Bigger Tires now no timing advance

[green787]

So we put bigger tires 12.50 x 35" on 17" rims, and lifted our 1994 Chevy 1/2 ton and now the computer doesn't allow any timing advance at higher RPM's. Or at freeway speeds and we are getting horrible mileage. Has anyone encountered this problem before and is there a way to adjust the computer or replace the chip or something to solve this problem... Any advice is greatly appreciated.:confused:

[sickrick]

How do you know your not getting any timing advance? Just curious.

[green787]

I used a timing light and watched as the mark stays steady at 8 degrees advance throughout the whole range of RPM and it has no power.

[amr126]

my only guess is the larger tires might have contacted the wiring harness. Overall I don't even think wheel speed is factor for the formula of timing advance-maybe the load measured on the trans, but not wheel speed. Even if that was the case, why would it make a difference? Personally, I would thoroughly check the wiring and make sure nothing is loose, and I bet you could find info on what controls the timing in your particular model and see if there's an easy solution.

[j10-401]

Ive never heard of tire size effecting timing advance, wheel speed isnt a factor. I would find out what sensor inputs your ecm uses for timing advance calculations. ect,tp,load,rpm signal, etc. Your vehicle is obd1, so chances are there arent all that many inputs.

[scottm]

Larger tires lower your effective gear ratio and increase your wind resistance and rolling friction at the same time. This means you have to apply more throttle to maintain highway speeds. Heavy throttle results in low manifold vaccum, which the computer measures with the MAP sensor. Low vaccum (high map) tells the computer to reduce timing to prevent detonation. The same thing happens with vaccum advance distributors, ei, when vaccum is low, less spark advance is produced. The easiest way to determine how hard the engine is working is to watch a vaccum gauge connected to the intake manifold while you drive. 15 inches of mercury vacuum is a light load, 8-10 inches is a moderate load. Any time you have less than 4 to 6 inches of vaccum, the computer (or carb) will go into 'power' mode, reducing spark timing and adding extra fuel. So bad milage is understandable!

This is a normal side effect of big tires, and is improved by changing gears in the diffs, from say 3.73:1 to 4.56:1. IMO its best to gear the truck to see 8 to 10 inches of vaccum at highway speed. This will improve milage and reduce engine wear. But I don't think it will effect computer behavior at idle in your driveway. There is a specific procedure for checking timing at idle, and it requires disconnecting a wire at the computer. Check a good shop manual for the procedure.

[green787]

Larger tires lower your effective gear ratio and increase your wind resistance and rolling friction at the same time. This means you have to apply more throttle to maintain highway speeds. Heavy throttle results in low manifold vaccum, which the computer measures with the MAP sensor. Low vaccum (high map) tells the computer to reduce timing to prevent detonation. The same thing happens with vaccum advance distributors, ei, when vaccum is low, less spark advance is produced. The easiest way to determine how hard the engine is working is to watch a vaccum gauge connected to the intake manifold while you drive. 15 inches of mercury vacuum is a light load, 8-10 inches is a moderate load. Any time you have less than 4 to 6 inches of vaccum, the computer (or carb) will go into 'power' mode, reducing spark timing and adding extra fuel. So bad milage is understandable!

This is a normal side effect of big tires, and is improved by changing gears in the diffs, from say 3.73:1 to 4.56:1. IMO its best to gear the truck to see 8 to 10 inches of vaccum at highway speed. This will improve milage and reduce engine wear. But I don't think it will effect computer behavior at idle in your driveway. There is a specific procedure for checking timing at idle, and it requires disconnecting a wire at the computer. Check a good shop manual for the procedure.

Now that's the answer I was looking for!!! Thanks Scott;)

[NIKAL]

I used a timing light and watched as the mark stays steady at 8 degrees advance throughout the whole range of RPM and it has no power.

You say your not getting timing advance and that you checked with a timing light? What Scott said makes since when your driving, but if your checking the timing with a timing light in the driveway while not moving and its not advancing then the tire size / lower RPM / vacume should have nothing to do with it.

[philofab]

It was probably like that before the wheels + tires... your just noticing it since it needs the power now. Check to make sure the timing connector is connected, someone could have set the timing and forgot to change it back. It disables the computer controlled advance. Connected = correct for driving, disconnected = base timing for setting the timing.

SUBJECT:
SECTION 6A - IGNITION SET TIMING CONNECTOR LOCATION

MODELS:
1994 CHEVROLET AND GMC TRUCK C/K MODELS WITH GASOLINE ENGINES

The correct location of the set timing connector is inside the cab where the heater box and the carpet meet on the passenger side floor. Pull carpet back to expose the single wire and connectors on the outside of the control module harness. The wire colors are tan with a black stripe.

[green787]

Thank you every one for the great advice... I am wondering if the computer is located behind the glove box.... There is a huge harness going to that area.... thanks Philofab, I will check that....

[loufish]

It was probably like that before the wheels + tires... your just noticing it since it needs the power now. Check to make sure the timing connector is connected, someone could have set the timing and forgot to change it back. It disables the computer controlled advance. Connected = correct for driving, disconnected = base timing for setting the timing.


There's your answer, at idle the timing should move around as you crack the throttle, turn on A/C and such, that's why there's a wire to disconeect to be able to check base timing.

[philofab]

Computer is behind right kickpanel just below glovebox.

[green787]

Thanks guys, Are there any good performance tune up shops between Orange County and L.A. that anyone would recommend???