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Starting to tune my 351C now. Recurving a distributor for the first time. What is a good max advance number for 91?


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My car has plates. registration and insurance for the first time in 15 years.

I am tackling tuning this thing and am looking for some solid advice.  Keep it in mind that I don't have any experience with a V8...

First thing I had was a bit of an off idle stumble.  That seems to be solved now as my timing used to be about 17 degrees advanced at idle, and is now 12.  Also double checked the setup of the accelerator pump and put a more aggressive cam on it.  

The engine...

A moderately spicy 377" Cleveland with CHI 2V heads.  Static compression was about 11:1 if I recall correctly.  Cam is a Howards flat tappet hydraulic (230041-12) 225/235@.050" .536/.542" lift.  112 LSA.  Carb is an old vacuum secondary Barry Grant Demon 750CFM.  Ignition is a full MSD complement, their distributor, and a marine box.  Shorty headers, 2.5" exhaust.  Plan is to run it on 91 pump gas, but for now it gets used so little that I put 93 ethanol free in it, or 110 depending on what I can get.

The MSD distributor shipped with garage door springs inside and a pretty conservative advance stop that limited it to about 21 degrees advance travel (.340" dia).  I swapped to the two blue springs (Not the garage door springs, nor the lightest springs) I also swapped the stop spacer to the silver one (they vary from .280" wild to .374" mild), it limits it to 25 degrees advance travel.  Attached is the MSD curve sheet so you can see what I am fooling with.

Currently my warm idle is about 860 RPM and I am at 12 degrees.  At 2500 RPM I am about 30 degrees.

What has me perplexed is max advance.  I have seen 32 - 36 degrees stated.  Right now I probably need to back off as I went a little advance crazy.  What is a reasonable number and why?  In theory I should be maxed out about 34 degrees as my distributor is set up this moment.

Thanks for your time - Peter

Pages from 8350.pdf

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Tuning is a science with a bit of luck in it.  Most engines perform at their best with about 34 to 36 degrees max advance.  The purpose of a higher advance at a lower rpm is to get you up and moving quicker than a lower advance will.

I have mine setup so all is in at about 2800 - 2900 rpm.  They way I set it is to have someone reeve the engine up until I no longer see any advance curve coming in.  At that point I then know how to adjust the springs and then make adjustments to get the full curve in at the rpm I want - in my case the 2800 to 2900 range. My engine is a 460 and runs best with 34 degrees total.  Once set I then let the engine come back to idle and that is then my idle advance setting for idle.   My distributor has 16 degrees in it and I run 18 degrees at idle for a total of 34.   

Below is a link to one of the best video's I have seen over the years explaining recurving  a distributor and  it advantages.   Also attached is a article on Duraspark distributors.

How To Re-Curve A Distributor, And Why! (badasscars.com)

Duraspark Timing.pdf

Hope this helps you.

Edited by Kilgon

Kilgon

 

 

"The only dumb question is the one not asked"

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I had my old '71 Autolite dialed in by a friend with his Dial-O-Dizzy-Apparature, here is the curve: 

IMG-20210515-WA0021.thumb.jpg.7cbfdc03514bb58f0c87394724f58874.jpg

 

Curve.thumb.jpg.dc0502a81cc24d7c625e63a206ad94ce.jpg

 

That means 12 degrees initial and 37 degrees total, on which the distributor will run at its best... 

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Tim

 

Never drive faster than your guardian angel can fly :runninpony:

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Well, for me, it's all about playing with it to get the best bang for the buck. Every engine will be a little different from what I've learned over the past few years. For mine, which is pretty much a stock rebuild, 10:1 c/r, quench heads, 4 V, slightly better than stock Melling MTF2 cam, and a 4 speed manual, I found the best curve on a stock Autolite dizzy with a Pertronix Ignitor II module and matching coil, was to weld and re-cut an advance slot to a 10L (.410") to give 20 degrees on the crank. I use 1 Ford heavy spring and 1 925D (I think it is). The heavy spring is set just loose and slight tension on the other. With an initial of 14 deg., this gives me 34 deg. mechanical no vacuum, all in by 3000 rpm. My vacuum advance is about 4-6 deg. on top. With the carb settings I have, this thing pulls like a train and zero spark knock on 91 premium, no ethanol. It will run so-so on 89, but crap on 87.

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Geoff.

 I learn something new every day!

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It looks to me like the blue springs and the green or silver posts should give you a good starting point without any vacuum advance. The green post will give you about 23° centrifugal advance plus your initial advance of 12° will put you at 35°. The silver post will be about 25° and 37°. And then you'll need to add in how much vacuum advance you'll get.

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“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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Bentworker,  3000 to 3400 all in is a pretty good standard and I would go with the bushing that allows the least mechanical (9 degrees dist. 18 @ crank. this will allow you some leeway to adjust base timing without overshooting total timing.

You may be able to tolerate more total timing with the aluminum heads, but that is by NO means where    your best power has to be.  You may find best power @ less than the typical non stroker is.

Boilermaster

 

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CHI heads need less total timing than iron heads due to the chamber shape. I'd stay conservative at 30° total and see what the engine likes. 

 

 

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I agree with Hemikiller. The chambers of the CHI heads are much better for flame propagation than OEM heads. Just guessing, without dyno testing, I'd start at 26 total, then 28, then 30. My guess is 28 will be the sweet spot. I'd use the "E" curve on the MSD curves page. Use whatever bushing gets you where you want to be with initial and total. Are you using a vacuum advance? What is the idle quality and manifold vacuum like at 860 RPM? Chuck

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I have a vacuum advance, not sure if I should use it or lock it out.

Idle quality at 860 RPM seems fair, when I place the car into gear it drops a bit but is stable.

As far as vacuum reads go - I have no idea what it is at idle.  I'm going to put a vacuum gage on it this weekend and see what I come up with as I am fooling with.  Wild guess is about 7"

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11 hours ago, Bentworker said:

I have a vacuum advance, not sure if I should use it or lock it out.

Idle quality at 860 RPM seems fair, when I place the car into gear it drops a bit but is stable.

As far as vacuum reads go - I have no idea what it is at idle.  I'm going to put a vacuum gage on it this weekend and see what I come up with as I am fooling with.  Wild guess is about 7"

I will guess your vacuum more like around 10-12"

20160929_171923_edit2_small.jpg

 

1971 M-Code Mach 1 w/Ram Air, 408 stroker, 285/291 0.558" roller cam, Blue Thunder intake, TKO600, Hooker headers with electric cut-offs, FiTech EFI w/ RobBMC PowerSurge pump

Strange center section with Truetrac, 3.5 gear and 31 spline axles, 4-wheel disc brakes

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