AMM resistor mod - 30lbs injectors [Archive] - SaabCentral Forums

: AMM resistor mod - 30lbs injectors

8th August 2002, 07:49 AM
I have a few questions regarding this, as without it my 30lbs injectors run waaaaaay to rich!
My questions are these:

Until what Rpm is the AMM consulted?

Does it make any difference whether you have a closed loop or an open loop type circuit? i.e. whether your car came or didn't come with a CAT as standard.

Does anyone know who's the Guru of this trick, as maybe I'd be best off speaking to them?

Oh and btw because the injectors only came in sets of eight, I have 4 spare if anyones interested.

Johann F
8th August 2002, 09:14 AM
Remind me of your fuel system - Lucas/Bosch?
TSN perf BB have questions like this concerning the Bosch setup everyweek.

8th August 2002, 10:33 AM
The air mass meter resistor trick was brought about by the Bosch 2.4 injection system in the Classic 900. In the 2.4 overboost is controlled by the voltage output of the air mass meter compared to an rpm map. When the voltage is higher than the map is programed for at a given rpm it shut the fuel system down.
To combat this a 300 ohm resitor is installed in the ouput wire of the air mass meter allowing for more boost. The resitor creates a lean condition. To counter the lean condition 30# injectors are installed. The fule system has a large enough adjustment window to make it work.
The side benifits of this is more fuel is provided at the upper end when the map seems to be only rpm based after a predetermied air mass meter output voltage. The combination works very well giving good drivablity and more power.
This mod with a Aquamist modified 2c water injection will let you run 21+psi boost with out running excessive amount of fuel. Lean is mean.

8th August 2002, 11:32 AM
Mines the Lucas system (without CAT) it is also the adjustable type of AMM, but I've yet to experiment with that.

Already had a look on TSN past posts. I've got some information but the answers to my particular questions were a little vague or there were conflicting opinions.:???:

Is there any way to get them on with this type of setup?

Dropping the fuel rail pressure is not an option. I've alreadi tried this, the pressure ended up at 12Psi :eek: to stop it running massively rich.

8th August 2002, 12:01 PM
I do not have any specific knowlegeof the Lucas system but would think that the principles are the same. Normally when doing the resistor we use a 2.5bar or 3bar pressure regulator. A rising rate makes everything carzy. Try using a 3.0bar regulator and 300 ohms in the ouput wire from the air mass meter. This would be the one that increases in voltage as more air is consumed.
There are conflicting opinions because most do not follow the correct method first they modify it be for they do it and upset the balance.
I ahve made a 2.2 LH work with 30# injectos and a rising rate regulator and a 470ohm resistor but it would have been better with out the rising rate regulator.
Use the adjustment on the AMM for fine tuning and the resitor for getting ti close.

Johann F
8th August 2002, 12:28 PM
You won't find any info from the otherside of the pond as it was only available in Europe. Also the Lucas doesn't have the high rpm lean out problems that plague the the Bosch system. Try adjusting the AMM and see what you get. Was there a particular reason you got bigger injectors?

8th August 2002, 01:29 PM
Thanks Brad, I'll have a go and see what I can come up with, that infos great!

I bought the injectors along with lots of other goodies I was having shipped over from the states, I wanted to cut down on the cost of shipping so I bought a load of stuff. and also as I was hiking the boost a fair amount, and didn't want to run a higher fuel rail pressure (and didn't want to stress an ageing pump) I thought I would need them (No factory lamda sensor so my ecu just squirts a preset amount). ECU's seem hugely overpriced and also I thought it couldn't be a bad thing to replace injectors that have run a few thousand gallons of fuel through them.

However, fueling is fine atm, but as I've bought the injectors I'd like to see whether there's any chance of fitting them. Plus I am planning within the next 6 months to possibly increase the boost further so maybe then I will lean out? - currently I do not.

I am pretty sure I could cope with 20Psi with my mods atm, so if I have to run 20Psi+ to utilise the injectors to an advantage rather than a disadvantage then so be it! :grin:

Engine Mods
Trent Saab APC (R23 removed)
Trent Saab Actuator
Trent Saab RRFPR
Norgstrom non-return valve
Bailey dump valve
30lbs injectors (not fitted/needed atm)
Adjustable solenoid restriction (prevents boost spike)
Gasflowed throttle body
Garrett turbocharger
Magnecor ignition leads (+50bhp at least!)
IC spray
IC fan
Abbott SS exhaust
K&N air filter

Johann F
8th August 2002, 01:37 PM
I'm not exactly sure if the Bosch injectors are at all compatible with the Lucas FI system. What I do know is that the early non Lambda Lucas system is always getting bad remarks on Rover V8 sites ( Lucas CU14 was in Range Rovers and TVR's etc) - however this could be a good source of info though - you may get Lucas compatible injectors with bigger capacity there.

8th August 2002, 01:38 PM

As you have a Lucas car, you are running low impedence injectors, the Bosch cars are high.

The Lucas car already has much larger injectors than Bosch as well. Converting all figures to 2.8bar (as used by our American friends) the Bosch cars are around 21 lb/hr and Lucas 26 lb/hr.

8th August 2002, 05:43 PM
Ackkkk! That really blows! I was advised on buying these injectors by my brother (not a great excuse but one nevertheless) as the second set were going to be his, but he decided on changing his car before he tried fitting his.

So by the sounds of it they are totally useless in my application as you can convert low impedence injectors to high impedence (via resistors), I belive, whereas converting the other way around, would be much harder. However, why was it then, that the engine ran extremely rich? Should it not run extremely lean if it's running on a low impedence system with high impedence injectors?

Maybe I could rig one of the eight injectors up as a fifth injector at some point if I can't sell them.

Johann...Thanks for the info, I might look that way if I decide to purchase some more injectors, I wonder what the complaints about the system are?

Alex...So that would explain why in conjunction with the RRFPR (@ standard rail pressure) the fueling is more than adequate for my boost of 17Psi, as it has the larger 26lb/hr injectors. Mine apparently are even larger than the larger injectors (see post My injector related past post ( - so I wonder what their flow rating is? the mystery continues....:eek:

That's a real shame, I was looking forward to the possibility of have flames shooting out of my exhaust on over-run. :grin: Maybe it's still achievable some other way!

I'm pretty gutted that I can't use them, but I guess next time I'll research the modification myself and not trust my brother, THE NUMPTEY!

14th May 2004, 11:37 PM
Sorry to revive an old post, but rather than starting a new one...

I have the LH 2.4 and am interested in the resistor mod.

Brad has explained things pretty well. What wire is fitted with the resistor? I know it is the one that increases voltage as air is increased, but without a digital multi-meter, I am looking more for a wire color.

I have also spoken with someone who said that this mod did not work. He fitted a 2.2 to get around the boost cut out.

I am trying to break the boost cut out threshold, and need help.


16th May 2004, 09:03 PM
Oil eater,

The wire at the AMM that Brad is talking about is orange in color.


18th May 2004, 09:32 PM
Thank you so much! Now I have a place to start!