|
Ignition Timing Curve Thoughts
|
Good article. Another vote for vacuum advance.
|
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. |
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.
|
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.
|
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 |
Thanks Tom.
|
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! |
Thanks for that link - a good site to nose around :)
LoBelly |
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.
|
| All times are GMT -7. The time now is 06:35 AM. |
Powered by vBulletin® Version 3.8.0
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Search Engine Friendly URLs by vBSEO 3.6.0
The representations expressed are the representations and opinions of the clubcobra.com forum members and do not necessarily reflect the opinions and viewpoints of the site owners, moderators, Shelby American, any other replica manufacturer, Ford Motor Company. This website has been planned and developed by clubcobra.com and its forum members and should not be construed as being endorsed by Ford Motor Company, or Shelby American or any other manufacturer unless expressly noted by that entity. "Cobra" and the Cobra logo are registered trademarks for Ford Motor Co., Inc. clubcobra.com forum members agree not to post any copyrighted material unless the copyrighted material is owned by you. Although we do not and cannot review the messages posted and are not responsible for the content of any of these messages, we reserve the right to delete any message for any reason whatsoever. You remain solely responsible for the content of your messages, and you agree to indemnify and hold us harmless with respect to any claim based upon transmission of your message(s). Thank you for visiting clubcobra.com. For full policy documentation refer to the following link: