Can someone explain this to me? From what ive seen, a stock bottom end engines power limits seem to increase with headflow. For example a non pi 4.6 is pushing it at 400 horses. A 3 or 4 valve 4.6 can live comfortably at 500 horses. An lsx motor which is made of the same materials pm rods, cast crank, hyper pistons can hang over 600 horses.

 

The stress on most engine parts (especially bottom end parts) is from torque not power. Increasing head flow increases engine speed. Increasing power through engine speed as opposed to torque is easier on the bottom end.

 

Well the LSx has a pretty strong stock hyper eutectic piston. They have lived at over 800rwhp, but the tune was DEAD on. Might be related to crown thickness on the piston too. From what I've seen in the LSx and SBC world, RPMs, wrong or not enough fuel, and bad tunes seem to kill stock motors many more motors than than torque. trbo355 says the biggest reason his cam only 355 grenade lived was the lack of RPM (I don't think that motor ever saw over 5,500 RPM).

 

I don't know, many people throw a set of arp rod bolts in the lsx motors and take them to 7 grand.......... including me.

 

I had a big cammed N/A LS1 and spun it to 7 grand daily for 40,000 miles without ARP rod bolts, but I was only make ~430rwhp. Most recent one I've seen (end of last year I think) is "98Z28CobraKiller" had an LQ4 stock longblock other than pushrods running regularly at 10.0x and it didn't blow till the 1st pass he turned the RPM up to try and break into the 9's.

 

Hi all, new to the forum, but I've seen a lot of engine failures over the years, and I think I might have something to contribute here...

The first thing is, do we really know why an engine broke? Mis-diagnosis of the root cause of engine failures is a common problem. Also, we need to keep in mind that "horsepower" is really just math. To get more horsepower, we have done one (or both) of 2 things: Increase peak torque output, (and) or increase peak torque rpm. Most modern factory bottom ends don't mind spinning to fairly high rpm, or pulling pretty hard down low. Asking stock parts to do both is asking a lot....

Today's factory engines are fairly small displacement compared to what was around in the musclecar days, yet power output can be right up there with the best of the big inch legends. The only way to get as much torque output from a small engine is to supercharge it. Small engines simply don't have the combination of pressure area (bore) and leverage (stroke) to produce big torque numbers N/A. If we can move the entire torque curve higher, we also get more horsepower through math. To get more torque N/A we have to go to a longer stroke, which puts a lot more stress on the bottom end components, and limits rpm through crazy rod and piston accelerations. We'll try to twist it anyway, so...FAIL!

The other way to get more horsepower is to move peak torque output higher in the rpm range. Since torque and HP are always equal at 5250 or so, if we are closer to peak torque after that point, the math is again in our favor for HP. This is obviously where your big head flow comes in. As rpm goes up, time to fill the cylinder goes down...duh...so we need a bigger hole to shove air through to keep the cylinder full. Now we have a good heavy charge in there at higher rpm, so it's easier to get those big numbers without twisting the thing to death chasing the downslope of the torque curve. But, ignition timing, fuel quality, fuel tune, etc. become critical to control detonation; because we have much less time to get the heat out of the cylinder before the next power cycle. Light it a tad early, run a bit lean...BANG...FAIL!

We haven't even taken into consideration hanging a valve and tagging the piston, although this is less of a concern with modern engines at "normal" rpm levels. Or oiling related failures from high g loads with a stock pan... Or....or.....or....

To sum this up, I would say find out exactly what broke first, then see if there is any sort of trend to base a power limit off of. We are experimenting with a turbo'd stock bottom end 350 that has put out 530hp / 605lb ft on the engine dyno turning 6500 with cast crank, cast pistons, and powder rods, with NO ill effects whatsoever. There's a thread on it in the turbo builds forum...project "no buck". We are going to see just what the limits of that stock bottom end are...

 

I have something to add myself..... for a turbo application.

IF the heads do not flow well it takes more boost to make the same power as a more appropriately sized head (generally speaking of course)
This is B/C despite showing higher boost the mass air flow entering the cylinder is the same.. but looking at the exhaust side there must be more exhaust pressure for more boost. So the exhaust is not breating as well, heat is trapped and heat is the enemy....

to put it simply, a head with lesser flow will require more boost and hence more back pressure and heat to make the same power... generally speaking