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Timing advance

 
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TITO



Joined: 09 Aug 2005
Posts: 31
Location: Brampton Ontario Canada

PostPosted: Wed Jan 18, 2006 5:14 pm    Post subject: Timing advance Reply with quote

I have been doing some readin and I understand that if I advance the timing on my 1985 GSL it would improve hp.

Now I am wondering how much i need to advance it, I hear 24L and 16T is good? I would also like to know what kind of hp gains I will be seeing on a bone stock 12a, with no a/c? I am furhter more curious about how this mod will effect fuel economy, and if I can still use the cheap gas??

Thanks TITO
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Blake
Been there, done that


Joined: 12 Jul 2005
Posts: 135
Location: Portland, OR

PostPosted: Wed Jan 18, 2006 7:23 pm    Post subject: Reply with quote

Well, with a stock car with a stock exhaust and intake, stock timing is probably best. If you want to improve HP, open up the exhaust and then bump the timing up just a couple degrees.

notes about timing:
There are two types of ignition timing advance utilized by the distributor. The first is mechanical advance, which bumps the timing up as the revs increase. The faster the engine revs, the sooner combustion needs to start to create max chamber pressure at the point of maximum mechanical advantage over the eccentric shaft. This is for performance. The second is vacuum advance, which advances the timing as vacuum increases (lower load). This is for economy.

When talking about ignition timing, it is important to know whether the numbers are given for idle or "full advance". Stock timing is set at idle. As the revs increase, the mechanical advance kicks in until it maxes out around 4K. If you rev the engine with the timing light on the marks, you can see the timing change. Most timing advice given by anyone other than Mazda is at full mechanical advance (engine revved up over 4K) with the vacuum advance disconnected. There is also an important distinction between Before Top Dead Center (BTDC) and After Top Dead Center (ATDC). The higher the number BTDC, the more advanced the timing, but the higher the number ATDC, the more *retarded* the timing. And vice-versa, of course. Everything is relative to TDC; not absolute.

Hope that helps.
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Blake Qualley
Pineapple Racing, Inc.
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TITO



Joined: 09 Aug 2005
Posts: 31
Location: Brampton Ontario Canada

PostPosted: Wed Jan 18, 2006 7:27 pm    Post subject: Reply with quote

I see. I think my vacume timing is off. I belive this because when I am at intermidiat loads the car is verry groggy, and consumes alot of petrol.

I know the car is also a little slower then it should be. The top speed with eagle 65/205 R13s is about 170kmh.

I am not sure about teh aspect ratio but teh width is 205.

If it is off how would I correct both timings?

Thanks for the info Ted.
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Blake
Been there, done that


Joined: 12 Jul 2005
Posts: 135
Location: Portland, OR

PostPosted: Wed Jan 18, 2006 7:58 pm    Post subject: Reply with quote

There could be many reasons for your performance problems. Timing is just one of them. If you want to rule it out, use a timing light to confirm stock ignition timing. Time it at idle, using the stock marks. With the timing light still on, rev the engine up to confirm that it advances, and with a cheap hand-operated vaccum pump you can check for vacuum advance as well.

When was your last tune-up? Not just oil change; I mean, new plugs, new pulg wires, new distributor cap & rotor, new fuel filter, new air filter, etc. If you can't remember, do it before you do anything further. That's just preventive maintanence and unless you keep on top of it, you cannot possibly troubleshoot other problems reliably. Also, after your next little drive, walk around the car and see if any of the wheels is hotter than the rest. A sticky or frozen brake caliper or wheel cylinder can really destroy your fuel mileage. Is your carb up to snuff? Perhaps it needs a rebuild. Or, it could just be a vacuum leak. Finally, if all else fails, get a compression check.
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TITO



Joined: 09 Aug 2005
Posts: 31
Location: Brampton Ontario Canada

PostPosted: Wed Jan 18, 2006 8:03 pm    Post subject: Reply with quote

Thanks. I changed the oil, and distributer cap and rotor recently. I am waiting on spark wires. I need to change the rear end fluid, and trany fluid. The carb was rebuilt in early summer. However, there may be vacume leaks, and it my not be tuned properly. I am having trouble find a shop that can tune my car where I live in Canada.

What should I plug up and what should i pump air into to check teh vacume advance?

Thanks TITO
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