19doug90

So i didnt get to touch the car last summer and its just getting registered to be back on the road this week. Motor in sig and the tune is all sorts of out of shape. Trying to get my head back into gear and make sure i have a proper understanding of the theory behind SA and VE curving so im not just taking blind shots in the dark. What i understand for spark advance: SA increases with rpm at low map values so lets say 10-40 or 50. Once SA reaches around where peak torque is made the objective is to pull the timing out to get back to around 20-24 degrees as quickly as possible. The reason for this being that as more fuel is introduced the burn becomes faster and needs less advance. This would leave us with increasing SA levels at low map and increasing rpm. Lowering SA at low rpm high map values and a curve through the middle of the chart where you hit peak timing somewhere around 50 or 60 bars map with it falling off, and the timing generally being lower at full throttle. What i dont understand about SA tuning: was talking with someone yesterday who has tuning experience with everything from mustangs to imports, and he was telling me that after you've pulled as much timing out just past peak torque there comes a point just before redline where you need to put timing back in? I see evidence of this on the stock 350 tune and dont understand it. What i'm thinking im going to do for a starting point: I'm thinking what will probably get me the best starting point is to go off the stock tpi curve and raise everything about 1000 rpms and go from there. Car made peak hp on the dyno around 6000 on a limped tune but i dont remember where my peak tq numbers were. As for VE tuning: Given what i understand about SA my thinking is basically where ever there is high SA levels the fuel trim will probably be a bit leaner and as SA falls off there will be a need for dumping more and more fuel in. This is a little bit easier aspect to deal with as i will have INT values as well as a WB to target it. Just trying to make sure i have a basic understand of the physics behind it so i have a clue as to where i should be looking to add or remove timing. i also have a bunch of other questions that will follow this.

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355ci motor, stock bottom end with forged pistons, dart irong eagle 180cc runner 64cc combustion chamber heads, 224/230 xe comp cam, stealth ram, LT1 BBK TB, double roller timing chain, aluminum valve covers, smog pump delete and more

My 1990 5.7L IROC-Z

Grim Reaper

You want to increase your spark advance as quickly as your motor can accept it. Peak Torque is a function of your cam and intake to a large extent. A cam with a long duration and short runners (a hot setup) will have it's peak torque at much higher rpms than an engine with a small duration and long runners (a mild setup).

The amount of spark advance will vary from engine to engine, but the amount of variation will not be a great as the variation with peak torque due to the wider range of cam shaft/runner combinations. I have seen 350 V8 engine with their peak torque below 3000 rpm, even in the low 2000 rpm range while hot setups have had their peak torque above 4000 rpm, some close to 5,000 rpm. And many race setups attain peak torque above 5,000 rpm and beyond.

Tyically when you look at a graph, the spark curve for all engines have a great deal of similarity when you look where you attain max spark advance. Only the max amount of spark advance the engine can handle is different, and even then, seldom will you exceed 36-38 degrees, regardless of your setup (excluding any super lean mixtures with highway mode...so let's ignore that) - let's only concern ourselves with a WOT scenerio. I find the majority of engines want max spark in before the mid 3000 rpm range (excluding a little extra tweaking of a few degrees).

Your "max spark spark" point will not match your peak torque in either the mild engine that attains it's peak torque low 2000 rpm nor the hotter engine that makes peak torque in the 5000 rpm. Your theory only happens to work co-incidentally with a TPI engine because they make mid torque around the low-mid 3000 rpm range, typically where you want the max spark advance in all engines, regardless of where it's peak torque occurs.

AC

"subscribed"!!! SA / timing has to be one of the more difficult subjects for me to grasp.

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1991 1LE 46K miles, 383 forged Eagle rotating assembly, Procharger P1SC, Dart Pro1 alum heads, 64cc/200cc,Harland Sharp 1.5 RR's, Comp Cam (Duration I:.520/E:.540 Lift I:236/E:248 113LSA), MSD 8336 Distributor, MSD 6+ Digital spark box, Holley HSR, 60# injectors, Hooker 2210's, Mufflex 4" catback,58mm BBK TB, Tremek TKO600, Spec Stage 3 clutch, Strange 12bolt w/3.73 gears, Spohn SFC,Spohn solid rear swaybar, Spohn LCA's, Spohn Adjustable TA, Denny's Nitrous Ready Driveshaft, $59 code, custom tune.
327rwhp / 485ft-lbs @5PSI boost. Both numbers having the timing 12 degrees retarded from where the timing needed to be. Re-dyno soon.

19doug90

i think my misunderstanding about "pulling timing out" had to do with pulling timing as you go from low map values as compared to full throttle passes.

My understanding of spark advance is simply that the more fuel you have the faster it burns, and the less spark advance you need.

My motor specifically didnt make peak hp until 6, not sure where it made peak tq but im thinking mid to low 4's. The only logic i heard behind pulling timing at peak tq is if you were going with a really agressive tune peak tq is where you're most likely to detonate.

i had a first tune that i took out yesterday, kept everything very rich and had a bit of timing pulled out at 4+.

Ran great in the low end and bogged where i hit 4, which isnt surprising. I'm going to take a shot at keeping static at the 80 bar map range at 3600+ rpms. 3400 at 90 map and 3200 at 100 map.

Thinking with that tune i should be safe enough that i can tune my fuel curve everywhere. And then come back and do some tweaking with my SA to get it close.

Probably get it on a dyno after that as from what i hear you will actually start to lose power with timing before you get detonation, and i dont really feel like tuning witht he knock sensor.

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355ci motor, stock bottom end with forged pistons, dart irong eagle 180cc runner 64cc combustion chamber heads, 224/230 xe comp cam, stealth ram, LT1 BBK TB, double roller timing chain, aluminum valve covers, smog pump delete and more

My 1990 5.7L IROC-Z

AC

So I mistakenly posted a thread VERY similiar to this one after I mistakenly missed this post that I subscribed for. I wanted to attach my spark map for comments. I'm thinking of doing exactly what 19doug90 was going to do and datalog "rpm windows" to base my spark adjustments from. My fueling is pretty darn good so far. Main diff is I'm running the $59 code and not the 8d so I have less resolution for adjustment.