So I've heard a whole bunch of stuff lately and none of it has helped me so I was wondering what exactly changes timing curve requirements for an engine.

Why does higher compression require more initial timing?

How does the compression affect the timing curve.

Everyone I've talked to says that with higher compression I need to run more initial timing. Well most people run 6-8* intial, whereas I have been told to run 14-16*.

So how doe that change the mechanical and vacuum advance timing curves?

Since the intial is higher, that means less mechanical and vacuum advance to keep total timing down. How does the change how the engine wants the timing then?

Is the rate of advance overly important? Most applications say all mechanical in by 3000RPM. Does it matter if all mechanical is in earlier? Is rate just as important total?

 

Best hp numbers are usually produced when peak pressure (from compression AND combustion), occurs around 20* after TDC.

1) advance needs to increase by rpm, due to the piston speed increasing, while flame speed doesn't. (20 degrees of rotation at 1000 rpms is obviously 6 times longer than 20 degrees of rotation at 6000 rpms ).

2) advance must be backed off when VE increases. This means both a) when nearing peak torque @ WOT as most are used to, but also b) when operating at WOT (idle and part-thottle have low VE, resulting in less compression and combustion pressures being generated). Remember vacume-advance? that's what this is compensating for... vacume typically means lower VE.

There's more to this topic like knock retard, coolant/oil temp adjustment (higher temps mean sooner detonation), barometic pressure adjustments, etc... and of course the ultimate barometic pressure swing: boost (or nitrous if you want to just focus on the pressure gains from combustion while excluding compression).

I look forward to the pro's response to this one. I imagine there's a lot of possibilities with computerized ignitions (like the 4 the 4th F-bodies use, or MSD stuff as well), but with a HEI design I'd wager you need to be a careful you don't push the edge of detonation as far as cars with knock sensors.

 

Sorry, almost forgot:

The static compression ratio of an engine is rarely a good factor for determining details like this. Dynamic compression (which is a better measure of VE due to it's incorporation of the cam shaft design) would be a better place to start.

A vehicle with higher calculated dynamic compression will have more pressure in the combustion chamber, and as a result, the flame front will travel faster, needing less advance overall. Typically a higher compression design also means more vacume at idle, and I'd imagine such an engine would need less vacume advance.

Not having set up a ignition tune from scratch that's about all I've got to contribute.

Good thread BTW.

 

Sorry to bring a death thread back to life but:

So in theory more dynamic compression should decrease the amount of timing required....

I'm just curious since my buddy and I have photocopy 350 buildups...same cam, he has ported 462 camelbacks, I have vortecs. My compression is 10.7:1 and his is 9.7:1 yet his engine idles rock solid at 8-10* initial yet mine wants 20*+ for a smooth idle. I just don't understand it....if I try and run my engine at 8* intial it will run rough, and response is poor.

Is it my compression that is affecting the idle quality so much? Is it the differences in our cylinder heads?

 

How's his fuel economy compared to yours? I've been able to richen up an engine enough to get it to idle fine with no vac advance at idle. If you set up the carb correctly, closer to stoich, it spits, sputters, pitches a fit.

Check his header/exhaust temp, compared to yours. I'd bet his are a few hundred degrees higher at idle. Not good.

Low 20s for initial timing is definitely in the ballpark for a decent idle, from my experience, at that compression level.

BUT, if you have "copycats," and his 462s are 64cc, and your Vortecs are 62cc (IIRC), then how are you a point higher?

 

He runs a flat top 9.7:1 piston, I run the 10.7:1 .125 domed piston....Sealed power makes both...I picked the higher CR trying to sqeeze more power out.

I noticed my engine water temp drops 15* at idle when I advance initial timing. Someone explained to me that overlap of my cam (224/230 duration XE268H) causes the lean fuel mixture to become diluted....requiring more advance to get a complete burn. I just don't understand why I need so much more timing then him considering compression is the only "major" difference.

He is getting about 14MPG city, I get about 12 MPG, my carb needs tuning, I've got a power valve and jets in transit already from summit along with some other stuff.

 

Your timing needs seem entirely normal to me. Virtually every street engine wants that sort of timing.

Why his doesn't is the real question.

You are running vac advance, right? You should be getting your idle timing by setting about 6-8* initial, and then adding manifold vac advance of whatever to get you to the mid-20s at idle. Total timing on your setup probably shouldn't be more than 30* at that compression, with those heads. Vortecs almost never want more than 32*, and you are above where I would have set my compression for that setup.

 

32* @ WOT doesn't ping, I'm running 94 octane.

His engine runs perfectly with 6* initial, on ported vacuum, with 24* mechanical.

He ran 14.1 @ 98 at sea level recently on a very hot/humid day.

 

Well,you left out a few things.What is intake valve closing point,quench height,combustion chamber CC's,rod length,cooling system efficency,dist advance curve.These aren't all that plays a part in total timing.The better the chamber the less timing you will need.
With that big of a cam your dynamic comp. is pretty low 8.5-8.6 (guess)and that is what the engine see's not 10.7

No two engines are the same---- just close--


The "old school" way is to baseline it with whatever timing you are running at the strip,then keep throwing timing at it 2* at a time until it slows down then back it off 1-2* and lock it down.You will be right for that day and that tank of gas you have that day,but pretty close overall.You will be suprised at the end results.
You need to start with the timing NOT to far advanced.

 

Well was hoping to get some theory on how engine characteristics change with compression not just specifics to my setup. Logically, compression is mechanical advantage....the more mechanical advantage the more fuel economy and power the engine should make. Now the cam controls dynamic compression, and therefore controls how the engine responds in lean burn conditions....so how would jumping from 9.7:1 to 10.7: CR change how the engine responds at idle and cruise....

 

Not enough for the SOP to tell.You could record it on an engine dyno.It won't make a daylight to dark difference.
They make ALL kind of books on theory and application.