Starter/Battery Cable (How Long?)

[PabloGee]

Polly Grigora said:

Relay fitted inside the car is best practice

Relay similar to the one below is made for the job, it has bigger contacts than the one you have and handles the heavy pull-in starter current better https://www.autoelectricsupplies.co.uk/p/12-volt-r...



Here's a socket for the above relay if you wish to use one https://www.autoelectricsupplies.co.uk/p/fixed-soc...



Cable heavy enough for the job that doesn't go brittle at high engine temperatures
https://www.autoelectricsupplies.co.uk/p/thin-wall...


Run 4/0.30, 3.0mm², 33.0A, - cable OD 3.3mm Thin Wall cable from battery pos to relay terminal 30, run same size cable from relay terminal 87 to starter solenoid

Job's then a goodun

Ordered. I'm thinking I'll replace the hot start kit with this, including the fuse to the battery +ve
Next to come, the fun bit of running a wire from the footwell...

[Polly Grigora]

Ok then, will be nice when completed

Bit late posting this, better late than never though especially as the job isn't in full swing yet

Covering the cable you'll be running in the engine bay to the solenoid with black fibreglass sleeving keeps the job looking good and adds extra protection

The following sleeving is very cheap for what it is https://undercontrol.co.uk/accessories/fibreglass-...

[PabloGee]

You lot just keeps spending my money...

I still need to buy the terminals.
Thankfully I've got a load of heat shrink stuff.

[Polly Grigora]

Don't be concerned about the money

Once you've completed the job and proven to the world that it's the dogs you'll be able to order hundreds of all the parts used, split them up, pack them into boxes and sell them here as Pablo-Wise Hot Start Kits

[PabloGee]

Ha. Good thinking… 😁

[Belle427]

If you want some cable for the solenoid i think i have some heat resistant 2.5mm and 4mm squared in the garage you can have for postage cost.
Think its black in colour.

[PabloGee]

That's very kind, I now have 3m red and 3m black of the wire mentioned above, so hoping that should suffice.

I have just taken a closer look, and discovered that the red +ve feed comes off the back of a 100A fuse, not a 20A fuse as the modwise kit includes.
I will order a 20A inline fuse (or should it be 25A?), as it seems silly not to since I'm changing the kit.

A bit bonkers how much of a botch this bit of wiring is in my footwell. I'm confident it's the only modification to the standard loom overall, but it seems like the worst version of the hot start kit I could have stumbled across.

[Belle427]

Think it was 20 amp from memory, mine blew once though maybe because the solenoid was a bit lazy.
Think I increased to 25 amp when it was rewired with better cable.

[PabloGee]

I had a good dig around the starter motor wiring, and remembered the previous owner had then removed a battery in the boot - but left the 25mm2 cables.
One from the starter to the boot running along the chassis with the fuel return line with a good length of excess in the boot.
So I’m thinking might take it out and attempt to route it alongside the current 25mm2 cable into the footwell and connect it up to run as a parallel feeder cable.

• the issue is that I doubt they’ll be the same length as it’s hard to measure in this situation*

I can then look at taking the length of 25mm2 black earth out the boot and using that between battery and block.

I haven’t suffered an actual hot start that required a cooling period, and what I can feel of the original cable feels flexible not crunchy.

Does a double run of 25mm2 equate to a single 50mm2, or is that boll***s?

Edited by PabloGee on Thursday 7th December 07:58

[Belle427]

Current carrying capacity will be about the same so yes.

[PabloGee]

So I went all in. Mostly.
TLDR - I have a Q at the end…

Took out the old starter feeder cable from the battery, followed it with a rope and then some 50mm2 with nicely heat shrunk covers and a 60cm length of heat shield wrap (the perfect length to get up to the bulkhead grommet).
Also replaced the whole hot start (basically a relay) with a heavier duty version properly fused and put together.
On first crank it popped the 20A fuse (though my immobiliser starter motor bypass has a 25A) so I put a 25A in and we’re good.

I took out the cable that ran along the chassis rail with the fuel return to the boot, as if I were to do a boot battery I’d run a thicker cable inside the car. It was a much longer run than the original cable…
It also meant I could get the shorter earth cable off the chassis bolt behind the fuel tank and run that through the engine bay grommet and onto the battery - the intent to put a direct battery to engine block earth.

Trouble is I can’t get to the earth bolt on the back of the block, nor can I get to any other stout bolt on the upper part of the engine without removing the plenum top.

So the length of earth cable I’ve re-used will basically reach an impossible bolt, or, I could put it on one of the rear plenum Allen bolts.

Is earthing to the plenum a good idea, or terrible?
What other points are recommendable?
I can run a longer cable through if it’s the right thing to do.

[Belle427]

Can't remember what access is like but maybe a starter bolt is the best option.
There is a threaded hole in the passenger side head at the rear but it's a bit awkward to get to, I'd personally rather see the block grounded though.

Edited by Belle427 on Wednesday 20th December 07:18

[PabloGee]

How’s about this one, it’s on the back of the head, above the head gasket, but below the rocker covers?
You can see a shiny bolt I managed to get into it, so clearly access is possible.
Or does the earthing need to be on the main engine block?

[Belle427]

That would do, I believe some cars had a ground here.

[PabloGee]

Excellent!
You can see the other ground below it, but I just can't get a spanner or wrench on it.

Again, thank you for the help!

[Belle427]

Just make sure its the right thread on the bolt as it isnt metric so may not tighten properly.
https://www.pistonheads.com/gassing/topic.asp?h=0&...

Edited by Belle427 on Wednesday 20th December 20:32

[PabloGee]

Yes, absolutely right.
I used a spare UNC bolt I had as a replacement on the power steering pump. Tried it in other empty threaded holes on the block, and it went into this hole perfectly.

Just acquired a spring washer to absorb the vibrations, so ready to finish the job.

[PabloGee]

Fitted. No spring washer, couldn’t fit it on, but will add a wave washer if it loosens.
It’s quite handy that these RV8s have unused threaded holes on them!


NS head at the back, just above another earth point.

[Belle427]

A job well done.

[PabloGee]

And to finally answer the original question on this post, I have two measurements.

Original 25mm2 starter feed cable - 2.1m - this was the one I replaced with 50mm2 cable and added a heat shield sleeve.
Cable run from starter motor along chassis rail into boot (also 25mm2) - 3.85m

The length of the earth from battery -ve to back of engine block is probably around 1m, I didn't measure it, just reused the 25mm2 one from the boot battery spot (on top of the fuel tank) to the chassis bolt behind the fuel tank. If anyone does this they will need to work out the right length for their application.

Makes me think the battery to boot change definitely requires a thicker cable to handle amp drop over the length, but the route can be made to avoid high heat engine parts, which reduces deterioration rate of the cable. Not sure I would run it along with the fuel return line again though if I were to do this again.