Sm4 Injector Driver ?

[Rolando]

What is the largest injector (lb/hr) the SM4 can control before the unit need to be modified and who would perform this mod? http://www.mrm-racing.se/forum/images/icons/icon6.gif

[Ian]

I think you are asking two questions here. 1. What is the highest injector current that an SM4 can drive? 2. What is the largest flow rate injector that an SM4 can accurately control?

1. SM4 ECU's have 4amp switching type injector drivers. This means that the driver can sink up to 4 amps of current before switching into its Hold mode. This is because virtually all injectors need more current to open than current to stay held open. You could run the injector in saturation (current limited by injector resistance or driver current limit) but it can overheat both the driver and injector. It is better to lower the current after the injector is opened to minimise power dissipation in the injector winding and driver.

However, saturation current is not related to opening current. A 3 ohm injector will have about 4 amp saturation current but it may only need 1.5 amps to open it and less to hold it open. That means you could properly control 2 of those injectors in parallel on the standard driver without modification. But if those same 3 ohm injectors had an opening current of 3 amps then you could not use them in parallel reliably.

2. In relation to accurate control of big injectors the limiting factors are the minimum injection pulse time required in the installation and, not only if the ECU can control down to that time, but also if that time is within the physical operating range of the injector. Injectors have a minimum time to close when the drive current is turned off. This time is due to electro-mechanical characteristics of the injector and some injectors have minimum reliable operating times of 2.5 msec while others can be less than 1.0 msec.

If a big injector on a small cylinder reaches its minimum operating time erratic mixture control will result as injectors stay open longer than commanded & matching may not be consistent with all injectors on the engine. Minimum operating time issues are usually seen on high compression N/A engines at 2000-2500 rpm with very light load with older style L-jetronic injectors (This was a real problem with batch fire ECU's as the injection time was halved due to firing twice per cycle compared to once per cycle for sequential)

I think an SM4 controls down to 0.7 msec and SMC / SM2 is 0.9 msec.

[Rolando]

Ian, Moran Motorsports racing fuel injectors operate linearly with a pulse width as small as 1.5 miliseconds and opens with the standard 4/1 amp peak & hold ECU drivers. Do you think these injectors will work fine with the SM4. I was considering using four of their 325 lb/hr injectors
instead of eight 160 lb/hr injectors. Thanks for your help.

[Ian]

Ian, Moran Motorsports racing fuel injectors operate linearly with a pulse width as small as 1.5 miliseconds and opens with the standard 4/1 amp peak & hold ECU drivers. Do you think these injectors will work fine with the SM4. I was considering using four of their 325 lb/hr injectors
instead of eight 160 lb/hr injectors. Thanks for your help.
Rolando, the SM4 will be able to drive the injector. Is this a 4 cylinder that you are considering using staged injectors with? The thing we need to calculate is if your minimum injection pulse width is less than 1.5msec.

To estimate the minimum injection pulse width for your installation as well as 325 lb/hr we also need to know;
Fuel type - petrol or methanol,
Cylinder size,
Compression ratio,
Camshaft spec - standard / big-ish / big / huge
Valve overlap - standard / bit more than std / lot more than std.

What we need to determine is if the engine will have a lot of dynamic compression above idle. The more dynamic compression the engine has the lower the manifold pressure will be when free revving and thus the lower the minimum injection pulse width. We can calculate the 100% VE pulse width for 100 kpa Load and we can then estimate the minimum from that and the other information.