Hey everyone. I recently got a 1986 F150 Stepside Shortbox Standard Cab 4X4 4-Speed and it has a seemingly aftermarket tachometer strapped to the steering colum. Theres no redline and I was wondering around what RPM the torque will peak at. It's the 4.9L I6 with original engine and tranny and im not hauling a thing if that makes any difference. Thanks!

 

Yes, you have an aftermarket tach. If it was factory it would be in the dash next to the speedo.
I am assuming you have the 4-speed manual with the 1st gear granny, and the one barrel carb on your motor.

On my '85 bronco with the C6 automatic it used to shift at 3500-3600 rpm if it was floored. I did have it up to 4000 rpm a couple of times, but it did sound like it was going to blow-up. I would say normal driving, you should shift around 2800-3000 rpm. You just have to listen to your motor. If it sounds like it is taking forever to gain more rpm, then shift.
That motor is a torque monster with a really long stroke, so it makes all its power down in the low end. I ran 40" Ground Hawgs for months with 3.55 gears no problem before stepping up to 4.56 gears.

IMO a worthwhile upgrade to that motor is a 4-barrel carb (365-450cfm), 4-barrel intake manifold and split long-tube headers. The motor won't rev higher but it will rev a hell of a lot faster b/c it will be making more power.

 

I'm looking at the Offenhauser high-performance 4-BBL. intake manifold on JC Whitney but it says its intended only for offroad use and racing and not applicable for highway use. Do I risk harm to the engine using it or are they just saying that so people in emission controlled states are discouraged from buying it?

edit: of course this intake manifold would be to match the Holley 4BBL Carb 390cfm