Ignition Timing Theory

[Guest]

I've got a couple theory-esqe questions about ignition timing and tuning and I thought they would be most appropriately answered in this section. I have a 78 GS750 and the stock setting was 37 degrees BTDC total. The reason for the ignition advance is to start the combustion process such that peak cylinder pressure is achieved about 7-10 degrees ATDC if I remember right? So then if we take a look at say a 1970's Small block/big block Chevy, it seems the factory total timing on those engines is between 28-34 degrees.

Here's where my question comes in:

Why do our engines, which make much more power per CC than the old muscle cars, require more timing? My thinking is that to make the higher power levels our engines would have more advanced combustion chamber design, and therefore require LESS timing because the combustion process happens faster. A couple ideas I have to explain this are

1. Higher rpm, which means we have to spark sooner
2. Less compression, making the process take longer
3. Some unknown reason having to do with them being air-cooled

How about the old school vacuum advance, would something like that on a GS improve fuel economy?

Last thing:

Is there any performance benefit on a mostly stock engine of advancing the timing a couple degrees as long as detonation is controlled (high octane). Or even on regular 87 because we have better fuel nowadays?



Sorry for the long, in depth, question but I know there are some very knowledgeable people on here that like to get into this type of stuff.

 

[GregT]

The reason for the disparity in advance between the 2V GS hemi head and the 2V bathtub chamber of the Chevy can be summed up in two words - Swirl and Squish.
The GS head is not a particularly sophisticated one. It has no squish to speak of and relies on the angled inlet port to achieve any swirl. The end result is a fairly slow burning charge.
The Chevy is actually more sophisticated than it looks. Harry Weslake laid the foundations for work on bathtub chambers and Chevy developed the canted valves. It has an area which promotes squish so is actually a fast-burn head.
I knew a SAE qualified engineer here who'd done two years at the Experimental Engine section of Chevy - after he'd been involved with Can-Am cars.
Talking to him about how the Chevy heads were developed was an eye-opener.

Most of the 2V GS engines respond to a bit more advance - but if you've built a serious one, the aim is to find how little advance you need to make max power...Time on a dyno pays.

 

[ArttuH]

I think RPM range is the key factor here. Peak torque and power on the GS engine are at much higher RPM compared to Chevy V8 so it needs more timing there since time available for burn process is shorter. And since ports and valve timing are optimized for high RPM cylinder filling and mixture turbulence are poor at lower rpms. And that causes slower burning and therefore higher optimal timing advance.

For increased advance on stock engine I would say maybe, maybe not Even if you can prevent detonation it doesn't necessarily increase power. This is very engine dependent. Some engines make best power at detonation limit and some at much lower timing.