Cold start rattle on LS3 - piston slap :) Engine replaced

[monstar]

It's probably a bit hard to compare your built engine to a run of the mill GM production line unit, I'd certainly hope your engine guy took a bit more care than the GM production line.

Yes full race build, bathed, tested, honed, bored, machined, balanced, overhauled. But pretty sure LS3 are made in 'Merica, not too much bad to say about the production facilities and protocols.

Stock GM engines are well known to breathe heavy and put lots of oil through the intake, I wonder what causes that? Worst part is it's not a problem restricted to the V8 engine either as the V6's are just as bad.

Poor oil / air separation (PCV) I think.

The other issue at hand is that some of these LS3 engines make horrible noises at start up where others don't. All things been equal (same engine/tune etc) they should either all do it or non do it. Just because a GM engineer says it's acceptable doesn't mean it really is, it just means GM doesn't care because to admit that this is a problem would potentially bankrupt the company repairing all those noisy LS engines out there.

Exactly. But one thing for sure this 2015 6.2 (340) tune is the only difference to previous, never had such an issue like this before now.

People need to remember we live in a capitalist society where bean counters regularly over-rule the engineers to save a few dollars where ever possible to the point that they would rather pay out for a few dead people than recall a million cars because it's the cheaper option. Do you think the bean counters really care if the new engine is a bit noisy sometimes because they removed a few guys from the engine line, the guys that double check those piston bore clearances for example, they just pay some engineer a big bonus to say "it's all 100% a OK".

True. Perhaps the solution in light of tune restrictions is to swap in high quality forged asymmetrical pistons:
View media item 346Cheaper than supercharging, huge efficiency gains, no noise.

 

[Immortality]

Yes via the PCV and the factory system basically has no oil separation at all. The issue I have is that it seems to happen from day one these LS engines and not only high mileage ones like my old V6 which probably still has less oil through the PCV than a newish LS motor.

My point been that for a new engine to breathe as heavily as the LS engines do there must be an issue with the piston/bore ring seal. It's a well known fact that GM runs a narrow low tension ring these days to reduce friction and improve fuel mileage, clearly they stuffed it up with the original LS1 engine as they were oil burning, heavy breathing piston slapping behemoths but did GM ever truly rectify it or just label it as "acceptable" with slightly higher tension rings fitted seeing as these engines still breathe heavily as they come of the GM production line?

 

[monstar]

Yes via the PCV and the factory system basically has no oil separation at all. The issue I have is that it seems to happen from day one these LS engines and not only high mileage ones like my old V6 which probably still has less oil through the PCV than a newish LS motor.

Since February 2011 the GenIV V8 doesn't have the same issue because the passenger rocker cover was revised with half decent air / oil separator. The LSA is a different kettle of fish, needs work per Camaro LE or Z/28. HSV guys go for brands mainly but there are fantastic MAN high efficiency diesel air/oil separators (coincidentally rebranded by Lingenfelter for LSA engines in the US) that actually work to TÜV standards.

My point being that for a new engine to breathe as heavily as the LS engines do there must be an issue with the piston/bore ring seal. It's a well known fact that GM runs a narrow low tension ring these days to reduce friction and improve fuel mileage, clearly they stuffed it up with the original LS1 engine as they were oil burning, heavy breathing piston slapping behemoths but did GM ever truly rectify it or just label it as "acceptable" with slightly higher tension rings fitted seeing as these engines still breathe heavily as they come of the GM production line?

I've not noticed the GenIV engine ever breathing particularly heavily checking via oil filler cap for example?

 

[Immortality]

I don't think you would particularly notice it via the oil filler cap, if the PCV system is working removing the filler cap will actually create a vacuum leak into the intake because at idle the manifold is at high vacuum and will be sucking on the crank case via the PCV valve.

I also doubt yours would breathe heavily regardless considering the engine work your has had. BTW, do you know what your piston/bore clearance is?

Yes I'm aware of the diesel oil air separators, out of Germany originally I think. Big black plastic thing if it's the same one you are referring to. It's a shame OEM doesn't fit a decent catch can system, would alleviate a lot of issues with engines in later life (not that manufacturers car much what happens with engines once they pass warranty mileage).

Just thinking out loud here for a minute, but we know GM likes to use narrow/low tension rings to improve efficiency, what effect if any do low tension rings have on piston stability in the bore? Taking that further, modern engine run these fancy short skirt pistons with a narrow ring pack positioned very high on the piston with the top ring very close to the piston crown which I believe is done to improve combustion efficiency but what effect does this have on piston stability? All of this is a mute point though because in theory, these modern hypereutectic pistons expand a lot less so in theory should have a much tighter piston to bore clearance than a forged piston (such as yours) and my old school Holden V8 donk.

 

[lmoengnr]

Both my Redline(L77) and Maloo(LS3) initially had oily intakes in the first 10000k's, but have since 'dried out' before 30000k's.

I think LS's need 20000k's to properly bed the rings in.

 

[Immortality]

You're probably right there. I know an ex Holden mechanic that used to do the warranty rebuilds on LS engines here. After rebuilding said engine it was then taken for a drive to properly bed the rings in......

It probably doesn't help that these engines come with very slippery full synthetic oils from the start. Much better starting with a run in oil and dumping after the first 1000km me thinks.

The problem is probably worse in cars that get driven to nicely and never really given a decent workout during the run in period.

 

[monstar]

I don't think you would particularly notice it via the oil filler cap, if the PCV system is working removing the filler cap will actually create a vacuum leak into the intake because at idle the manifold is at high vacuum and will be sucking on the crank case via the PCV valve.

About 65-68 kPa vs 101 baro (arr, I'm by the sea).

I also doubt yours would breathe heavily regardless considering the engine work your has had. BTW, do you know what your piston/bore clearance is?

No, but those details on file with builder. Pistons are asymmetric per JE video description above.
View media item 355View media item 353

Just thinking out loud here for a minute, but we know GM likes to use narrow/low tension rings to improve efficiency, what effect if any do low tension rings have on piston stability in the bore? Taking that further, modern engine run these fancy short skirt pistons with a narrow ring pack positioned very high on the piston with the top ring very close to the piston crown which I believe is done to improve combustion efficiency but what effect does this have on piston stability?

I know what you mean, check this pic of the GenV (LT1) piston from underneath:
View media item 304Vs mine:
View media item 158This gives an idea of how it looks from underneath (Wiseco symmetric vs JE asymmetric):
View media item 347View media item 350Whereas there's a fair bit of meat of course with my compression height but rings well below crown:
View media item 3492nd Ring: 1.5mm
Bore: 102.41mm (4.032 in)
Oil Ring: 3.0mm
Top Ring: 1.2mm

All of this is moot though because in theory, these modern hypereutectic pistons expand a lot less so in theory should have a much tighter piston to bore clearance than a forged piston (such as yours) and my old school Holden V8 donk.

Yes, changed with Gen V but Gen IV was a compromise with inexpensive cast materials (silicone being cheaper than alu) using 40 year old piston design.
View media item 354View media item 356

 

[Paulie81]

Mine is pretty much the same and has never worried me. Im guessing the Aluminium block probably makes it sound louder.

 

[426Cuda]

I found these links which may be of interest: (chrisp yes there is a lot 'out there' and from my understanding, we are lucky having our LS V8s over the other comparable cost options for both performance and longevity).
The following link refers to LS1 and is reassuring to all LS owners and yes to my understanding the LS3's have seen improvements to what is a basic problem to most manufacturers; the links describe this popular forum 'problem noise' repeatedly discussed here and the US (other most likely have better info but I hope this info helps alleviate some concerns - I personally find just letting the car idle in the garage for a short time (couple of minutes) before driving off helps to quickly stop this driving cold noise than just driving slowly for a few minutes. I am not a mechanic, now only use 95 or 98octane fuel to reduce knock. I don't race the car so use the oil the manufacturer recommends (another popular forum topic) - Delco 1 5W30 oil and I prefer not to go thicker as there are more parts in the engine to consider than just catering for a few minutes of piston clearance chatter.
https://ls1tech.com/forums/generati...791-piston-slap-explaination-gm-engineer.html
SS Forum http://www.ssforums.com/forum/engine-technical-discussion/63498-piston-slap.html
AMW SS forum member provided this opinion: Okay, what worries me is that some of you don't have any "slap" in the morning, upon cold start-ups. However, I found this quote online from a GM engineer: The noise is an annoyance but will not hurt anything and the engine is fine. (see link for full comments).
Also re "Especially in cold weather starts, isn't piston slap *expected* and *unavoidable* in engines such as LS3 with hypereutectic pistons?
The whole point of hypereutectic is to increase the thermal stability...which is to say to REDUCE the coefficient of thermal expansion. In other words hypereutectic don't expand and contract very much based on temperature."
If you are getting piston slap in an LS3, it can be a worst case scenario of a piston on the smaller side of acceptable tolerance and a cylinder bore on the larger side of acceptable tolerance."
Hope this helps but if not, suggest do a search yourself and lodge any key recognized professional opinions found on this forum. Worth it to ease concerns I also had but are now comfortable with.
In brief as explained in the links above: "Light weight pistons are great but the skirts, by necessity, are short making it hard to make them both strong and flexible and the shorter skirts make them more prone to rocking.
Unfortunately, when the performance and fuel economy oriented pistons are run cold they are very prone to "slap" until they warm up to operating temperature.
The piston designers and development engineers are always treading the fine line between piston slap cold and friction and power/fuel economy loss when the engine is warm."

Excellent post VCoz. Thanks!
One comment though. The non expansive thermal properties of hypereutectic pistons, sounds, prima facie like a contradiction, when followed by the "prone to noise/slap, rocking before they reach temperature". Of course the piston do expand a little, no doubt, as do the liners. Did you read anything like this in your research?
Cheers

 

[monstar]

If if were the pistons flapping and slapped like an LS1 then it would straight up make the noise on startup. It would clack and carry on like it does on the hundreds of videos on YouTube.

With LS1 piston slap the noise is there from get go and gets louder and relative to engine RPM until induction / engine case takes over. In other words free revving no load you can definitely hear it.
This noise described multiple times in text and video in the original thread, occurs only under load when cylinder pressure is high on a new engine - cold and between 1200 and 2700, in open loop, with depressed accelerator pedal position.
It does not affect all GenIV V8, only the 6.2, and only those with the 2015+ OS in the ECU.
I propose this is a case of excess cylinder pressure versus massive heat entropy whilst ECU is in warmup mode. If that sounds too wanky and hard to grasp, heat build up during compression vs heat loss to the chamber environment causes crap combustion - knock. As per the previous thread, this cold load knock can be tuned out but this tune is more emissions compliant. Therefore I propose the VFII tune allows the ECU to compensate within more restrictive fuel / spark parameters.
The VFII noise is matched per RPM, doesn’t get louder with more RPM. Level is easily overcome with induction and crankcase noise 2500+ RPM. Commanding less power via accelerator, or ECU commands transmission shift, the noise is non-existent. In other words doesn’t knock when commanded spark retard...
So before this turns into a monstar vs the googleverse debate and the sheer weight of info from a decade or so ago dragged in to bear, let’s be clear this is not piston slap under light / typical operating conditions, only under peak cylinder pressure “cold load”, specifically an issue with the manner warmup is handled in the last iteration of the emissions compliant tune (VFII OS only).
I welcome discussion regarding Pistons, bores, rings, in adequate detail because I’ve studied a bit about these things in order to deal with this stuff.
Also I whole heartedly believe that knowing what’s making which sound is really important to maintaining the car. Bottom line is it’s noted with Holden as being typical, and Aus Cons Law has your back.