Optimized Advance Curve for the 246GT

Hi,

i'd like to share an optimized advance curve for the 246GT 2,4l with standard compression and a S125AX/BX/C with you. The advance curve has been created deploying an programmable electronic ignition as test bed and a S125BX using a Sun 404 distributor machine, using standard 95 octance fuel (european 'Super' fuel). I can also post the Megajolt configuration file with the advance curve if anyone is interested.

This curve has a more aggressive advance in the lower RPMs and give a bit more torque starting at 1500/2000 RPM, the engine revs up more willingly and the curve removes a good amount of the misfires on overrun and idling down a hill (if you like the misfires, ignore this post .

The aim was not max out power and torque but rather to create an optimized curve for rebuilding a distributor or setting up an electronic ignition, so it should be quite safe for a standard engine. If you have a higher compression engine or use less knock resistant fuel than 95 octane, you need to retard the timing accordingly.

RPM Advance
500 10º
800 15º
1000 18º
1500 23º
2000 28º
2500 34º
3000 39º

Enjoy,
Adrian
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Great work and a very interesting thread, Adrian! Thank you for doing the work and for generously sharing the results here.

Your conclusions seem to match my own subjective feeling as I drive my (stock US) Dino: The top end power is as good as it can be, but the throttle response from low/mid rpm is not as quick as it should be. Subjectively, the engine feels as if it could use a bit more ignition advance, and now you are confirming exactly that.
I have two questions:

- Did you stop at 39 degrees advance because you did not want to exceed the maximum factory advance, or did you verify that nothing was gained by increasing it further to, say 40 or 41 degrees?

- More importantly, how do I implement your new advance curve in my distributor? Since the slope of the advance curve is steeper than the stock version, I imagine it would involve adjusting the springs and/or counterweights in the distributor. Can it be done accurately enough in a mechanical advance stock (US) distributor, or do you recommend going to an electronic advance system? I presently have the Superfomance electronic ignition system which is built in to the stock Dinoplex box. Since I enter the car in concours events sometimes, stock appearance of the engine compartment is important to me. What would be the best/easiest way to modify the advance curve to match your chart?

Thanks again!
Lars

 

No problem to go up to 40-41º with good fuel from what i have seen but no real gain either, and you don't want induce knocking when driving in cold humid weather or with bad fuel.
IMHO, 39º is already quite advanced for such a small engine capacity, but the Dino engine has not really a fast burning combustion chamber design so it needs some advance at high revs.

You need softer springs for the two pins installed near to the weight axle, the amount and thickness of the responding washers required depends on your distributors condition and wear. A good workshop doing marelli distributor rebuilds should be able match this curve within a degree on a distributor tester. For a stock appearance this is the best approach and the S125C can handle this curve quite nicely.

Here are the cam advance values for the distributor tester setup:

RPM Advance
500 0º
800 2.5º
1000 4º
1500 6.5º
2000 9º
2500 12º
3000 14.5º

 

Will try to have that done.
Thanks,
Lars

 

For the less technical among us (me!) what is the purpose / impact of making this kind of change? Any downside?

 

Thank you for sharing this!

For mid range, I think this should help, because I have a bad flat spot off idle, but one other observation: I played with 6 to 10 deg adv at idle, and noticed very little difference. I see a bigger difference just when the motor starts to breathe. Have you noticed a big difference in throttle response just off idle? Also, do you have any idle CO figures that seem to work well for you. I have a feeling I've got it way rich.

 

The purpose is to optimize the idle/low to medium RPM range for torque, power and engine response. The factory timing curve is quite conservative, probably because of the inferiority of fuel in the late sixties/early seventies and also because Ferrari was probably thinking you would spend most of your time in revs higher than 3-4K where the advance curve would nearly be at its maximum anyway.

With the attached curve you should get a better response and a faster revving up engine in low and medium RPMs and it removes the flat spots around 1500-2500 RPM. Also the backfires in overdrive between 2000-2500 RPM are reduced as the fuel/air mix burns up more completely. There are no downsides except that you need a competent workshop to recurve your distributor (it seems that many workshops nowadays dont really cater for distributor recurving or the advance curve itself, this is kind of a dying art as it seems).

2dinos, from my experience 15-18º crank shaft advance works nicely for the idle, the engine is more willing to rev up and idles a tad higher (between 900-1000 RPM depending on your carb setting), but the more interesting change is indeed between 1500-3000 RPM, where the engine now feels kind of 'refreshed' when applying the throttle.
Sorry, i don't have actual CO figures, have to confess that i evaluate richness by inspecting the plugs rather than measuring the exhaust gases

Best,
Adrian

 

I found that my car was happiest when I set up the timing, ran the car to get temperature up, then, rotated the distributor slightly to and fro to get the highest revs at idle. Tightened up drove away. Plugs were a good color, temperatures were OK.

 

Adrian,

How much would this change for US's 91 octane fuel?

Thanks,

Manny

 

Sorry, i have no experience with 91 octane (RON) fuel, in europe 95 RON is the standard fuel.

Best,
Adrian

 

I was suffering rather significant hesitation at 2000 RPM. Car ran well at RPM above this. Double checked everything...distributor cap, plugs, wires, etc. Removed carburetors, cleaned jets, reset idle mixture, checked float levels, everything. Running optical pickup (Crane) and MSD. Finally, carefully checked timing curve. Found advance was at least 10 degrees greater than factory specification all along the curve (not unlike that suggested as the optimized curve). Rotated distributor to correct advance and hesitation completely resolved.

One observation only.

Jim S.

 

Great work there!

 

Jim,

this is interesting, i found that the hesitation at 2000-2200 RPM in three different Dinos was completely cured by the posted advance curve, but that was with 95 RON fuel. Do you remember what fuel you have used, and which ignition (MSD 6A?) and spark plugs you were using? Did you experience knocking?

Hesitation starting at 2K usually has two reasons, if you have a noticable hole in the engine RPM progress while crossing that area, it can be the second set of springs which is sticky or the spring progression rate is too high. (The second set of spring takes over at around 2000 RPM in the factory curve). This also happens when revving up without a gear engaged.
When the engine hesitates/acceleration feels sluggish starting at 2K and feels normal above ~2500 RPM then the advance is too retarded (as in the factory curve while driving 95 RON), this is more pronounced when you drive.

Best,
Adrian

 

Adrian -
MSD-6A, I believe I have NGK plugs in, rebuilt distributor, and carbs carefully tuned to try to avoid hesitation. No knocking on local high-test fuel - 92 Octane.

Do both sets of springs have the same spring constant and length? When rebuilding I could not discern a difference.

I checked timing and found significant advance - greater than the factory curve. When I retarded back towards factory curve the hesitation dissipated.

No hesitation in neutral. When gently accelerating from a stop the car would bog-down at 2000-2500 RPM. Above this speed it ran fine. Now all better!

Thanks for your study and publication of your experience. It is a great help.

Jim S.

 

Jim,

thanks for the update! My best (only) guess would be fuel quality if the distributor advance mechanism and carb jets have been checked, as there is no other difference to our engines. Do you remember if the resulting curve was nearly identical or somewhat steeper than the one in my chart?

There are two different set of springs, two shorter ones (14 mm, near the weight axles) and two slightly longer ones (17 mm, at the end of the weights). The spring constant/rate is the tuning factor (together with the washers), by inducing softer springs you'll have a steeper advance curve and the other way round.

Best,
Adrian

 

Adrian - this is very interesting. Do I understand correctly that in the standard, factory configuration, that the two springs (on each of the sides) are different. The shorter spring sits closer to the pivot point, and the longer spring is located at a greater radius to the pivot point? What would be the effect if they were reversed, as I did not appreciate a difference at the time I rebuilt my distributor? Indeed, I may have asymmetric lengths on the two sides. Not difficult to fix, but of interest.

Jim S.

 

Correct, the springs are different. Each weight has a short spring/long pin near the weights axle, and a long spring/short pin near the other end of the weight. If you reverse them, in theory the advance up to ~1000 RPM would be probably more progressive until the (now longer) spring next to the end of the weight would set in, but i never tried the swapped combination so this is just a guess.

Best,
Adrian
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When i took my distributor apart for the first time a few years back, I found that the each weight had springs and pins of differnt length providing five inflection points in the advance curve not the three in the manual.

In my experimentation to understand and improve, I did try reversing the long and short pin in each weight. I found that the first pin acted almost all the way to full advance before the second pin had a chance to act at all. Also the second pin would rattle around as too much clearance to the fly wheel rim when not in use. I reject this option as it did not look right.

To help with the experimenting and in order not to destroy the original components, i bought the superformance weight and pin kit. The components and dimensions are annotated for referenece. I ended up not using this kit for the final set up as managed to tune the original components.

Unfortunatley i did not and still do not have access to a SUN or similar distributor testing machine, so to get the initial inflection points I made up a metal bar that enabled me to turn the advance plate with the distributor mounted in a vice and watch the spring/pins start to act on the flywheel. Playing with the shims and springs allowed me to tune the inflection points. Determination of the actual shape of the curve including between between inflection points was carried out on the engine while running with a snap-on strobe. Some of the results are shown.

The final set up provides good running and acceleration although i do feel the freedom of the engine to accelerate between ~4000 and 6000rpm could be improved. Perhaps the higher advance is what I am missing. Interesting thread and thanks for sharing.

Kevin (2 slides to follow)
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Very nice work, thanks for sharing! You might want to set your idle advance to 11-12º instead of 9º, this lifts the curve up by 2-3º and might help with the advance at high RPMs as you then reach 39º in full advance.

 

Adrian,

yes easy for me/others to try and test on a road run. If the idle on R1 (I have the 6 lobe cam) is too rough I can set that back further to compensate. At the moment the idle is ideal. However, the fact that your advance curve has a steeper gradient means i would have to change the pins/shims/springs which obviously is more involved again. A programable advance system like yours is what is needed to do this efficiently and properly. That touchies the boundaries of originality vs modification, which of course is a sensitive separate subject. Alternatively a spare distributor to play with and swap out would help....luxury!

An addtional coment; while it may be a useful guide to show example pin and spring lengths, the dimensions of the bob weights relative to the pivot and the location and depth of the piston cylinders needs to be known. I noticed that the superformance ones were different to my originals. Furthermore my originals appeared to have worn bases to the cylinder bottoms and each depth was a slightly different. Yet another set of parameters to worry about. The mechanical approach is indeed fiddley to work with and not ideal.

Kevin

 

Adrian and Kevin - your contributions to Dino science are worthy of the "Reynolds Chain Award". (I just made that up). I believe that the timing chain (I believe manufactured originally by Reynolds of the U.K.) is the only component of the Dino engine that has never failed (someone will surely set me straight).

Anyway, thanks for your expert input. You are the first co-recipients of the Reynold's Chain Award.

Jim S.

 

Jim thanks for the comment,
I wasn't aware that the chain was manufactured by Reynolds, so felt the need to check it. I scanned my cam shaft removal slides and zoomed in. You are indeed correct. Reynolds are probably better known in the UK as a bicycle manufacturer. So far the chain has held up, hopefully the case for others as well. Sorry for the temporary diagression of your thread Adrian.
Kevin
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Just dwelling (Hee Hee) on the chain diversification another moment, Do you think that the Superformance replacement chains are Reynolds?
G P

 

Jim, thank you