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Discussion Starter · #1 ·
Like the title states, my car wont rev past 6300-6400, it's like it's hitting a rev limiter. Even if I ease into it to that rpm, it still wont go past it. I noticed that in my datalogs, when I map out rpm and vbat, there is a correlation.

when my rpm's peak, my vbat goes down and the peak of the rpm and trough of vbat coincide. Tonight, at 6409 rpms, my vbat was down to 12.06. when cruising, vbat stays around 13.3.

I have had my alternator and battery tested. The battery was fine, but low on charge, 12.3V iirc. The alternator tested out fine as well. I have datalogs of the car going up to 7200, so I know it used to rev up that high.

Could anyone offer up where to start looking for issues? The car doesn't stumble or do any kind of other missing anywhere else in the rpm band. I'm still thinking alternator issue.
 

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Well, it sounds like you have a couple of minor issues, but they are not directly related to your root high-rpm voltage problem. Generally (and I have no idea which car or alternator type we are talking about here), low voltage at higher rpms are most commonly poor cables or connections, slipping belt (bad tensioner), poor brushes or brush spring pressure, poor slip rings, or bad regulator. It can be other stuff - but those are most common.

I would start with simple, like checking for belt glaze indicating slippage. If no joy, test for voltage drop through wires and connections - e.g., max 0.2V drop from alternator stud to battery post. Note that voltage drops affect the system worse as voltage and loads increase, so it might do OK at idle but have issues as seen. This all takes a few minutes so might as well give it a shot.

If no joy, and you say it's been bench tested (?), then I would do a field grounding to verify if the alternator brushes and slip rings are capable at high rev's (as opposed to the regulator). For this, the RPMs are raised enough to see the drop occurring, then ground the field momentarily to bypass the regulator (see pic below). You should see the voltage jump unregulated. Do not hold the grounding for long or allow it to exceed 18V. This is just a check to see if it will jump and that it's mechanically OK. If so, it's most likely the regulator is bad, and you can pull it for separate testing to verify. If not, and voltage does not jump, your alternator is mechanically toast and needs attention or replacement.

HTH. At least it should give you some direction to find clues for your issue. Good luck!

David

Field grounding on a typical Ford IR setup. Be careful doing these tests, as it's easy to get stuff (or you) wrapped into the spinning parts and belts:
 

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What are you logging with? I'm just collecting information because I need something for my car.

The new mustangs all have rev limiters unless you have defeated it somehow. I presume this isn't your new car.

From my own past experience buzzing mine over 7000 I have rattled a couple of alternators to death. I finally got an under drive pulley and haven't had to replace it since then.

Is it possible the alternator works ok until you wind it up and then it starts to go south or could the voltage regulator be crapping out? I'm not sure how you would test either of these theories except to replace them.
 

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Could it be his springs are going south? I normaly turn my car 9300, sence the springs are going weak 72 she starts to fall down. I shimmed them all the same , ran twice and 3 have fell back down . Not gona run again till I get new ones. Just a thought
 

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Discussion Starter · #5 ·
I use Binary Editor with a Moates Quarterhorse on the A9L ecu in my '66. Great program, allows you to change anything, real time datalogging with dashboards, real time tune changes. I like it a lot.

I haven't ruled out the possibility of the voltage regulator either. Guess I could just get a 1 wire and rule them both out. Alternator is rather old, and it's the 60amp. Wanted to upgrade it anyways since adding the EFI.

Gonna try one more thing in the tune, then I might be buying a new alternator.
 

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Tonight, at 6409 rpms, my vbat was down to 12.06....
That's not enough of a voltage drop to cause a misfire.You need a lot more of a drop to cause a misfire that limits RPM. Check the spark plugs first. Don't over think it just cause its EFI.
 

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My first thought is that often the BatFac (injector ms/V correction) is off, and the injectors are not getting the correct increase in open time for the reduction in voltage. They get 'lazy' as the voltage drops. It's especially common with upgraded injectors. Some systems require an unusually high compensation factor - more than the actual increase in ms to get the fuel in.

Easy to see it in logs though. The O2 should go leaner approaching misfire. If it remains proper AFR and suddenly spikes dead lean, that's an ignition misfire, unless you are running very high fuel rail pressure. Then it could be injector pintle threshold (sticking or slow pintle un-seat) with the combo of low volts and high pressure.

As Gary indicates, ignition misfires are not likely at that voltage as they also have compensating dwell to make up for it, assuming the coil is fresh stock spec or better. There's less than 2.3ms for the coil to charge for each firing at 6300 assuming a .1ms firing period, so it needs to be in good shape.

David
 

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This might be 2 unrelated issues perhaps.
Sounds like it. Root problem is voltage drop as revs increase. I'd just do a Ford 95 to 130A 3G or 6G conversion (1, 2 or 3-wire) for cheap, easy, stable, and efficient. Do the testing to verify. If so, do it and move on.
:tup:
David
 

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Discussion Starter · #10 ·
I'm not overthinking it at all. That's why I first took my battery and alternator down to have them checked. Coil was a brand new unit, stock ford, roughly 2 - 2.5 years old. Voltage regulator and alternator are of unknown age, at least 5+ years each.

current alternator on the car is whatever advance auto would give you if you went in and asked for one for a '66 mustang v8. All's I know about it, is that it's a 60A one.

I do think i'm going to upgrade the alternator, as i've been wanting to do it for awhile now. Car license plate renewals ate up my '66 cash for this month though. Guess I'll get to it in a couple weeks. Still drives fine, everywhere else, just not above 6300.
 

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Does your engine already reached higher RPM than 6400?

If not, it could be valve float (had the same at 5500 with stock heads)
 

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Discussion Starter · #12 ·
it will/has been past 6400 multiple times. Have Vic Jr. heads and K-Motion K800 valvesprings.
 

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Discussion Starter · #13 ·
double post
 

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Does your engine already reached higher RPM than 6400?

If not, it could be valve float (had the same at 5500 with stock heads)
x2 As mentioned valve springs are starting to sack out, slack is building up in valve train and allowing lifters to pump up.
Readjusting your lifter preload from zero to 1/16 turn will help out and allow engine to rev out.
 
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