why does the auto mach red line at like 5800 and the manual at 6800?

 

the auto has a regual crank (non forgded) so it cant take the higher rpms. in other words you got the better bottom end on your car.

 

I've been told that it deals with the trannys itself. That the auto shouldn't rev that high due to wear and tear.

 

um.....cause autos SUCK!..?

 

you don't want to rev an auto up real high when the torque converter flashes at a low rpm... it'll just tear it apart. You need to loosen it up or you risk balooning it or foaming the fluid which always ends badly.

If you want to rev it up really high then you need to put a higher stall converter in it. As far as I know there is no difference in the crankshaft from 5sp to auto but you'll want to double check. I'd call up a ford parts counter and ask for the part number for a crank for an auto and for a 5sp. if they're the same, then that answers that.

Last I checked it was the valvetrain and connecting rods that determined your max safe RPM, not so much the crank. As much as I hate citing anecdotal examples, there are gobs of guys out there turning 6500+rpm on stock GT cast cranks after tossing in some h-beams and cams with more duration, so if that was a worry... you'd think we'd have seen more broken crank shafts. I've NEVER encountered such a thing on modular motors. Never, ever. And I come in contact with a lot of mustang owners.

Edit... BTW.. auto's don't suck. they have a purpose just like 4speeds and 5 speeds and 6 speeds. You pick the tranny that's appropriate to your application. If you want really killer run to run consistency then there's no substitute for an auto for most people. If you can make a stick run consistent times, then great for you.

 

I did a search on the Mach 1 registry and Scott Hoag the designer of the Mach 1 stated.... "the auto mach 1's have a cast iron crankshaft and the 5 speeds come with a steel crankshaft....the manual revs to a higher limit and needed a steel crank".

Here is a link of Scott Hoag talking about auto's vs 5 speed. It doesn't get into great detail, but here you go
http://www.mach1registry.org/forums/...ead.php?t=2808

 

I stand corrected on the different crank. Good info. Thanks.