I have an aluminum 6L that has made over 740 rwhp. I have been told repeatedly that these engines should not be run at over 1000 HP. Is this a real issue/limit? I want to make 850-900 next time out. What is the HP limit of an aluminum 6L? What goes wrong?

 

You would definitely be at the limit.
The reasons is flexing of the crank and also the block not being able to take the pressure.
If you are running stock internals at that power level, it's just reckless.
It will give some time.
If you want to run something with high HP get a dart shp iron block or if you want to stick with aluminium get an ls9.
Rated to 900hp from the factory.
The dart block is rated for 2000+HPAnd combined with good forged internals it will live a long time at your power level.

 

There are plenty of people making 1000rwhp on stock Gen4 bottom ends (the better factory rods). Not saying it will last forever; however there are people running low 8's like that in 4.8/5.3/6.0/6.2's. There are plenty of people making that power with Gen3 internals; however they are on much more borrowed time due to the weaker rods. I pushed a factory cast crank to 1000-1200rwhp for years with hundreds of 1/4 mile, street pulls to 160mph+ and a bunch of 1/2 mile racing making about 1200rwhp in 5th gear. It was initially a stock LS3 block,crank, good forged rods/pistons/LS3 ported heads and Precision GTS76 turbo that made 1000rwhp. Then a year later an LS9 block was build with stock LS3 crank, Compstar rods/Wiseco pistons, and MAST LS3 heads (380cfm) upgraded the hot side to a Precision 88mm T4 (1450) and eventually the crank failed after hundreds of hard passes showing off doing a 1st to 4th gear rolling burnout and in 4th I let off abruptly showing about 150mph on the speedo going 70-80mph and the tires hooked up which did nasty driveline shock and snapped the 7/8 rod journal clean thru in a diagonal line. I would say the limit could have been pushed higher but 1200rwhp it took it with no whining. The crank journals still looked great all other intact ones.

Most people feel under 1000rwhp with a turbo a stock crank will live with good rods/pistons. Blower setups create more crank snout tension and can introduce weird harmonics into the motor/bearings which wears stuff out quicker if it is a cast unit and not as easy on parts like a turbo setup is. The 4 bolt block at around 1200rwhp will have sealing issues with LS9 gaskets so people run O-ring or Athena style for better sealing and more power.

I would say run it the power you are talking about and plan to build up something better ASAP before it goes boom and you can still use some parts off of it.

 

You're obviously not a turbo guy are you.
Stock LS2 block has been 1000+ HP without failure.
HP ratings on parts are almost always bogus.

 

740 isn't bad and 850 isn't the limit of a Gen 4. I had a SBE 6.0 that's seen more action then a meth head in a Pasco county trailer park. Its been PD blower, twin turbo, procharged, single turbo and been in 4 different cars and won't give up smoke, I lost track of the miles but its well over 100k at this point of constant abuse. Recommending a Dart block for under 1000hp is pretty silly unless someone just has a hole in their wallet and wants to add weight. It's been proven numerous times that SBE 4.8, 5.3, 6.0, 6.2 can make 900+ for years on end as long as it has proper support with fueling and tuning.