injector impedance

[Lurksalot]

Following a previous post I think I have tyo change the Impedance of the Injectors to suit the MBE ecu just in case

Whats the best way

fit resistors in series or new injectors

also what are the consequences with regards fuel mapping etc of this change

many thanks

Kev

[corsechris]

Series resistor obviously considerably cheaper. There will be some 'slugging' of injector response so you'd expect a slight change...but I'd be surprised if it's significant enough to force a re-map???. Waiting to be corrected on that, mind. Chances are, changing the injectors would give at least as big a difference as putting in a series resistor, if not more.

Just waffle though, never done it myself.

[Kevin Wood]

Low impedance injectors with series resistors will still have better open / close times than high impedance injectors so definately not worth binning the injectors.

A resistor of about 2-3 times the injector's resistance in series with each injector will do the trick - so about 6 - 9 ohms. Alternatively, parallel all the injectors and use a resistor of about 2-3 ohms in series with the lot.

In theory the power rating of the resistor needs to be about 10 watts for each injector. You'll probably get away with using a lower rated resistor (say half this) unless you're running it full chat for long periods of time though, since the duty cycle will be low.

Edited to say: Regarding the impact to fuel mapping I'd say it'll make a pretty negligible impact. The open / close times will change slightly. This will impact fuelling a little at light load where the pulse widths are short (Idle / cruise) and not at all at the critical areas (high load) where the puls widths are long enough for the open / close time to become insignificant - assuming it's been mapped accurately to start with.

Kevin

[Lurksalot]

Many thanks

Kev

[Lurksalot]

1) Ok so I am off to maplins to get 4 Qty     10 ohm 7W wire wound resistors , a solder gun and some heatshrink tube .

2) I will solder 1 resistor into each of the wires from the injector to the ecu

3) My potentential ECU fry up is averted

Does That sound about right    

(Plenty of yes responses would really help  )

Kev

 

[corsechris]

I'd be tempted to go a bit lower with the resistance and a bit higher with the wattage. Say an H8R2 which is 8R2 and 10W. Even 10W is a wee bit marginal anyway - if you assume a 3R injector and an 80% duty cycle, you're still dissipating 10W or so (depending on how close to 12V you get across the injector/resistor combination and assuming a simple resistive model to calculate power). But they do have a big (10 times) short-term overload margin so you might be OK. Be tempted to just try it, but if you are paranoid, try using a 3R9 and a 4R7 in series in place of each 8R2 - that would be well over-rated. Only costs an extra £1

[Kevin Wood]

I'd be tempted to go a bit lower with the resistance and a bit higher with the wattage. Say an H8R2

Yep. Me too. That'll do the job. Remember the battery voltage will get dragged down quite a bit when cranking so erring on the low side will give you some margin without giving the injectors an unduly hard time.

if you assume a 3R injector and an 80% duty cycle, you're still dissipating 10W

True but that's at full RPM and full load. The duty cycle drops off quickly at lower revs and lighter loads. Depends on how the car is going to be driven, I guess.

You will need to make sure the resistors can dissipate the heat though. Wrapping in heat shrink will require them to be derated quite a bit because their rated power will be assuming free air. If it were me I'd be tempted to use metal clad wirewound resistors and bolt them to an ali. panel under the scuttle or under the bonnet.

Also remember the ambient temperature under the bonnet will be much higher than the 25 C that the resistors are likely specced at.

Kevin

[Lurksalot]

I'd be tempted to go a bit lower with the resistance and a bit higher with the wattage. Say an H8R2 which is 8R2 and 10W.

Sorry guys I don't understand your number and letter codes    they don't write them like that at maplin

do you mean 8 ohm and 10 watts

Is there anything ready made that I can use or is this still the best way

maplins  hipower wirewound 10 watts are rated to 70 dgrees  if that helps

cheers

Kev

[corsechris]

Hi Kev,

I'm sure I got that code straight off their site just to avoid confusion...sorry!

Anyhoo - convention is to indicate a decimal point in a resistor value with an R, so 8R2 is 8.2 Ohm.

You'll probably be OK with the 10W chappies and as Kevin said, the maximum dissipation will only occur at WOT and Max revs (or at least max power), so give them a go if you can't find anything else.  If it was me, I'd make up some 20W chaps by using 2 smaller ones in series (use the values in my previous post), or ideally perhaps, get some 25W types that you can bolt to a chunk of ali as a heatsink. Some may consider this OTT.

Maplin don't give the thermal data for the resistors so it's tricky to be accurate...but if you assume stated rating is in free air @ 25C (which is pretty typical), you can make a stab that the resistors get hot @ roughly 4.5C per W, so you'd need to de-rate them some for being wrapped up (increases the temp rise per W dissipated) and some more again for being in a hot environment (reduces the headroom you have before you reach thermal damage), which would reduce their effective rating quite a bit...hence the suggestion to play safe and either 'make' a 20W resistor or buy one. Looking at the old Maplin cat, they used to do some nice 50W resistors in bling style gold anodised packages with bolt-tags.

Do hope this helps - never sure how much background is useful or just plain confusing.

[Lurksalot]

Do hope this helps - never sure how much background is useful or just plain confusing.

dead right, but I dont' mind learning , I just have to read carefully until it sinks in

And yes Thanks it does help ,

Kev

[corsechris]

Cool.  The basic equations really are simple - I=V/R  and P=IV.  Resistors in series  - add the values, resistors in parallel - result = product/sum. The rest can be worked out from these. Where it gets more involved is in having to think about power dissipations and what they do to the components in question. Best in this case to assume all the components are pure resistors (no such thing in practice, but a fair approximation here).

But waffle aside...let us know how it goes and what you decide to do for the resistors.

[Lurksalot]

Ok

Ive gone for the 8R2 10W wirewound but I have another question with regard temperature

Is the temperature rating question with reference to the current generating the heat within the reisistor or the external influence of the heat under the bonnet.

Kev

[Kevin Wood]

QUOTE

Is the temperature rating question with reference to the current generating the heat within the reisistor or the external influence of the heat under the bonnet.

Both have an influence. If the resistor is rated at 10 watts at an ambient temperature of 25 degrees c, for example, it means that the body of the resistor can dissipate enough heat to limit the temperature of the resistor to a safe value (maybe 100 or more deg c) when the surrounding air is 25 deg c. Raise the surrounding air to 60 deg c and the resistor will have to run hotter still to dissipate 10 watts of heat. That may result in it running too hot.

TBH every installation is different and a "suck it and see" approach is probably best in this case.  Just keep an eye on the temperature of the resistors until you know they're coping OK. If they discolour at all in use it may eb an indication that they're running a little hot.

Cheers,

Kevin

[Lurksalot]

that makes sense .

thanks for the help

Kev