Originally Posted by MU71L4710N

muzzy, he will not be fine with the stock maf if he is running over 369rwhp (i fudged my math the first time around). thats why YOU have a lightning maf in your car right now...

bettersn, the breaking point is NOT 420rwhp. the rods are rated to 475 MAX horsepower (and that is not an "i can take 475 horsepower ALL DAY EVERY DAY number", it is a MAXIMUM number that can be sustained before breaking. the longer its at those numbers, the quicker its going to snap). 475 horsepower is roughly 403rwhp on a 5sp. anything over 403rwhp, is exceeding the manufacturer's reccomendations for any sort of safe power levels. the pistons are good to 550 or so, and the crank to 600+, just FYI. and the reason people go higher is things are rated to a number, but can take more. the transmission in these cars are rated to 360tq, which is a bolton gt. however theres plenty of nitrous and boosted gt's running well over that with trannys that still work fine.

NOW. the problem with the 2v's comes into play that in order to get those 400+ horsepower numbers, you have to significantly increase boost and timing compared to the 3 and 4v motors that have the same exact internals. check out numbers on the 3/4v community, most are running well over 400whp and it is considerably safer than doing so on a 2v (though still not anything i personally would call safe). ox has said this several times that if you do head or cam work on the 2v, it (atleast should) make your "higher" hp levels more safe. i still dont completely agree with it, but statistically less 3/4v motors are blown at higher HP numbers than 2v's even though they have the same internals. i've seen more than a few 3v's running 440+ rwhp and have been for over a year now, but i rarely see a 2v with that power level for that long.

bottom line is these motors were NOT designed to run 400+ rwhp with the weak *** internals they are built with. these motors were NOT built to be boosted. pretty much any boost number your running is risky on the stock motor (wheather its 340 or 440 whp). some people have great luck with theirs, others don't.

Originally Posted by Hangwire

I feel lucky every damn day haha. And considering the very minimal changes ford had to make to this engine when they moved it over to the Cobra platform, its safe to say the motor itself was engineered to handle boost and extra power. Its a low displacement, low compression motor that makes it ideal for boost. It just wasn't very practical to throw forged rods into a motor that 80% of them probably will never see enough power to come close to blowing. It wouldn't be nearly as "cheap" for us to buy and build these cars if they built them up that much. Notice Cobra price tags 7 years down the road.....look at the way GT prices have plummeted.

Originally Posted by cliffyk

Yup, all machines are like that--there's a point at which the design life is slightly affected, then moderately affected, then significantly affected--by loading them beyond the design's original specifications.

When Ford's engineers designed the '03 Cobra, which was to be a production run "sold to the public" vehicle and thus expected by that public to meet typical reliability and service levels, they decided that a forged bottom end would be needed to meet that expectation. You can be assured they didn't do this just to make the car cost more to produce. It was done because the cast steel crank/sintered metal rods used in the GT could not support the 390fwHP design goal AND meet the level of reliability and service life customers expected.

So if to you "really safe" means "last as long as a stock unmodified engine" (let's say 200k miles for sake of argument), then 360rwHP is the practical limit for the stock bottom end. If however "really safe" means "won't blow up in the immediate future", then 400rwHP is an acceptable limit--albeit flirting with the breaking point, which brings up another issue.

Statistically the failure-points for machines follow a typical standard distribution bell curve. This means that although 425rwHP is the point at which most engines will fail, it's not a on/off switch. There will be some that fail at lesser loads and some that fail at higher loads. The points below and above 425rwHP at which 68% of the engines fails is called the standard deviation (SD).

Based upon nothing but some years of observation and anecdotal reports I would guesstimate the SD for our engines to be 25rwHP or so. If you accept this then it means that 68% of stock bottom end 4.6L 2V engines will fail between 400 and 450rwHp. Statistically, at -/+ 2 standard deviations, another 27% will fail between 375 and 400rwHP, and between 450 and 475rwHP.

If you look around the various forums you'll find these guesstimates to be pretty close to what really happens--also keep in mind that pulling 450rwHP on a dyno, and then doing nothing but cruising around town making the hotties wet, is quite different from putting down that number and spending every waking moment at the strip using it.

So, in the end we have a classic Dirty Harry situation when pushing a GT engine up over 400rwHP--"Do you feel lucky today?"...