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is it possible that a motor with slightly more compression and a bigger cam than stock would need LESS timing advance at crusing speeds than the stock settings?
*edit*
oh yea.. and I'm looking for the LT4 main spark advance tables (stock) does anyone have those handy or know where I can get them? Thanks!
It is possible but more or less unlikely. While compression does have a major role. Cylinder combustion chamber design and camshaft design plays major roles as well. A chamber that produces lots of swirl, has the correct quench area to produce lots of turbulence would make for a highly effiecient burn. A head such as the GM fast burn head perhaps would not require as much timing as a conventional small block head. Raising the timing numbers creates more cylinder pressure and the reverse is true. A LT4 head has 4cc less chamber volume as well. This in itself raises the cylinder pressures.
I have seen the LT4 timing tables and they are not much different than a LT1. Another thing that affects timing is what camshaft is installed in the car. The LT4 had a slightly larger cam than the LT1 as well. A cam with longer duration normally requires more timing at the bottom end to bring the cylinder pressures up to help stabilize the idle characturistics of the engine.
Tom, the reason I ask is I feel the ever so slightest surge at cruising speeds (about 45-50 mph).. I was thinking of taking 2* of timing out at that MAP/RPM to see if it cleared it up.. its at the stock setting in that RPM range now.. what do you think?
More importantly, what gear and what RPM are we running at that speed? Get a data log of what its doing when she is surging, maybe we can figure something out.
A cam with longer duration normally requires more timing at the bottom end to bring the cylinder pressures up to help stabilize the idle characturistics of the engine.
Good, hopefully this will bring my idle vaccum up.
Ok, question .... i just drove my car around for the first time with the HOT cam and when taking off from first it nearly died unless i gave it lots of rpms... i checked the datamaster logs and it went down th 5* of spark advance when it did that... to fix this, i just need to bump up the timing table in the lower rpm areas.. right?
Never mind ... i've got it figured out. Had to raise the low rpm, high MAP spark values quite a bit.
RRealize this is old, would like to look at a stock lt4 Base ignition timing table for comparison as I am mapping things in my car with holley hp. Do you maybe still have this info, or a screenshot of it?
Here are all the choices for timing tables on the stock LT4's PCM, just so you know what's available:
The following is for the stock LT4 calibrations from the factory, no mods. I screen shot the first two that reference timing against RPM and MAP, which is what I think you want. I also included the "minimum advance vs RPM" table in case that is useful. If you want any other screenshots from the menu of tables, let me know.
Here are all the choices for timing tables on the stock LT4's PCM, just so you know what's available:
Every so often a zombie thread comes up with some neat stuff.
For example, thanks to this I was able to confirm that the "LT4" BIN I have converted to run on a 94-95 PCM does in fact contain the stock Main/Extended spark tables that you posted above.
Interestingly the minimum spark advance table you posted is the exact same one all the LT1s use, so it doesn't look like GM messed with that. Whoever converted that "LT4" tune to $EE did though.
Here are all the choices for timing tables on the stock LT4's PCM, just so you know what's available:
The following is for the stock LT4 calibrations from the factory, no mods. I screen shot the first two that reference timing against RPM and MAP, which is what I think you want. I also included the "minimum advance vs RPM" table in case that is useful. If you want any other screenshots from the menu of tables, let me know.
yes, exactly what i was looking for this is a good baseline for me to work from. I am using the holley hp and the miniram does not have the plenum design for the iac passages like the stock intakes do. Tip in, ae vs map rate of change and some other stuff will be different. I've got it to idle good around 850 (bigger cam). Thinking if i had started with a converted ltx intake it would make things easier to tune. the reversion i have at idle and tip in wouldn't affect the idle air passages in the stock stuff as much as it does with the miniram. Your tables do confirm that some of my lower map areas of the timing table are very conservative. Orr pointed that out to me 4 months ago. makes sense seeing the stock tables.
THANK YOU THANK YOU THANK YOU!
does the factory stuff give a table with map vs rpm axis' and desired air fuel ratios? that would be extremely helpful.
Last edited by Bill Chase; Apr 1, 2021 at 05:20 AM.
Reason: TYPOS
does the factory stuff give a table with map vs rpm axis' and desired air fuel ratios? that would be extremely helpful.
No, here are all the choices of fuel-related tables:
I think Volumetric Efficiency vs RPM vs MAP is where tuners of the stock system play. There is a constant for "Stoichiometric AFR" that is set at 14.7, so that may serve as some kind of reference for the PCM to target, at least in open-loop/WOT. I don't know how to "convert" VE tables to give you plain AFRs, but here's what the VE tables look like:
Thanks, most of this is just a sanity check for me. The ltx intakes once ported can be pretty lazy airspeeds until rpm gets up a little bit. Notice the factory pumps up the timing at tip in and cruise. It is giving me a decent reference of what my map should look like. Most of the base holley maps are designed for single planes or longer runner stuff. This helps
spark table quesiton & LT4 spark table needed
