Hi guys. I still frequent some other message boards where more old school racer guys who run carbs hang out. They all curve mechanical distributors to have all timing in by around 2500 rpms. None of the factory TPI 305 or 350 bins have the timing in this quick (then again mechanical factory distributors probably did not advance this quickly either). An 89 305 TPI doesn't have all of the timing in until over 4000 rpms. Would more power be made if I bring timing on a slightly under 10:1 compression ratio car in significantly sooner? What do you guys do on the WOT end of your timing tables?

 

From my understanding, the rule of thumb is, the hotter the cam, the sooner you bring in all of your timing. In other words, with a stock engine (small cam), the existing spark calibration is about right, with the total SA coming in late. Only if you cam your engine would you want to bring it in much sooner. If you tried to bring in all of the SA by 2500rpm on a stock TPI (torque peak area), I'm sure you'd have some serious knock retard.

EDIT: I didn't see your engine combination before. Obviously you wouldn't want to run the factory spark calibration on that setup. Definitely bring in the total SA sooner. 2500 sounds like an ok place to start...

 

Well, compression, octane, weight, gear, converter, heads are all going to effect what kind of timing it wants.

Like Grump says, do what the car likes.

Basically shoot for the lowest amount of timing consistent with the best performance.

Meaning of it runs the same at 32deg(example) that it does at 28deg(example), run the 28 deg.

Use something to measure(track mph or g-tech) and pay attention to the plugs and the knock counts and timing pulled, wideband helps for finding flakey spots in the AF ratio too(if lean spot causing knock)


later
Jeremy

 

I don't know but it seems logical to me that the OEM calibrations didn't bring in the max WOT timing until high because of the commanded AFRs. At peak power OEM was usually on the rich side meaning you can add timing to gain SOME power. Ideally a tune that isn't concerned about the cat would run a leaner AFR and probably make more power with less timing than SOME of GM's tunes.
Other than that, I've never found a decent 250+hp 350ci+ motor that didn't feel and perform better with all of the timing in by 2800.
When racers ramp in the timing too fast it's usually matched to a torque converter and gears that hide the "too much timing" at low RPM. An engine can feel really good with a lot of timing in too soon. To get a good feel for the effects I stray from Grumpy's tuning style in that I'll take out a large amount of timing just to really get a good feel of what is going on. I bounce back and forth. As an example; I'll go from one extream to another just to get a feel of the engine and what it wants, then hit middle grounds, from those 3 tests I come with a narrower range to make changes. It's like that game where you guess a number between 0-100 and you pick 50 then it tells you either higher or lower . That's how I do the WOT timing when I'm not at the track.

 

Jon,

Sounds like you tune the way you play "BATTLESHIP".

 

When all you have are hammers, the solutions all use nails, and brute force.

For WOT, and short sprints you can get by with what would only be discribed as terrible timing curves. But the nature of the beast forces that.

If they're happy you sure ain't gonna convince them otherwise. Logic only goes so far with some people.

My spark is pretty much in by 2,500......
But there are at least 1/2 dozen modifiers to that....

 

Thanks everybody. I was in no way suggesting that I agree with the "brute force" method some of the carb/racer guys use. I had one guy telling me there was "no way" a small block chevy would survive with 40+ degrees of timing (and we were talking about low load cruise only situations). He said the efi tuners were "crazy" to be running that much timing. When I tried to explain to him that even factory setups from the 60s and 70s saw 40+ degrees of timing just due to the vacuum advance on a mechanical distributor he didn't know what to say. Oh well.

 

327,

As Bruce would tell you, more advance isn't a sign of better anything. The better the cylinder head and combustion chamber design, the less advance required to burn the charge more completely. I think he runs 25-28d total in his GN, maybe even less. The LS motors have very "conservative" total advance.

 

You will always need spark advance with gas as your fuel. This is because gas burns at a certain rate, that rate is influenced by many factors but if you looked at a cylinder pressure vs crank angle it would be easier to understand.
Yes more homoginious mixtures require less advance but those LS motors are also more volumetricly efficient and if you looked at the VE vs SA compared to the same for an older sbc you'll notice a trend .

 

Jon,

From what I have read, the combustion chamber design of the LS heads encourages much more rapid flame propogation(ie Fast Burn heads). So the mixture burns faster (and hotter) releasing more energy for the same amount of charge in a shorter time span. IOW, need less advance timing. Regarding your point of Crank Angle v Cylinder pressure, you're less likely to have mixture combustion working against the piston as its coming back up. In fact, mixture ignition can be timed to take advantage of piston position for greater momentum.

 

Dom, correct, but I was just pointing out that there are a lot of factors going into how much timing the engine wants.
As for momentum, that's a loaded word because it can mean a lot if not described correctly.
Things like rod ratio and cam specs also drastically effect the required timing so saying that the LS series needs less timing than gen1 because of the combustion chamber design would be incorrect. That is a large factor but not the only one. It does lower the required timing
Factors for any IC engine;
1. Mixture quality
2. Port designs
3. VE
4. Cam specs
5. Quench
6. Static compression
7. Combustion chamber design
8. Piston top design
9. Rod ratio
10. Bore stroke ratio
11. Intake/Exhaust pressures (engine load in MAP and Atmosphere)
12. Temperature of parts in contact with intake charge
13. Weight of vehicle or more advanced, time spent at a specific load (builds heat)
14. Weight of rotating mass
The list could go on if I sat here and took more time.

 

Jon,

No argument from me on the many variables that go into timing.
However, other items being constant, the combustion chamber design, as you have agreed, is a big factor. As for the momentum,
I had read an SBC Vizard article about an optimum point of piston travel where the mixture combustion adds to piston travelling back down the cylinder bore, rather than fighting it or slowing it down as it comes up during the compression stroke. IIRC, it was something around 10BTDC.

 

Yup, that value isn't set in stone because of the different burn rates of various engines and the cam leaving the valves open . The ideal for power and fuel efficiency is to make the most torque. This is done by reducing wasted energy (going into loading of various parts) and by getting the most pressure from combustion to press down on the piston at certain crank angles. With the crank and rod making at 90* this is where the potential for making the most power is greatest BUT it doesn't mean anything unless you look at the integral of the function generated by pressure vs crank angle. It's so much easier explaining all of this with visuals than it is with text

This might be of use; http://www.engr.colostate.edu/~allan/engines.html