Initial timing recommendation please

[JohnJL]

I am trying to fine tune my webers on a new engine build. I've seen many threads but none with the same combination as mine.

Here are my variables;

3.0 SC Case
11:1 98mm JE pistons
Twin-plugged
7bpe plugs (for tuning, will switch to 6's for normal)
DR24 cams, 108 lobe separation
Weber 40IDA3Cs
34mm vents
60 idles
160 mains
180 airs
JB Racing twin-plug distributor set at 20 degrees advance beginning at 1800 to 3000 rpm
No vacuum advance/retard
Twin MSD 6T and blaster 2 coils

I've seen everything from 10 btdc to 5 atdc. Expert suggestions very much appreciated.

[911pcars]

Not an expert.

I'd start at somewhere between TDC and 5ºBTDC. For setting up Webers for idling, the ignition timing is a negligible variable - timing does affect idle speed but not idle mixture.

I'd also suggest a trip to a dyno shop where they can plot ign. timing as well as monitor A/F ratio and EGT to arrive at a optimal combination of jetting and ign. timing.

Your Weber settings seem reasonable. Maybe larger venturis, especially if used on the track. There may be setup specs for similar engines in the archives that can point you in the general direction, but who knows how optimized those settings are, as well as how identical they are with your engine and its operating conditions.

Sherwood

[JohnJL]

Thanks. My portable LC-1 rig died so now I either pull the other out of the track car or borrow one. I'm trying to do the second.

Any more tips on initial timing? Given the slope on the timing curve is a straight diagonal one, I would think I'd want initial timing at say 10 btdc so that when the 20 degrees is in at 3000 I've got 30 deg btdc. This engine developed peak power on the dyno at max 34 degrees with EFI, same intakes, pistons, rods, etc.

I can change the total advance to get more so if 10 deg btdc at idle is too much I can move it back towards TDC and dial in more advance to the distributor so total advance can stay the same or get even more advance.

[RoninLB]

Originally Posted by JohnJL

Any more tips on initial timing?

, I would think I'd want initial timing at say 10 btdc so that when the 20 degrees is in at 3000 I've got 30 deg btdc.

peak power on the dyno at max 34 degrees with EFI,

I run 10 initial. When exploring carb and ignition initial settings I moved dizzy around when winter cold starting over the course of a few drives. Hardly any difference between 5 & 10 initial but seemed a large difference from 0 to 10 initial. It would run cleaner at the 10 for warming up. After a drive i couldn't tell any difference when fast throttle at 5 or 10. Also E-cam and 8.5 : 1

So experminent



And the carb may want different settings that EFI. Chances are EGT will run richer ie cooler. This graph is my curve. I would think you need more conservative #'s ?







john-cramer put this graph together

[Grady Clay]

John,

There is a LOT that goes into these decisions. None trivial.

With twin plug, the general principal is there isn’t the need for extreme (approaching 30º) spark advance. In general the 26º-29º range works well and the lower can make more power. Yes, you can use 30º and more but it eats power. The dyno will tell.

Idle timing is only to get it to idle properly. It has little to do with ‘on cam’ performance other than the two are connected by the same distributor. One reason to use vacuum advance is for better start-off from a stop. The vacuum advance and distributor advance can advance the timing when the engine rpm go below idle and the vacuum changes. This is what allows you to simply let the clutch out and ‘idle’ off from a stop. Very desirable and doesn’t diminish performance on the cam.

I recommend you run your twin-plug distributor on a distributor machine and post the actual advance curve here before you run the engine. Many distributors adapted from single plug can have more advance than desirable with twin plug.

Originally Posted by JohnJL

7bpe plugs (for tuning, will switch to 6's for normal)

The NGK numbering system goes the other way. BP7ES may be great for initial run-in and tuning but you will need BP8xxx (cold) or even BP9xxx (colder) with that compression ratio. BP10xxx are usually only suitable on track. (I assume you have 14 mm plugs at the exhaust spark plugs. I prefer 12 mm plugs there.)


Help me by likening the DR24 cams to stock Porsche cams. It would be useful to post the profile compared to ‘E’, ‘S’ and ‘RSR’ Porsche cam profiles.


I agree with Sherwood about the 34 mm venturi size for initial testing. Only after you have everything running properly, you might experiment with 36 mm venturi and possible 46 mm PMOs with 36 mm and 38 mm venturi (38 mm venturi don’t work well in 40 mm carbs).

Best mate,
Grady

[JohnJL]

Thanks Sherwood, Ronin and Grady.

The distributor I have is specifically designed as a twin-plug replica. Its similar to Henry's. Its curve begins at 1500 rpm and is fully in at 3500 rpm.

On the bottom end of this motor on its EFI intake and other cams, it continued to make power up to 33 degrees. It topped out there so we dialed it back to 32. We were never able to instigate detonation.

I am still dialing in the webers and know from previous experience you have to have the ignition sorted before trying to chase any mixture issues. I note that throttle response isnt quite what I had on the 2.7 and it wants to idle very low and almost die when coming out of gear with the clutch in or in neutral coming to a stop. Blip the throttle and it settles into a solid idle again. I suspect that is an overly-rich idle mixture but before chasing that I wanted to be sure I had initial timing set. As I understand it, incorrect initial timing in particular can cause both of those conditions (poor throttle response and bogging when returning to idle.)

You are of course correct on the plugs. I forgot bosch and ngk heat ranges run reverse. I have NGK 7's in there now for tuning. I'll leave them or go to 8's for Targa Tassie.

I agree Dyno time would be ideal, I need to find someone open on the weekend or borrow another LC-1. The car is an occasional driver, weekend tourer but my goals right now are tuning for a tarmac rally, Targa Tasmania. That event is less like track driving and more like cop-free mountain driving. You are on full throttle 10-15% of the time as the course is very twisty, usually wet and always slippery. So I'd like to stick with the 40's and get those dialed in. I had 57 idles, 34 vents, 180 airs and 145 mains dialed in on my 2.8 so comparing that with someother posters in this board led me to those initial settings. I've got a box of spare jets...There's a thread here somewhere with someone else having a hotted-up 3.0 and similar cams with the same settings and a posted dyno/A/F shart that looked very good.

The cams are from John Dougherty. They are DR24, with a narrower lobe separation of 108 degrees. They are set at 2.4mm. Intake lift is .49" 250deg, exhaust is .455" 240 deg. I gues that makes them similar to GE60 or a bit more lift and duration than an S cam. It has less duration but more lift than the 3.0 RSR (278deg .464" intake, 267deg .45" exhaust.) We spent several conversations planning specifically for this setup.