What amps does a starter motor draw on a 911 engine - re selecting a battery fuse size/amps to protect electric harnesses?

Starter motor cable is not typically fused on vintage vehicles. Current draw can be enormous (several hundreds of amps) of inrush and even worse when cold or motor has high compression.

The problem with trying to fuse is that extended (extreme use case) cranking is actually more likely to melt the insulation before you would blow the fuse and you don't know which side of the fuse it will melt on 1st; the fused side or the feed from the battery. Batteries are rated for Cold Cranking Amps and 600A isn't an uncommon number. in cold winter with sluggish oil, and a high compression motor, you'll draw nearly that 600A. Not a problem in CA I know.

There are 175A bussman type automotive fuses (actually go much higher) for modern power distribution panels in vehicles but that is a different application than putting a fuse on the starter motor feed from the battery.

Best bet would be to use an inductive current clamp (Fluke is best but HF actually sells a serviceable one time use unit cheap!) and measure your current draw on a crank cycle when dead cold. Up size 50% from there. Try that for a bit and carry a spare in case it blows so you don't end up stranded!

Are you maybe referring to the fuse for a separate starter relay?

Found this article an article this

https://www.12voltplanet.co.uk/fuses-guide-uses.html

I think the 911 starters are 1hp, Not sure what that equates to in amp requirements.

800 volt ampere (VA) is approximately 1 HP.

12 volt system would require 66.66Amps to = 800 VA.

You will draw more than 66A when cold.


Measurement with inductive current clamp is the way to go if you do this.

No fuse + battery to the starter, important to have a good grommet through the engine tin.

Can't say I've ever worked on a car with a starter fuse, and modern cars have a separate fuse for practically everything.

I have placed a 300 amp fuse between the negative battery cable and the ground connection to the chasi. The battery is now upfront and this fuse is intended to blow if the long positive cable insulation is accidentally compromised causing a serious potential fire. My starter motor is not affected by this 300 amp fuse in series with the battery.

914, 911 used .8hp starter motors until 1973. The 1974 they started using 1.5hp starter motor when the engines went to 2.7 L.

Well, there is your starting point - 300A.

What you need to do is measure what you actually draw and then look up the exact fuse that you are using.

As an example:
https://www.grainger.com/ec/pdf/1DC69_1.pdf

for this particular fuse type, if you look at the chart for the 300A fuse, you will see that if you pull 1000A though this fuse, it will still take about 2 seconds to melt the fuse.

Most 12v batteries will not provide 1000A even when shorted. The reason is that internal to the battery cells there are jumpers that go from cell to cell. if those melt (due to high current draw) or break (due to vibration or crash impact) that is effectively an internal "fuse"

Now look at the same curve if the "short" were only pulling 400A. You'll see that it will take about 100 seconds (1 1/2 minutes +) to blow. Due to the shape of the curve at that region the 100 seconds could easily go to 200 or 300 seconds since the slope of the curve near a vertical in that area.

The question is during the time that the excess current is being pulled by a potential short, will the insulation melt first? That is a bit tougher to answer and you will need to know more about the insulation specs of the cable you've chosen and the gauge of the cable.

To be effective you would need to drop your fuse down to maybe something like a 100A fuse. From that curve you'll see a current draw of 500A would take about 0.5 seconds to blow the fuse.

Now the question is if you are cold cranking the engine what is your actual current draw? If you exceed the 500A (which is possible for battery to deliver) for more than 0.5 seconds, you'll blow the fuse.

Sorry this isn't easy but it demonstrates why fusing the battery cable in not done in practice.

Great feedback - thanks. I will get an amp reading while cranking the engine as my next step. My fuse looks like the picture in the link you gave.

I went with a marine circuit breaker instead of a fuse

Find the chart for your exact fuse. The example I posted is for illustration only and won't necessarily pertain to your fuse.

There is a wide variation in the current vs. time to blow among fuse manufacturers and even within a manufacturer, they often offer different types (slow blow, fast blow, time delay, etc.).

As another idea, you may want to research BMW's a bit. A number of their production vehicles locate the battery in the rear trunk. I don't know if they fuse the main battery feed. Quite often in modern cars, what is going on is they fuse sub-systems on smaller fuses to improve that amp vs. time to blow. However, I don't recall seeing a full B+ type fuse but could be mistaken.

Forget about a fuse. The starter acts as a short circuit until it starts turning. That means the current is limited by the ability of the battery to deliver current and the resistance of the wiring. The shorter the time period you are measuring the higher the current. Digital current meters take too long to measure an instantaneous starting current and are useless for that purpose. On the other hand, a fuse needs time to heat up before it blows; but why ask for trouble.