Ignition curve for a centrifugal blown SBC

[DorianL]

Hey all,

I plugged a laptop in to my 6AL2. Turns out, my ignition curve was very sluggish: 27 degrees at WOT.

Anyway... What would you guys suggest as timing curve? Do I keep vacuum advance?

My cr is 8.0:1. 406SBC. Forged rotating assy. Water meth injection. Centrifugal super up to 10 PSI.

Limit RPM to 6200?

 

[Indycars]

Wish I had an opinion for you, cause that would mean I had experience with a blown motor !

 

[larrym]

I'm not really familiar with SMC timing curves or blowers, personally I don't run much more the 25 degrees in first gear in my Buick and about 21 in the rest (its nice being able to adjust it from the drivers seat). Find if I add too much timing I can't run as much boost without getting detonation, its trying to find that sweet spot that is the trick.

 

[Big_G]

I am running a ProCharged 383, 20 lbs. boost. No water/meth. Attached is the timing map, FAST efi.

 

[larrym]

Fast sure looks nice to work with thanks for the screen shot!

 

[DorianL]

WOW! Thanks!!!!

That's a BIG help. It's tough to convert KPa into inches of Hg. Converting into boost is pretty straight forward.

From what I read, mechanical is all in by 2500 rpm.

Initial is 20 degrees with vacuum. As it gets closer to atmospheric pressure, another 10 degrees of advance pop in there at idle.

It seems that there is about 0.5 degrees of advance pulled for every pound of boost all the way from full vacuum to full boost.

Soooo, I am going to aim for 20-4 degrees initial. No vacuum advance. At atmospheric: 35 degrees total... all in by 2500.

As for boost retard, it will be half a degree per PSI as of 2 PSI... So at full boost it will pull 4 degrees at 10 PSI.

Let's try that out first. Water/meth should offer a good safety margin.

 

[mathd]

Big G,
You are running 20 psi of boost without meth/water injection? please school me on how you can manage that safelly, what is the secret beside boost retard ?
Thanks for the Screenshot its gonna help me some too

Usually since you can convert easily from kpa to psi.. just multiply the in.hg by 0.5(0.49) so you get the psi and convert more easily to kpa(since 1 in.hg = 0.5 psi, 1 psi = 2 in.hg) that is rounded off to give a ballpark.
kpa to in.hg = kpa * 0.295 = in.hg
bar to in.hg = bar * 29.53 = in.hg
in­.hg to psi = in.hg * 0.49 = PSI
kpa to psi = 14.7/101.3 * kpa = PSI AND if you substract - 14.7(or your absolute atmospheric pressure) from the previous result(MAP) = boost pressure
1 bar = 101.3 kpa, 101.3 kpa = 14.7 PSI (sealevel pressure)

I know you probably know the last part, but am posting this in case it can help other people convert the unit when running a boosted application.

 

[DorianL]

Ummmm. Let's say I'm pulling 14inHG at idle. What is that in KPa?

Isn't 1bar 0inHG?

 

[mathd]

I fixed my previous post, it was rounded a little now its more accurate.

1 bar is almost the absolute atmospheric pressure at sea level(1 bar = 0.987 atmosphere). Most people will round 1 bar = 1 atmosphere.
1 atmosphere = 1.013 bar = 101.3 kpa = 29.9 in.hg = 14.7 psi
1 kPa = 0.295 300 586 467 in.Hg
Rounded a little.. 1 bar = 100 kpa = 29.5 in.hg = 14.5 psi

Lets make it accurate and not rounded.
kpa = in.hg / 0.295
14in.hg / 0.295 = 47.45 kpa (or 6.885 PSI)

Or you could do it the other way around...
psi = in.hg * 0.49
14 in.hg * 0.49 = 6.86 psi.

kpa = psi * 101.3 / 14.7
6.86 * 101.3 / 14.7 = 47.27 kpa

thats enough accurate for what we are doing, if you want more accurate do the math and add more number after the comma.
For more acuracy see:
http://www.engineeringtoolbox.com/press ... d_569.html

 

[Big_G]

Big G,
You are running 20 psi of boost without meth/water injection? please school me on how you can manage that safelly, what is the secret beside boost retard ?
Thanks for the Screenshot its gonna help me some too

I see many others running so little timing, most have much higher static compression ratio. Mine, at 8.3, is giving up some low end torque, but allows higher boost without needing water/meth. Note via the timing chart that my ECU does pull a modest amount of timing at higher boost levels. Also, under high boost, my air/fuel ratio is at 12.0 at moderate boost and 11.0 at the highest levels (>15 lbs. boost). Running a highly efficient inter-cooler is mandatory. Be sure it has good air flow through it. Doing all of these fends off detonation. Be sure to select the proper heat range spark plug. This can only be done through trial and error. The engine is too loud to hear detonation, and can be seen through examination of the plugs.

 

[mathd]

Big G,
You are running 20 psi of boost without meth/water injection? please school me on how you can manage that safelly, what is the secret beside boost retard ?
Thanks for the Screenshot its gonna help me some too

I see many others running so little timing, most have much higher static compression ratio. Mine, at 8.3, is giving up some low end torque, but allows higher boost without needing water/meth. Note via the timing chart that my ECU does pull a modest amount of timing at higher boost levels. Also, under high boost, my air/fuel ratio is at 12.0 at moderate boost and 11.0 at the highest levels (>15 lbs. boost). Running a highly efficient inter-cooler is mandatory. Be sure it has good air flow through it. Doing all of these fends off detonation. Be sure to select the proper heat range spark plug. This can only be done through trial and error. The engine is too loud to hear detonation, and can be seen through examination of the plugs.

I have a 8.5:1 cr, i run 32° total timing at 2500 rpm with no more then 5 psi of boost(i dont use boost retard "MSD 6AL"), i did read(or try to) my spark plug for detonation and i think it did not detonate.. its hard to tell since it was running sooo rich/dark spooge(gonna fix that and the complete tuning next season, just got my LM-2 yesterday )

Wish i could run an intercooler with the roots style supercharger :/.

Just to make sure, we are talking about pump gas and not race gas?

 

[Big_G]

Big G,
You are running 20 psi of boost without meth/water injection? please school me on how you can manage that safelly, what is the secret beside boost retard ?
Thanks for the Screenshot its gonna help me some too

I see many others running so little timing, most have much higher static compression ratio. Mine, at 8.3, is giving up some low end torque, but allows higher boost without needing water/meth. Note via the timing chart that my ECU does pull a modest amount of timing at higher boost levels. Also, under high boost, my air/fuel ratio is at 12.0 at moderate boost and 11.0 at the highest levels (>15 lbs. boost). Running a highly efficient inter-cooler is mandatory. Be sure it has good air flow through it. Doing all of these fends off detonation. Be sure to select the proper heat range spark plug. This can only be done through trial and error. The engine is too loud to hear detonation, and can be seen through examination of the plugs.

I have a 8.5:1 cr, i run 32° total timing at 2500 rpm with no more then 5 psi of boost(i dont use boost retard "MSD 6AL"), i did read(or try to) my spark plug for detonation and i think it did not detonate.. its hard to tell since it was running sooo rich/dark spooge(gonna fix that and the complete tuning next season, just got my LM-2 yesterday )

Wish i could run an intercooler with the roots style supercharger :/.

Just to make sure, we are talking about pump gas and not race gas?

Pump gas.

 

[Big_G]

Hey all,

I plugged a laptop in to my 6AL2. Turns out, my ignition curve was very sluggish: 27 degrees at WOT.

Anyway... What would you guys suggest as timing curve? Do I keep vacuum advance?

My cr is 8.0:1. 406SBC. Forged rotating assy. Water meth injection. Centrifugal super up to 10 PSI.

Limit RPM to 6200?

When I was running a carburetor, I did retain vacuum advance. Now the ECU simulates the vacuum advance.

 

[DorianL]

Big G,
You are running 20 psi of boost without meth/water injection? please school me on how you can manage that safelly, what is the secret beside boost retard ?
Thanks for the Screenshot its gonna help me some too

I see many others running so little timing, most have much higher static compression ratio. Mine, at 8.3, is giving up some low end torque, but allows higher boost without needing water/meth. Note via the timing chart that my ECU does pull a modest amount of timing at higher boost levels. Also, under high boost, my air/fuel ratio is at 12.0 at moderate boost and 11.0 at the highest levels (>15 lbs. boost). Running a highly efficient inter-cooler is mandatory. Be sure it has good air flow through it. Doing all of these fends off detonation. Be sure to select the proper heat range spark plug. This can only be done through trial and error. The engine is too loud to hear detonation, and can be seen through examination of the plugs.

INVALUABLE!!!!! Thanks mate!

 

[mathd]

Big G,
You are running 20 psi of boost without meth/water injection? please school me on how you can manage that safelly, what is the secret beside boost retard ?
Thanks for the Screenshot its gonna help me some too

I see many others running so little timing, most have much higher static compression ratio. Mine, at 8.3, is giving up some low end torque, but allows higher boost without needing water/meth. Note via the timing chart that my ECU does pull a modest amount of timing at higher boost levels. Also, under high boost, my air/fuel ratio is at 12.0 at moderate boost and 11.0 at the highest levels (>15 lbs. boost). Running a highly efficient inter-cooler is mandatory. Be sure it has good air flow through it. Doing all of these fends off detonation. Be sure to select the proper heat range spark plug. This can only be done through trial and error. The engine is too loud to hear detonation, and can be seen through examination of the plugs.

INVALUABLE!!!!! Thanks mate!

Yeah, that and the correct spark plug reach.
I assumed the guy that buy my spark plug the first time to be the "suitable" item, i had dieseling/engine run-on problem just because of that(too long a spark plug reach). Now i know to check and re-re-re-check over and over again.
I got to re-check the heat range once i get the A/F set correctly. that will be in about 5 month from now(next summer).
Right now the heat range is just fine, but am running excessively rich.

 

[mathd]

delete: sorry that was offtopic

 

[DorianL]

Right-o! Here's the plan...

The Digital Programmable MSD 6AL2 ignition curve can really only be plotted in terms of retard. If you stab in a locked distributor, it cannot be advanced from that point - only retarded. So, you need to stab your distributor in with the maximum anticipated advance and remove degrees from there.

So it is tricky because things are all versa-vice in yer mind.

I am shooting for 20 initial, 8 vacuum + 15 mechanical (all in by 2500 rpm).

Boost retard -0.5 degrees/PSI as of 2 PSI.

This translates to stabbing the distributor in at 43 degrees of advance.

Vacuum and boost retards are plotted on the same absolute PSI/RPM graph with PISA ranging from 0 - 30.
- 0 being absolute vacuum (never going to happen);
- 14.7 being atmospheric pressure (WOT N/A); and
- 30 being 15.3 PSI of boost.

RPM retard plot:
0 - 1000: 43 -15 = 28
1000 - 2500: 43 -15 to -0 ≤ 43
2500 - 6200: 43 -0 = 43

The above is modified by the vacuum/boost retard plotted as follows:
At full vacuum at idle: no retard
As one reaches 14.7 PSIA or atmospheric pressure: up to 8 degrees will be pulled.
As of 16.7 PSIA or 2 PSI of boost: pull 0.5 degrees/PSI

They way I figure:
- At idle, I'll be have 28 degrees of advance
- Cruising at say 1700 with high vacuum I'll have 35-ish degrees of advance.
- Moderate acceleration at 1700 will land me 27-ish degrees. (-8 degrees of vacuum advance lost)
- Stronger acceleration (no boost) will gradually lead to 35 all in by 2500 (-8 degrees of vacuum advance lost)
- At boost say 4300 rpm and 6 PSI: 32 degrees of advance

The only kwestion that remains is: as of what PSIA do I start pulling the vacuum advance? 10 PSIA?

Cheerios,

D.

EDIT: also ordered one of those adjust-a-jet plates. Not convinced that was a good idea.

 

[grumpyvette]

maybe its just me, but I can,t see having more than about 35 degrees advance total at any time with a combo like that and Id be having significantly less advance as the boost came in, below is the ignition advance curve I generally start with for NON-supercharged engines, so if boosted it should be even less aggressive, and have less total advance
on my 383 with nitrous I need to pull a full 11 degrees from the total advance when running a 175 hp doss of giggle gas

 

[mathd]

The way you have it set is prefect i think. Too bad i cant help with the last point(at what rpm vac-advance kick in and out.)
Wish i had a MSD BTM or a better unit then the 6AL i have. I may look into adding a boost retard unit if that can be done with this ignition box.

About the adjust-a-plate, i got the holleyquick change jet kit. If you want picture i can send some or want to ask question.. only badside right now is they dont make a chrome/silver unit(well they make it but i cant get it nowhere..) so i will use the quickchange jet from holley to tune the carb and once am done tuning ill remove those non-chromed float bowl and have the stock chrome unit back on.
I did not go with the adjust-a-plate because this a change of the metering block and i dont like that since those are tuned specific to carb model, and as i said before they are aluminium :/.

 

[northrnyankee]

I don't know if this will be on any interest or use to you guys but I run an e-curve distributor. I set the curve and limits of the advance curve with 2 dials and the rev limiter with another dial. I have an excell program where I can punch in numbers to set any curve I want and it tells me what to the set the dials at