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The plot is very good data. It looks 5.5 to 6k is the right shift point. It just feels scary when it suddenly shuts down. Moving the limit to 6.5k makes good sense and would take away that awkward stall when you shift at 6k.
... I drove stock muscle cars back in the 60s and 70s that red-lined at 7,000, and manufacturing technology has to be better today...
The metallurgy and manufacturing are better, as is the engineering... but the accountants are also better, which means the rods have been value engineered to operate perfectly at under 6250 RPMs on the 4.6L 3-valve, which is why almost no-one has thrown or bent a rod on the stock tune. However raise the redline or go with an unsafe fuel mixture (detonation), and you're on borrowed time.
Don't get me wrong, the 4.6L engine loves power when it comes in the form of boost. It will handle 450 RWHP all day long at the stock RPM (around 12 PSI), and if you go with forged rods you can push past 600 RWHP (18 PSI) reliably and raise the redline... Then you have to look at all the supporting mods.
The stock powdered metal connecting rods have no problem handling 7000rpm N/A, but the stock phasers don't like sustained rpm over 6500 and the valve springs are rated to 6800.
Stick to 6000 as the maximum continuous operating speed (maximum HP rpm) with only short bursts to 6500rpm higher to be safe.
The stock powdered metal connecting rods have no problem handling 7000rpm N/A...
I disagree.
It's the problem of momentum and changing it abruptly. When the piston goes from an upstroke to a downstroke and vice versa, as the RPM increases, it takes exponentially more strength to handle that transition. The stock rods weren't made with the best processes out of the best material, it's pressed powdered metal, not forged, not billet. They are also quite spindly.
It's the problem of momentum and changing it abruptly. When the piston goes from an upstroke to a downstroke and vice versa, as the RPM increases, it takes exponentially more strength to handle that transition. The stock rods weren't made with the best processes out of the best material, it's pressed powdered metal, not forged, not billet. They are also quite spindly.
That's correct but it's a combination of cylinder pressure and rpm that breaks the rods, not rpm alone. As long as the engine remains N/A, the cylinder pressures won't be high enough to break the stock rods at 7000rpm. On a FI engine, forged H-beam rods are better able to handle the compressive forces that would otherwise break the stock powdered metal I-beam rods.
I'm happy with 6,000 RPM as a shift point coming out of first, I just want to eliminate that momentary nose-dive when the rev limiter kicks in. I'm not racing - just enjoying the power of a hot V-8 and a stick-shift for the first time in a long time. I don't think I have the marbles (or a safe place) to run at 6k in 5th gear.
From a technical standpoint, Sintered Metal is an excellent process for making precision metal parts because it eliminates the internal stresses that causes a part to warp. Piston rings and engine bearings are Sintered Metal. What is really important is their post-processes that achieves crystal alignment and hardness.
Maximum rod loading is in tension as the piston in that cylinder goes through TDC during the overlap portion of the cycle (or when the exhaust valve has just closed and the intake valve is just about to open if there is no overlap at all). It's an inertial acceleration thing. Rod tension is what makes them come apart.
IIRC, my '08 GT has just under 47k miles on it, and it puts up with being run up to 6000 just fine. This is at a road course where for 20 - 25 minutes at a time it's seeing 6000 rpm about twice each minute, in 3rd and 4th, times four such sessions. I'm not really sure where the fuel cut is set (FRPP tune, which might be 6200 or even higher).
I do use Motorcraft full synthetic oil and don't do track days when daytime temperatures are predicted to run much above about 75° (heat stress limiting me more than the car; next month I'll have completed my 69th trip around the sun).
You should be fine with just occasional blasts up to 6000 or a bit more. But I would put my oil change interval no higher than the "severe service" schedule (I think it's 5000 miles, but do check this).
Norm
Last edited by Norm Peterson; 10-09-2016 at 12:34 PM.
I'm happy with 6,000 RPM as a shift point coming out of first, I just want to eliminate that momentary nose-dive when the rev limiter kicks in. I'm not racing - just enjoying the power of a hot V-8 and a stick-shift for the first time in a long time. I don't think I have the marbles (or a safe place) to run at 6k in 5th gear.
Trust me, you would need way more power or a seriously high gear ratio to be able to hit 6K in 5th gear. The 4.6L is governed at 140-145, and really just peters out above that, it may hit 150 on a long enough road with a tailwind. with 3.55 gears, you'd won't have enough power to overcome wind resistance at about 4500 RPMs in 5th. Even with 4.1 gears You probably won't get much above 5200 before hitting the wall. But you'll easily wind be able out 4th.
10-08-2016, 11:42 AM
