Starter motor

[lowact]

Trying to decide if I will need to upgrade my starter motor.

Upgrading my (matching number) engine to HE spec., this will increase cylinder pressures from 160 to 250 psi, making the starter motor’s job a lot harder.

The starter motor from my donor (XJ12S3 HE parts) engine (Lucas 27457E) appears very similar to my E-type starter (Lucas 26327D), 10.6 kg, only the pinion dia. and gearing is different. On the E the flywheel ring-gear/starter pinion tooth ratio (= torque multiplier) = 134/9 = 14.89, on my donor engine this ratio is 160/9 = 17.78. In other words, due to the gear ratio the HE engine starter arrangement delivers ~20% more torque (therefore 20% slower cranking speed?) than the E-type. If Jaguar thought it necessary …

Option 1 is to use the E-type starter arrangement, rely on my new, super-duper ECU to start the engine in less than 2 revs.

Option 2 is change to the HE engine starter arrangement; by using donor engine starter with a new, matching, 160 tooth flywheel ring-gear (AUD 365) plus fitting and balancing. Or I could go all out and get a lightweight Fidanza flywheel with 160 tooth ring-gear already fitted (AUD 840).

Option 3 is a new starter with internal gear reduction, either XKs Unlimited’s 37-4003 (0.9 kW, 4.2 kg, AUD430) or Powerlite’s RAC404A = SNGB USM005/1, (1.4 kW, 4.2 kg, AUD500).

I’m preferring Option 1. Just wondering, has anyone else had experience using an early E-type starter motor on a 12.5:1 compression engine, any suggestions, thoughts or issues?

[MarekH]

The standard starter motor will spin the engine at 120rpm.
The super duper starter will spin the engine a bit faster than that and use less electricity doing so.

The track record of the standard option goes back ~30years reliability.
The newer option has less reliabilty running to its name.

The newer super duper gear reduction motor is lighter and thus easier to fit.

The only reason I can see changing to the super duper option is if when you need to replace or repair the original, the cost is the same.

The reason for this is simple. With a fuel injected motor (as you will have), you will inject exactly the right amount of fuel to start the engine directly at the throat of the cylinder opening. The carburettored crowd, by contrast, have to fill the float bowls, atomise cold fuel, get airflow at 120rpm to carry this poorly atomised fuel past the water jacket housing, into the black manifolds, up and over the runners over the cam covers and then down to the cylinders which have been opening and closing and pumping since God knows when to create the airflow to bring that petrol to them. Put bluntly, most of the fuel condenses onto the cold metal surfaces along the way and it's a very inefficient way to get started. That's assuming you guessed how much choke to add.

Running a fuel injected car which is datalogged shows this very easily:- my car starts quickly on petrol, but even quicker if started on LPG. I can see the starter motor rpm on the graph and sometimes the engine comes to life after just half an engine revolution - that's on just the third cylinder to be fired. The starter motor simply doesn't come into the equation. Here's a graph which demonstrates how unimportant the starter motor is on a well tuned engine:-

start on lpg graph.png (29.69 KiB) Viewed 4863 times

At t=9.313, the idle valve opens (light blue trace, top graph)
At t=10.031, revs are zero, pulsewidth is zero (pink trace bottom graph)
At =10.141, revs are 663rpm, injector pulsewidth is 12.188mSec
At t=11.203, revs peak at 2461rpm, idle valve is already closing - it's job is done
By t=16seconds, revs settle at 1100 and the idle valve has closed, pulsewidth is 5.7mSec

We went from 0 to 660 rpm in 0.11 seconds. I think the super duper starter motor isn't going to bring anything to the table if you put the right amount of fuel into the cylinders. In this case "12 minus 5.7 is the amount of choke needed" if all of the fuel is atomised and reaches the cylinder.

Essentially, most carburettor starting problems are fueling problems, because the process is so sub-optimal. You won't have that, so you'll never need to thrash the starter motor and battery to get started.

This is a skimmed HE engine, ~13.5:1 with a standard Lucas starter motor.

kind regards
Marek

[madjack4]

Hi just a quick note the early 72 v12 has a different starter motor pinion size to the later etype v12 when i bought my 72 etype it sounded awful when cranking when i fitted a clutch i could see the starter didn't mesh if u have the engine out you can bolt starter on with no gearbox fitted and check if it meshes correctly

Regards

[lowact]

Thx Marek – exactly the experience, rationale/reassurance I was hoping for. I interpret you have the later 160 tooth ring-gear with the 25 mm dia. pinion (my option 2). I have the earlier 134 tooth ring-rear with the 29 mm dia. pinion. Means you have the 18x torque advantage while I will only have the 15x. I guess this means my starting current will be proportionately higher and I’ll get more of a voltage drop when starting. Still, from what you are saying I reckon it will be ok, so thx again.

Jack, my car is also ’72. The teeth on the flywheel (ring-gear) and starter (pinion) are larger. After Engine No. 7S7000 a smaller tooth size was used. As a result the later flywheel ring-gear is very slightly larger and has 160 teeth instead of 134, while the 9-tooth starter pinion gear is smaller, 25 mm dia. instead of 29 mm dia. In yr case, sounds like PO had fitted a later (smalerl-tooth) starter to mesh with the original (larger tooth) flywheel. I guess the solution was to change the starter back to one with a 29 mm diameter pinion?

[madjack4]

Hi thats correct i had my original starter reconditioned and managed to get the larger pinion from a jag breaker and sent it away to be rebuilt been fine for 4 years now

Regards