 PM

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#1 _jklumpp_

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Posted 14 May 2013 - 11:10 PM

Pulled the engine out of the LJ today to check it over after we had intermittent oil pressure warnings at Eastern Creek last weekend.

Whilst pulling the ancilleries off, I found the alternator bracket was broken at the front bolt hole (to the block).

The when we got the engine out, and removed the sump, we found the oil pickup 'bell-end' sitting inside the sump - it had sheared off through metal fatigue.

Then when I unbolted the pickup pipe from the block, I found that bracket had snapped too!

Whilst being tuned a few weeks a go, the oil filter & sump plug also came loose....

The engine (VK black 6cyl) is only a few months old, and has only done 3 race meetings plus a few dyno runs, and some road K's. ...

It has a knife edged blue motor crank, which was the subject of some debate at the build stage, with a local balancer refusing to touch it because he didn't believe it could ever be properly balanced. My engine builder said it has less meat off it than the knife edge cranks he normally uses, and in the end it was all balanced up by his balancer in Sydney. It also has JE forged pistons, Yella Terra ultra light flywheel & romac alloy balancer.

But now with all this metal fatigue in such a short period of time, I'm wondering if the harmonics are the cause..... and do I just reassemble the engine & see what breaks next, or do I swap the knife edged crank for a counterweighted blue/black crank now, before any more catastrophic damage is done?

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#2 dattoman

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Posted 15 May 2013 - 02:40 AM

Its a 6 ... they fall apart often

I'd say a light flywheel and balancer could be causing your issue especially with that crank

Was everything balanced together ?

Crank,flywheel,balancer ?
Were the pistons balanced in their weights

Were the rods set to all the same weight and correctly balanced ?

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#3 warrenm

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Posted 15 May 2013 - 07:18 AM

I'd say your pretty close with your diagnosis. My belief is if you're going to use a counter weighted crank, leave the weight on the crank or use a red motor crank. I haven't had much success keeping an ultra light YT flywheel on the end of the crank, red or blue, so now use a std weight wheel & 9lb balancer & a girdle. It's torsional vibration that is causing the fatigue cracks in most of the parts that you refer to. One other method that can help dampen things is to grout fill the block to the bottom of the welch plugs, I know it adds weight & reduces water capacity but but helps dampen things.

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#4 orangeLJ

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Posted 15 May 2013 - 07:59 AM

What sort of HP did you end up with (Only if you don't mind sharing)

That's definitely a little worrying for such a fresh engine build.

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#5 _oldjohnno_

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Posted 15 May 2013 - 09:09 AM

Short answer: you need more flywheel weight to shift the node further towards the front of the crank, where the damper can do something about it. Damper selection will take a bit of trial and error to suit your rpm range.

I'm not at all a fan of cut-down counterweights for anything other than short duration work - why use counterweights at all if you're gonna cut most the most important bits off? But I know others disagree and have used knife edged cranks without problems. But basically I agree with Warren, the problem is all to do with torsional vibration and has little or no connection with balance.

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#6 _jklumpp_

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Posted 15 May 2013 - 11:49 AM

Thanks Guys.

The crank was a source of concern from day 1, as per my thread here http://www.gmh-toran...crank-decision/ and now it seems that the predictions from those 'against' have come true....

Yes Datto, everything balanced together, and AFAIK rods & pistons also matched.

Warren & Johno, your comments on taking weight off the counterbalanced crank are the exact comments made by my mechanic, who was against the use ot the knife edged crank - and funilly enough my engine builder who uses the knife edged cranks, grout fills his blocks....

Chris, engine got 215rwhp on the dyno, with a very nice flat torque curve.

I have a low K HZ red motor in the shed, where it has been for over 20 years.... I'm pretty sure it will be a virgin crank, and the suggestion from my mechanic this morning is to have that machined, and get everything balanced again, and put it back in with some new bearings... comments?

Unfortunately when the engine build was all finished, I ebayed all the old parts which included a heavier 'lightened' flywheel, and steel balancer.... isn't it always the way!

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#7 orangeLJ

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Posted 15 May 2013 - 12:25 PM

I know Col's engine (built by dave) is running a knife edged crank but like you say, it's grout filled (group Nc yellow/green LJ) but that from memory was more a precaution, rather than a cure for anything.

I'm running a YT flywheel in the purple track car and my orange LJ too, never had an issue (using a romac "race" steel balancer) both run to 6-7k rpm reasonably regularly without issues, BUT both are running red cranks

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#8 Litre8

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Posted 15 May 2013 - 12:52 PM

Isn't there a balance 'factor' that, when increased/decreased, allows the bad harmonics period to be moved around, ideally out of the rpm range you normally race at?

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#9 _oldjohnno_

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Posted 15 May 2013 - 04:17 PM

Isn't there a balance 'factor' that, when increased/decreased, allows the bad harmonics period to be moved around, ideally out of the rpm range you normally race at?

No, or not directly anyway. From a torsional vibration point of view the crank doesn't really care what the balance factor is. Though you could argue that making big changes to the B/F would also change the overall mass of the crank, and this in turn would change the crank's resonant frequencies.

The usual way to tune around the rpm ranges is to adjust the flywheel and damper mass. The mass of the flywheel and damper relative to each other is also important - too much damper or too little flywheel moves the vibration node rearwards which is very hard on the crank, block and clutch. Too much flywheel and too little damper is hard on the crank snout and cam drive/valvetrain. I don't think the issues of balance factor and balancing are anywhere near as critical as some people make them out to be.

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#10 _jklumpp_

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Posted 15 May 2013 - 04:56 PM

Possible 'Dumb' question - would changing from the knife edged crank to a std red motor crank change tune/power delivery characteristics of the engine at all?

Just wondering if I'd need to revisit that if I swapped to the red crank?

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#11 _Ned Loh_

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Posted 15 May 2013 - 05:22 PM

What RPMs? Consider a 'Pro Race' damper, and stock weight steel flywheel, with your current crank, or with a new one.

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#12 _Ned Loh_

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Posted 15 May 2013 - 06:09 PM

Ps. I went through something similar a year or so ago.... http://www.gmh-toran...aft#entry675951

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#13 _jklumpp_

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Posted 15 May 2013 - 07:26 PM

What RPMs? Consider a 'Pro Race' damper, and stock weight steel flywheel, with your current crank, or with a new one.

6000-6500 max

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#14 _oldjohnno_

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Posted 15 May 2013 - 09:02 PM

What RPMs? Consider a 'Pro Race' damper, and stock weight steel flywheel, with your current crank, or with a new one.

6000-6500 max

I'm with Ned. At the very least get rid of the ultralight flywheel. A lightweight YT wheel might be ok, stock weight would be fine but the ultralights are hopeless even with only 6500.

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#15 _Bomber Watson_

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Posted 15 May 2013 - 09:05 PM

What RPMs? Consider a 'Pro Race' damper, and stock weight steel flywheel, with your current crank, or with a new one.

Personally I would start there to, but guys like OJ, Warren and Howard are smarter than me....

edit, OJ beat me to it.....

Cheers.

Edited by Bomber Watson, 15 May 2013 - 09:05 PM.

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#16 _jklumpp_

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Posted 15 May 2013 - 11:07 PM

So, you're all suggesting one of these http://www.ebay.com....=item3367e371d0

With one of these http://www.ebay.com....=item2a279696d0

on the knife edged crank should solve the issue?

and if I go to the red crank, still replace the balancer & flywheel?

...annoyed, because I ebayed my old parts which included a Powerbond steel balancer, and 'lightened' flywheel!

... and what about my 'dumb' tuning question guru's?... too dumd to answer? :-D

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#17 warrenm

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Posted 16 May 2013 - 06:51 AM

Possible 'Dumb' question - would changing from the knife edged crank to a std red motor crank change tune/power delivery characteristics of the engine at all?

Just wondering if I'd need to revisit that if I swapped to the red crank?

With the set up that you have now, the revs would rise very fast but also drop quickly when you drop out of your torque range. If the crank assembly was heavier the revs should hold on better. My original LJ motor was red crank 8.6kg wheel powerbond balancer, I fitted an ultra light wheel, the result was broken pressure plate straps,loose & sheared flywheel bolts & always tightening loose bolts on the engine. Fitted the 8.6kg wheel again & it all stayed together. With the motor I'm using now, I have blue crank, 10.4kg wheel plus the heavier balancer, I've used the heavy crank setup so it doesn't bog of the line.

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#18 _oldjohnno_

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Posted 16 May 2013 - 09:01 AM

Possible 'Dumb' question - would changing from the knife edged crank to a std red motor crank change tune/power delivery characteristics of the engine at all?

Just wondering if I'd need to revisit that if I swapped to the red crank?

The lighter the rotating assembly, the quicker it will accelerate, but as Warren pointed out a light RA will also be liable to bog off the line, so for a drag racer a bit of rotating weight is usually a good thing with a small engine. For a circuit car the lighter bottom end would probably be a bit quicker, but if it's beating itself to death it's not really an advantage overall.

The advantage of the blue/black crank is that it's much easier on the bearings and block at high rpms. Considering the low rpm range (6000 - 6500) of this engine a red crank might be a good option. It'd allow you to run a sensible flywheel/damper combo while keeping the total RA weight down.

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#19 _jklumpp_

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Posted 16 May 2013 - 09:54 PM

makes sense.... But overall 'tune' of the engine wouldn't need to change? ie. carby setup, dizzy advance, timing, etc?

I imagine the dyno graph with the 2 different cranks (and no other changes) would be the same, but the way, or time taken, to deliver the power/torque would be slightly different - correct?

as a PS, I spoke to the engine builder today who runs the knife edge counterweight cranks in the NC cars, and I asked him what balancer/flywheel combo he uses - turn out he uses the exact same combo as I have (romac alloy/YT RPM).... says he's never had a problem, though they did nearly lose the flywheel at Bathurst last year!.... but as I mentioned before, he grout fills the blocks.

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#20 _STRAIGHTLINEMICK_

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Posted 16 May 2013 - 10:51 PM

If the crank is still ok then i would use it again .As was mentioned by johnno and warren the problem is with the flywheel and balancer weight and not the crank .We have customers run knifedged cranks in Nc ,speedway and drag racing with heavy flywheels and balancers without a problem .Ian Tate uses them also . But in the end you MUST ONLY liisten to your engine builder because he knows what works for him .

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#21 _TorYoda_

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Posted 17 May 2013 - 12:00 AM

In my own personal experience and opinion the only place those super lightweight cranks, flywheels and balancers are any good is for a drag race engine. Endurance motors always seem to suffer similar high frequency fatigue failures when too lightweight components are used.

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#22 _oldjohnno_

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Posted 17 May 2013 - 08:36 PM

turns out he uses the exact same combo as I have (romac alloy/YT RPM).... says he's never had a problem, though they did nearly lose the flywheel at Bathurst last year!

I'd class nearly losing a flywheel as a problem...

Tune-wise, I doubt whether you'd have to change anything much at all.

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#23 _jklumpp_

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Posted 18 May 2013 - 09:46 PM

^^ yeah, I saw the problem with that statement too Johnno!

...OK, so doing some thinking on all the comments that have been made - Johnno, you said I need to move the node oward the front of the crank, and we do this by adding flywheel weight. BUT we don't want to go too far forward, as this is hard on the snout/timing gear.

Doing some research on weights of Balancers/flywheels, my current combos is 1.75kg Balancer with a 6.05kg flywheel.

Romacs all steel balancer is 3.6kg, and the pro-race is 4.16kg

YT's flywheel weights are RPM:6.05kg, L:8.6kg, and STD 10.6kg

So would it be logical for me to say that going back to the 8kg flywheel, with my 1.75kg balancer would be best next step?

If I add more balancer & more flywheel, aren't they just cancelling each other out, and leaving the 'node' pretty much where is it?

Unless I guess if I went to the 10.6kg flywheel with a heavier balancer ... Does the heavier balancer mean it will deal with the vibration better?

I also note your comments johnno on your web-site about the usual 6cyl crank harmonics coming in in the 6000-6500rpm range. Down the straight at Eastern creek in 3rd gear, revving the engine to 6500 for the gear shift, it was in that 6000-6500rpm range that the car felt like it was gonna shake to pieces!... I just kept looking at the tach thinking "can I change yet!"

As a side note, I pulled the crank out of the HZ motor I had in the shed - still had the original bearings in it, and was clean as! Mechanic is measuring it up to see if it needs any machining, and his opinion is that the red crank with the flywheel & balancer I have will be OK - but I'm not convinced I won't end up with the same problem....?

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#24 _oldjohnno_

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Posted 19 May 2013 - 09:01 AM

I wish I could give you a nice simple ratio for flywheel : balancer mass but I just don't have that data. Besides, it's not that simple - the way that the mass is distributed in the wheel is just as important as its overall mass.

What I can tell you is that the ultralight flywheel hasn't worked for me and many others at the rpms you're talking about. As well as the problems around 6200 or so there is often some roughness felt just over 3000 as well. You might get away with the ultralight and a red crank - the shaky rpm range will be a bit higher - but whether it would be beyond 6500 I don't know. And if you raise the rpm range with further engine work you could be back to square one.

My gut feeling is that a 8.6kg or 10.6kg flywheel with a balancer of around 2 to 3kg would be a good place to start, at least close enough that you could fine-tune it with balancer changes. With drag racing it's not such a big deal cos it's all over so quickly, but as you've found out prolonged running at the shaky speeds can be problematic.

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#25 _STRAIGHTLINEMICK_

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Posted 19 May 2013 - 05:52 PM

Yep i agree with johnno ,if you are using a lightened 12 counterweight crank use the 8kg flywheel with 7/16 " bolts, one dowel and 3kg balancer ,if you are using a red crank you may need the heavier flywheel also with 7/16 " bolts and a dowel. There are no calculations for these weights ,they are what seems to work on a circuit car .On a drag car with manual trans heavier rotating mass is needed to get off the line when bottom end torque is compromised for top end power . Flat shifting gears will also take advantage of the increased inertia .This worked on Harrops fj as i know a racer who saw the extra thick machined steel flywheel in the car at a castlereagh meeting in the early seventies .A couple of blokes on this forum might be doing this in their drag cars as well ,please let me know .