Car is a 1991 6spd vette.
Combo is:
Forge rotating assy.
9.87 comp
Lt4 hotcam
Canfield 197cc heads
Converted LT1 intake.
Headers.
3 inch exhaust.

Abow 2000rpm, all is fine. Under, and during light cruise the car bucks, just like a missfire. I can hear in the exhaust that the engine did not runt smooth at those RPM.

Good part is that after 40 .bins Its fixed so its 95% perfekt
What I did whas..

Disabeld DFCO at low rpm.
Alot of AE, to keep the WB happy.
Also I increased the VE at high MAP and low RPM.
And this is interesting, I adjusted the TB blades so the iac counts increased from 10 to 40 at hot idle, that made crusing better.
And the major diffrence where in the timing tabel.
This combo whants ALOT of timing, its whery sensetive to changes at low rpm.I cant imagine why it loves timing so much. But I go no knock, and the driveability is better and better.
The more timing, the smoother the engine runs.
I need to ramp up the timing at once from idle to 1000rpm.
ANyone else experince this kind of "trouble" with manual cars and hotcam?

This is what I ended up with to keep driveability at is best.

 

I get that bucking/jerking during a low speed crawl at speeds below 1000 rpm.

though, my cam is about 6° less duration than the hot cam and I'm running a Miniram intake (similar to LT1).

The benefit of the increased IAC counts is easy to explain. By using the IAC port in the manifold (which is where it's supposed to go), you're using the built-in IAC air distribution that GM put into the manifold. Do a search on the phrase split-BLM to find out details.

I'm curious what disabling the DFCO accomplishes. If you put it back in, what happens to the 95%? Does it go down to say 75%?

Also, IIRC, the LT4 SA map is similar to what you have.

 

THanks for the answer.
Good point about the "split BLM issue". I also modified the stock TPI TB so all the air from the IAC goes to the built-in IAC air distribution of the intake. Instead of it "leaking" in to the plenum.

I will try to switch on the DFCO, and see what happens.

Yes I also noticed my timing tabel is simular to the stock LT4 tabel. But I "have" to run even more timing at low rpm. I thougt this much timing would give me knock. But I have not noticed any, and the engine loves it

Thanks for the feedback..

Devilfish

 

Good to hear



/N.

 

What's up with your 400-800 rpm 100 kpa SA?

 

There are a couple of items that can cause issues. These can be seen in a data log.

The first is that the closed thottle SA table needs to match the main SA table. Use a data log and look at the MAP at various RPMs as the ECM switches between the closed and main SA tables. Then make the closed throttle SA values the same as the main table at that MAP value.

This is to prevent jumps in timing as you ease on & off the throttle. You can see the timing jump in a data log when they are too far apart.

The other item is the proportional gains. Look at the data log and check how high & low the O2 sensor is swinging. Does the RPM change in concert with the O2 changing? Usually the engine RPM will jump up a little when the O2 swings lean. If so, then the proportional gains need to be reduced.

RBob.

 

To help off idle with the 6spd. So it wount take off so hard. Most in "stop and go" traffic.

 

Thanks for the feedback.
I changed the closed tps spark tabel to more match the main spark. This made the witches between the closed and main SA tables more "smooth". I Cant say I could "feel" any major diffrence in driveability. But then again, I just tested this a short time ( 5 min driving ). I will test and log some more. Thanks for the tips

Im sorry, but could you explain a bit more about the " proportional gains", i dont follow you there.

Thanks for all the help.

Devil...

Cheers..

 

The proportional gains force the O2 sensor to oscillate. It works on the PW to rasie and lower the AFR. When there is too much gain then the oscillations in AFR affect the engine. This is the table I usually play with:

Reduce the gain at the airflows of the issue.

RBob.

 

Does this exisit in the 8D code?

 

Yes, the table shown is the $8D calibration. In the BIN that table will be at $4CF (drop off the $8000). It shows how the companion table at $4DE is used. Which is the prop gain vs O2 error. The O2 error table I don't usually mess with, although it can be changed if required.

RBob.

 

oh ok.. I thougt you saw someting in my spark tabel that could cause probleme..

But nice tips anyways, I will look it up.

And I enabeld DFCO again, no change so I keep it active.

THanks