Fresh forged engine-piston slap?

[dac69er]

Finally got round to starting my freshly built forged engine up and am a bit disappointed as I have some piston slap when cold

It lasts about 30 seconds at this ambient temperature (16°c)

Having a read, it is pretty common on forged builds. It wasn't something I was expecting with the wossners tbh.
I appreciate it's a done job now and I can do much about it apart from rebore to the next size and another set of pistons.

Is it pretty normal? Should I just run it and not worry? I don't have alot of choice tbh, I've sunk a fair bit of money into the engine.

Will it improve once run in? I've only run it through a heat cycle to bleed the coolant through and run it about 20 meters down the road to make sure it drives.

 

[gorganl2000]

i had the exact same experience and thoughts with my forge engine which has in the same pistons as yours. I was told it was normal and would settle down when it was ran in after a few hundred km, and it noticeably did. However, I noticed it is never as quiet as a stock engine on first start up, but the piston slap/noise is significantly less and goes away as the engine warms up and reaches operating temperature. In fact, listening to the forge engine via diy det cans, its pleasantly quite during operation once up to temperature. Based on my research, It appears to be related to the piston material and the bore tolerances needed for operation/expansion/boost

After the run in period, when i changed over the fully synthetic oil, in my opinion that was a key factor in significantly reducing the start up piston slap/noise.

if its well built, i'd say you have nothing to worry about.

my ambient temperature is higher than yours--on average 30c

 

[Jay]

I would concur with that. Pretty much any forged build I've heard has that increased noise from cold start. In some cases it's been pretty bad (as in alarmingly loud slap), others not so much, you just know it's a bit louder than stock. All seem to improve a bit with mileage.

If you're happy that it's within tolerances on assembly then I would try not to let bother me.

 

[dac69er]

I would concur with that. Pretty much any forged build I've heard has that increased noise from cold start. In some cases it's been pretty bad (as in alarmingly loud slap), others not so much, you just know it's a bit louder than stock. All seem to improve a bit with mileage.

If you're happy that it's within tolerances on assembly then I would try not to let bother me.

Thanks both.
Tbh, it wasn't something I was overly expecting. It never really gets spoken about in the starlet community so wasn't sure if it was normal or not!

It sounds like a noisy tappet, but a bit louder. It lasts a very short period of time. Once a little bit of heat is in the engine, it's is nice and quiet (well, it sounds ok to me).

I was just a bit disheartened as I have spent ages on this build making sure everything was just right for what I wanted and the noise doesn't sound great. But if it is normal, then I won't worry about it. I will just finish off the boost piping and start running her in

 

[WhiteOverGoldMK1GT]

cant recall that mine had it, she definately had some blowby though.

 

[gorganl2000]

blowby is relative...it depends on what you and your engine builder want from your engine...more boost/performance will usually imply larger/wider ring tolerances being used to account for heat/expansion, which in turn is more blowby

 

[Calum122]

My freind had a GC8 Impreza which was forged and that was pretty bad.

And I always said, if that was a bone stock engine, you'd say it's about to fall apart. When it comes down to forged builds, my amatuer mechanic ears are just not good enough.

I can't recall what my Starlet sounded like, I don't remember it being that bad. But I am about to refresh the engine, so I'll expect a noisy start up then!