Cheap fuel cut defender

When you increase the boost pressure, by for example shimming the wastegate or installing a T bleeder, you can hit fuel cut. The fuel cut is applied by the computer as a 'protection'. Something might be wrong the computer will think, most probably the wastegate. As a protection of the engine, the fuel will be cut off for a few seconds.

The amount of air flowing into the engine is measured by the AFM, Air Flow Meter. The AFM works using the Karman / Vortex system, which will be explained on my site some day later. If you take a good look at the inside of the Supra's AFM or the one of the Lexus 400, you see one big passage where the air flow is actually measured by the AFM electronics and you see a little passage, where unmeasured air flows through.

The 'unmeasured passage' can be blocked with an adjustment screw to lean out or enrich the mixture a little.

This is an air flow meter, but the one from the Lexus (used for the Lexus 550 upgrade). You can see the big air passage for measured air (with the black measuring electronics) and the smaller passage with the unmeasured air. The adjustment screw can be seen on the right in the unmeasured air area.

The whole trick about 'defending fuel cut' is telling the ECU less air is flowing into the engine, than there is actually. You can do this by flowing more air through the 'unmeasured' area. Adjusting the unmeasured area by making it bigger, could be a good solution, but I'm not a real fan of drilling or milling the AFM.

A better solution is letting unmeasured air flow into the turbo intake, after the AFM. You can do this, by putting another hole in the accordion hose going from the AFM to the turbo and filter the air going into this hole with a nice small K&N alike filter. However, it is quite hard to drill holes in rubber, so let's think of a better solution. 

If you take a good peak at the accordion hose, you see three holes on top of it. These holes would give you the excellent opportunity to fit an extra breather filter, so unmeasured air comes into the whole air system and you can delay fuel cut a little.

Now let's check what these holes are used for:

One hole is used for the BOV, Blow Off Valve. This valve is opened whenever you release the throttle and the turbo is still spooling. It prevents too much pressure being build up in the vacuum system, so the hoses don't get too stressed and the turbo can spool freely. This prevents the turbo from damage and will help the turbo spool up quicker when you're shifting gears. A solution for getting this hole free for this modification would vent the BOV to air, instead of back into the accordion hose (and thus the turbo). However, this is not a very good idea. Since the air that is being vented to air is already 'measured' you will have a very big boost leak. This will result in the engine running way too rich for a little moment, but since the car is running in closed loop (you've released the throttle) the computer will adjust the fuel mixture leaning it out completely. This can cause damage or engine stalls. Unless you have dedicated electronics like a piggy back fuel computer, I would suggest to leave this hose alone.

The second hole is used for the ISC, Idle Start Control Valve. This valve controls the idle RPM by bypassing air from the throttle plate. The ISC is in fact a stepper motor and opens or closes a valve so air can be bypassed. This would be a quite good option to use, but you will need an extra filter on the ISC. Furthermore I don't really like the idea of screwing around with the whole system too much. Although it's a fair option, let's see what's next.

The last big hole in the accordion hose is used for the PCV system. Positive Crank Ventilation. This system catches the oil vapors coming from the camshaft covers and leads them back into the whole air system. The PCV system in fact is connected to the throttle body as well. Under unboosted circumstances, the PCV vapors go straight thru the throttle body to the engine, but under boost the PCV vapors go through the turbo first. This is done, because you do not want your full turbo pressure (1 bar or so) on your valve covers. Well, if you tighten the valve cover bolts a little bit more, maybe ;-)

The whole PCV system is just an environmental issue. There is a very good 'replacement mod' for this, in the form of a catch can.  This catch can is in fact a good and environmentally justified solution. Search the web for the catch can modification, or check back later when I've put the catch can mod online.

So, the decision is made. We replace the whole PCV system with a catch can alike system, and use the newly made 'spare' hole in the accordion pipe to let unmeasured air flow into the vacuum system.

The build up is quite straightforward. Measure the hole diameter and buy a hose. Connect at the end of the hose the K&N air filter. Make sure the air filter does not breath hot engine air only. Put everything together and of you go.

Here you see the breather modification on one of my visitors car, the 88 Turbo of David Henry.

What can go wrong?

Yeps, that's the question asked many times when doing modifications. What can go wrong. Well, a lot of course. The main thing about this modification is we have more air with the same amount of fuel. Fuel is used under high boost for cooling the engine, hence it's running rich under boost or WOT. We lean out the mixture by getting more air in, so this can go wrong. I strongly suggest to watch your EGT values (Exhaust Gas Temperature) and optionally your O2 sensor values.

A good solution of getting a little bit more fuel into the engine, would be increasing the fuel pressure using a AFPR (Adjustable Fuel Pressure regulator). More about this modification soon to be read on my site as well.

The last thing, of a minor issue, is I can not believe this modification is 'linear'. I mean, it could be quite possible about 20% air is supplied under low throttle conditions, but over 40% of air is taken in under WOT. Or visa versa. Again, it's very important to watch your EGT, or get your car to the dynojet to program your fuel computer.

Again, watch out with this modifications. Just to overcome fuel cut by a couple of PSI is ok, but don't use this modification when you want to run 18 PSI on the engine. You need better equipment for that!

Special thanks must go out to David Henry, with whom I discussed this modification, who did this to his own car and at last: who supplied me the pictures of the modification.