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Great Article on Ignition Advance Curves
Here is a very good article on ignition advance curves.
http://www.gnetworks.com/v4files/bar...withimages.pdf Here is some more info: http://www.aedperformance.com/Tuning%20Tips.htm |
Thanks for sharing the links, Tom. I bookmarked them for future reference.
You never run out of stuff to learn about these cars! |
Some more articles:
This article is done on kit Cobras http://www.innovatemotorsports.com/p..._Sept08low.pdf More technical: http://www.enginebuildermag.com/Arti..._equation.aspx |
Henry Olsen (working with John Bishop) has done much research and written quite a bit about carb/ignition tuning. Henry is the author in links that Tom posted above. Here's another link to Henry's work tuning a Roush 427 in a Brizzio Hot Rod: http://automotiveu.com/tuningperformance.htm
For those in the SF Bay Area, Henry is available on Saturdays (by appointment) to fully tune 4 cars per Saturday. His contact information is in the links... I highly recommend him! |
More great articles on timing:
ported vs manifold vacuum advance http://www.lbfun.com/warehouse/tech_...uum%20advance/ read them all |
Thanks for sharing. There was a lot of good information in there.
I am hoping someone can answer some questions that were not talked about. With centrifugal advance most people say it should start just above idle, around 1000 rpm and that it should be fully in by 3000 rpm give or take 500, depending on who you talk to and what engine it is. What I have never heard discussed is should the advance be linear with rpm through that range or is it a curve? Say you want intitial at 12^, and the advance to start at 1000 rpm, and at 3000 rpm you want to add 24^ to the 12^ for a rotal of 36^. If it were linear the advance would add 6^ at 1500 rpm, 12^ at 2000 rpm, 18^ at 2500 rpm, and finally the 24^ at 3000 rpm. Intuitively I would think it should be linear. But intuitively it seems to me that an engine turning 10K rpm would need more advance than one turning 3K. Anyone ever map this out and see. I would, but everything I own is EFI. |
Thanks for posting.It was interesting to note they said most Performance aftermarket dist. were set for a slow timing advance makes me wonder if I shouldnt have mine run on a Dist, machine.Thanks again WB3
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Quote:
Lars' and John's articles are the gospal in the GM circles. Jim |
MSD says flat out they use heavy springs to retard/slow down the mechanical advance on their distributors when shipped new. That's what I would do if I was them! Maximum protection from detonation for those who don't know what they want and yet easy to change the advance curve and advance limit for those that do.
By the way, thanks Tom for your thread on reading spark plugs. An interesting approach but potentially fraught with some downfalls. The new gasolines make reading the plugs tricky, at best. Also MSD says because of the mulitiple spark with the box units it is difficult to rely on plug reading alone. It's getting tougher and tougher for a backyard kind of tuneup these days! Still good basic advice on plug reading that certainly has it's place. |
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Thanks for sharing the great info Tom. Most timely in my case, as I am just installing a new carb and dialing everything in now.
Great stuff!! |
What would you have to have on your engine to be able to connect up one of the scan tools? Something like on board diagnostics (OBD) to allow you to digitally watch your timing, temps, etc. Does anyone do that sort of thing?
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It's really "just" a matter of installing the various electrical sensors required and feeding that information to your diagnostic device. You could still run a carb and classic (weights and springs) distributor. The wiring would be a bear, I wouldn't want to go there myself.
Many of these readings can be taken with a digital voltmeter and the data interpreted IF you know the voltage range of the sensor. |
I don't usually talk about my vacuum advance unit because it's some what controversial with "rodders". In fact I was strongly urged to NOT install one, I did anyway. I use manifold vacuum, adjust idle rpm accordingly, it's been working well for me for about 7,000 hard miles on my FE.
I recently picked up the car in Oakland and drove it up to Redding. I got a decent 14 mpg with a top loader and was running at normal speeds the whole way. With SOME hard through the gears along the way as well. I was very pleased with the perfomance and the mpg. Mainly because the mpg increases my range with the small tank. |
Use a Vac Adv Module and Manifold Vac as Source
I use a vac adv module and connect it to manifold vaccum for better idle mixture burn (my wife complains less about the gas smell), trouttle response and better mileage. I using an MSD 8478 distributor that came with a vac adv module but you cannot limit the amount of vac advance. The Crane vac adv unit is also completely adjustable. It uses a small stepping plate designed to mount within a GM distributor to physical stop the advance. This did not fit the MSD easy. The Accel vac adv is adjustable is the amount of degrees of advance and at what vaccum the advance starts.
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Most of the early Ford distributors have an adjustment within the port. You remove the outer vacuum nipple fitting and can reach inside ( I believe its an allen screw) and adjust the vacuum. I think they went to the wayside when emmision standards got stricter. There was a short periood of time when the mfg. were mandated ( I love that word) to make engines "tamper proof". I can remember having to pull/drill restrictor caps to adjust idle mixtures enough to allow a good idle. It was usually after the engine got some miles on it and the circuits were no longer pristine. I bet I have a thousand restrictor caps from when I worked at the Honda (motorcycle) dealer. Hmmmm...may be needed in some "restoration"...sounds like e-bay material.:LOL:
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