Rover V8 Trigger Wheel – Megasquirt

Fitting a Rover V8 Trigger Wheel, has been one of those jobs I’ve been putting off. A Megasquirt ECU requires a 36-1 trigger input. Many people fit a trigger wheel to the front pulley, however I wanted to use one similar to a Range Rover P38 automatic flex-plate or a 4.0 Discovery Manual flywheel.

I thought the trigger wheel on the front pulley looked a little Heath Robinson. OK, there was greater opportunity for adjustment but it looked very prone to damage from debris and a pain to adjust. Keeping in mind this car sits very close to the deck and that a piece of stray fence wire had speared our family hatch’s oil filter a month back, I wanted something else.

Fitting a trigger wheel to a P38 Serpentine bottom pulley, isn’t straight forward. Apart from getting the pulley off, the rear face needed machining before the trigger wheel will sit flat. By the time I’d bought the trigger wheel, bracket, sensor and connectors, had the trigger wheel machined etc. I’d be looking at similar money to having a flywheel converted.

The reluctance sensor mounting plate for a front trigger wheel on a 4.6 Rover V8 is a little more tricky than other models as the convenient mountings on the timing cover were removed from the casting.

There may be many out there that swear by the front pulley system as being the best and most flexible way, but all I want is tucked away and forgotten about.

I have a Discovery 4.0 Manual flywheel, that comes with a 36-1 trigger wheel as standard but it weighs 14.1Kg / 31lbs. My 1986 Rover Vitesse one doesn’t have a trigger wheel but still weighs 13.9Kg / 30.6lbs. The Discovery one looks really chunky with a lot of it’s mass at the outside edge, effectively making it even heavier.

I knew a couple of people had machined flywheels, so I contacted Ray at V8 Developments to see if he could help. A week latter I had a lightened and balanced flywheel complete with the 36-1 trigger plate from an automatic flex plate.

Other peoples blogs:

This is a standard Rover Vittesse Flywheel.

This is the clutch face – complete with rather worn surface. – This one was not mine, as it’s actually in better condition. Twenty years of damp storage and a tuned V8 hadn’t done mine many favours. When Ray said it needed to be re-faced, I wasn’t even slightly shocked. I’m guessing simply removing the rust from mine would have saved me 1Kg.

This is the rear face, the side that faces the engine. There is a large raised central mass. The trigger wants to sit right on the angle section, meaning either a groove needs cutting like in the photo below or much more material needs removal.


(center bottom = custom, left = standard )

In the above shot you can see a modified trigger ring, removed from an automatic flex-plate (top) has been modified and recessed.

Above is a standard Discovery V8 flywheel. Notice the 36-1 trigger sits in a narrow groove.


Custom Megasquirt Flywheel

Ray at V8 Developments done the one below for me. A lot of material has been removed and a trigger wheel from an automatic flex-plate attached. Similar to the MGB in the link above, or further down.

I’ll think you’ll agree, that’s a beautiful job! It definitely didn’t look that nice when I sent it. 

The new recess now houses a standard flex-plate 36-1 trigger wheel, furthermore much of the mass from the centre outwards has been removed.

Should ever I need to, I can actually rotate the trigger wheel 45° by detaching it and bolting it back on.

The tips of the trigger wheel sit 4.5mm from the mounting flange surface, whereas the Discovery ones sit further away at 12.5mm. Therefore, I might have to space my reluctance sensor away 8mm. When I remove my automatic flex-plate I’ll measure that and decide.

Look! No rust or deep grooves in sight! It really does look factory fresh. All this for half the price of a new one!

Lightened and Balanced Flywheel

I guess some of you would like to know how much it weighs. Well…..

10.4Kgs / 23Lbs. That a saving of 3.5Kg / 7.6lbs.

That’s not as light as my JEE one at 8.5Kg, but that hasn’t got a trigger wheel and 8 associated bolts, so I’m quite chuffed.

It’s claimed that 1Kg from the flywheel equates to 40Kg from the chassis in first gear (depending upon tyre and flywheel diameter). As most of the mass removed was from the outside portion of the flywheel, this could be closer 1Kg = 80Kg from the chassis!

Therefore, my chassis just lost between 140Kg and 309Kg (280lbs and 617lbs) in first gear.
This 4.6ltr V8 will rev like motorbike!

This guys done loads of sums:

If you look at the P38 Manual flywheel, the automatic flex-plate above and this MGB flywheel below you’ll see an extra dowel hole. I’ll need to do this on my mill.

You can see that on this flywheel, it initially looks like less of the central mass has been removed. However, the trigger wheel has been set into a groove 9.5mm (3/8″) deep, whereas mine is only 1.5mm deep. – That’s difference is the 8mm thick my reluctances sensor mount needs to be.

Personally, I think a shallower recess, spacing the reluctance sensor and removing more central mass; as what Ray at V8 Developments did; is a better bet, as a 3/8″ recess will only leave a clutch face 7.5mm thick.

The clutch face on an SD1 flywheel is 17mm thick and a Discovery one is between 18.5mm and 28.5mm thick.


Talking of clutches, I’m using a 23 spline, 241mm (9.5″) Clutch from a rwd Ford Transit (MT75 model). It’s identical to the Rover SD1 item. If I get clutch-slip, the 8510309, 267mm (10.5″) from a Discovery can be fitted by machining off the SD1 mounting dowels and adding new wider diameter ones. Given that the 4wd MT75 gearbox I’m using was only designed for 200Bhp, maybe a little slip will save me modifying another box.

The standard Rover V8 reluctance sensor is narrow and passes between two vertical trigger markers, whereas if I were to use a front pulley trigger wheel each trigger lobe passes close to the tip of a reluctance sensor. The gap between the sensor and the trigger wheel is a forum talking point and has been know to cause issues. 

In first gear, at roughly 700Kg and over 400Bhp per ton, this thing will spin all 4 wheels without even trying, so clutch slip probably isn’t going to be an issue.