Best options for regearing (or replacing) a rear end

[ratdude747]

Max torque for an '84 is at either 1400 or 1600 depending on the model. You can see that here: Documentation/Engines/300 Six and then the Specifications tab. Scroll down to the page on 1984.

I see. Per another thread, right now I'm running a Manual Trans (N2B) ECU in it (it's supposed to have a Z1A, and what the stock unit was)... so I guess that should have shifted it down from 1600 to 1400? AFAIK that's the only engine specific difference between the two setups... The carbs are different, but I have a YFA from a manual '86 in my junk pile; there were some minor porting changes but otherwise the only difference I could find is that the manual lever had no provision for a TV/kickdown cable connection (only the ball for the throttle/cruise cables). Performance wise, no change from the old ECU (which I suspected was having TPS and O2 input issues)... so this thread's issue is still existing (I drove it on the highway through a lot of hills for 6 hours today, 12MPG and a lot of downshifting!).

So theoretically it's where it's supposed to be for economy... except that it can't put out enough power at that low of an RPM to overcome high speed wind resistance and/or anything resembling a hill... which forces it to kick down to 3rd gear, mostly nullifying the benefit of overdrive. It pulls just fine out of overdrive (turning 2800-3000 RPM)... but that's only with no or moderate load (I haven't had to haul a trailer or anything close to the weight limit), so it doesn't count. Short of a dyno test, I can't say I really know how I'd tell if the engine is actually meeting spec or not. Maybe it's supposed to be this gutless at modern highway speeds???

[Gary Lewis]

I see. Per another thread, right now I'm running a Manual Trans (N2B) ECU in it (it's supposed to have a Z1A, and what the stock unit was)... so I guess that should have shifted it down from 1600 to 1400? AFAIK that's the only engine specific difference between the two setups... The carbs are different, but I have a YFA from a manual '86 in my junk pile; there were some minor porting changes but otherwise the only difference I could find is that the manual lever had no provision for a TV/kickdown cable connection (only the ball for the throttle/cruise cables). Performance wise, no change from the old ECU (which I suspected was having TPS and O2 input issues)... so this thread's issue is still existing (I drove it on the highway through a lot of hills for 6 hours today, 12MPG and a lot of downshifting!).

So theoretically it's where it's supposed to be for economy... except that it can't put out enough power at that low of an RPM to overcome high speed wind resistance and/or anything resembling a hill... which forces it to kick down to 3rd gear, mostly nullifying the benefit of overdrive. It pulls just fine out of overdrive (turning 2800-3000 RPM)... but that's only with no or moderate load (I haven't had to haul a trailer or anything close to the weight limit), so it doesn't count. Short of a dyno test, I can't say I really know how I'd tell if the engine is actually meeting spec or not. Maybe it's supposed to be this gutless at modern highway speeds???

You didn't say what speed you drove, but my 460 will return ~12 MPG at 65 MPH, so I believe something is wrong with your setup. And that's turning ~1800 RPM with no shifting, regardless of the size of the hill.

I wouldn't have thought the ECU could make the carb rich enough to drop you from ~18 MPG, where I think you should be, to 12. That's burning 50% more fuel, and that is a lot to attribute to AFR.

Assuming you've checked for codes, the only thing I can suggest is that the ECU isn't advancing the timing. That would cause it to be gutless and the MPG to go away.

[ratdude747]

You didn't say what speed you drove, but my 460 will return ~12 MPG at 65 MPH, so I believe something is wrong with your setup. And that's turning ~1800 RPM with no shifting, regardless of the size of the hill.

I wouldn't have thought the ECU could make the carb rich enough to drop you from ~18 MPG, where I think you should be, to 12. That's burning 50% more fuel, and that is a lot to attribute to AFR.

Assuming you've checked for codes, the only thing I can suggest is that the ECU isn't advancing the timing. That would cause it to be gutless and the MPG to go away.

I'll recheck on the new ECU, but on the old one, I was getting timing advance (30 degrees at idle). Based on how it's idling, I'm pretty sure I'm getting timing advance on the new one too. The old ECU kept self testing as lean (no stored codes), yet my O2 would show 0.8V (rich)... which is what had me perplexed. Pretty much the entire system has been replaced (or rebuilt, in the case of the carb).

I meant to bring my code tester on the trip, but bozo here left it at home.

[ratdude747]

You didn't say what speed you drove, but my 460 will return ~12 MPG at 65 MPH, so I believe something is wrong with your setup. And that's turning ~1800 RPM with no shifting, regardless of the size of the hill.

I wouldn't have thought the ECU could make the carb rich enough to drop you from ~18 MPG, where I think you should be, to 12. That's burning 50% more fuel, and that is a lot to attribute to AFR.

Assuming you've checked for codes, the only thing I can suggest is that the ECU isn't advancing the timing. That would cause it to be gutless and the MPG to go away.

We got codes... and it's in limp mode.

Posting about it in the other thread, as not to further derail this one.

[Gary Lewis]

We got codes... and it's in limp mode.

Posting about it in the other thread, as not to further derail this one.

I don't think you will derail this one. I think you will close it out.