i have an 89 f150 that had a 4.9 300 in it with a 5 speed transmission. I recently put a new f250 351w motor in the truck and want to change a few things. 1.) i tack around 2500 rpm going about 55-60 mph going down the road. I would like to change the gears in the rear end so the truck isnt geared so low. Im not sure whats in it but would like to know whats a good ratio to put in it. 2.) I want to put a auto transmission in it as well and wanted to know whats a good one to install. Is it worth it? and how much trouble is it to convert? 3.) ever since i put the new motor in it sometimes idles about 1500rpm and if you smack the pedal real fast itll idle down to about 800rpm. i was thinking 2 things. one the throttle cable might not be right or positioned right or the butter flly valve is sticking. I took the throttle body off to clean it while i was seeing if it was the tps or the other sensor and soon as you try to open the valve its a slight sticking motion. 4.) whats a good set of headers to install on this motor (nothing to expensive just a hunting truck) thers a "flowsound" muffler already installed but i may change it im not sure yet. Any advice or opions on any of these things would be highly appreciated.

Thanks,



Scott

 

To answer your questions in order:
1) I'm not sure what the stock ratio was for the 4.9L, but 3.73s or 3.55s would be a good choice.
2) If you want overdrive (might help with #1) I'd use an AOD. If you don't care about OD, a C6 would be best.
3) Not sure about.
4) Search around for a used set of shortie headers. Install fairly easy without having to modify exhaust.

 

Thanks for the advice

 

I would like to go in order also...

1. Depends on what size tires you're running. It sounds like your gear ratio is already pretty low. You could swap gears OR possibly run larger diameter tires, maybe even put a small body lift on it. The larger tires should bring that RPM operating range down a bit and give you a little more juice to play with.

2. What Big_Red_Brute said. Personally, I like my M5 for the same reasons I like manual locking hubs. Simplicity and reliability.

3. You could be right. Either of those things could cause unusually high RPM's at idle, but perhaps look at cleaning up the IAC valve. If it's not closing all the way you will be allowing more air into the engine. Is this just happening when its cold or does it happen all the time? The ECM may be thinking that there is a cold start condition.

4. Definitely shorty headers. Passenger side can be a bear to install, but no modifications otherwise. Go for it.

 

The fast idling issue is due to the thottle body being worn out. Chased this problem on a 91 with the same 5.8 engine. Changed and replaced everything, but in the end it was the throttle shaft sticking due to being worn out. Put a new aftermarket throttle body on it and problem solved. Idles at the correct RPM every time now. Take the intake hose off next time the idle is stuck high, then take a screwdriver and push on one of the plates to force it closed more. I think you will find it moves and the idle comes down to normal.
Stay with the 5-speed--better fuel mileage and more reliable.

 

Unless you changed the gauge cluster that reading is not accurate, 2500rpm at 55 would have to be in 4th gear not overdrive assuming 3.55 gears and stock sized tires.

Check the axle code on the calibration decal on the drivers door pillar and compare to this chart.. Ford Axle Code Chart .: Articles
But before you go changing gears fit the tires you're gonna want and decide on if you're keeping the manual or getting an automatic

An AOD would be relatively easy and durable if it's a late model or a fresh rebuild with a shift kit.

Could be a worn throttle linkage or the bushings on the shaft for the butterfly valves, may be easier to just replace the whole throttlebody.

Pacesetter.

 

I have 33's on there now with a 3" body lift and I've been wanting to throw some 35's on there but I haven't really had the money for another lift and I wasn't sure how much it would affect the gas mileage and power? Because it already drinks gas and I don't really feel any noticeable power until about 2300-2500 rpm according to my tacometer (which could be wrong). And you can forget about takeoff, I don't even have to hit the gas to take off with the 5 speed. This is a hunting truck but my main goal of this truck is to get most of the torque to the tires (want good get up an go) while also being able to cruise around 60-65 without the motor screaming and to get the best mpg if all possible. And for the trsnsmission, if I had the time and money I would probably put a auto in it but for the time being the 5 speed is prolly gonna stay and I may change my mind if I get a new throttle body and fix my the idling problem. I really hope that's all it is cuz its bout to get on my last nerve and it isn't helping the mpg.

 

If you are planning on going 35's, I would recommend 4.10's or 4.56's for gears. That will help the mpg situation out allot. Not to mention take off.

 

What would you recommend the best rear end would be for off road? I believe I have an open rear end now and it takes nothing for one side to just sit there and spin then I have to get out and lock the hubs just to pull up wet surface.

 

OK, if the truck is going to be an off road only truck, a locker or spool would be a good idea for the best traction possible. But if you plan on driving on the road, lockers can be harsh in curves. My recommendation for you would be what's called a trac loc carrier. It was Ford's factory limited slip. Here's a link to the carrier on JBG.

8.8 Inch Trac Loc Carrier, 31 Spline

 

Thanks for all the help I'm going to look into it

 

Another word to the wise, if you're planning on changing the carrier you might as well change the gears while you're at it if you are still wanting to do that. And while you've got it torn down it would be a good idea to do the bearings/seals too.

 

alright will do but i have another question. So far as headers goes are there much different between the shorties and the long tubes? I wanna get a set and i know the shorties would be easiest to install but i heard there wasnt much difference between the manifolds and shorties.

 

Few other questions. When I get ready to change the gear ratio could I use some used gears or would I be best to buy new ones? And if I changed the rear end ratio would I have to change the front end as well yo match?

 

The shorties are not that much easier to install. There are two major advantages to having them, one being that the flow rates are much better than stock manifolds. The second is that they bolt right up to your stock exhaust.

It could never hurt to go with the long tube headers if you also plan on replacing or modifying your existing exhaust.