5 replies to this topic

[coupe-r]

Can anyone with an MPi Mini confirm if the cam trigger peg is supposed to sit in-between the trigger lobes or should it point at ONE of the lobes?

Setting up a Link MoonsoonX ECU and we aren't getting a cam trigger voltage at all when cranking... However its also an aftermarket cam thats should have been exactly copied off the MPi cam.

 

[Ethel]

I've no idea really, but it's thin & concentric so I'd guess it goes between the gap. I wouldn't have thought the position sensing is too critical as it just differentiates when which of the two injector patterns are required & they'll be timed by crank sensor. Just guessing from your piccy & cam lobes position - it looks like number 2 cylinder might be firing when it's triggering the sensor, so the next injection event would be number 1 cylinder. That sound boringly conventional, so may well be right.

[coupe-r]

Yeh from what we can see the trigger goes in between the lobes, which is against all best practices for triggering a VR sensor. Should be a sensor with a small air gap to a distinct TOOTH passing close by it. Especially when the cam is moving so slowly when cranking... But hey British engineering right....

Its an aftermarket cam so prob made of different material and not as magnetic for the sensor as the factory steel camshaft...

"I wouldn't have thought the position sensing is too critical as it just differentiates when which of the two injector patterns are required & they'll be timed by crank sensor."
The cam sensing is CRITICAL for the ecu to know WHEN to fire injector 1 and 2, if its done off the crank only, then all you have is batch firing. Meaning u can't control injection timing for each bank (it would spray them together) then one cylinder rob MOST of the fuel cause Siamese port head.
Thats why they added a crank sensor on the MPI minis as its actually sequential injection with injector 1&2 tied together and 3&4 tied together.

They then fire injector 1 twice and injector 2 twice, but at SPECIFIC crank degrees (usually 390-410 BTDC start of injection)

Edited by coupe-r, 10 September 2024 - 02:17 AM.

[Ethel]

Yes it will need to know which cylinders to inject, but it only has to choose between 2 pairs that are 360 degrees out of phase by crank angle. Outer will always follow inner in each pair. 

 

Its logic only needs to be:

 

Was there a cam signal since I last squirted?

 

Yes - squirt in to pair A twice according to the crank position

 

No - squirt in to pair B....

 

It's similar to the missing tooth (or teeth) for the crank sensor. It can't possibly tell exactly where the missing tooth wasn't, it just knows the gap passed the sensor, so exactly which tooth will pass next.

 

 

... In practice I bet the cam sensor initiates a full 4 cylinder sequence of crank timed injection & ignition events, so it always starts in the same place off the starter motor.

[Steve220]

It also depends if the ecu is seeing a rising or falling trigger in its settings, as it makes a significant different due to the size of the trigger lobe on the cam.

[Ethel]

I don't think that will matter much. Isn't the Mems timing disc 60 tooth? That's 6 degrees per tooth &  18 degrees across the (2 tooth) gap. I expect both the gap & crank sensor are 90 degrees off TDC, where nothing happens, so the ECU knows what's required well before it has to do something sparky or squirty according to the crank position & speed.

 

The cam "teeth" look intended to give a strong pulse more than an accurate pulse. The sensor  has to sit in an oily crankcase, so whizzing past both sides of it could help clear metallic debris and counter cam endfloat by making the gap to one lobe smaller if the gap to the other gets bigger.