Voltage drop during cranking

Now that I have a data logger (Sniper EFI system) I can see stuff that I haven't seen before. For example, I can see the system voltage while cranking. When the engine is hot the start slows down a little bit so I looked at the data logger and the voltage dips down to about 9 volts before the engine fires. That seems like it might be a tad low since the ignition system might get a little weak at that point.

Anyone else with a data logger know what their voltage is during a hot start? I'm not sure if I have a problem or not. If I do have a problem I'm not sure if it is starter heat soak or perhaps a voltage drop in the wiring.

are you using a relay connected direct to the battery for the ECU ?

Install a double 00 thickest cable from starter to the battery. That helped my hot start issues!

I believe the general spec for the starter system which is the battery cable's (pos & neg) and starter solenoid contacts is no more then .5 volts. Test the voltage drop of the pos and neg cables individually and see what they read and go from there. Ron

I've done this with a grid, like yourself we had around 9v when cranking at best. I put this down to being just the way things are and played with start rpm timings etc to get it as good as I could. Motor would turn lazy, kick back etc.
I've since totally rewired the car and run an earth "ring" around the car with bus bars to pick up earth points for stuff. I've also run a heavy gauge earth cable direct to the starter. The difference is amazing, motor spins quick hot or cold and last time I checked it we had 11V + cold cranking. I have to put this down to a better earth and less voltage drop on the earth side, bad earths also get worse when hot. I'd at least run a good ground wire to the starter to see if it makes any difference.

Andy, that's too big of a drop. Preform a voltage drop test and find out where the issue(s) are located.


Joe

When I was having voltage drop problems, it was the battery cable connections. When trouble shooting, you might want to bypass or check voltage drop across the battery cutoff switch too.

Done this many times with a meter, around 9v is approx. what I always got. Seems about right to me. Got to give up something to get the work done. Without the alt. the battery is simply discharging to spin the starter.

I large positive battery cable to the starter and good grounds make a big difference.

Just for Info, the MSD grid is only measuring and logging supply voltage (to the grid itself). So theoretically the voltage could have been different across the starter motor.

I have the battery in the trunk with a 1/0 battery cable which should be plenty. But my ground side isn't as large so I'll try adding a second ground cable to see if that changes anything.

Nice thing about having a data logger is I can save the two files and overlay them to see if makes a difference,

My TS car was having issues until we ran a ground cable up to the starter itself. Just was not happy any other way. Likely the 60MM cam huge spring load and babbit bearings are all partly the cause. Ive never had to do this before but this engine is just not happy any other way

Yeah I might do try that too. It is easy to do in the shop by just laying a cable on top of the car and hooking it up at both ends. If I find a place that makes it happy then I can do a permanent install.

Andy - you should measure the voltage drop on the ground circuit. Measure from the negative battery post to the starter housing. I personally would use my Fluke 87 and use the min/max memory function at the fast sampling rate, but just watching it is fine. Between your ground cable and the body ground, the drop should be minimal. I would expect the drop to definitely be under 1 volt. Unfortunately I don't remember what mine came in at.

Voltage drop is the voltage lost from the battery to the starter. It can be measured by actually measuring the voltage from the hot post on the battery to the starter stud while cranking.

Check the voltage at the battery while cranking. If it is also that low, the battery is weak. If the drop is actually a couple of volts, that's something other than a cable being a size or two too small.

9 is pretty low. I maintain a lot better than that running 2ga. from the back of the car. Check connections and check the drop across the master switch if you have one. Those things don't last forever. Also check starter current draw.