Should My CJ7 Be Able to Go Faster?

[ThisGuyUKnow]

Just finished a 900 mile trip with my 82 CJ7 Limited . I found that the fastest I could go was 64-67 in 5th and the pedal to the floor board. I am used to my Waggy with a 360 and 727 which can creep up to 90+ if I am not watching my speed.

I know the CJ7 isn't exactly the most aerodynamic vehical out there so I don't expect it to be speed racer in 5th and go 120 mph but what I was getting seemed a little low. My question is: Is this normal for a AMC 258 i6 / 4.2l powered CJ7 with T-5 or should I have a mechanic check it out?

1982 CJ7 Limited , 31" Mud Terrains, GM HEI, Ford Taurus Elec. Fan,Stock Gearing?(not 100 percent on that)

 

[LongHorn]

Just finished a 900 mile trip with my 82 CJ7 Limited . I found that the fastest I could go was 64-67 in 5th and the pedal to the floor board. I am used to my Waggy with a 360 and 727 which can creep up to 90+ if I am not watching my speed.

I know the CJ7 isn't exactly the most aerodynamic vehical out there so I don't expect it to be speed racer in 5th and go 120 mph but what I was getting seemed a little low. My question is: Is this normal for a AMC 258 i6 / 4.2l powered CJ7 with T-5 or should I have a mechanic check it out?

1982 CJ7 Limited , 31" Mud Terrains, GM HEI, Ford Taurus Elec. Fan,Stock Gearing?(not 100 percent on that)

Was there a "Headwind"?:laugh:
But seriously what was your rpm?
When's the last time you tuned that sucker up?
Is your Catalytic converter plugged?

 

[ThisGuyUKnow]

Was there a "Headwind"?:laugh:
But seriously what was your rpm?
When's the last time you tuned that sucker up?
Is your Catalytic converter plugged?

HAHA yeah if the wind was blowing north I could get 75 and if going down hill maybe 76. Up hill I would dive to 54 if didnt down shift. Just before the trip the cj received a New HEI, New plug wires, new bosch 1+ platinums, and a taurus fan.... all in working order. It may be the cat is that an easy/cheap fix or to get checked? I have no idea what the RPM is the CJ doesnt have an RPM gauge but it seems with the peddle to the floor in fifth(by ear) that the RPMS are around 1900-2200? if i had to guess definitely around red line... its almost as if i were at like half throttle but I can take it in 4rth up to the high 70s before the motor starts getting agitated at me

 

[LongHorn]

HAHA yeah if the wind was blowing north I could get 75 and if going down hill maybe 76. Up hill I would dive to 54 if didnt down shift. Just before the trip the cj received a New HEI, New plug wires, new bosch 1+ platinums, and a taurus fan.... all in working order. It may be the cat is that an easy/cheap fix or to get checked? I have no idea what the RPM is the CJ doesnt have an RPM gauge but it seems with the peddle to the floor in fifth(by ear) that the RPMS are around 1900-2200? if i had to guess definitely around red line... its almost as if i were at like half throttle but I can take it in 4rth up to the high 70s before the motor starts getting agitated at me

If your motor is internally stock as I imagine. Your redline should be in the 4500- 5000 range. Most vehicles cannot achieve redline in O/D, your 5th gear is suppose to decrease your rpm at highway speeds to reduce wear & increase mpg. It's normal to down shift to maintain speed on a grade.
What is you mpg now? Is this a new problem?
On the "Cat" issue. Does your exhaust sound restricted? I don't know a easy way to check. Do you know how old it is?

 

[W7NB]

HAHA yeah if the wind was blowing north I could get 75 and if going down hill maybe 76. Up hill I would dive to 54 if didnt down shift. Just before the trip the cj received a New HEI, New plug wires, new bosch 1+ platinums, and a taurus fan.... all in working order. It may be the cat is that an easy/cheap fix or to get checked? I have no idea what the RPM is the CJ doesnt have an RPM gauge but it seems with the peddle to the floor in fifth(by ear) that the RPMS are around 1900-2200? if i had to guess definitely around red line... its almost as if i were at like half throttle but I can take it in 4rth up to the high 70s before the motor starts getting agitated at me

I would worry about the HEI. Unless it was curved for a 292 I would bet you advance is way off. Where did the HEI come from? Was it this way before the HEI?

Also, what gear set in the diffs? Look for the letter code on the diffs and one of us can figure out RPM based on tire size and ratios.

 

[CJ]

Yep - The weight curve is not working or is off - get it checked...

 

[ThisGuyUKnow]

I would worry about the HEI. Unless it was curved for a 292 I would bet you advance is way off. Where did the HEI come from? Was it this way before the HEI?

Also, what gear set in the diffs? Look for the letter code on the diffs and one of us can figure out RPM based on tire size and ratios.

I think we ended up going 8* below top dead center. We got about 15.9 MPG running HWY 49 in the high 60's.

I know that the OD is supposed to lower RPMs but even so putting my foot all the way in the gas would do something wouldn't it?

As far as Perf before the HEI I don't know. When we got it the CJ lost spark after like a day and had a whole slew of problems pop up, the starter was falling out and bad, the alternator cable was snapped and therfore not charging the bat, no spark was getting to the plugs and the resistance in the bat wires was high. SO the CJ received an optima red top, Brand new Bat wires and wiring to the alternator, and the HEI and a new Starter.

So it sounds like Either My exhaust system is plugged up(I was told to check this by a member here and on IFSJA as well as a Mechanic at gas station) Or my gearing is horribly off. Where on the CJ7 AMC20 is the Ratio stamped I'll take a look. Or maybe it is both.

Last night there was a loud pop and then the engine under acceleration start making a loud puffing noise. One of the bolts on the Exhaust manifold had backed out. Looks like it just got loose cause the threading is still there but makes me seriously consider the exhaust issue. It sounded like the exhaust pipe was right in the engine compartment....

 

[W7NB]

I think we ended up going 8* below top dead center. We got about 15.9 MPG running HWY 49 in the high 60's.

This is called base timing, and determines the advance without mechanical advance from the flyweights. Make sure this was set with the vacuum hose to the distributor disconnected and the hose plugged, or your base timing is already off.

Inside the distributor is a set of weights and springs. These determine how far the timing is advanced based on centrifugal force. If the distributor was not specifically set up for a Jeep AMC 258 i6 / 4.2l liter, the advance curve will not match the engine. The most common oops I see on this is folks using HEI's from early Chevy 4.3 V6's. With a gear change they will fit, but the timing curve was very different on these two motors.

Who put in the HEI? Is it new out of the box and set up for a Jeep, or was it a junkyard HEI someone adapted for you?

I know that the OD is supposed to lower RPMs but even so putting my foot all the way in the gas would do something wouldn't it?

OD was meant to lower RPM into the most economical range for cruising, based on factory original tire size, gear ratio etc. The AMC 258 i6 / 4.2l 's even in factory configuration were not exactly high horse rockets, so even before lifts and bigger tires shifting back from OD to 4th gear on hills was not unusual, but in 4th gear you should be able to run up a hill at 60 or 65 no problem.

If the engine is lugging (below ideal RPM range) the timing is off (not enough advance) or the fuel mix is too rich or lean you won't have the power.

So it sounds like Either My exhaust system is plugged up(I was told to check this by a member here and on IFSJA as well as a Mechanic at gas station) Or my gearing is horribly off. Where on the CJ7 AMC20 is the Ratio stamped I'll take a look. Or maybe it is both.

The ratio is not stamped on the housing, but rather a letter from A - D or two letters (indicating a diff with factory Limited slip) from AA-DD. The letter will be stamped on the back of the housing on the passengers side adjacent to the differential cover.

Tell us three things and I bet folks on the forum will offer good advice.

1. Where did he HEI come from? Was it set up for a Jeep AMC 258 i6 / 4.2l ? Who installed it?

2. Letter off differential cover.

3. Other Mods. I'm assuming someone has done a nutter bypass on the Jeep since it has a non-factory ignition system. Is it the original Carb?

Last night there was a loud pop and then the engine under acceleration start making a loud puffing noise. One of the bolts on the Exhaust manifold had backed out. Looks like it just got loose cause the threading is still there but makes me seriously consider the exhaust issue. It sounded like the exhaust pipe was right in the engine compartment....

This is also a good possibility. With the other mods done the catty could be clogged up. After you fix the manifold gasket/bolt issue I would try pulling the cat and putting in a temporary bypass pipe to see if it clears up the problem

 

[Old Dog]

A vacume gauge will tell you if the exaust is pluged up.
If it's a 82 it probably has 2:73 gears. That would make Overdrive pretty worthless.

 

[ThisGuyUKnow]

A vacume gauge will tell you if the exaust is pluged up.
If it's a 82 it probably has 2:73 gears. That would make Overdrive pretty worthless.

Maybe this is where I am getting lost here. I don't think I fully grasp the concepts behind gearing. Assuming that the axle ratio is 2.73(I have no reason to beleive it has been changed from stock) then according to the calculator on Novak-adapt at 80 MPH the RPMs would 2035.77 and 100mph would be 2544.71. So I see how the the gearing would not be Ideal.

I am not an expert so I am not trying to tell you guys that your wrong or anything I am just trying to learn so please don't take it that way. What I dont understand is even if the gearing is too tall why in 5th with my foot to the floor is it not piccking up speed. If I push it in 4th to 75 and up shift and then put the pedal to the floor the CJ gradually loses speed(on a flat surface) till it hits about 65 and the engine sounds like it is spinning at around 2000RPMS(guessing by ear-no guage) In my 98' F150 or my dad's 77' Celica, both of which I have been driving since I was 16, if you shift to early into 5th the engine will bog down and lose alot of power but if you keep in the gas it will eventually spin up to speed. I don't know the gearing in those two vehicles.

So what I am gathering aside from the gearing is that I should:
-Check the exhaust for blockages
-confirm that the timing is correct on the dizzy
and
-Possibly make sure the Carb is correctly tuned

Has anyone here with a T-5 had a similar issue I dont want the Cj to go 100mph but I would like to beable to hit 80 on the interstate at the least.

 

[LongHorn]

Maybe this is where I am getting lost here. I don't think I fully grasp the concepts behind gearing. Assuming that the axle ratio is 2.73(I have no reason to beleive it has been changed from stock) then according to the calculator on Novak-adapt at 80 MPH the RPMs would 2035.77 and 100mph would be 2544.71. So I see how the the gearing would not be Ideal.

I am not an expert so I am not trying to tell you guys that your wrong or anything I am just trying to learn so please don't take it that way. What I dont understand is even if the gearing is too tall why in 5th with my foot to the floor is it not piccking up speed. If I push it in 4th to 75 and up shift and then put the pedal to the floor the CJ gradually loses speed(on a flat surface) till it hits about 65 and the engine sounds like it is spinning at around 2000RPMS(guessing by ear-no guage) In my 98' F150 or my dad's 77' Celica, both of which I have been driving since I was 16, if you shift to early into 5th the engine will bog down and lose alot of power but if you keep in the gas it will eventually spin up to speed. I don't know the gearing in those two vehicles.

So what I am gathering aside from the gearing is that I should:
-Check the exhaust for blockages
-confirm that the timing is correct on the dizzy
and
-Possibly make sure the Carb is correctly tuned

Has anyone here with a T-5 had a similar issue I dont want the Cj to go 100mph but I would like to beable to hit 80 on the interstate at the least.

You don't have enough horse power to over come the drag.
If you were running 3.31 or 3.54 gears you maybe able to continue accelerating in 5th. The reason we need gearing is match the power curve of the engine to the load.

 

[LongHorn]

Maybe this is where I am getting lost here. I don't think I fully grasp the concepts behind gearing. Assuming that the axle ratio is 2.73(I have no reason to beleive it has been changed from stock) then according to the calculator on Novak-adapt at 80 MPH the RPMs would 2035.77 and 100mph would be 2544.71. So I see how the the gearing would not be Ideal.

I am not an expert so I am not trying to tell you guys that your wrong or anything I am just trying to learn so please don't take it that way. What I dont understand is even if the gearing is too tall why in 5th with my foot to the floor is it not piccking up speed. If I push it in 4th to 75 and up shift and then put the pedal to the floor the CJ gradually loses speed(on a flat surface) till it hits about 65 and the engine sounds like it is spinning at around 2000RPMS(guessing by ear-no guage) In my 98' F150 or my dad's 77' Celica, both of which I have been driving since I was 16, if you shift to early into 5th the engine will bog down and lose alot of power but if you keep in the gas it will eventually spin up to speed. I don't know the gearing in those two vehicles.

So what I am gathering aside from the gearing is that I should:
-Check the exhaust for blockages
-confirm that the timing is correct on the dizzy
and
-Possibly make sure the Carb is correctly tuned

Has anyone here with a T-5 had a similar issue I dont want the Cj to go 100mph but I would like to beable to hit 80 on the interstate at the least.

The reason it won't eventually wind up in 5th is because 5th reduces your 2.73 by 30%.

 

[Old Dog]

You don't have enough horse power to over come the drag.
If you were running 3.31 or 3.54 gears you maybe able to continue accelerating in 5th. The reason we need gearing is match the power curve of the engine to the load.

The reason it won't eventually wind up in 5th is because 5th reduces your 2.73 by 30%.

X2. Couldn't have said it any better myself.
Check your gear's to see what you have, there could still be a power issue with the engine.

 

[CJ]

There is a reason the Bugatti Veyron needs all that HP to reach it's top speed. It has to over come the drag, Jeeps are no different... Except they won't hit 250 mph +... :laugh:

 

[ThisGuyUKnow]

You don't have enough horse power to over come the drag.
If you were running 3.31 or 3.54 gears you maybe able to continue accelerating in 5th. The reason we need gearing is match the power curve of the engine to the load.

Interesting I would like to learn more about how to properly match gearing. Not only so I can correct the problem but also I plan on putting a NV3550 behind my 360 in my Grand Wagoneer in the next 6 months or so and I have no Idea how to calculate the proper gearing to go with in the axles.(Looking for a good middle ground between primarily on-road DD and occasional weekend trails)

 

[LongHorn]

Interesting I would like to learn more about how to properly match gearing. Not only so I can correct the problem but also I plan on putting a NV3550 behind my 360 in my Grand Wagoneer in the next 6 months or so and I have no Idea how to calculate the proper gearing to go with in the axles.(Looking for a good middle ground between primarily on-road DD and occasional weekend trails)

I've not seen that Transmission behind a V8, what's torque rating on it?
What size tires are you planning?
This is important to know when choosing your rear end gears.
Your Jeep might have achieved what your looking for if it had stock tires, as taller tires effectively reduce your gearing.

 

[Thorwon]

So it sounds like Either My exhaust system is plugged up(I was told to check this by a member here and on IFSJA as well as a Mechanic at gas station) Or my gearing is horribly off. Where on the CJ7 AMC20 is the Ratio stamped I'll take a look. Or maybe it is both.

Last night there was a loud pop and then the engine under acceleration start making a loud puffing noise. One of the bolts on the Exhaust manifold had backed out. Looks like it just got loose cause the threading is still there but makes me seriously consider the exhaust issue. It sounded like the exhaust pipe was right in the engine compartment....

I think the exhaust is the place to start. When the cat went on my 97 Dodge ram (360 engine) I couldn't make it go over 50 mph. It would start and run at low speeds fine but would not pull at speed.

 

[IOPort51]

it's rated at 300 ft/lb. translate this is it good or bad or just ok?? It's a long sucker, almost 17" and weight of 100 lb.

I've not seen that Transmission behind a V8, what's torque rating on it?
What size tires are you planning?
This is important to know when choosing your rear end gears.
Your Jeep might have achieved what your looking for if it had stock tires, as taller tires effectively reduce your gearing.

 

[LongHorn]

it's rated at 300 ft/lb. translate this is it good or bad or just ok?? It's a long sucker, almost 17" and weight of 100 lb.

IMHO it's a Bad idea to spend the money, sweat & time it takes to do a Transmission swap to change to a weaker unit.

 

[ThisGuyUKnow]

I've not seen that Transmission behind a V8, what's torque rating on it?
What size tires are you planning?
This is important to know when choosing your rear end gears.
Your Jeep might have achieved what your looking for if it had stock tires, as taller tires effectively reduce your gearing.

Any NV3550 or AX15 swappers out there? - Page 2 - International Full Size Jeep Association
Yeah one guy here went with the ax-15 and another the 3550 and there are two other members on the forum that have gone with AW-4 Automatics as well as one other member who had installed a 5.3l Vortec in his Cherokee and ran the GM version the NV3500 with 35s and I think 4.1 gears?

It probably wouldn't hold up to hard core offroading but if you were going hardcore I would think an OD tranny wouldn't even be on your to do list. For a DD rig with the occasional Offroad adventure should be a good choice.