With your correction of 10 volts and 10.5 volts (not amps), we can proceed.

If during cranking, the voltage drop falls below 10 volts (yes, the starter motor as well as all electrical loads, uses available source voltage), either the battery capacity isn't sufficient and/or engine and/or other mechanical friction is excessive causing the starter motor to drag.

However, you said you installed a battery with a 800 CCA rating. That's good, so let's assume the battery has sufficient cranking capacity.

Is this a newly built engine? Whatever. Check cranking speed (and voltage drop) with and without the spark plugs installed. Engine should crank freely w/o the plugs. Voltage drop should also be less since there's less cylinder compression (0) to overcome. Is your engine trying to squeeze A/F above 12:1 compression?

As Mahler9th suggests, prime the engine by first activating the injectors. Introducing some air/fuel before cranking (Ign. OFF) should be enough to start the engine (w/Ign. ON) and allow the now-operating alternator to take over and provide source voltage to all electrical accessories and recharge the momentarily discharged battery.

Another thought is that the starter might be faulty or under-capacity. The starter motor might have a short, bad connection or internal mechanical drag (armature to field coil) which can also draw excessive battery current as well as reduce cranking torque/speed.

Hope this helps,
Sherwood

I think this may be all about the EFI/injector circuit and performance design, not the battery, available power, frictional losses et cetera, et cetera.

In my case, the injectors won't spritz below a certain voltage, and that level is easy to exceed when cranking and trying to start.

In my case (albeit with different variables), my car was an unreliable starter before "intervention," even with a PC-925 battery. My starter is a ubiquitous y2k era lightweight, and my engine was and still is a high compression mill.

The solution I described worked immediately in ~2002, and has worked ever since.

My new race class requires >200 pounds of ballast in my car. So I could go to a larger battery. But I prefer to put my ballast in strategic places, and stick with my small motorcycle battery.

Starts every time with a squirt into the cylinders.

check the battery with a MM while cranking to verify volts.
if the ECU has bad power it only reports what it sees.

if the battery DOES drop that low then work the starting part of the problem to figure out why it is dropping that low.
poor connections
bad starter
hi compression + other issues
a lot of advance at idle can make it hard to start.

Thanks to all for the ideas and theories. I finally got the car to my new shop and installed the lift, been in transition for a couple of years. I'll take a look at the starter, grounds and connectors. I've also contacted Clewett Engineering for a wiring diagram to momentarily ground the injectors and or increasing the wire size to the injectors. I'll post back when I solve the problem. Thanks again.