jojobanks

I've heard that some great things can come from properly setting up your advance curve, though I'm not really familiar with the process. Any tips for tuning the curve on an HEI distributor?

kalli

well you need to find out what maxmimum timing the engine likes best. only way to find out is a dragstrip
depending on the engine you have max advance maybe already at 3000rpm.

so what you are interested in tuning is the area below that (for the advance curve), and that again is done with trying and testing on drag strip

if not sure you can always work with the numbers from ford.

2+2 posted them a couple of times and for my engine the Boss curve works pretty well.

if you don't have a machine for curving a distributor you can translate those numbers.
1 degree at cam is 2 degrees at crankshaft,
1 rpm at cam is 2 rpm at crank.
at all times have the vac advance disconnected to not upset the results

so if it says for example at 400rpm: 0 degrees and 800rpm 4 degrees,
than this means if your cam is at 400rpm (engine at 800), you should have a certain amount of advance and at 1600 rpm (800 at cam) you should be 8 degrees further advanced

so In an a real world example
according to 2+2 and Ford for 2V and 4V stock
http://i29.photobucket.com/albums/c2...GF-12127-A.jpg

for performance engine and Boss:
http://i29.photobucket.com/albums/c2...ZF-12127-E.jpg


it says for the BOSS:

centrifugal advance:

distributor rpm --- min --- max
0350 --- 0.00 --- 0.50
0500 --- 0.00 --- 0.50
0750 --- 0.00 --- 1.75
1000 --- 2.50 --- 4.75
1500 --- 7.75 --- 10.0
2000 --- 8.50 --- 11.0


the above values are for when you have a distributor setting machine
you can shove the distributor in there and it will spin the distributor for you, you have a **** there where you can adjust speed and with that machine you can measure the advance
there's not many shops left that have such a machine. If you know someone, then simply bring your car there they'll happily do it for you.

if you don't you can try yourself. I usually do it this way


re-calculate the table for values at crankshaft so you can use a timing gun


crankshaft rpm --- min --- max
0700 --- 0.00 --- 1
1000 --- 0.00 --- 1
1500 --- 0.00 --- 3.5
2000 --- 5.00 --- 9.5
3000 --- 15.5 --- 20
4000 --- 17.0 --- 22

Looking at this table you can see that if the engine likes 32degrees and the max centrifugal advance is 22 at 4000RPM, then obviously my initial timing must be 10 degrees

lets say my idle speed is 850, timing there must be initial 10 degrees (0 centrifugal advance)
at 1500 anywhere between 10 and 13.5 deg
at 4000 I should be at 32 degrees and if I rev higher I should be not advancing any further

In most distributors you can set the curve with springs and the maximum (limit) with tabs. First thing you do is to limit, then set the idle, verify total.
once that is correct you can adjust the curve using the springs

In my car I found that 10 degrees is not enough with my cam, but it doesn't perform better with total at 34 or even 36
so what I did was to have the initial higher (14 degrees) and limit the centrifugal to ~18 max, all in at 3000rpm already
but again this is for _my_ car

here's both of them graphed, you can see the difference between typical stock and typical performance on where the curve is kicking in





Kalli

2+2GT

You have put a lot of time and thought into this.

Typical un-corrected distributors are far from the optimum power curve. I have done a lot of distributors, even GM/Delco, and I've never seen one that was in spec, whether it was an untouched original or fresh "rebuilt" from a parts store. Simply going to stock is usually a performance upgrade. Vacuum advance diaphragms, especially the aftermarket type, are usually way off, I mean performance and mileage-killing off.

jojobanks

thx kalli for all the info!! who needs books when you got people like that around?

the main question I still have is how do you adjust using the springs? do you buy new springs and swap different rates in and out or do you modify the spring itself?

how do you adjust the vacuum advance? is it as simple as modifying the arm to the diaphram? Or do you have to buy an adjustable unit?

when you're reading the advance at each rpm, I'm assuming you're simply holding the engine at the rpm and writing down the value. One fellow had told me (for drag cars) to use a video camera, and hold WOT through the RPM range, then use the video to go back and map your curve at WOT (since a drag car will generally be WOT). Does this sound like a sound technique? I'm building a street car, but it still intrigued me.

jojobanks

bump for info

2+2GT

Typically centrifugal advance is adjusted by slightly bending the post at one end of the spring. Ford vacuum advances are adjusted by changing the spacers inside the advance, and aftermarket units are adjusted by inserting an Allen wrench in the hose nipple.

kalli

yep, some kits like the pertronix come with a set of springs (light/medium/heavy)

for the thing with WOT ...


a) WOT has absolutely no affect on the vacuum advance
the vacuum advance reacts to ported or manifold vacuum. At WOT you'll have 0 vacuum at manifold (and ported pickup). So therefore it's like a disconnected vacuum advance

b) WOT has absolutely no affect on the centrifugal advance. The centrifugal advance reacts only on engine rpm. it does not care whta your foot does
the centrifugal advance works in such a way that theres two pieces of weight in the distributor and the higher the rpm in distributor (related to engine rpm), the further they will be thrown away from the centre of distributor
only the springs hold them back. so stiffer springs will cause to hold them back more and a loose spring will cause that they can easily swing away from center
since they're attached to the baseplate that will cause the ignition to advance
I hope that's clear enogh

c) what does vacuum advance do ...
when you are cruising on freeway at 60mph in 4th gear you
- have the foot barely on the throttle (that means high manifold vacuum)
- the cylinder is hardly filled with air/gas mixture
- you only need a few horses to keep you at that speed. from your 200HP+ or 300HP+ you only use lets say 10% at this moment
since the cylinder is not properly filled it needs a long time to burn this mixture (longer than a proper fill).
So for that you want to advance the ignition that the spark happens exactly at the best time possible. if you don't you just cause: higher engine temperatures, loss in torque at cruise rpm (you reallistically have to give a bit more pedal), ****ty emissions
on that note: this is one of the few thigs that cause better emissions _and_ better gas/mileage. So on a street driven engine you don't want to be without it
This is the reason why distributors without vacuum advance are sold as racing distributors. A lot of people think "that sounds fast" for my street engine.
So for a car that sees cruising (freeway, highway, town) you'd want vacuum advance
A car that lives on the dragstrip only knows only one throttle position and that is WOT. no need whatsoever for vac advance

d) centrifugal advance
well of course with higher engine speeeds the piston travels faster. and when you fire at 10degrees before TDC at idle it might be ok (the full flame front might be there at 5 past TDC to push the piston)
but at 4000 rpm firing at 10deg before TDC will cause the flamefront to be fully there at lets say 30 degrees after TDC, which means you loose a lot of torque and you just shove fire through the exhaust. engine is getting hot and headers red
centrifugal advance wants to adjust that

e) racing
if you are drag racing the car only the first thing you can remove is vac advance. As said it will never see any vacuum at WOT
the next thing is: if your centrifugal advance is all in at 3500rpm and the engine is always above that ... then there's no point having a centrifugal advance. you just set the timing where it should be at 3500+ and lock it out


g) so what your friend said, let me be more specific on how to adjust all this

the first thing you need to find is "what is the total timing my engine likes best". Initial timing (lets say 10, +maximum adjust of centrifugal advance (say 22) =32 degrees total timing).
This is the timing the engine will see with vacuum advance disconnected and engine revving high

Find total advance
1. disconnect vacuum advance, connect timing light. start revving the engine slowly, watch where it stops advancing (lets say at 34 degrees at 4000rpm)
that means if you rev any higher than 4000, the total will stay at 34, it's not further increasing
2. take the car for a spin on a dragstrip, go through all gears at WOT. or do that on a dyno.
3. try the same with 32 degrees etc. see where you get best speed/ET or HP

find idle advance (initial advance)
4. now that you found the total timing your engine likes best, lets say 34 degrees, check your idle advance. (initial advance)
lets say it idles at 8 degrees, but you need 12 because of your cam, then obviously your distributor is having a centrifugal advance of 32-8=24 degrees, but you need 32-12=20 degrees.
So you need to find how you restrict that in your distributor (check manufacturers manual). in most cases for limiting you can use tabs
once you have them in place you will have the total where you want it and the initial where you want it

Adjust curve
Then you can adjust the curve to the specs provided (changing springs etc)


if it's a stock 289, then the best thing to do is to time it to manufacturers specs or lets say 10 degrees initial and then adjust the curve from there. that's why there are specs for it


That above is just the general idea, but realistically when you are on the dragstrip you want to do some plugreading etc as well.

And i'm pretty sure a good dyno shop can set your total timing and if needed adjust the rest as well if its all over the place

groho

how come no-one mentions the use of an adjustable timing light, their less then $75 and you can validate total, centrif, and initial advance with simple math. The best I could do for my 289 was 36* total by 3200RPM, with 10* initial. See if this webpage helps..http://www.hotrod.com/techarticles/s...g_curves_tech/ BTW, this is a time consuming process so don't expect to nail the bullseye first time out, but the reward is noticeable for a $20 investment and half days work.

kalli

not a bad article, but "find best accelleration between 1000 and 3000 rpm _in first gear_" is laughable :-) might work for a 40HP car
I'd say most of us are members of the traction limited club ...

2+2GT

OK, the first thing would be to add genuine Traction Masters™. When I did that, I started getting a black stripe from both tires, and no wheel hop. Later I added Traction-Lok™, and the stripes were the same length.