Spoolin

iTrader: (8)

Was logging some runs on a buddies truck with HPT and noticed during his romping around how much the timing changes depending on driving conditions (Duuhhhh). My question is I noticed when he got out of the throttle and the engine was doing most of the braking the timing went to negative degrees which told me it was lighting off the air/fuel mixture before TDC which sounds to me like knocking no?
So when we do engine braking coming down a mountain and we drop it into 2nd or 3rd gear is the engine purposefully causing knocking to slow down the truck? (which doesn't sound right to me) There's a lot I'm probably not understanding here so that's why I ask.

alanderson1978

iTrader: (1)

I need to know more about this as well. I'm working on a 2-Bar tune, and I have so far found out that when timing goes negative, your power goes to almost zero. The problem I'm having is trying to advance the timing to keep it positive after the IAT, ECT, AFR adders and such, but not getting knock.
Just taking little steps for right now.
I was tuning my spark last week (before I found my IFR trouble), and was logging every advance/retard parameter to figure out what was pulling all the timing.

Stoichiometric

iTrader: (10)

You might have been seeing that timing as a result of DFCO (deceleration fuel cut off). I know they have certain tables with special timing when the DFCO is enabled, not sure what they are though. Usually my timing is very advanced 40+ degrees when there is a lot of manifold vacuum (engine braking) or very light throttle. The only negative timing I have seen with my truck is under high load or manifold pressure at low RPM. I have my DFCO disabled so it does not interfere with my data logging, so I don't know what my timing would do then.

Stoichiometric

iTrader: (10)

alanderson1978, I zerod almost all IAT, ECT, ect. tables when I was beggining to tuning the spark. It was adding complication when I was commanding 20* and getting 8* because of some IAT multiplier or other table. I tried to disable all sources of timing input besides the high octane table (I also make the low octane table the same as high octane for tuning). Once you get the basic spark figured out you can add the other tables back in one at a time and see there effect as you add them.

Spoolin

iTrader: (8)

Wow, thanks Henry! Kinda grasping it but your way ahead of me on knowledge but basically when I see negative timing it is in fact lighting off the spark prior to TDC. When you mention timing at 40 and 8 degrees your talking about degrees after TDC right?
Also I assume the lower (number wise) the degrees where you set off the spark the more you can maximize the full potential of the combustion right? What is the ideal spark? Like 1 degree?
Also 40 degrees of timing which is what I was seeing when truck was not under any kind of load basically means the engine is not doing much of anything power wise?

Spoolin

iTrader: (8)

Also, knock, detonation, pre-ignitionis all the same right?

parish8

iTrader: (6)

you have it backwards. when we say 40deg or 8 deg that is advance, or before tdc. you need to light it off eairly to get the max cylinder presure while the piston is on the way down.

if you see negative timing then that is after tdc. some turbo guys do this to try and light the fuel mixture while it is in the exhaust manifold and build boost faster but i dont know of any ls1 guys doing it. under decel i dont think it matters what is going on. if thats how the factory programed it i wouldn't worry about it.

Stoichiometric

iTrader: (10)

'Timing advance' is refering to how many degrees before the piston reaches TDC that you get spark. So 20 degrees means the spark happens as the piston is approching TDC (coming up the bore). 0 degrees is at TDC and anything negative is spark after TDC. More power is made to a certain point by increasing the advance. The flame that is created in the cylinder takes time to grow to a usable amount. You are trying to maximize the amount of energy pushing the piston away from TDC.

If you light the mixture too early (too much timing) then the flame front can burn too quickly in the cylinder and hit the cylinder walls. This happens especially at low engine speeds where the piston is moving more slowly away from TDC. When the flame front hits the cylinder walls it creates excessive pressure that causes an explosision of the rest of the air fuel mixture. That is pinging, I think. These terms are often interchanged incorrectly (I do this myself and am not exactly sure which is which).

Another type of knock that can happen and that I have heard is in audible is pre-ignition . This can happen due to hot spots in the cylinder (possibly caused by carbon build up). These hot spots can remain hot enought between firings to pre-ignite the fresh air fuel charge as it enters the cylinder near BDC. The burning air fuel is then compressed on the compression stoke. One or two cycles can be enough to burn holes in pistons/valves.

There is a lot of good reading around, If I come across some of the articles I read I'll share them.

Spoolin

iTrader: (8)

Ok, well this makes a little more sense! So obviously negative timing isn't gonna do much damage other than if done too much at which point it will glow the headers, and in turbo applications turn the turbo into a jet engine (I think we were talking about that on Saturday).

So on a typical engine when does the intake valve close? I can't be sure but I recall learning that they close a little past BDC no?
I guess the faster you spin your engine the more timing your gonna need to allow the flame front to develop no?
I remember you saying you're running 40 degree's throughout WOT but how does that work when your in the lower RPM band, you still running 40 degrees at that point?

Stoichiometric

iTrader: (10)

The intake valve does close a little after BDC, the later it closes the more top end oriented the engine is. It does seem logical that the faster the engine spins you need more time to get the flame going. I don't know the answer to that one. I run WOT 2500rpm+ at 24* across the board on my truck. I run less timing at lower RPM. I'll get some screen shots of my timing table later on, it will be easier to see what is going on.