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Re-curved distributors on "modified engines" What do you mean???

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THE FRENCHTOWN FLYER
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Re-curved distributors on "modified engines" What do you mean???

Post #1 by THE FRENCHTOWN FLYER » Tue Jan 29, 2019 2:58 pm

I frequently hear mention of getting your distributor re-curved if you modify your engine. What does that mean? Please explain how your distributor would need revisions and what revisions are needed. Specifically, address initial advance, centrifugal advance, and vacuum advance.

OK, ok, I know this question could be a subject for a graduate degree thesis. I want your opinions about how to specifically change, initial, centrifugal, and vacuum advance and why. Full disclosure: I have my own theories but I'll let the conversation get started with your opinions before I comment. Any takers?
FORD 300 INLINE SIX - THE BEST KEPT SECRET IN DRAG RACING

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Re: Re-curved distributors on "modified engines" What do you mean???

Post #2 by MechRick » Tue Jan 29, 2019 4:31 pm

Isn't this like students lecturing the teacher? :D

Opinions I have, limited experience I can share.

I've always heard performance engines should be 'all in' at 3000 RPM. Tried this on a blow-through turbo on pump gas. Detonated like crazy. Conclusion? Pump gas engines with stock combustion chambers (timing requirements) need stock timing curves.

As an observation, engines with efficient combustion chambers need less mechanical and vacuum advance (Chevy Vortec for instance).

Race engines don't need vacuum advance at all, since they don't need to be efficient at part throttle.

Camshafts that lessen dynamic compression tolerate more aggressive timing.

I would think increasing an engine's quench or installing better aftermarket cylinder heads would lesson timing requirements. I bet pmuller9 has some input here, and others who have accumulated some serious dyno time.

As an aside, my F250 will not tolerate more than 10 degrees BTDC of initial timing. Runs fantastic at 14 degrees, but pings down low until the tach reaches 3500 RPM. Wondering if Ford missed the timing curve a bit.
1994 F150, 4.9L/ZF 5 speed, C-Vic police driveshaft
EFI head w/mild port work, 3 angle valve job
1996 long block, stock pistons, ARP rod bolts
Stock cam, aluminum cam gear
Hedman header, full mandrel bent duals, crossover, super turbos
http://fordsix.com/forum/viewtopic.php?f=2&t=73244
Bronco II with a 2.3L swap http://fordsix.com/forum/viewtopic.php?f=13&t=72863
1988 F250 2x4, 460 ZF 5 speed.

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Re: Re-curved distributors on "modified engines" What do you mean???

Post #3 by bubba22349 » Tue Jan 29, 2019 4:40 pm

Recurving a distributor is a term that can mean lots of different things. To me and my performance experiments over the years it has to do with first the distributors mechanical advance rate, base timing, and the amount of total advance, then later also the vacuum advance amount. Ford distributors advance systems have been fairly standard on the V8's since 1957 and the inline sixes finely also got this better system around 1967 through to the end of the DuraSpark II system in 1984 or 5. Looking inside these distributors you find that Ford gave us two different amounts of the mechanical advance to use for tuning. In the old days I also used different springs or by adjusting the posts to bring in the advance quicker. I never had the use of a distributor machine or could afford Dyno time, so I did my tuning the hard way using extra timing degree marks (or with some timing tape) on the dampner and a timing light and making a pass on the strip or a timed run on street, doing plug reading, jetting, etc., not the best but for low budget operation and with quite a lot time it worked somewhat for me. Of course today its all different, we have all those great electronic control boxes, weather stations, narrow & wide band gauges, and completely different fuels too. :thumbup: :nod:
A bad day Drag Racing is still better than a good day at work!

I am still hunting for a project car to build but with my current low budget it's not looking so good. My Ex- Fleet of Sixes these are all long gone! :bang: 1954 Customline 223 3 speed with O/D, 1963 Fairlane project drag car with BB6, 1977 Maverick 250 with C4, 1994 F-150 a 300 with 5 speed.

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Re: Re-curved distributors on "modified engines" What do you mean???

Post #4 by powerband » Wed Jan 30, 2019 12:09 pm

another' $.02's for discussion:

Early stock and modified small block six Fords can benefit immediately with an upgrade to modern electronic ignition and advance capable distributor . The inexpensive DSII distrib already is designed to have a reasonable advance curve with built in internal weights and external vac. The advance weighting, max and curve do vary with dist' application year and can be verified or modified pretty simply. I've used DSI and DSII setups on the NA Tri-Power 250 and Forced Induction Maverick project with minor mods' to the weights/window, spring tension and vac reaction.

Undoubtedly a build-specific advance curve set on a distributor machine is preferred for the absolute performance edge with a matched overall build for performance edge .

have fun



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Re: Re-curved distributors on "modified engines" What do you mean???

Post #5 by Dr Jay » Wed Jan 30, 2019 12:47 pm

My Comp Cam 252, 240 head, 390 Holley combo
pinged like crazy on 91 octane pump gas.
New small gap brackets, heavier springs, and a cranked vacuum diaphragm spring, really helped. My newer 300 headed combo with the 268 Comp Cam lead me right back to stock everything. Much better rpm reach!!
10 btdc or so initial on all of it.
PS This is all known stuff.
Blessings,
Dr Jay

1978 F100 Shortie, Lowered front and rear, 300 .030 over, 300 carb head, Clifford, Holley 390
268 Comp Cam, Cloyes, Fuel Inj. exhaust manifolds, MSD 6A, TOD, 9"

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Re: Re-curved distributors on "modified engines" What do you mean???

Post #6 by Lazy JW » Wed Jan 30, 2019 3:58 pm

I question the value in sending my distributor to someone far away and having them "recurve" it when they have never so much as seen my engine/vehicle.
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Re: Re-curved distributors on "modified engines" What do you mean???

Post #7 by sdiesel » Fri Feb 01, 2019 6:44 pm

Lazy JW wrote:I question the value in sending my distributor to someone far away and having them "recurve" it when they have never so much as seen my engine/vehicle.



Amen. off the race track. i can understand track cars that see the same service every time they go. but not my ranch trucks.
i've seen people who should know better recurve in hopes of covering a design flaw or poor choice in components hoping for a simple fix to an ungainly problem.
We all can't be Smokey Yunick, after all.
a long love affair with the 300 six.
my lastest and final fling is a fresh 300 in an 88 ford f350 dually 4X flatbed

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Re: Re-curved distributors on "modified engines" What do you mean???

Post #8 by THE FRENCHTOWN FLYER » Mon Feb 11, 2019 9:32 am

Has anybody bought a re-curved distributor and put it on a distributor machine and compared it to a stock one they had? How did the centrifugal advance change? How did the vacuum advance change?
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Re: Re-curved distributors on "modified engines" What do you mean???

Post #9 by powerband » Mon Feb 11, 2019 10:55 am

again, ... Undoubtedly a build-specific advance curve set on a distributor machine is preferred for the absolute performance edge with a matched overall build for performance edge .

With the advent of programmed-dwell modules and Hall effect 'trigger' (per HEI / Duraspark) and a much more precise firing and final advance control, tuning electronic ignition has less variables to contend with for a steady power output curve. Matching ignition optimization to the fuel optimization solely with springs and weights takes a Sun machine, feedback information and experience ...

good 'modern' ignition article per Hemmings':

https://www.hemmings.com/magazine/mus/2 ... 19426.html

have fun

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Re: Re-curved distributors on "modified engines" What do you mean???

Post #10 by Firepower354 » Mon Feb 11, 2019 1:01 pm

IMO, a verified set of timing marks, decent timing light, and accurate tach, make a fine alternative to a Sun dizzy-spinner.

We used to buy the generic kits for HEI GM and usually end up with the light weights, and one each soft & medium spring, on mild SBC with lazy heads and loose quench. Vac can adjustment varied to keep from part throttle rattling.

Without a dyno and some fancy equipment, it's gonna be a best-guess scenario anyway. I don't see the mail order recurve as being an awful thing, if one hasn't the time or ability. I like to set my total, then work backwards. Worn weights and pins have to be addressed early in the process or it'll be erratic.
Factor in the RPM range, load, temp, application, altitude, chamber, swirl/tumble, squish, ad nauseam...
OE curves have to do their best within the parameters of mileage, emissions, reliability with performance way down the list.
Look at the nightmarish 70's Detroit iron. Low compression, retarded cam and ignition timing, lean carbs, tall gears, and little wonder why mileage and performance were dismal. An SP2P and a curve kit did wonders.

Racing, max-power tuning is far easier, just make peak cylinder pressure happen at 15-20 degrees ATDC depending on R/S, offset, etc
I think that too far advanced is a common mistake, adds heat and hurts power
It gets 10MPG, but goes up to 14 if I lie.

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Re: Re-curved distributors on "modified engines" What do you mean???

Post #11 by wsa111 » Mon Feb 11, 2019 1:52 pm

It's known as experience on what you need for your application.
I have a Sun Distributor Tester & i can make sure what i want is what i get.
I have been recurving distributors for over 50 years.
Every engine & its combination require a different total, rate of advance, when the advance starts & when its all in.
A lot of forum members use my recurves & i get feedback on the positive results.
If you think you get the proper curve from just purchasing a reman Cardone you are totally wrong.
DUI's HEI's that come out of the box are way off, some worse than others.
All my recurves are tailored strictly for you engine, trans plus other factors.
67 mustang,C-4, with mod. 80 hd, custom 500 cfm carb with annular boosters, hooker headers, dual exh.-X pipe, flowmaster mufflers, DSII dist. MSD-6al & MSD-Blaster 8252 Coil. Engine 205" .030" over with offset ground crank & 1.65 roller rockers. 9.5 comp., Isky 262 cam.
2003 Ford Lightning daily driver. Recurving Distributors. billythedistributorman@live.com
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Re: Re-curved distributors on "modified engines" What do you mean???

Post #12 by powerband » Mon Feb 11, 2019 5:13 pm

I am re-learning what I thought I knew RE ignition advance curves with the turbo and supercharger projects, all the variables of NA and then there's while in boost #.

The 250 with supercharger currently runs stock mid 70's DSII with the advance "13" weight interpreted :: 2 X crankshaft = total advance of 26 Degrees plus initial setting. Vacuum is active providing additional vacuum advance which is (of course) inactive under boost. This was working without detonation with 5-8 lbs @4K rpm boost. With new pulley ratio and additional boost available 10-15 lbs@4K rpm, 250 now needs need an advance retard referenced to boost ignition or fuel related solution to prevent destructive detonation. (Blow-Off can be set to prevent high-boost and detonation) . I haven't moved to laptop programmable ignition system but it might be inevitable.

My Daily Driver '63 with 70 Maverick 170 I swapped in @ 15 years ago, still has OEM Points ignition - IF it has a problem I'll swap in a Duraspark - IF it ever has a problem ...


have fun


IMO, a verified set of timing marks, decent timing light, and accurate tach, make a fine alternative to a Sun dizzy-spinner.

Ignition theory and tuning application is not in everyones 'toolbox'. Timing light operation isn't even common knowledge. Nice option available to have an advance curve verified as compatible with the engine build goals. I don't know anyone locally still sets up a distributor . ...
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Re: Re-curved distributors on "modified engines" What do you mean???

Post #13 by Firepower354 » Tue Feb 12, 2019 3:43 pm

powerband wrote:I am re-learning what [i]






Ignition theory and tuning application is not in everyones 'toolbox'. Timing light operation isn't even common knowledge. Nice option available to have an advance curve verified as compatible with the engine build goals. I don't know anyone locally still sets up a distributor . ...



Yep, just one more part of the difference between "builder" and "assembler", like cam degreeing, squish setup, ring end gap, rod side clearance, and so many little details. :beer:

Timing light use is still exponentially more common than the antique Sun dizzy-spinner. Last time I used one was 11th grade when I sorted the HEI on my 292 swapped C10 after the integrated head 250 cracked the head, like they do. :rolflmao:
It gets 10MPG, but goes up to 14 if I lie.

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Re: Re-curved distributors on "modified engines" What do you mean???

Post #14 by wsa111 » Tue Feb 12, 2019 8:27 pm

Firepower354 wrote:
powerband wrote:I am re-learning what [i]






Ignition theory and tuning application is not in everyones 'toolbox'. Timing light operation isn't even common knowledge. Nice option available to have an advance curve verified as compatible with the engine build goals. I don't know anyone locally still sets up a distributor . ...



Yep, just one more part of the difference between "builder" and "assembler", like cam degreeing, squish setup, ring end gap, rod side clearance, and so many little details. :beer:

Timing light use is still exponentially more common than the antique Sun dizzy-spinner. Last time I used one was 11th grade when I sorted the HEI on my 292 swapped C10 after the integrated head 250 cracked the head, like they do. :rolflmao:

A timing light can be used as a fast check, but the Sun Tester is king for precise setting & the time is takes to achieve what you want.
67 mustang,C-4, with mod. 80 hd, custom 500 cfm carb with annular boosters, hooker headers, dual exh.-X pipe, flowmaster mufflers, DSII dist. MSD-6al & MSD-Blaster 8252 Coil. Engine 205" .030" over with offset ground crank & 1.65 roller rockers. 9.5 comp., Isky 262 cam.
2003 Ford Lightning daily driver. Recurving Distributors. billythedistributorman@live.com
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