I find it very unlikely a bad VR would cause this. During cranking, the alternator (and hence VR) has little to do with anything....its 99% battery voltage until the motor starts. If the VR was bad the alternator output may be out of whack (which it isn't 13.8V hot). I also don't see if a VR was bad , how it wouldn't show up on the ammeter. Even so, I had another VR on this car for a time and the issue was still present.

I would find it more likely to be a bad/undersized cable then a charging system issue. The battery is pretty much new and has given me no indications on being bad.

I don't mind stepping up to a heavier duty cable but nobody seems to make them for my car and I don't want a generic cable spliced in all over.

I wish I could do a voltage drop test along the cable but no way I can get a probe on the starter terminal with hot headers. Aren't the terminals on your mini-starters up against the block basically? I can't even get the terminals on/off the starter without pulling the starter first and rotating it.

Probing for voltage does not require any current handling, so put a ring terminal on some 16ga wire that is long enough to get one end out safely away from the heaters and somewhere convenient where you can then put connect to the probe on the meter. Of course the ring terminal will need to be mounted on the starter stud. Tape up the free end while warming up the vehicle.

Some terminals are close.Mine were so I trimmed some off of them.Its still encased with the headers.I can still get a alligator type clip on mine.Its tough but can be done.Rocky

**UPDATE**

I took the car out for a 15 minute drive and got it pretty warmed up. I parked it and did some testing. During the hard start condition, I did the following:

Measure voltage across the battery during cranking with a remote starter so ignition disabled. Battery voltage was 13V (fully charged), and when I fired it, voltage dived (expected) and recovered at 11.4-11.0V during about 3-4 seconds of cranking. I would note that during this time, the starter began slow and then sped up to almost normal.

One thing I did notice that seems to be making this marginal situation worse is my electric fans. These are tied to the ignition circuit so when the car is hot enough to have the fans engaged, I have a 20 amp draw and .5-.8V drop with the car off.

Where as the started turned over slower (but fast enough to start the motor) with the fans off, the fans on seem to push it over the edge. I know these newer starters take less current than the old style starters but are sensitive to inadequate voltage.

I may need to wire a toggle switch in series with the sender unit so I could disable the fans, if need be.

I still have a slow-ish start so the fans aren't totally my problem but it is making this worse for sure.

Aside from the fans issue, does the battery voltage during cranking seem in-line with what you would expect?

Put an oil pressure switch in the ground side of the relay's coil circuit. This will keep the fans from coming on till the engine has oil pressure and you're less like to forget to turn it on as it's automatic.

Again, I reiterate, the contacts inside the starter are likely the culprit. I linked to a replacement set above. Cheaper than a new starter.

I have been working with Denso starters for over 30 years and I can tell from the symptoms it has NOTHING to do with the contacts. The starter would click and not start every time. His car is cranking slow so the contacts are completing the circuit like they are designed to do.
As far as the OP's thought on heat soak I have seen these starters so hot the decal melts off and they still start the car. I run an 11" flywheel and the starter in my Savoy is actually almost against the starter and it cranks perfect.

Gus

I removed the terminal block on my starter and I have plenty of room to get at it on my 63 as you can see it has alot of room on the drivers side. I can go right between the header tubes and get on the starter terminals. But I am lucky to have the room as you may not as you have stated. The 11 volts while cranking is good but you said it dropped more then that first and then came back to 11 or so volts. I would want to see the starter draw when it first drops and when its cranking showing 11 volts. I have seen starters with worn bushings do that where on the hit of the key they drag at first and then once they turn some they will center some in the bushings and crank better. From what you said it sounds like it could be in the starter but you can tell more if you see the starter amp draw at first hit and while its cranking. If it draws way to many amps at the hit it I would suspect starter dragging causing to much draw. And if any way to get to the starter terminal where you put the voltmeter positive on the battery positive post and the voltmeter neg on the starter hot 12 volt feed terminal and should have no more then .5 volt drop when cranking. I have seen them as high as 7 volts with a bad cable or connection. Is there any way you can put a 1 to 2 ft jumper wire on the starter hot stud when the eng is cold and leave it on there and put the other end of the jumper off to the side but insulated so it dont short to ground so that when its hot you could just hook your voltmeter to the jumper wire so you dont have to get around the hot header ?? Then do a voltage drop test also when hot. The voltage drop test and the starter draw test will tell you a ton about the starter circuit. Ron

you mentioned lengthening the leads to the starter by 6". how was this accomplished, what ga. wire and terminals was used for this, and do you have any pics how this was done/routed ? possibly something here to investigate.

The ammeter reads current, not voltage. Voltage can drop even if current stays high. Without enough of both when running, your battery won't fully charge. Add to that the engine is easier to turn over cold than when hot. Also, like stated by others, a bad ground, poor connection or corroded wires can cut your voltage at the battery or starter.

If the VR failed open and the alternator was allowed to overcharge 15+V, that would show up as a constant charge on the ammeter. Obviously if it failed closed and kept the alternator from charging enough (below 12.5V), the ammeter would show a constant discharge most likely.

Yeah voltage can drop and you can have lots of current, in the wrong direction (from the battery). Either way the ammeter tells me this.

BTW I had the cable custom made so it's not spliced together to achieve the 6" extension. The leads are 4/10 ga if I had to guess...whatever the factory used. Cables aren't corroded and grounds are fine. The starter grounds to the block I would assume and I have a 4ga neg cable that goes to one of the A/C compressor brackets on the WP housing. It's aluminum, not painted, and free of corrosion.

I had a slow crank hot situation on a 496 with a Denso starter. Battery in trunk originally using the car chassis as the ground/negative cable. Adding a dedicated 1 gauge ground wire from battery to block helped a lot.

Heat does slow the minis down but as many have said, with heavy cables and a good battery they usually get it done.

One option for the cables is to make your own. NAPA has all the stuff... heavy wire, ends, heat shrink. I crimp the connections as best I can, heat with a propane torch then feed in rosin core solder until the crimp is full. When cool, slip the heat shrink over the connection and warm it up. Done, neat looking and functional.

Summit has 2ga premade wires I may be able to work with. Would that be enough extra capacity over a 4ga to make a difference?

I also agree the problem is not the starter, your cables are either too small or you need a better ground cable. Is your battery located in the trunk ?

Nope. Battery sits in the tray right where the factory put it.

Double check your cables and the terminal ends. You may have a corroded cable? Make sure that your engine and battery are grounded good to the frame. Redundant grounds are good, you can't have too many grounds. Check the connections at the starter relay,and make sure the connections are good at the starter.

When we were running the dirt track car, we had a single 0 copper strand welding cable with both ends soldered and shrink wrapped for both battery cables. We never had a heat sink issue. A 4g battery cable is too small in my opinion. I have #2 gauge cables on my V6.

Auto Zone should be able to test your voltage drop and amp draw on your car at their store.

A hot crank and start of a big cube motor needs all the battery voltage available. Isolate those cooling fans during engine cranking, grounding the fan relay through the op sending unit so the fans only run when the motor is running is a great idea. I was never happy to hear fans running 7 minutes after I shut the car off, they pull a lot of voltage out of a resting battery. Voltage needed to start the motor. Gene

Yes...

Try taking a few degrees of timing out of it and see what happens.....

Or it could be like mine, show nothing unusual on the ammeter and not fully charge the battery. Also, 4 gauge cables are vastly undersized for power and ground. We have 4 gauge wire here at the shop and the only thing we use it for is wiring in 1500w power inverters.