Originally Posted by CrazyAl

You're right that a modified motor tends to shift the powerband into the higher RPMs. However, that doesn't change the fact that rods tend to break on the intake stroke at high RPM.

Here's the details:

A connecting rod has pivots (bearings) on either end. One is the journal bearing on the crankshaft and the other is where the piston pin is. Becasue of these pivots, only tension or compression forces can be applied to the rod. The rod never sees a bending load. Now then, here's the kicker--most materials are stronger in compression than in tension. Think about it. You might have a rope that you could pull on untill it broke. But have you ever sqeezed a rope until it broke? Let's say we had a given metal bar. Suppose it could withstand 1000 lbs of force in tension before it broke. That same bar might be able to sustain a load of 1500 lbs in compression. This applies to rods too. The rods are stronger under compression than they are under tension. Of the 4 strokes in a 4-stroke engine, three of them place the rods under compression: the power stroke where the expanding gas is pushing the piston down, as well as the exhaust and compression strokes where the rod is pushing the piston upwards. However, on the intake stroke the rod has to pull the piston down. This is the weak spot.

A piston doesn't weigh a whole lot, but when your engine is running at 6,000 RPM (that's 100 revs per second), the piston has to travel the length of the engine's stroke in a very short time. Becasue it has to move so fast the tension in the rod is extremely high...and this is when rods tend to break because it's typically the "weak link" in the system.

You'll notice that this is more or less indipendant of the HP of the engine. Moral of the story--watch your RPMs. Race engines that are built for high-rpm duty have super-lightweight pistons, which are designed to reduce the loading on the rods in this situation. They also have stronger rods, of course.

Originally Posted by lol_vtec

Hello all,

First off I'm new to this forum and just purchased my first Ford Mustang. I've had the car for a few weeks now and have already added intake and exhaust. I have many more plans for my new baby. My question is......What can I get away with, power wise, before I need to build a stronger bottom end? I have a cai new throttle body, long tube headers, x-pipe, and exhaust. I want the frpp intake manifold, hot rod cams, and frpp stage 1 heads. Beyond that I don't know what the stock block can handle. Any pointers would be appreciated. Thanks in advance!

Originally Posted by Lem-06

what about if you have a light peppering of FI in the equation?
Say running boost to get 440hp on a very safe tune.
Would the intake stroke still suffer the same limitation in regards to pulling the piston down, or would the boost pressure in the manifold help in pushing the piston down the cylinder?

Originally Posted by danzcool

It's all about RPM, I personally think that the weak point is at the transition at the end of the exhaust stroke and beginning of the intake stroke before the intake valves open, where the piston wants to keep going up.

Originally Posted by UPRSharad

There's a thread (with video) on S197forum where a shop tried to blow up a stock 3V before installing the customer's built motor. On twin turbos and race fuel, I think they gave up trying to blow it up on the dyno after around 8 or 9 pulls and making something ridiculous like 7-800rwhp. Your mileage may vary, but the point is, it's all in the tune.

Originally Posted by Lem-06

Thanks sharad I forgot to mention that.
I believe their final run was 919hp iirc.
After that they gave up and took it out to inspect.
Granted that's on a dyno and not hurling a 3800lb car down the 1/4. It's still impressive