SC dizzy re-curve graph for use with 40IDAs

[MdR]

I see lots of threads talking of re-curving the advance of the SC distributor to make it more compatible with carbs but I can't find any recommendations as to what the new curve should look like i.e. what advance at what revs?
I see some mention of a modded 2.7RS curve or a modded 67S curve.
I have a 1983 ROW 3.0 SC with 40IDAs jetting supplied by by PMO.
Can anyone help?

[psalt]

Martin,

The comments you have read are about the US version of the later 911 SC. When Porsche raised the compression ratio of the final version of the SC, they had to seriously detune the ignition timing curve to avoid detonation on the then mandated 87 octane US unleaded regular fuel. The ROW engines have more advance, but were still detuned from ideal for a wide VIA hemi headed engine. The 911 engine design is knock limited on road fuel , so the ideal max advance will be determined by your climate, engine temp and fuel octane. The factory setting for your engine was 25 BTDC max vs out to 38 BTDC for earlier 911's. Another factor overlooked in carb conversions is that the long narrow CIS runners boost torque in the mid range where detonation probability is highest, so on paper your engine should tolerate a bit more advance with carbs. How much, no one can tell you over the internet. Since every engine is different and fuel local, the only way is to experiment. In setting up EFI systems, I usually use race fuel to find where the power drops off, then retard for the fuel that will be used. If you are not comfortable in detecting detonation, you may want some help. A session on a rolling road can help. If you can get into the 30's BTDC without detonation, you will be within a few % of ideal. If the resulting initial timing is OK, there is little benefit to recurving the distributor.

[Gunter]

Carburettors are not the main criteria to set a curve, AFAIK.

Your distributor curve would be set for CR, octane used and general average air temps.

Since you don't have vacuum anymore, the pod would be removed, the internal vac advance plate would be locked and the weights/springs manipulated to match the above criteria.
It's not cheap if done correctly by a professional, ~$300 Dollars.

To get the maximum benefits from carbs, hotter cams and different pistons are used. Your current pistons would have CIS domes. Headers or SSI's add horses.

Without any other mods, carbs will not add much performance but give you pretty bad gas-mileage.

[Steve@Rennsport]

Originally Posted by MdR

I see lots of threads talking of re-curving the advance of the SC distributor to make it more compatible with carbs but I can't find any recommendations as to what the new curve should look like i.e. what advance at what revs?
I see some mention of a modded 2.7RS curve or a modded 67S curve.
I have a 1983 ROW 3.0 SC with 40IDAs jetting supplied by by PMO.
Can anyone help?

Martin,

In general, re-curving SC distributors on carbureted cars offers a marked improvement in throttle response and fuel economy. Actual values depend on the grade of fuel being used so I cannot offer anything specific here, however I've done lots of these over the years and everyone has been quite happy as long as they had 92 (95RON) fuel or better.

Don't remove the vacuum cannister, even though its not used in these applications, since locking the plate disables the critical rotor phasing function.

[MdR]

Thanks for the replies.
Here in the UK I run on minimum 95RON unleaded, my SC engine is backdated running SSIs & early style 2 in 1 out exhaust & is nailed inside my '69 911T chassis.
AFAIK the engine internals are as factory designed.
I have set the max advance to 32deg using an advance reading Snap On timing light.
Although I understand the CIS pistons & cams aren't the best for the carb job my finances say otherwise
I am running the vacuum retard can unplugged but in situ.
It's not the maximum advance that I'm really querying it's how & when the advance gets to the maximum.
Any more information gratefully received.

[psalt]

Martin,

Take a look at this thread

Best advance curve?

Keep in mind that the factory curve charts are in distributor degrees and you have to double the numbers for crank degrees. Notice how the early curves are steep to 20 crank by 2000 rpm and a lesser slope to 6000 rpm max. The later SC curves are clipped much earlier to avoid high speed detonation from the higher compression and lousy fuel. Your original distributor should have around 20 degrees of crank advance and if you set the max to 32, your idle timing should be around 12 BTDC, or +5 advanced from the factory spec. If you are OK running 32 BTDC max with a ROW distributor, any improvement from a recurve will be minor and largely imaginary.

[Gunter]

Originally Posted by Steve@Rennsport

Martin,

In general, re-curving SC distributors on carbureted cars offers a marked improvement in throttle response and fuel economy. Actual values depend on the grade of fuel being used so I cannot offer anything specific here, however I've done lots of these over the years and everyone has been quite happy as long as they had 92 (95RON) fuel or better.

Don't remove the vacuum cannister, even though its not used in these applications, since locking the plate disables the critical rotor phasing function.

Steve, when you recurved my '82 distributor to run without vacuum, the pod was removed and the plate locked with a threaded screw through the case?

What am I missing.

Thanks.

[Steve@Rennsport]

Originally Posted by Gunter

Steve, when you recurved my '82 distributor to run without vacuum, the pod was removed and the plate locked with a threaded screw through the case?

What am I missing.

Thanks.

Its been a few years (and a LOT of distributors through here) so I do not remember your unit, but I think you wanted the vacuum cannister removed,..........

Call me.

[Gunter]

Thanks Steve,

Yes, its been a few years and the distributor runs nicely.

Correct, I wanted the vac canister off and supplied a small cover for the hole.

Specs were 9.5 CR, min 92 octane and moderate air temperatures.
CIS with 964 cams for a ss 3.2.

What confuses me is that your reply above says: "Don't lock the plate" because of "critical" rotor phasing function.

MdR has carbs and no vacuum, so, how can rotor phasing take place if the vac canister just sits there doing nothing?
Also, as I understand from Paul (psalt), vacuum would not have any effect anyway at high RPM's and therefore, wouldn't the mechanical advance do rotor phasing?

My understanding of "rotor phasing" is to insure that at high RPM the spark doesn't jump to the wrong pole inside the cap. Is this correct?

You locked the plate in my distributor and it performs very well, no problems red-lining.

I'm really interested in fully understanding the issue and appreciate your patience.

[snbush67]

Martin,

I think that you have the appropriate distributor set up for your engine as is. As long as the distributor is functioning normally with a decent advance then it would be a waste to re-curve it.

I have read that the ROW distributors have the same curve as the early US SC's with the large intake ports.

I have been advised that the best setting for performance is to set the distributor so that it fully advances at while the engine is at 3000rpms, then allow the idle to fall where it may. Of course if it is a daily driver that is used mainly on the street this may not be an optimal set up as your mix at idle may have to be rich to keep it running smoothly at idle.

I have a similar set up and have found a decent middle ground around the stock setting. I have two stock early SC distributors that I have cleaned and serviced and they both advance exactly the same, I do not buy into the re-curve hype kool -aid that many sell.

Good Luck,

Shane