When is a lot of spark timing too much?

I'm looking at the 90-91 Corvette spark timing map and seeing upwards of 46° BTDC at low maps. See table below.

I hear that too much timing can be hard on rods and bearings, but is that only if you get detonation?

Yet, GM programmed this much spark advance on factory cars that run for 100,000 miles or more... Does that mean that this much timing is not necessarily detrimental? Maybe at low engine load it's not as bad as at high load?

I currently run about 42 in these low map areas, and I tried 46 and the engine seems to like it. Part throttle feels better (likely the short runner Miniram intake likes a lot of spark timing). I'd like to maintain that if it's not a problem.

I'm not seeing any knock counts of spark retard kicking in, nor do I hear any knocking.

It's only a 4 deg increase, but getting up near 50 gives me some pause.

Thoughts?

 

Every engine is different, and timing requirements vary wildly depending on air fuel ratios. It takes a lot of advance to light off lean mixtures on the older sbc chambers. Too many variables for anyone to properly make a solid suggestion sir. Best answer is get an ottoscope and learn how to read plugs. They tell the story.. oxygen sensors and knock sensors can mislead you. And at best they are reading what's happening in the cylinder with a several millisecond delay. Spark plugs do not lie to you.

 

Thanks. Yeah, its not so much about what is right for performance. Agreed 100% on your comments regarding that aspect of it. Looking at plugs, seat of the pants feel, AFR readings, etc I'm sure all play into it.

I'm more questioning at what point high spark timing can start to potentially cause excessive wear and tear and/or actual damage to the engine.

However, perusing around online, it seems that it's not unheard of to be cruising around at close to 50° BTDC...

https://www.motortrend.com/how-to/0604rc-ignition-timing/

https://www.motortrend.com/how-to/set-ignition-curves-create-optimal-performance/

These guys are running ~28 deg even at idle (14 initial + 14 vacuum)?

All I know is I see a marked improvement in throttle response an the added 6-7 deg of timing in these low map areas.

 

I'm seeing 40s for light throttle and cruising for spark advance. Like said a lean mixture burns slower so it'll take more. Under any decent throttle accelerating 38 is about the limit I've seen on mine. It stops making power at wide open around 33 degrees but I have much better chambers.

 

That is very typical for an SBC. Mid to high 40s is common under cruise, decel, very light throttle.

 

This looks normal to me, mid to high 40s was what my 86 ran stock. If you're not knocking, I would not worry further.

On a modded engine you would have to change those timing modifiers.

 

Thanks guys. Yeah, that seems to be the consensus... I've posted on a couple of different boards regarding this subject.

What was happening was that for my Miniram setup, I copied and pasted the '96 LT4 Corvette timing map out of the $EE mask into my $8D and used that as my baseline and tweaked a bit here and there. But it's maximum is around 42.

But after a warm startup, I would get much improved part-throttle throttle response for about a minute before it would "decay" back to where it was. So my 42's would end up around 48 for about a minute and I really liked how the car felt during that period.

After racking my brain for a while trying to figure out what was happening, and then digging through the $8D code in detail, I found that there's a startup spark timing adder that decays out over the course of about (lo and behold) ~1 minute. It adds about 5-6 degrees on top of the existing timing table. I then went back and could see that timing being added in the Tunerpro data.

That got me to thinking...

I then did some digging through other bins and it surprised me when I saw the older L98's running that much timing on a continual basis, which then got me to wondering if I can run that kind of timing as well... and it sounds like the answer is yes.

 

Wow.... so in addition to the increased cruise timing, I added 5 deg to my idle timing. Picked up a 0.5" Hg of vacuum and it's noticeable smoother, and even smells better. Also noticeably better throttle response during cruise.

Must have been some minor combustion inefficiency previously. Overall very nice improvement.

 

At light loads (low MAP kPa) the combustion process (flame front) is slower, so the timing needs to be advanced higher to keep the peak cylinder pressure (LPP) at the desired 15-20* ATDC. Also at light loads there's a higher percentage of residual exhaust gas stuck in the cylinder between cycles. This residual EGR slows the combustion process, requiring even more timing advance to keep the the LPP at that 15-20* ATDC.

 

Yeah that makes sense. I guess the old adage "give the engine what it wants" definitely holds true.