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More Advance Timing


Nova lately falls on face from dead stop for split second. This week end I played with pump cams and shooters [no luck]. 350 with 750 Holley V.S. My distributor was done re curved while back total timing 32*. Got thinking that my advance wasn't coming in soon enough. I run 20* initial and 12 mechanical. I had two silver springs with stock weighs. So I replaced one silver spring with gold to get advance in sooner. This helped a lot. 



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I would go for 34º total on that.

Centrifugal advance assy. on the HEI is pretty darned good.  The stock weights and advance plate are perfectly acceptable for all but the highest HP/RPM engines.  ALL stock HEIs that were installed in V8s are designed to have a total centrifugal advance of 20 degrees, +-1 degree due to production line tolerances.  This is as measured at the crankshaft (10 at the cam).

It is susceptible to old age, though.  Typically the centrifugal advance weights wear their pivot holes into an "oval" or eat a trench into their pivot pins OR BOTH.  This is bad and no attempt to change the advance curve should be made on a distributor that suffers from these problems- fix it first or get another HEI to start improvements on.  Also, the centrifugal advance plate (that pivots around the main distributor shaft as the centrifugal advance moves it) up at the top of the distributor sometimes gets gummed up and sorta "sticky"- slowing the advance curve and generally preventing it from working correctly.  If your centrifugal advance doesn't "snap" back to zero when you twist it with your hand and let it go then you have this problem.  You need to pull the distributor shaft apart and clean everything out, especially up top, before you proceed with upgrades.

Centrifugal advance:  Stock the advance mechanism is pretty good but the stock springs are usually way too strong, causing the advance curve to come in too slowly, if it ever gets fully advanced at all.  All you need are the right springs and the right initial advance setting.  Most Small Block Chevys like about 32-38 degrees total advance at WOT.  Since we know already that the HEI has 20 built into the stock mechanism the first thing we need to do is set the initial advance correctly- that means you need an initial advance between 12 and 18 degrees (you might want to retard it 2-4 degrees for daily street use just to build in a little safety margin).

Now all we need to do is make sure the centrifugal advance comes in at the right RATE relative to engine RPM.  You want it "all in" by about 2800-3200 RPM for a street motor.  You do this by changing the centrifugal advance springs to lighter ones.  If you use the Crane advance kit like I do you are looking to install one Blue (heavy) spring and one Silver one (medium).  These springs are located directly under the rotor and are easy to remove/replace by hand or with needle nose pliers.  These springs will give you an advance curve that starts at about 800 RPM and ends at 2800.  Easy so far, right?  If you don't have the Crane kit then install whatever springs you have and check the advance curve with a dial back timing light and a tachometer, swapping springs until you get it close to these specs.  It doesn't matter if the springs are not "matched" side to side- you can install one heavy one and one light one.

Vacuum advance for the street:  You want about 12 (crankshaft) degrees total vacuum advance if you run WITHOUT a functional EGR system, 16 degrees if you run WITH a functional EGR system.  Regardless, you want it to come in between about 5 and 15 inches of manifold vacuum.  I have found the most expedient solution to be the Crane advance kit once more.  Install their can with about 9 turns on the adjustable advance can spring. IMPORTANT!- Also, use the little "lockout" cam that comes with the Crane vacuum advance to lock out AS MUCH ADVANCE AS POSSIBLE. This will still leave you with about 12 degrees of available vacuum advance. If you set it with 2 notches LESS lockout than maximum you will end up with about 16 degrees available vacuum advance 
for those of you running an EGR valve.

The vacuum advance canister should be plugged into a "manifold" vacuum source on the carb or intake manifold.  This is a vacuum port on the carb that provides full manifold vacuum at curb idle.  This is best for cruising/part throttle and will give the best performance overall.

WATCH YOUR IDLE RPM WHILE YOU SET INITIAL ADVANCE TIMING!!!  You note that the centrifugal advance curve that I recommended above starts at about 800 RPM.  If you try to set your initial timing with the engine idling above this RPM point you will NEVER get a true initial advance reading.  Set it with the idle temporarily slowed WAY down if you have to but DO IT RIGHT!



-- Edited by DennyW on Monday 10th of June 2013 01:14:06 AM

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Denny I have points distributor which was set up on sun machine while back. Has 12* mechanical and 20* initial with vacuum advance unplug. Also been using ported vacuum. Only real problem was hard start. With full vacuum get extra advance right away. I have change things around set it up run full vacuum. With full vacuum can run less initial timing making easier start correct. Just trying understand how this works.

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I went switched to full manifold and set intial at 12 degrees. With vacuum advance plug into full manifold got 30 degrees at idel. Just want see how throttel response was. It was night day differance. Need get adjustable vacuum advance for sure. Plus car starts like dream.

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smile Glad to hear it.



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I always tell the truth...even when I lie...

Head, or gut...

I give the same as I get...

-->>My truck project<<-->>My Personal Site


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cop chase

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