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by Tim Turpin

This is a modification to 240 Turbos that can allow you to run higher boost while lessening the risk of fuel starvation. It is a very simple modification that anyone can do in less than an hour. As always- do this modification at your own risk! I claim no absolute knowledge and make sure you have taken the precautions necessary to ensure your car's safe operation. I would strongly suggest that you use both a graduated boost gauge and an air/fuel gauge to monitor your system.

Some Technical Background and Theory
All 240 turbos have a boost enrichment switch on the firewall. This switch activates at 2.9 PSI by grounding terminal 7 on the ECU. Terminal 7 causes the duty cycle to go up to 64-70 degrees of dwell (71-77% duty cycle meter).

1984-1985 turbo models also have a cold enrichment switch. This switch grounds terminal 11 on the ECU when the coolant temp is under 131 degrees F. It is activated by vacuum not pressure. It gives enrichment for a maximum of 1.5 seconds when the accelerator is pressed. The vacuum hose that goes to this switch is cut off by a vacuum valve when the coolant temp gets higher. Thus the factory terminal 11 enrichment is triggered only during cold starts. If you have an older Volvo without the cold enrichment switch don't worry, your ECU is the same as later models (terminal 11 is unoccupied) and you can still do this trick. This modification will ground terminal 11 at a preset boost level you choose (warm AND cold) allowing enrichment when you need it the most.

Terminal 11, when grounded, puts the duty cycle up to 82 degrees of dwell or 91% duty cycle meter reading- quite an improvement over the normal boost enrichment (71-77% duty cycle meter). This extra fuel can help prevent detonation when running higher boost levels (12 PSI-?).

Detonation can be the result of many factors. Heat, octane rating, improper timing, and fuel starvation can all play a part in detonation. This modification only addresses one element- fuel level. And, as such these other issues should be confronted as well to help avoid engine meltdown. To learn more about detonation and how to avoid it check out the 240 mods section on the Turbobricks website.

This is a primitive method that meets my main requirement for a mod- cheap. There are better, more sophisticated ways of providing fuel (additional injectors, EFI) but, this is the cheapest. I figured it was worth a try. Another good element of this modification is that it works within the OEM system by retaining airflow measurement as a means of metering fuel.

For This Modfication You Will Need:
  • An overboost pressure switch from a Saab turbo (or another Hobbs style adjustable pressure switch)
  • A vacuum cap for one nipple on the switch (1/8")
  • A terminal and length of wire from a junkyard ECU (unless you already have the cold enrichment switch)
  • A small vacuum Tee (1/8")
  • Short length of 1/8" vacuum hose
  • A few small hose clamps
  • Several lengths of wire and various electrical connectors

The Step-By-Step

  1. Set your pressure switch for the level you want to activate the fuel enrichment. You will need a multimeter and air pump to determine how to adjust your switch. Mine is set for 12 PSI. By using a multimeter or A/F gauge you can home in on a good starting point. Once wired up you can then tune the switch to activate after the point of stumbling, but before leaning out. More on tuning your fuel system and finding an enrichment point follows this section.
  2. Find a good place to tap into the vacuum lines for the boost signal. Use the vacuum Tee to tap into the line. I put mine right next to the overboost switch (under the dash, driver's side). If you happen to have an '84-85 model with the cold enrichment switch it would be even easier to tap into the vacuum line there because as a bonus the wiring to terminal 11 goes right through this switch as well (should be the white wire).
  3. Wire one spade on the Saab switch to a good ground. Wire the other to terminal 11 on the ECU (passenger side footwell). To find terminal 11 all you need to do is unscrew the ECU plug, the terminals are numbered. If yours is occupied you can splice into it either at the ECU or the cold enrichment switch (again, the white wire). If you have an earlier model slide your junkyard terminal plug into the unoccupied space. Make sure your OEM overboost switch has been adjusted to allow for elevated boost levels. If you don't know how to raise your boost safely do not pass go, go to the Turbobricks site and ask the mailinglist.
  4. When your switch activates it will allow the ground to go through to terminal 11 and thus initiating the fuel enrichment. That's it, pretty simple huh? I would recommend you thoroughly read the Turbobricks modifications page to get a good base understanding of where this modification fits in the scheme of things.

What Can This Mod Do For Me?
At this point you may be wondering what to expect from this mod. I have a few observations of my own as well as impressions from others. First, grounding terminal 11 should allow you to run about 3 or 4 more pounds of boost after the point you would normally start to lean out. In terms of horsepower this would mean a gain of about 24-32 ponies. This, however, might be a generous assumption considering heat will limit power and the stock turbo will be less effecient at these levels.

How much more boost will this mod allow YOU to run? Well, every car is different. The state of your fuel system will probably be the largest determinant (although other systems will play a part as well). How old is your fuel pump or distributor, when did you last change your fuel filter or O2 sensor, clean your injectors? How new is your control pressure regulator and what fuel pressures are you running. These will all effect how your car runs now and how much you can benefit from the addition of more fuel. Currently a stock clutch limits me to running only 13 PSI so my benefits from this mod are yet to be fully realized. Others have found it allows them to run as much as 15 PSI with no ill effects.

In addition, grounding terminal 11 does not seem to affect drivability negatively. The addition of fuel can be as smooth as the stock enrichment switch if the activaton point is adjusted properly (more below).

Fuel Tuning Tips
First find your car's wide open throttle (WOT) fuel enrichment. Use a voltmeter spliced into the O2 sensor wire (in the engine bay, not the thicker cable at the ECU). A good place for the ground location is the engine block (I choose the valve cover where the ground strap is located). Place the voltmeter´s selector in the 0-1000 mV. scale.

Test: 4th gear, 2500 RPM, full throttle, maximum stock boost. It should read between 850-925 mV. (each car varies) If not, go back and check your fuel system. The goal is to maintain this level as you increase your boost. In a modified engine with over 12 psi, shoot for 920-935 mV.

Here is a helpful chart provided by Angel Morales:

850 mV.: almost lean but acceptable at moderate boost levels, best power.

920-935 mV.: the "good and rich" mixture. Safest mixture, cooler combustion, less detonation-prone.

945-950 mV.: too rich, visible black smoke, engine hesitation.

Adjusting Your Enrichment Point
After finding your WOT reading slowly start increasing boost. When the voltmeter begins leaning out (probably around 12-14 PSI) you need more fuel. Adjust the new pressure switch to ground terminal 11 at this point. How much more boost this enrichment will allow depends on the particular car, Again, keep your eyes on your voltmeter to see when the system starts leaning out.

Once you have reached this point an air/fuel ratio meter (Intellitronix, Cyberdyne, etc.) should be adequate for monitoring the system. Just remember what the gauge reads when the car is running nominally. Most members have found that an LED based A/F meter reads 9 or 10 LEDs at WOT under normal conditions. It is important to note however, that these meters should just be used as a guide as they are not accurate enough to rely on for specific measurements.

Also remember to monitor other detonation variables when running this higher boost. Prime among these variables is timing reduction. The stock retard device in the distributor only removes 6 degrees at higher boost levels. Ideally to prevent ignition related pinging you should either dial back timing a few more degrees (down to 10) or run an aftermarket ignition system that allows for adjustment of retard in relation to boost. Another listmember is reportedly working on a modification that will allow the retard device to take out more timing under boost. I will try to link to that info when posted.

Technical Assistance:
Writing this article was made easier with the help of my fellow Turbobricksters- Philip Bradley, Dave Barton, Angel Morales, Trent James, Milo Whitson, and Jake Sternberg.


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