Am I getting everything out of my 258?

[Flex BT]

I have an 81 with the inline 6 AMC 258 i6 / 4.2l . There is no computer and the vacuum system is minimal. I have done the teamrush upgrade and have a Weber 38 DGES, 33" tires and a significant lift. My jeep is geared at 2.73 and my main question is at highway speeds (probably around 65ish) in gear 4 and 5 (T-5 ) there is no response after pushing the throttle down somewhere around halfway. Gear 5 is pretty much useless, especially if any kind of incline is encountered. Is this normal for my setup? I've done so much work on the jeep, the only thing I can suspect is maybe an exhaust obstruction or something, or maybe with my gearing there just isn't anything more the I6 can throw at it. I've replaced the fuel system front-to-back, tuned and replaced the carb multiple times. I'm thinking about either putting a 4.0 head on or trying fuel injection but I'm not sure.

 

[Sasquach]

Glad to see you not highjacking someone else's thread but...
http://www.jeep-cj.com/forums/f2/getting-most-out-my-258-a-20840/
In my opinion the 4.0 head is the first place to start, then go from there with whatever fuel system you prefer or can afford.The multi port either Mopar or junkyard or the throttle body choices. I went with a mild cam upgrade and fresh timing gears and chain to make a good running motor, street and trail.
oh ya and it will pull a gear higher on any grade.

 

[gert]

The no horsepower/torque sounds like my old AMC 258 i6 / 4.2l on the interstate…… I would be looking at changing your gear set a 2.73 with 33 inch tires is to much I just did the calculations and it says you would be at 1500 rpm at 65 in fifth gear you want to be closer to 2000 rpm to have usable torque/horsepower 2.73 / 33 / 65/ 0.86 = 1554 Calculators
3.55 gears would get you where you need to be 3.55 / 33 / 65 / 0.86 =2021
The Borg-Warner T4 & T5 Jeep Transmissions - Novak Conversions
I swapped 3.55s into my jeep long time ago when I still had the stock AMC20 in the back with 34s on it and it was alot more fun than the "emissions friendly" 2.73s I had I still had great highway and was able to play in the mud just fine just for giggles 3.73 gears gets you 2123 rpms and 3.91 gets you 2225 rpms wikipedia says you are putting out a whopping 129hp net @4000 rpm and 218 ftpd torque @1800

 

[IOPort51]

The tire the system was designed for was 27 inch. you could run 2.72:1s with those and more than likely run 65-70 at about 2000-2500 RPM.

I decide what highway speed and what size tire I want and choose a ratio to let me cruse @ about 2500 or the power/torque peak. then for off road it is up to 1st and the low range in the TC. fortunately the 3.54:1 that came stock put me in the ball park. The next big project is a tera low for the Dana 20 TC.

I always thought that an over drive tranny in a Jeep was kind of out of place, but thats just me.

 

[Petescj]

I always thought that an over drive tranny in a Jeep was kind of out of place, but thats just me.

Thats true. I think the same should be said about gears numerically lower than 3.54. Remember times were changing in the , 80s Cjs and new government regulations on fuel mileage and emissions were the main cause of the pathetic 2.72 gears. I didnt care for them with 31s more a less 33s.


Gear Ratio Calculator

 

[ThisGuyUKnow]

OP you need to either get smaller tires or regear/ find lower gear axles ASAP. You said you have done alot of work to your jeep and with 2.73 and 33" tires and with that setup with a T-5 its not a matter of if but of when the T-5 will go. You dont want to add installing a Transmission to that list if you can avoid it.

If you google grimjeepercalculator you will see that your setup will not hit 1500 RPMS in 5th until 63 MPH you would not hit 2500 rpms until 105 MPH and you would not hit 3000 rpms until 125 MPH. This is why 5th is pretty much useless to you as you would be operation somewhere in between those RPMS while in fifth gear. This set up is putting alot of strain on your Transmission .

Id address your tire/gear ratio and see if you get some more oomph out of your engine.

 

[davey25]

Pretty sure it's just your gearing..I have the same gears in my 5.0 H.O mustang..obviously with a smaller tire..but you don't get much once in 5th

 

[moranda]

I had the exact same set up only running 31's. I didn't even try to use fifth because it was pointless. I regeared to 4.56's which is stupid low now cause I still have the 31's but now I stay in fifth most of the time. Its so low I typical just use second to start out in. The plan is to get some 35's on there once I can afford the lift and tires but I would strongly suggest a regear.