Knock On Start Up

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I live in the bahamas and we have a relatively active drag racing seen here in Nassau, and unfortunately we go through alot of rb25det motors, it not uncommon to see 400 whp skylines,.

recently though there has be a shortage of rb25det motors and when i blew 3 pistons a month back i decided to rebuild mine, i should mention that there are no proper shop here to blue print the motor so we just took it down replaced the pistons, retorqued the motor and dropped it back in.

now i have a knock on start up, it last for about 2 seconds and then the motor runs fine, we have determined that the knock is a result of clearance issues, i have trashed the car around at 12 psi without any problems but want to increase the boost to 20 psi, just want to know if you well advised experts think its going to be a problem or will the knock get worse.

[Lock_to_lock]

Could be bottom end bearings.

If thats the case i would defiantly not boost it up until its stored.

Also, are you running the stock turbo? If so, it wont take 20psi.

[NateR33]

If it goes away once the its been running for a bit i would say its a bit of piston slap, which is when the piston shifts slightly in the bore. By the sounds of it anyway. Did you measure/hone the bore or did you just slap pistons in it and throw it back together ?

Minor piston slap goes away as the motor gets hot and piston size increases slightly.

[The Mafia]

are they forged pistons? Don't forgies usually slap a bit when they are cold?

[GeeTR]

...we go through alot of rb25det motors, it not uncommon to see 400 whp skylines...

Maybe instead of destroying the dwindling world supplies of RB's you should try sending it off to be rebuilt properly.

[Finchy33]

Maybe instead of destroying the dwindling world supplies of RB's you should try sending it off to be rebuilt properly.

[GTRsean]

Bah, they are a dime a dozen aren't they......compared to RB26's!!!?!

Probably buy ten stock bottom ends for the price of a fully built one.