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Requesting Spark Advance Advice – CI vs Al Heads 2 Attachment(s) Greetings, TGO – This time, I’m requesting spark advance tuning advice from the assembled intelligensia. In my one-owner ’85 Camaro IROCZ28, I’ve a FT-cam’d, LM1 350ci block sporting a pair of ZZ4 aluminum heads to which an L98 intake manifold is bolted. The TPI intake and fuel injection are stock to the ’85. The ‘85 ECM is the stock ‘6870 and the mask is $1F_updated. While the engine builder installed a Hypertech Thermomaster 155332 PROM, there’s room for improvement IMO. In pouring over various threads in fora on spark advance – cast iron heads vs aluminum heads, I’ve compared the SA tables between the 85 Camaro (HLK bin) and the 85 Corvette (HLH bin), both sporting CI heads at the time (delta #s in red signify higher SA in the Corvette) – please see the 1st attachment. In a similar fashion, I’ve compared the SA tables between the same FT-cam’d ’85 305 (HLK bin) and a rocker-cam’d ’88 350 (ABTT bin) Corvette – please see the 2nd attachment. In both cases, the SA tables in the HLH and ABTT bins are higher than a CI head-equipped 305. While I get that Al heads can handle more SA over CI heads, it appears that AI heads over a rocker cam can handle even more SA that Al heads over an FT cam – does my understanding make sense? Questions: a) Since I’ve ZZ4 Al heads over an FT cam and I figure the Hypertech PROM’s SA table to be not much different than the stock HLK bin, would it be reasonable to import the HLH SA table as a 1st step to increase power in the mid-range? b) Since I’ve a ‘6870 ECM and the $1F_updated mask does not make available knock counts in the datastream, would I be wasting my time on this exercise? |
Re: Requesting Spark Advance Advice – CI vs Al Heads Couple things to note here 87+ L98's and Lb9 tpi engines received roller lifter cam types. Not rocker cam. Just wanted to clarify that. Second, cam type doesnt matter that much. What matters is cam specs and how that relates to your compression ratio, both static and the actual valve closing point compression aka dynamic compression Next is combustion chamber design. These are the main things that determine what an engine wants for timing given a specific fuel. Zz4 heads are basically corvette aluminum L98 heads. These heads for some reason do not seem to have real fast burn chambers. They seem lazy and actually tolerate alot more timing. Gm actually gave them alot more timing than the iron headed f body motors Also last note, those are main spark tables but they do not include timing addition from power enrichment mode. Fbody and vettes can have different values for pe mode timing so cant directly compare the two main tables until you look at the pe table, and also the coolant temp based timing adders and subtractors Using the vette based timing table is advised but i would pull a few degrees to start and work it up slowly, datalogging to see how it goes. Its really kinda hard to determine optimal timig for cruise and part throttle. Kinda have to monitor throttle position, intake vacuum, rpm, load variable, and gas mileage to see if its being most efficient Wot is done on the dyno |
Re: Requesting Spark Advance Advice – CI vs Al Heads Those heads are typically in the area of 28* BTDC once at WOT with RPM and load. They can like more SA in lower gears (less load). Set up the lower end of the SA table for driveability. Then as Orr mentioned get the WOT areas via a dyno or track. RBob. |
Re: Requesting Spark Advance Advice – CI vs Al Heads Many thanks, RBob and Orr89RocZ, for your suggestions and I apologize for my tardy reply as I've been wrestling with internet trollers here in the PRC. Orr89RocZ - Thanks for the correction roller vs rocker. For certain, an inaccurate choice of words on my part. Upon my return from business travel and no longer annoyed by PRC internet trollers watching every key stroke for seditious language, I've a few follow-on questions. |
Re: Requesting Spark Advance Advice – CI vs Al Heads Good afternoon and, now that I've safely returned from the PRC, I've two follow-on questions.
Originally Posted by Orr89RocZ
(Post 6192956)
Second, cam type doesnt matter that much. What matters is cam specs and how that relates to your compression ratio, both static and the actual valve closing point compression aka dynamic compression.
Originally Posted by Orr89RocZ
(Post 6192956)
Using the vette based timing table is advised but i would pull a few degrees to start and work it up slowly, datalogging to see how it goes. Its really kinda hard to determine optimal timig for cruise and part throttle. Kinda have to monitor throttle position, intake vacuum, rpm, load variable, and gas mileage to see if its being most efficient. You also mentioned that you would "...pull a few degrees to start and work it up slowly, datalogging..." Since I'm extremely inexperienced when it comes to tuning, does "pulling timing out" refer to adding or subtracting, say, 3-5 degrees of SA uniformly across the entire table or only at part throttle and cruise? Secondly and referring to a generic RPM-LV8 table map like those above, how would I compartmentalize the table for part throttle, cruise, and WOT? Idle's easy, I reckon... |
Re: Requesting Spark Advance Advice – CI vs Al Heads Thats a small cam so you'll have higher cylinder pressure. It will want less timing in general. Pulling timing out means subtracting some. I would do it across the table for now. Slowly working into wide open throttle Idle LV8 usually 40-60 depending on cam. Small cam it will be less. High rpm deceleration can be 32-40. Light part throttle 60-112 Half throttle stuff 120-160. Wot 190 and up depending on how powerful motor is. Stock L98 is usually 190-210 ish. Big hp stuff will near max out at 248-255 or whatever the limit is. |
Re: Requesting Spark Advance Advice – CI vs Al Heads Many thanks, Orr89RocZ, for your reply. I'll take yours and RBob's suggestions, what other information I've been able to glean from other similar threads on this forum, and mock up an initial SA table. Even though I've a '6870 ECM and using a $1F_updated mask, I managed to find all the SA-related scalars and tables from the bin. In addition to the base SA vs RPM vs LV8 table, here are the other bits: - Scalar: Max Knock Retard Spark (not in WOT) = 17.93* - Scalar: Min Coolant Temp to Enable Knock Control = 40.25C - Scalar: Max SA Limit = 41.84* - Scalar: Coolant Compensation Spark Table Bian = 20* - Scalar: HW Mode Spark LV8 Disable Threshold = 255 counts - Scalar: HW Spark Coolant Temp Disable Threshold = 63.5C - Scalar: HW Mode Spark RPM Disable Threshold = 800 RPM - Max Knock Retard vs RPM (in WOT) - PE Spark vs AFR - Coolant Compensation Spark vs Load (LV8) - HW Mode SA vs Load (LV8) - TCC Locked Spark Retard vs RPM vs Load (LV8) - Knock Retard Attack Rate vs RPM - Knock Retard Recovery Rate vs RPM - Time Out Spark vs Coolant Temp - Spark Time Out Decay Rate vs Coolant Temp - Spark Time Out Decay Delay vs Coolant Temp - SA Correction vs MAT If anyone on this thread has insight on how the SA logic operates based on these scalars and tables, I'd appreciate any tips and suggestions on how to interpret and then implement what I've learned. Flow charts would be wicked useful.:thanks: |
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