spark timing vs. cylinder

[icicleboy]

Some of the newer pcm's have a control for retarding or adding timing vs. individual cylinders past a certain g/cyl of airflow. A nice way to add more boost, or timing while keeping knock at bay on certain offending cylidners.

I'm looking to modify this table to verify a problem asap.... but the table shows cyl #0 - cyl #7. I'm pretty sure that cyl #0 does not mean cyl#1 necessarily. It's usually to do with code in the pcm referring to specific drivers. Does anyone know the cylinder co-relation? I'm trying to reduce timing on cylinders 2, 4, 6, 8.

Any help is appreciated.

[EC_Tune]

Cyl 0 is the first in the firing order.

[icicleboy]

thanks for the reply,

firing order for this motor is : 1-8-7-2-6-5-4-3


Does that mean that for affecting cylinders 2, 4, 6, 8.... I would need to adjust : 3,4,5,6?

Your help is appreciated.

[EC_Tune]

Fueling is more of an issue than spark in most cases. 5 & 7 are the leanest in general.

[icicleboy]

my goal here is to actually reduce overall backpressure in one of the exhaust manifolds of the motor, to diagnose for an extreme backpressure issue causing a single cylinder misfire.... so keep revving the motor up, while the offending side gets less power (pull 6 deg timing across the bank) and see if the issue goes away or not.

[tahoe]

while the offending side gets less power (pull 6 deg timing across the bank) and see if the issue goes away or not.

Doing this will not help you diagnose an exhaust back pressure issue.
Since all cylinders get the same amount of fresh gas/air mixture, and the spark plug still fire the charge, the amount of exhaust gas that has to go out is still the same.
You could even get higher back pressure by moving the ignition point closer to TDC (example 15 deg) since the exhaust gas will be hotter than if the burn starts further away from TDC (example 21 deg).


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[icicleboy]

thanks for the response tahoe. I guess there is no way to diagnose this issue then? We're thinking that a single cylinder misfire is happening because backpressure is so high on that one exhaust runner that during overlap exhaust gas is filling the cylinder and not allowing fresh air to enter.

The misfire happens only on one cylinder (reported as cyl 5, but suspected to be cyl 6). It doesn't happen until 5700rpm. Motor is a boosted LS4, making about 470whp. swapped so far is injectors, plugs, coils, coil packs, ign wires, etc. Dwell was cranked, no luck. Springs were changed as well. Leakdown to be done tonight, but it runs perfectly otherwise, and pulls really hard until the issue happens. Any ideas?

[tahoe]

It could be a case of a leaking valve.
There could be many reasons for that ocuring at high rpm/load, and not in normal driving.

A lifter could be pumping up. (are you using a heavier oil than recomended?)

The pushrod could be bottoming in the lifter when the valve get realy hot and expand in length.

Check you lifter preload/valve lash.

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[BBA]

You could have a y pipe problem, a cam with a bad lobe, or a bad lifter, bad injector or any of the valve seat/spring related issues as well as any other cause. In short, it's hard to diagnose without doing a cylinder balance test as well as cranking pressure and leak down tests.

Just build a bored out stroker monster with a huge cam and new heads...that should fix it.

[icicleboy]

we've done a leakdown test showing all cylinders at 9% except #2 and #5 at 5%... this was on a cold motor. The motor only has 34k km's on it... rods are all perfectly straight, rockers are also perfect, springs upgraded (not float). It doesn't happen until 5700 rpm or so (around 470whp) and it's as if the cylinder shuts off. Does that sound like a lifter? This car does have DOD... so I guess it's possible that one of those fancy lifters f'd off.