PeachParts Mercedes-Benz Forum - View Single Post - Modification for D-Jet M116 and M117

Greg in Oz · #10 04-11-2005, 04:26 AM

Tomguy,

I appreciate your thoughts and I believe you raise an interesting question. I've been thinking some more about this so I'll try to put it to words. See what you think of this reasoning.

With the D-Jet, the only inputs are (I hope I haven't missed any):
1. Timing signal from the trigger points,
2. Coolant temperature,
3. Air temperature,
4. Manifold pressure,
5. Opening throttle (temporary enrichment only while throttle opening is increasing),
6. Closed throttle (for fuel cut on overrun),
7. Wide open throttle (4.5 only, you are correct the 3.5 does not have this).

From this I deduce that for any throttle opening apart from closed (or wide open on the 4.5), the ECU does not actually know what the physical throttle opening is. The mixture is therefore determined entirely by manifold pressure (vacuum), coolant temperature, air temperature and engine speed. At all but wide open throttle at high engine speeds, the only significant restriction to airflow will be the throttle itself. Is it therefore reasonable to say that it matters little what the ambient pressure is at the air intake? Under steady cruise conditions, even at high speed, the relatively small throttle opening will be the only significant restriction to airflow and the manifold pressure will be determined by the throttle opening. For a given cruising speed, a certain amount of engine power will be required. This power will be dependent upon the amount of fuel and air consumed. If the air pressure at the air intake is slightly higher, then a correspondingly (if only slightly) smaller throttle opening will result in the same manifold pressure giving the same signal from the MAP sensor and hence the same fuel mixture.

At wide open throttle, then I agree that other restrictions apart from the throttle, such as the filter element and intake pipe (snorkel) become more significant. You are correct in suggesting that I felt there must be some benefit to improving air flow. I have made use of an intake pipe with a larger cross sectional area in the hope of reducing restriction to the airflow at wide open throttle and high engine speed. My primary reason for taking that air from ahead of the radiator however, was to allow cold air to be inhaled rather than any possible "ram" effect. If there is any ram effect then I believe it will only be of any significance at wide open throttle and at high road speed. Hopefully, should this situation occur, the resulting manifold pressure will still be within the working range of the MAP sensor and that the resulting fuel mixture will be correct. The colder air will be accounted for by the air temperature sensor on the intake pipe and the mixture slightly enrichened as necessary.

The greatest benefit I have realised (and this would be due to colder air) is the elimination of pinging (knocking or detonation) under load and during acceleration without having made any other changes. I have also noticed improved throttle response (probably primarily due to ellimination of pinging) and the engine also seems more willing to pull under load in the upper rev range at wide open throttle. Previously it appeared to be somewhat "strangled" above about 5,500 rpm (even though it could still pull to the 6,500 rpm recommended limit). Now it continues to pull strongly and quickly to this limit with no indication of "running out of breath". Whether this is due to less restriction to airflow, lower air temperature, or a combination of both, I cannot be sure. Possibly it was previously pinging in the upper rev range as well (but not audible due to masking by other engine noise).

Hopefully you can follow my logic and it offers further food for thought.

Greg

PS. You could make use of a water manometer to measure air pressure at various locations in the engine bay as well as forward of the radiator, just as a water manometer can be used to measure pressure drop at various points in an air intake and air filter system ahead of the throttle body. I am just not sure that it is very significant due to the reasons given above. Readings from an exhaust gas analyser under real driving conditions before and after the modification would be worth seeing. Unfortunately, I do not have access to this equipment to try it.