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07-12-2013, 09:05 PM
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CC Member / Sponsor
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Join Date: Jan 2002
Location: Provo,
UT
Cobra Make, Engine: Daytona Coupe
Posts: 1,359
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Not Ranked
Ignition Timing Curve Thoughts
Good article on ignition timing strategies.
Classic Inlines Spark Advance Strategies |
07-12-2013, 09:34 PM
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CC Member
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Join Date: May 2009
Location: American Fork,
Ut
Cobra Make, Engine: 66 Cobra
Posts: 930
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Not Ranked
Good article. Another vote for vacuum advance.
__________________
Wayne
"Everything is alive. If you get angry at a vehicle or the trans, it won't fix until you apologize and say you are sorry." "The vehicle always knows what it is doing and what the cause of it's bad feeling is. If you ask it humbly what the problem is, it will tell you. Then you and it will both be happy."
Gil Younger
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07-13-2013, 06:58 AM
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CC Member
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Join Date: Jul 2009
Location: Westerly,
RI
Cobra Make, Engine: Fordstroker 408w custom solid roller-Craft ported Brodix 17*heads-CFM ported Vic Jr. intake-1 3/4 primaries- 575hp-TKO-600RR Liberty upgrade- -Moser 8.8 trutrac-McLeod Street Extreme--QA-1-Wilwood brakes, Classic Chambered 3" Cobrapacks, Avon's
Posts: 645
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Not Ranked
I was getting a hanging idle and looked into it extensively. It appeared to me that the vacuum canister was hanging up a bit. I tried to lube it but I could not get it to change.
Just for a quick fix I pulled off the vacuum line and put in a quick advance curve. ( 2 light silver springs). I have the black 18* stop and run a 14* initial with my modern combustion chamber design.
My timing is all in at my cruise speed (2200 rpms)
I see no difference from the seat of my pants. Temps- same, Power-same, idle-perfect and rock steady.
At this time I'm just going to keep it where it is.
If I lost 1or 2 miles per gallon on the highway I'm not going to worry about it right now.
__________________
Lou
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07-13-2013, 08:32 PM
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CC Member
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Join Date: Sep 2009
Location: Camarillo,
CA
Cobra Make, Engine: SPF #2608, Roush 427SR T-W
Posts: 911
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Not Ranked
I thought if you had more advance in your cruise speed you would get better gas mileage. Maybe the difference is not enough to measure. It is really difficult to detect a small change in gas mileage.
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07-13-2013, 11:02 PM
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CC Member
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Join Date: Dec 2004
Location: Kansas City,
KS
Cobra Make, Engine: jbl
Posts: 2,291
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Not Ranked
I did an unexperimental analysis of with and without vacuum advance and figured it was worth 2 mpg. I am a believer in using it, quieter and snappy and can be easier on valves & exhaust. I had the carb tuned to run on the ragged edge of leanness at cruise, when I hooked up the vacuum advance I had to richen the transition slot at least one size, maybe two.
Last edited by vector1; 07-13-2013 at 11:04 PM..
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07-14-2013, 07:50 AM
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CC Member
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Join Date: Sep 2006
Location: No city...only 118 residents in Manter,
KS
Cobra Make, Engine: Cobra Auto Works body, Ron Godell Racecars chassis, 1989 Mustang GT 5.0 HO (converted to carb), W/C T-5, 3.73's in a Ford 9" Traction-Loc.
Posts: 812
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Not Ranked
Great reading....thanks, Tom!
That article really brings home the issue of compression ratio's effect on spark timing.
I have always been a fan of vacuum advance....however, I think my Mallory Hi-Fire system is mechanical, only. I may look into the possibility of adding vacuum advance....if it needs it. Right now I think the long block is box-stock 1989 Mustang GT/5.0 HO.
Cheers!
Doug
__________________
YD,E./PNB
No names were changed to protect the innocent! 
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07-14-2013, 02:49 PM
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CC Member
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Join Date: Jan 2008
Location: Nashville,
TN
Cobra Make, Engine: Backdraft, SBF 351w (463 CI)
Posts: 272
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Not Ranked
Thanks Tom.
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07-14-2013, 07:32 PM
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CC Member
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Join Date: May 2006
Location: St. Louisville,
Oh
Cobra Make, Engine: A&C 67 427 cobra SB
Posts: 2,445
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Not Ranked
Well finally my question has been answered.
The question: Why is it that the timing is only increased until 3000 rpm give or take a 1000 or so?
Experts (so called) have always explained that as the rpm increases there is less time to burn the fuel, so the timing must be advanced as the rpm increases. Some of these experts state that the burn rate of the fuel does not change, with rpm.
I have always said there is more to it than less time to burn the fuel, or you would need to keep advancing the timing all the way up the rpm band. I also calculated the time difference with respect to rpm and the advance curve does not keep up with the loss of time.
Finally someone explained it. The burn rate of the fuel is increasing with rpm do to the increase in mixing with rpm. The burn rate is NOT constant, with respect to rpm!
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07-17-2013, 07:40 AM
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Senior Club Cobra Member
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Join Date: Feb 2006
Location: Melbourne,
VIC
Cobra Make, Engine: RMC, carb 347 TopLoader and Jag running gear ~ so old school I time it with an hour-glass :D
Posts: 1,293
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Not Ranked
Thanks for that link - a good site to nose around 
LoBelly |
07-18-2013, 09:15 PM
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CC Member
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Join Date: Sep 2009
Location: Camarillo,
CA
Cobra Make, Engine: SPF #2608, Roush 427SR T-W
Posts: 911
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Not Ranked
I just changed the springs in my MSD mechanical advance from the heavy silver to the light silver springs. This will move the full advance up 3,000 RPM. Not sure which "advance stop bushing" I have, but if the red one, the full advance was at 5,500 and now it will drop to full advance at 2,500 RPM. Not sure what to expect.
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