davepl

I've been pimping this basic theory without much success for a year now (because I've done nothing to prove it):

- Base tune is too aggressive, causes detonation on 91 octane
- Timing gets pulled
- Engine runs hotter because timing is pulled
- Engine detonates more easily because it's running hot
- Go back to start, heating and pulling timing until it overheats

I bring it back up because there are two things that have come to light recently that sort of line up with this:

- The in-depth research G-Speed did revealed intercooler pump cavitation, which would no doubt exacerbate or trigger the above

- GM altering the angle of intercooler bricks to try stave off detonation on the rear cylinders as part of their cooling package solution

I'm not an engineer, but I could rule this all out by running a set of hard laps with C16 or some other high-octane fuel and seeing if it gets different results.

Has anyone done a full track session with 100+ octane? If so, did the car hear any less than it normally would with 92?

dar02081961

Dave your theory has a flawed assumption in step 3.
The engine wouldn't run hotter because of less timing (timing being pulled) the reason for pulling timing is to reduce detonation and cylinder temps.

And step 1 is questionable because it is based on the assumption that the tune enforces a certain amount of timing to run. This isn't the case.

The advance ("timing") is set based on the average learned threshold of knock. This threshold is learned from continuous probing (or referencing the base timing table) until knock is detected then set slightly shy of that point regardless of octane (90ish through 96ish). Attack recovery time/speed tells the PCM how long after a knock event to wait before probing the threshold again.

Point is the car will run fine on 91 at reduced timing and wont overheat because of the reduced timing.

Reduced timing is a resulting solution to high cylinder temperatures and the associated knock, it's not the cause of the high cylinder temperatures. Elevated cylinder temps come from a combination of a bunch of factors which is another whole discussion in itself.

davepl

I have to disagree there - not with the reason, but with the result. Reducing engine timing DOES increase the engine temp and the amount of heat dumped into the cylinder heads especially.

It pulls timing to prevent detonation, but that actually increases cylinder temps. Hence the cycle.

http://www.nastyz28.com/forum/archiv.../t-154952.html

Dabigsnake

"Point is the car will run fine on 91 at reduced timing and wont overheat because of the reduced timing."

I can attest that my car will ruin all day with reduced power, without overheating anything. I am pretty certain it has to do with 91-93 octane. I don't know if anyone has ever verified that the reduced power is eliminated by running a higher octane? I gave up, as GM agreed to replace my car, and the rumored "fix" for the '17 came to be. Would love to hear from anyone that has proven this 91-93 octane= reduced power. and cured by inducing higher octane.

Would I have to reset the EMC when going from lower octane, reduced power, (with no dash messages shown) and then trying 100 octane. or will it reset the timing and fuel tables on it's own?

I bought a dashlogic to try and trace it down, but, gave up.

davepl

You lost me - sounds like you're saying Zs don't overheat at the track, but I think it's well established that some do.

All I'm trying to find out is (and it sounds like you're curious too) whether higher octane has helped people survive longer without overheating.

dar02081961

Dave. Lot of respect for you. But a there is a lot of partial information on ignition timing and cylinder temps out there. Bunch of general statements floating around as the law. Problem is there are many truths.

Your statement is true when discussing "excessive" reduced timing. Excessive reduced timing does increases cylinder temperatures. Long discussion. Let it suffice to say when you excessively reduce ignition advance the intake charge doesn't have time to completely burn at TDC and the fuel remaining after the incomplete burn can end up causing a rise in overall cylinder temps. So yes you are correct.

However that doesn't apply to this scenario and isn't the same as backing down advance (timing) a few degrees from the knock threshold. Or said another way readjusting advance to time ignition perfectly at TDC.

Knock is where some of your highest cylinder temps occur. This is because the charge ignites (to soon or burns to fast) while the piston is still traveling upward towards TDC. You can imagine the extra friction (stress and heat) created from and explosion trying to drive the upward moving piston downward. Yea this is a catastrophic situation and will kill an engine quick.

The above situation represents a scenario where not as much advance is needed to "time" the burn to occur at exactly TDC and the fire is being ignited too early in the cycle. This is what is normally happening when your LT4 experiences knock. And in this scenario a reduction in advance (timing) by a few degrees will reduce knock and the associated elevated cylinder temps. (exactly what the PCM does)

This is because allowing the charge less time to burn after its ignited allows the piston time to reach TDC before ignition. Or said another way this reduced timing readjust ignition to occur perfectly when the piston reaches TDC.

Timing requirements to perfectly ignite the charge at TDC change with temperature and boost.
As engine temps and boost levels increase, less ignition advance is required or tolerated. This is because the intake charge can burn faster and quicker at higher temps. So less ignition advance is required or tolerated. And if advance isn't reduced as engine temps increase then knock and cylinder temps will in fact increase until a catastrophic failure occurs.

davepl

I agree almost entirely, but I guess I always felt that the amount of timing that gets pulled in response to a knock event was "excessive" relative to the timing it -should- have been running to avoid that knock in the first place. It's not just optimizing spark - once it senses detonation, it pulls quite a whack of timing out and re adds it back in slowly.

To make up numbers, if it's going to detonate at 32 and the timing map calls for 33 and it detonates, it doesn't go down to 31... I think it pulls quite a bit more.

I have no metric or benchmark to know if the amount of timing that gets pulled is significant enough to cause the chamber to heat up though... which is why I called it just a theory.

ktoonsez

I have done 4 sessions on a 93 degree day with 101 octane. There was no difference in how quick I overheated if that was what you were asking in the OP. I had zero KR on all 4 runs. Here is run #1 and #2 of the day:

#2

#1

davepl

Yes, I think that answers it. Thanks!

jdmdohcpower

Random bit of thought.
So I ran a tank of 100, did nothing. Overheated the same. However by my third tank of 100 (my car was only tracked and getting trailered so only running 100 consistently) I noticed a difference. My thoughts are that 1 tank is not enough to change the octane tables of the tune. The car needs more time to recognize the higher octane. Who knows if Im right or not but I felt that I wasn't overheating nearly as much or as fast after a few tanks of 100 and nothing else

GSpeed

When we had the OEM style supercharger heat exchanger, knock was through the roof. There were times when we actually saw negative timing. It was all the engine could do, to keep itself happy.

When the blower temps were under control and pump shut down was not an issue, we saw actual, and commanded timing very close.

The race gas, in our humble opinion helps the situation, because its cheap insurance. The knock control in these cars is very good, and very fast, however it doesnt help much with performance if the car has blower pump issues.

We run a mix of MS109/93 in all of our testing.

Dabigsnake

[QUOTE=ktoonsez;1592964239]I have done 4 sessions on a 93 degree day with 101 octane. There was no difference in how quick I overheated if that was what you were asking in the OP. I had zero KR on all 4 runs. Here is run #1 and #2 of the day:

So, are you saying that you seem to overheat at the same rate with 101 octane, as you did with 91-93 octane? You didn't seem overly aggressive on the track. Did you notice any reduced power?

ktoonsez

Yes it was at the same rate. The car only had 800 miles on it so I did not beat on it like I do with my C6. But after about 3 hard laps I believe in video #2 you can here me say car overheating or limp mode when I got the DIC message about reduced power, car needs to cool down.

Dabigsnake

Sorry, that was dar0208.... Quote, about 91 octane. Trust me, my car overheats when it has full performance, but going into reduced power (loosing 20+ mph on straight) you cannot overheat it, but Z51's pass you up!! I'm curious as you about higher octane helping w overheating.

Dabigsnake

Ok, just to be clear. You got a reduced powere message, or just a idle overheat message? I've never gotten a reduced powere message, but certainly felt the power loss and no response at times w pedal to the floor.

ktoonsez

Reduced power message on DIC.

David@MMS

i figured they angled the bricks to compensate for the new higher lid now being flat vs. angled. also figured this move was related to something other than water temp

i have experience running a FI LS car pretty hard one way and then again same everything but with 20* less timing, on accident actually, basically zero timing overall and quite negative timing in boost.

the later the spark, the less combustion takes place in the cylinder, so less heat sent to the cylinder. it also lost little over 200hp this way (1100 with the timing)

its pretty easy to do this if one wanted to know for sure.

davepl

Over in this thread:

https://www.corvetteforum.com/forums...r-results.html

They said:

"Conclusion: As water temps go past 212 timing begins to be pulled and reduced power begins. In the 248+ temp’s logs show timing reduced to only 3deg, if spirited driving continues car will go in to limp mode."

If timing is being pulled down to 3 total, that's THIRTY DEGREES of retard and it starts as early as 212F.

If I'm reading that correctly (and I don't mean it's all out by 212, it happens over that range) then I'm starting to like my theory again.

Or do folks still maintain that pulling 30 degrees of timing out of an engine that you're beating on won't heat it up by a significant additional amount?

This is not all to say timing causes the issue - but the lack of system cooling forces it into a state where it's hot, pulling timing, getting hotter, pulling more timing, and so on. At least that's my theory still :-)

Still, in the thread I cite above, they try it with 98 octane and do not avoid the problem, so preventing knock retard clearly isn't the solution.

Sorry, but that's backwards. Yes, if you retard it an INSANE amount you can get the combustion still happening in the exhaust headers. But it's really about where the peak pressure occurs relative to the crank angle that determines the efficiency of combustion. You can experiment pretty easily and prove it to yourself on a car old enough to have a rotatable distributor (or a laptop on a modern car).

I've even done it by mistake when I was trying to phase a distributor by locking in a timing value and watching it heat.

David@MMS

though i never experienced it, seems more plausible after some morning research.

check out this guys graphs.
https://www.physicsforums.com/thread...engine.701885/

and this resulting thought: the later the spark (to a point) the longer the cylinder is exposed to what i will call 'higher' than existing water temp, the more heat will transfer into the water vs. captured as physical work.

i was concentrating on peak temperature vs average temperature. i see here that peak temperature can drop while average temperature can rise
so i decide now you could sure be on to something

GSpeed

We actually saw -1.5* timing at one point, under wot.

The Ecu pulls very little timing off an increase in water temp. However, 3/4 of the timing removed, is in a table we can't see, based off of IAT2. We verified this with our intercooler pump shut down. We originally thought the timing reduction was oil temp based, but once we were able to control all the temps, timing was within 2-4* of commanded high octane table, at all times.

So, to summarize- coolant to 250, only retards timing up to 5*. However, IAT 2/manifold temp (post intercooler temp) can remove up to 17, that we saw.

Base timing is roughly 23* when everything is happy.