Cany anyone give me a ballpark figure on what the max safe RPM is on a stock I6-250 w/ auto tranny?
According to Ak Miller (the Ford Man) 6000 is the top RPM for the inline sixes unless you put some exotic parts into the engine to raise the strength of the block. PaulS
With the stock carb they can`t rev up very high because there just isn`t alot of cfm going through the carb (depending on the year ~ 215 or so) . Once you get a better carb header and a little head work you would want some APR rod and main bolts depending on the year the rods should be fine, any time you think you`r gonna be pulling high rpms you`r want to get you`r engine balanced and this is where it gets pricy if you want to go past I think its 6,500 rpm you`ll want forged pistons but it might be 6000 ? but you`ll need a BIG cam to be making power past there in a 250 . Hope this helped a little.
elliot, As I understand from my contact with Ak Miller the problem with the in-line sixes is the long crank. Hamonics travel back and forth along it and at some RPMs will cancell each other out but at just above 6000 RPM the vibrations (from the firing cycles) actually become dangerously amplified and will crack the crank and block unless a steel crank and block girdle are installed. The block girdle is attached to the pan rails and replace the main bearing caps in order to stiffen the entire lower end to handle the harmonic vibrations created by the firing of the engine. I am not an expert in this area but Ak Miller is - a racer himself and in charge of the muscle parts for Ford for years before and during the inception of the motorsports division. He was the one who told me that the 289 I was building could take 7200 RPM in stock configuration but that over that RPM crank and block mods were necessary. When he says that the sixes should only turn 6000 without mods to the crank and block I am going to accept that. To turn a six to that speed you will have to make changes in intake and exhaust systems as well as balance all rotating and reciprocating parts. To go over that RPM (and keep it together you will have to stiffen the block and use a stronger crank. BTW: I believe the first mod you make to any engine should be to balance it - it runs smoother and revs easier because you don't lose HP fighting imbalances. That goes for the sixes too - they may be "inherently balanced" by design but until the factory tollerances are removed they are not ready for pistons, cam, or any other mods. PaulS
I never heard that before , I realy don`t think that you are gonna be going over 6 grand any way with a 250 unless you made it an all out race engin but thanks for pointing that out
In short, then, I should be safe keeping the engine under 6000 or 5500 or so?(like you say, I doubt it could get that high in RPM anyway; about the highest I've had it is 3700)
Well, I've been looking into the 2v aussie heads, but I'm poor college kid with not a whole lot of money- any cheap mods that you can think of that deliver more power? I purchased a K&N filter, a rebuilt carb, and put electronic ignition on the car, but those are the only mods.
