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Old 04-23-2013, 01:20 AM   #11
Lem-06
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Originally Posted by CrazyAl View Post
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.
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?
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Old 04-23-2013, 01:55 AM   #12
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Originally Posted by lol_vtec View Post
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!
It's mostly about the tune and keeping the RPMs under redline. With a good tune (and stock redline), you'll have to upgrade the fuel system before you can grenade the motor. Raise the redline and rev it too high and a stock car will greenade. Try to push the tune too lean or too aggressively on the timing and boom.

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Originally Posted by Lem-06 View Post
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?
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.
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Old 04-23-2013, 02:04 AM   #13
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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.
Even with a cam that has overlap between intake/exhaust?

either way i can see how going from compressive force to tensile force can lead to damage, especially at 6000 rpm's
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Old 04-23-2013, 03:20 AM   #14
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Im a noob with engine but isnt the 3v 4.6l rev limiter just cut everything for like .3 second when it hit the rev at 6200 rpm (stock rpm limit)? I was told I couldnt damage anything this way since there was no fire once u hit the rev limiter opposite to shelby that can definitly bounce the rev limiter and still hear it back firingcrazy (forged engine must help).. what is the difference between both theses reactions? I also got told that hiting rev with a blower isnt good cause the blower is still boosting air in the engine even if the rev is hit, what are the simple fact? I honestly do sometime hit the rev a couple time when im doing a powerful rolling burnout from 1 to 2nd..dont blame me, is it that bad even if the engine get to shut power at redline
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Old 04-23-2013, 03:30 PM   #15
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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.
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Old 04-23-2013, 05:53 PM   #16
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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
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Old 04-23-2013, 09:06 PM   #17
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Quote:
Originally Posted by UPRSharad View Post
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.
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Originally Posted by Lem-06 View Post
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
Yeah, that's my car. It was shut down at 6k rpms every time, which furthers the "RPMs kill" argument. The last # was 819, and that was due to running out of fuel pressure (there were some issues with the fuel pumps and the injectors were only 60LB).
It may never have a 1/4 time, it was built for standing mile trap speed (should see WOT into 6th). Will probably do some road course on reduced boost also.

On the overlap cams, I would think that the fact that the exhaust and intake are open would also limit the positive pressure in the cylinder.
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Old 04-23-2013, 09:48 PM   #18
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Ahh 819. I knew it was still an insane high stock number.
Even not doing hard launches that's still damn good. Can't wait to see your new ##'s

Ahh true. Just wondering how, it at all it would affect it
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Old 04-24-2013, 12:45 AM   #19
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Anyone can enlight me up on my previous post?
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Old 04-24-2013, 05:58 AM   #20
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Thanks guys. Makes mn feel better about my plans for my baby!
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Old 04-24-2013, 05:58 AM
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