How timing changes with engine compression (CR)

[Frank818]

Hey guys,

Trying to find, here and google, a good explanation on this.

Let's say you run at 9.0:1CR with 25-deg advance (don't bother on RPM and load yet unless you say it makes a huge difference in my question) and that is the max before detonation. Then you change your CR to 8:1.

How much more timing can you put in before detonation?


I know this varies A LOT depending on everything (engine, air, fuel type, etc..), but I'm looking at some generic numbers just to give a general idea.

For example, maybe on a stock LS7 if you lower CR by 1 without changing anything else you can add 5-deg and go up to 30-deg.
On a 2.5 Subbie doing the same maybe you can add 3-deg and go up to 28.

That's what I'm looking for. Can you generally add 10-deg or 1-deg?
How sensitive is the timing increase with regards to the lowering of CR?

This will help me start up with something, cuz I needed to drop my ratio by 1 point due to failling to find some parts for my rebuild.

[NAZ]

If only it was that easy. Not sure what you're after as you question brings up several more just to prepare an answer. There is no one right answer when the only variable is compression ratio.

In simple terms, our objective is to set the ignition lead to accomplish peak cylinder pressure at the point that it exerts maximum leverage on the crank (typically at 15 - 18 degrees ATDC) without detonation. Since fuel has a burn rate we need to set the timing before TDC to adjust peak pressure where it does the most work. For an engine that runs at a constant speed such as a piston powered airplane or engine powered generator timing can be set at a fixed number of degrees BTDC. For a street driven car that sees varying loads and RPMs a timing curve is used and many times a load sensing supplemental timing control is also used (like old school vacuum advance). The optimum timing will depend on many variables including the fuel, AFR, load, RPM, volumetric efficiency, and engine configuration and that's just the start of the list.

Perhaps your first step is to do an internet search on why we set the ignition lead to some point before TDC and follow the bunny trails from there to see what all affects optimum timing. Or if you're more specific with your goals folks would have an easier time providing you suggestions to accomplish them.

[NevaLift2Shift]

That's a good question. Hopefully someone here has the answer if not, join the "Guild of EFI Tuners" group on facebook. Lots of knowledgeable people on there. I don't have enough experience to give you a correct answer.

[Frank818]

I thought that would be it. On the other end I also thought that if only CR is changing and no other variables, that a theory did exist on the change of timing in order to keep peak cyl pressure.

My goal is simly that I had a timing curve in 240 points at the moment, based on 9:1CR, but I'm going to change to 8:1CR and I was wondering if I should increase timing and if yes should I start by 2-deg only or 10-deg, etc... or maybe increase by 10-deg under vacuum and 5-deg under boost, that's what I'm trying to understand.

So for example if you run at 80KPa, 3000RPM, 9:1, you have 32-deg. Then you change to 8:1, still 3000RPM and 80KPa, what should happen to timing in order to have maximum cyl pressure (may not be the same number as with 9:1, but still maximum pressure possible for 8:1).

[STiPWRD]

Frank, I would hold off on modifying your timing curve until you're actually tuning your engine and getting real data from sensors or a dyno. It sounds like you already understand the theory. I suggest starting at safe timing numbers and making small increases until either power levels off or you begin to see knock. I would just go off the real data.

[Frank818]

Frank, I would hold off on modifying your timing curve until you're actually tuning your engine and getting real data from sensors or a dyno. It sounds like you already understand the theory. I suggest starting at safe timing numbers and making small increases until either power levels off or you begin to see knock. I would just go off the real data.

Yeah true, that was one possible plan, I think I'm trying to go too quickly and get back that year I lost. lolll
I'll first get the car on the road and then play along with timing. I still need to re-do my fuel map cuz I'm changing injectors so I'll be plenty overwhelmed with everything at the same time (including rear bumpsteer) it's not a good time to think about timing. loll You are absolutely right.


Once I get there and see the results, I'll post back here.

[DMC7492]

Hello I concur on the holding off! Evertime I get a chance to take the 818 for a burn, I’m logging and making adjustments for three summers now, but the build is still in progress and I’m constantly changing fuel mapping or timing because other changes to the build, So I always keep telling myself that I’m not ready for tuning to peak numbers ! It’s so tempting but wait until the build is ready for “THE TUNE”!
That’s my experience and I’m sticking to it! DMC