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Distributor Advance Curves


bnam
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While looking up some info for another thread, I looked through the page with the curves from the Factory manual. It struck me that the Tii distributors (both US and Euro versions) actually have the least advance.  The 003 distributor seems to have a curve that matches the Tii up to 1500 cam rpm (3000 crank rpm) and then continues to have more advance.

 

So, why is the Tii distributor considered an improvement for non-Tii engines?

 

Byas

 

12-00-009_zpsb6145845.jpg

 

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No -- all 2002 distributors have centrifugal advance and these are just the centrifugal curves.  Some have (in addition) vacuum advance and or retard. These curves should apply to all if you disconnected the vacuum can.

 

I'm surprised that not many have responded to this thread since a common advice on this board seems to be to put in a Tii distributor.  Even though factory manual seems to suggest that Tii disti could actually be a step backwards as it does not have that much advance -- and moreover its curve is the same as the others.

 

Am I interpreting this incorrectly?

 

Byas

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Yes, I think you are. Why do you feel more advance is supposed to be an upgrade? For a higher compression engine too much advance too fast will start to pre detonate. You want to tailor the advance based on the compression of the piston spec, actual compression, cam overlap and driving style/rpm range. If you look at the information forms or speak to someone who curves and plots dizzy advance curves these are only some of the specifications they are concerned with. You want the ignition timing to match the footprint of the engine output. That is why stock dizzy works well on stock engine and a mech only dizzy may not be an upgrade for a smogged cali '76. But a vac disconnected dizzy will flaten a high compression cammed twin dcoe setup becuse it will not let the engine rev up in the right spot. It will work but this is the difference in fine tuning or just adjusting for max advance.

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No -- all 2002 distributors have centrifugal advance and these are just the centrifugal curves.  Some have (in addition) vacuum advance and or retard. These curves should apply to all if you disconnected the vacuum can.

 

I'm surprised that not many have responded to this thread since a common advice on this board seems to be to put in a Tii distributor.  Even though factory manual seems to suggest that Tii disti could actually be a step backwards as it does not have that much advance -- and moreover its curve is the same as the others.

 

Am I interpreting this incorrectly?

 

Byas

I for one agree with you.  The reason the Tii doesn't have as much advance is because it is fuel injected and the fuel burns faster.  Carb'd fuel takes a bit longer and so more advance.  My 02 cents. (I'm FI'd and that is what I have seen in my research as to what advance to set for the EFI).

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Just about every street motor would benefit from vacuum advance. The idea is that you want more advance when the motor is running under light load with lean intake mixtures (like when you are cruising at mid-engine speeds [3-4K RPMS], such as freeway driving), which increases fuel economy and driveability. Vacuum advance works by using a source of vacuum derived from just above the carb butterfly(ies) (so-called "ported vacuum") to advance the timing at low to mid engine load conditions.  Vacuum advance abates as vacuum near the throttle plates is reduced when the throttle plates are opened (or closed). This is of course a greater problem with non-carbed motors- never seen it done on an FI motor, but presumably these systems have more sophisticated ignition systems with more advanced sensors.
 
Literally, the only reason some carbed motors don't use a vacuum advance mechanism is because users don't understand the foregoing, or there is no good source of ported vacuum (like on 02's with sidedraft carbs). So far, there has been no good solution made available to derive ported vacuum from a sidedraft setup (I am sure some intrepid engineer could figure something out, if sufficiently motivated). Although I am using a tii dizzy at present, I would go back to a dizzy with dual advance (vacuum and centrifugal) in a heartbeat if that problem was solved.

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Chris,

Good point.  I'm still trying to figure this out - so let's take your distributor as a case in point -- why the Tii vs. the 73 and older 003 distributor?  The 003 has the same centrifugal advance curve as the Tii (if you left the vacuum can unconnected) -- and with the vacuum can connected, you would have the Tii curve plus the part throttle vacuum advance, plus greater advance than the Tii at 3500+rpm.  So what advantage does the Tii disti give your 73 dual carbed setup?  Even if the carbs have no vacuum port, could you not leave the 003 with vacuum can unconnected and still have the same or better curve than the Tii? 

Byas

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Oodles of information have been written and discussed on this topic (not just on this forum either).  Sticking another distributor into the engine may not help the situation.  I purchased a Bosch remanufactured mechanical advance distributor from Pelican Parts and after installing it in my tii, the engine detonated no matter what I did to adjust it so I pulled it out and put in a rebuilt (recurved) tii distributor.  Problem solved (I'm holding on to the new one for my 69).

101409008.jpg

 

 

Not sure about the DHLA sidedraft version, but my DRLA downdraft Dellorto carburetors on the VW had a vacuum port for connection to a distributor vacuum canister. The port had a rubber stopper.

 

Here is some older material from the May 1974 Roundel

TimingPart3page1.jpg

 

Timingpart3page2.jpg

 

Timingpart3page3.jpg

 

 

 

 

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The article you posted does not list the Tii distributor curve as an improvement for any non-Tii.  This is point I was trying to get clarity around since I've come across several threads in the past where the Tii distributor is posited as an improvement for carb'd cars.

 

Byas

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these are just the centrifugal curves.

 

Ah.  I missed that in your first post.

 

Jimk's fundamentally correct- the reason the tii curve has less advance is

that the kfish really IS the shit.  There hasn't been

a better fuel injection since- if all you're looking at is atomization. 

 

The 500+ psi vaporizes the fuel really really well-

so when the plug pops it, it burns in a real hurry. 

That means that you don't need nearly as much advance

as the carbs, which kinda puddles it in in compariason,

or even EFI, which works at only 1/10th the pressure.

 

So why stick one on a carbed engine?  I dunno.

I think the conclusive answer to your question is-

 

ignorance.

 

It's recommended for sidedrafts because then you don't have

to think about vacuum advance- but as you say, there are

other distributors that have better mechanical advance curves

even not using the vac advance.

 

It works fine for race cars, because it gives enough 'retard'

to get the thing fired, then it goes to full advance

by the time you're in the power band, and that's all you need.

 

So if it's good for tiis and race cars, it must be better- right?

 

Plus, you can get them new, which hasn't been the case for most of the other

curves, and there's a pretty decent argument that even the wrong dizzy that's

working is better than one that's not working at all...

 

My conjecture,

 

t

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The article you posted does not list the Tii distributor curve as an improvement for any non-Tii.  This is point I was trying to get clarity around since I've come across several threads in the past where the Tii distributor is posited as an improvement for carb'd cars.

 

Byas

Sorry - I should have added some comments about posting that old article mentioning distributor upgrades and not necessarily using a tii-specific distributor with carburetor(s).  I'm with Toby on this - seems to be a "jump on the bandwagon" effect.  I did it when I purchased the reman. unit but have since learned from the experience.

 

Jim

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  • 2 years later...

To complete the package, and for sh*ts and giggles, I looked in the barn and came across the first two Roundel articles in the 3-part series on timing that JGEROCK posted above from July 2013... 

 

First part is from the Jan 1974 Roundel; second part is from the Feb 1974; and the third part (JGEROCK posted above) is from the April-May 1974 Roundel.

  

Enjoy.

-Bob

 

Jan1974Roundel Timing Pt1 .pdf

Feb1974Roundel Timing Pt2 .pdf

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