Spark timing question

[ULTM8Z]

If adding in extra advance (vacuum) at cruise, (low throttle angle/high vacuum/lean AFR) feels better as in crisper throttle response, slightly lower temps (water/oil) and slightly better mpg's, you are going the right way, and the engine is telling you this.

That's pretty much where my head is at on this issue.

BTW, the reason the Corvette spark map shows such low timing at WOT is because (probably emissions related) GM reserved about 6-7 deg for use in a Power Enrichment spark advance table. You don't get that extra advance unless you meet the criterion for %TPS and MAP to enter into power enrichment mode. Even with that, it still seems a bit low for a SBC. But maybe the L98's heads and TPI manifold preferred only about 30 total at WOT. Or the small cam and 9.8:1 produced too high of a DCR and they were guarding against the ECM engaging knock retard during normal operation.

 

[FS87LT]

If the engines were supposed to be able to use 87 pump octane fuels (about 91 Research octane), that might explain the low numbers. Seems like some of the ECMs were calibrated to advance the timing until the knock sensors retarded it, if higher octane fuels were used? So, do the calibrations "on the safe side" to minimize knock sensor intervention (which I read had some time limits on it before full advance was re-allowed).

Just some thoughts,
FS87LT

 

[G72Zed]

That's pretty much where my head is at on this issue.

BTW, the reason the Corvette spark map shows such low timing at WOT is because (probably emissions related) GM reserved about 6-7 deg for use in a Power Enrichment spark advance table. You don't get that extra advance unless you meet the criterion for %TPS and MAP to enter into power enrichment mode. Even with that, it still seems a bit low for a SBC. But maybe the L98's heads and TPI manifold preferred only about 30 total at WOT. Or the small cam and 9.8:1 produced too high of a DCR and they were guarding against the ECM engaging knock retard during normal operation.

FWIW, I've worked on several of those "ZZ4" GM Crate 350's that used the "fast burn" 58cc Corvette L98 based aluminum heads. Hyd. roller cam and 10.1 CR, they say 32* and from what I remember they were all rattle monsters and could not take the timing even with good (91-93) pump fuel, none E, very timing sensitive !!! Customers were quick to ditch those heads or built other engines with more power and less detonation issues, again, carb application, not EFI.

The new Iron Vortec heads GM uses on the Crates now can take 34-35.5+* and not bat an eye with 91 pump, smaller FT cam and make 380-395HP, all with 1 full point less compression and at 64cc. GM learned there lessons and updated there head program since the early-late 90's for the SBC.

 

[sandlapper]

IIRC, ZZ4 sbc motors (long discontinued) came with alum ZZ4 heads aka L98 heads aka x113 casting number (also long discontinued). And, while those heads have 58cc chambers, their chambers are a small, closed chamber "bathtub" design and not "fastburn" --- fastburn design serial production heads first began with advent of Gen II reverse-flow heads (aka second gen LT1 & LT4) --- which did precede serial production of iron fastburn L30 & L31 Vortec heads.
AFAIK, fastburn ZZ motors first began with introduction of ZZ6 and its alum fastburn ZZ6 head (casting # 19417568) having 62cc chambers --- that head not a serial production piece, but only performance CPP fka GMPP. No beef; just ZZ4/L98 not a fastburn --- and so true, ZZ4 Not detonation resistant at all !

 

[G72Zed]

Thanks sandlapper for the added details an response/clarification on the ZZ4/L98 heads. I had one customer literally toss the heads in the trash can, the other customer put in a modified LS based 525CT Crate, with a carb of course LOL.

 

[FS87LT]

Seems like I first noticed the "fastburn" nomenclature applied to the LS NASCAR heads in GMPP?

 

[sandlapper]

Seems like I first noticed the "fastburn" nomenclature applied to the LS NASCAR heads in GMPP?

Perhaps something like this

NASCAR-Approved Spec Engine

block and heads based on LS2 --- ARCA Menards, NASCAR Whelen, NASCAR Pinty's
LS2 is Gen III (after Gen II LT1 & LT4) --- LS motors' chambers have what I think is a fastburn design.

 

[Tokyo Torquer3]

You don't indicate the details of your engine so hard to comment on your timing map. Timing as high as 50 degrees can help the engine in many ways under deacceleration when there is no load on the engine, vacuum is high, and hardly any fuel is going into the chambers. There is little risk to hurting the engine. Under load with lots of fuel in the cylinders is a different ballgame.

Use this link and scroll down to the "Tuning Guide" and the section on timing under deacceleration.

Progression Ignition

Fully electronic timing control. Tuned with your smartphone or tablet. Wireless Bluetooth connection

progressionignition.com

 

[2ndGenCrazy]

That's pretty much where my head is at on this issue.

BTW, the reason the Corvette spark map shows such low timing at WOT is because (probably emissions related) GM reserved about 6-7 deg for use in a Power Enrichment spark advance table. You don't get that extra advance unless you meet the criterion for %TPS and MAP to enter into power enrichment mode. Even with that, it still seems a bit low for a SBC. But maybe the L98's heads and TPI manifold preferred only about 30 total at WOT. Or the small cam and 9.8:1 produced too high of a DCR and they were guarding against the ECM engaging knock retard during normal operation.

There are actually no emissions requirements or testing at WOT. There do exist algorithms to protect the piston and cats under heavy load.
There are established procedures each Auto maker has for developing the engine control software. My GM division had several dyno cells devoted to the calibrators providing the software for fuel injection systems we manufactured. As a side note, fuel varies quite a bit depending on season and location. Is this weeks fuel the same as last weeks fuel? Even though fuel is required to have a minimum octane, How is it this week? I have no idea if this is "spot checked" to verify fuel companies are not cutting corners now and then. Maybe try a different brand if you all of a sudden experience some ping.

 

[FS87LT]

Back in the middle 1960s or so, Texaco used to advertise "blended for your part of the country and time of year". This was back when each major gas company had their own specific refineries and delivered the gas in their own-labeled trucks, rather than "a contractor" to do that.

In the current state of affairs, there are a LOT of different fuel blends which cover the USA. A map of such used to be in the back of the Mobil website. Varying depending upon ozone concentrations of various areas, or other air quality "non-attainment areas" involved.

FS87LT