Knocking sound on a new built engine (JE PISTON) slap or somthing else?

[fangcatoo]

Hey people, I'm the proud owner of a 2018 RS-3, lately i've blow my engine and i rebuilt everything, new block new crank and everything, I've bought the JE pistons with the plating, and Tuscan Intergreated Rods, and the car is now done 1500KM of calm driving and I've noticed from the first start of the car a strange noise, sounds almost like a knock, I've found a video of an RS-3 with the exact same sound, my mechanic told me it's probably piston slap and it's normal for these pistons to sound like that, I'm asking you people before I go run on the dyno if I should worry about that noise,
also forgot to mention if I turn on the AC at IDLE the sound dissappears in the moment i turn it on!
This is the video of the RS-3 with the exact same sound :
Please share you thoughts!

 

[Sequence]

Piston slap is common on built daza/dnwa annoyingly, some piston setups are more common than others, the higher load and rpm from the ac help smooth it out. Not the end of the world but worth keeping an eye on.

 

[flybynite]

Unfortunately generally the pistons made for high boost pressures are made from an alloy that has higher coefficient of expansion so need more clearance when cold to have the correct clearance when warm. They tend to slap when cold

Pistons made from something like 4032 are low expansion but only of use for N/A engines as they tend to fail at high boost. People like Mahle are supposed to have an alloy that is a mix of both but not sure they are available for the RS£ and are generally moon money.

If it is due to piston expansion the noise should go when it is warmed up. Just warm it up properly before the pull

 

[fangcatoo]

Unfortunately generally the pistons made for high boost pressures are made from an alloy that has higher coefficient of expansion so need more clearance when cold to have the correct clearance when warm. They tend to slap when cold

Pistons made from something like 4032 are low expansion but only of use for N/A engines as they tend to fail at high boost. People like Mahle are supposed to have an alloy that is a mix of both but not sure they are available for the RS£ and are generally moon money.

If it is due to piston expansion the noise should go when it is warmed up. Just warm it up properly before the pull

Hey, thank you for your response, the problem now is, the engine has 2500KM at the moment, it was noisy from the start and it only went away when turning AC on, and after the dyno even with the AC on it's noticeable and with the AC off it's really bad, even when warm, and that makes me thing that it's not a piston slap/I got older JE ultras with a bigger clearance, tell me what you think, Thanks!

 

[flybynite]

Hey, thank you for your response, the problem now is, the engine has 2500KM at the moment, it was noisy from the start and it only went away when turning AC on, and after the dyno even with the AC on it's noticeable and with the AC off it's really bad, even when warm, and that makes me thing that it's not a piston slap/I got older JE ultras with a bigger clearance, tell me what you think, Thanks!

I don't think it is piston-slap if it does not change when warm. Does it change when you put another load on the engine? Like turn all the lights on and load the alternator or put it in drive against the brake?

You need less clarance to stop piston slap (tighter in the bore) but for that you need low expansion pistons or the engine will grenage when hot. My experience with custom pistons is on older N/A engines, boost always adds problems.

I would be talking to the guys that built the engine, could easily be something missed or not tight.

 

[fangcatoo]

I don't think it is piston-slap if it does not change when warm. Does it change when you put another load on the engine? Like turn all the lights on and load the alternator or put it in drive against the brake?

You need less clarance to stop piston slap (tighter in the bore) but for that you need low expansion pistons or the engine will grenage when hot. My experience with custom pistons is on older N/A engines, boost always adds problems.

I would be talking to the guys that built the engine, could easily be something missed or not tight.

Yeah, thing is the car at full idle makes the noise hard and when I put drive wite full brake it reduces abit, and when I turn off the AC it goes wild and when it's on it quites down, I've noticed my accesory belt is pretty worn with a white line in the middle, I'm really confused of the situation because the car ran the dyno and runs just fine, that noise cannot be heared when driving whatsoever, only when coming to a full stop, when you deaccelerate from lets say around 30KMH when the car downshifts you can hear the rattle abit.

 

[fangcatoo]

bump
I checked somthing yesterday and found it interesting
When the engine is running at idle with AC/OFF the noise is terrible.
When the engine is running at idle with AC/ON Low speed the noise reduces but still pretty loud
As you increase the level of the AC the noise almost dissappears for good.
(I've noticed my sereptine belt has a mark on the middle of it along the belt and I've noticed wear (a few ripped hairlines from it, not sure if it can make a ticking noise)

 

[Paulbean]

Could it be a tensioner for the aux belt breaking up? This is purely a guess as I've not seen the belt configuration.

 

[8YARWY]

bump
I checked somthing yesterday and found it interesting
When the engine is running at idle with AC/OFF the noise is terrible.
When the engine is running at idle with AC/ON Low speed the noise reduces but still pretty loud
As you increase the level of the AC the noise almost dissappears for good.
(I've noticed my sereptine belt has a mark on the middle of it along the belt and I've noticed wear (a few ripped hairlines from it, not sure if it can make a ticking noise)

If you suspect the belt then why not remove it then test again........