I've been having problems with my car knocking at higher speeds.

I have a 1990 SD, stock 305 with exhaust work, and a heated O2, added a T56

When I go WOT in 1st gear, O2 is around 980-1000 mV. In 2nd its around 960. Still no knock counts.

I did a test run at 100+ mph in 5th and the O2s were down to 880 and getting leaner with more speed. It was taking out up to 12 degrees of timing at 4000 rpm and I was only running 22 degrees total(before knock retard).

Should I start playing with the delta MAP accel enrichment? Or is there something else that could cause this effect?

 

A couple things come to mind ....

1) Did you datalog so that you can compare the MAP readings between the high mV in 1st gear and the lower mV in 5th? I'm curious as to if the MAP values were different.

2) Did you datalog so that you can compare the MAT readings? You may be getting a lot more cold air at speed ... cold = denser ... = leaner.

3) Was it an extended run in 5th gear (did you go WOT in 1st and take it all the way through to 5th?). Or - did you just go WOT in 5th?

Tim

 

You never should use a O2 sensor as your only tuning aid.
They are subject to EGT, and backprssure errors.

Delta MAP accel enrichment is just for fast rpm changes.

If your lean on top the add more fuel thru the PE AFR, or VE table.

What's your injector's duty cycle, and what do the plugs tell you?.

 

I'd zero in on this suggestion. On some engines, I've seen the MAP value drop 5kpa on hard load, like you'd get in 5th gear. This causes too effects in SD: 1) your VE Table may be significant enough to cut fuel and 2) your Spark Table will increase as often the lower MAP readings have higher spark advance (which also leans the engine too).

If you are indeed getting significantly lower MAP values, then you need to adjust/tweak the VE & spark table that correspond to the MAP reading in the higher gear.

 

OK, I went and compared some data logs from different days.

I went WOT in 5th gear, not starting in 1st and going through the gears. Had it at full throttle for about 30 seconds. I'm also running open loop right now to eliminate the extra variable of the BLMs.

The MAT temp did not seem to make much of a difference, I looked at two high speed runs, one at 80F and the other at 150F. On both runs the O2 showed significiantly leaner than at low speed, also getting more knock counts and spark retard.

The MAP drops from 100 @ 3000 RPM to 98 @ 4000 RPM. It seems lean over the whole RPM range including before the MAP begins to drop (also knocking more through whole range).

Injector pulse time for the low MAT run was 10.5ms @ 4000RPM and just beginning to decrease. I didn't go past that in 5th.

Is it possible the MAT sensor has gotten sluggish and is not responding to the change in temperature quickly enough?

I still have to go outside and check the plugs, I know the knock counts are real. I disabled the spark retard once (oops) and yup there was knock alright.

Thanks for the suggestions guys, I appreciate it.

 

I would get a WB O2 sensor first before I would start basing any more tuning decisions or believing you have a problem. You are using the stock O2 sensor for a purpose it was never designed for (WOT tuning). It's strictly a switching device designed to oscillate around 14.7:1 to comply with emissions - nothing more.

In fact, when you start to see how the stock O2 sensor works compared to a WB, you quickly see that the stock O2 doesn't even do the 14.7:1 very well sometimes.

 

Another item to consider is the length of time at WOT. The engine is starting to build up some heat. Cooling becomes another consideration.

My gut tells me that the fuel pressure should also be checked.

RBob.

 

I,m curious about how can this guy know how much fuel is delivered for each injector when his car get in 5th gear at 100 mph?.

Since the kpa fluctuates (drop) when the car (or the driver) changes from 4th to 5th gear, there are some variations in spark and VE values, but, what about fuel volume into cylinders?.

Anyone knows any measure method/deduction?.

(Please note: no fuel pressure/volume into rails. I means fuel quantity delivered for each injector at the "peak" moment).


Denis V.

 

Look at the BPW via the data logs. Knowing the injector flow rate and opening time can calculate the volume of fuel.

RBob.

 

Well I changed the commanded AFR from 12.5 down to 12 and did some more driving.

In 1st and 2nd I get 0 knock counts. In 4th and 5th I get up to 15 degrees of spark retard!! It feels like I'm letting off the throttle when that happens

The VE table is around 90-92% for the powerband. Maybe I'll keep adding fuel and see what happens.

I'll try and find someone with a fuel pressure gauge. And yeah, that O2 never worked too great. The car always idles smoother in closed loop.

I included sections of the data log with the interesting bits. The extension has to be changed to .zip

 

All else being correct I would change the code by adding a SA correction based on transmission gear.

RBob.

 

It looks like you may be getting somewhere cheezx!!!
Just remember that the car is under much more load at higher gears than lower - so Detonation is much more likely, you may want to try only adding fuel in the RPM ranges where knock occurs in the F/A ADDER VS. RPM Table and/or I just use the VE for most WOT fuel changes.
Also as someone else stated - do not rule out Fuel pressure drop!
And after you figure that out , GET THOSE HEADS ON YOU CAR