Victa

Hey guys, I have a single dizzy 1981. On ebay there is a twin distributor. Can I upgrade? To a twin distributor? Kind of like the idea.

JET951

Hi Victa , being that your'e 1981 ( 4.5 or 4.7 ) is K -Jet ( Euro version )with two dimensional ( conventional ) electronic ignition system ( turn the dist to change the ignition timing with a timing light ) this system was used in the 928/S up to & including the 1983 year type , the simple answer is no

Because }

The twin ( piggy back ) dist was used in the 1984 & 1985 year types ( and the UK 1986 ), you can not turn the distributor to adjust the ignition timing with this system , meaning the dist is locked in one position ( the ECU ignition computor does the large changes in ignition timing ) via imformation from the crank angle sensor & different flywheel to suite & this ignition ECU was used in conjunction with the electronic fuel injection system , meaning different wiring & injection harnessses + a stack of other changes

Victa

Thanks jet951. Makes perfect spence. Was dreaming because a friend upgraded his Ferrari dino to twin dizzy. Thought I could do the same.

Wisconsin Joe

Well, you could change over everything to the 4.7 S2 motor. Engine, ECU, fuel injection system, everything.

It would probably be simpler and cheaper to buy an S2 (84-86 ROW/Euro)

123quattro

A twin distributor won't make any more power than a single. The only benefit is you have control over a larger swing in spark timing.

Victa

And while I was there..............

danglerb

Point of the twin dist is that you can use two coils, the coil I believe being the weak link at higher rpms.

123quattro

There are lots of cars making double the 928s power with 1 coil.

hacker-pschorr

I converted my 81, it wasn't that big of a deal.

I just had to drill a hole in the upper bellhousing to hold the crank sensor and have 85/86 fuel rails modified with custom hold downs. I also rebuilt the harness with updated weatherpac style Tyco / Amp connectors. I also had a custom bracket made to hold the ignition modules to have them in the stock location.

Running the wiring harness so it didn't look like a back woods cob job was the most challenging part since I had to re-do part of the front harness for the ignition system.

Z

At 6,000 RPM, the engine is making 100 revolutions per second, and one revolution occurs in 10 milliseconds. The engine is a four stroke eight cylinder. That means it fires four cylinders per rotation. Since each rotation requires 10 milliseconds, that means a cylinder has to fire every 2.5 milliseconds. With a single coil system, that single coil has to also fire every 2.5 milliseconds, once for each cylinder. After it fires a spark plug, the coil being used doesn't have enough time to fully saturate before it has to fire the spark plug for the next cylinder. There's not enough available time, so the dwell period is reduced, reducing spark output from what the coil could produce if it had the time to fully charge, like it does at lower RPMs. As cylinder pressure increases, it becomes harder for the spark to jump the same sized spark plug gap. At some point, increased cylinder pressure and the reduced coil output result in there being insufficient voltage to jump the spark plug gap, and the cylinder isn't firing anymore, and not making any power.

That's what was happening on Hacker's car at the boost levels being run on it, and was one of the main reasons his car was switched to the later two coil ignition system.

With the two coil ignition system, each coil only fires half of the engine's eight cylinders. There are still four cylinders firing every engine rotation, but each coil is only responsible for half of those four firings. The coils alternate firing, and each one only fires two cylinders for every engine rotation. At that same 6,000 RPM, each coil only has to fire once every 5 milliseconds. That gives it more than enough time to fully saturate, and build up it's maximum possible spark potential, before it needs to fire again.

Another advantage of the dual distributor ignition system compared to the single one is ignition timing accuracy and consistency. The twin distributor system uses a sensor that reads the position of the crankshaft directly from the toothed wheel mounted to the crankshaft. The single distributor system uses a sensor inside the distributor, which is driven by the camshaft, which is driven by the timing belt, which is driven by the sprocket mounted to the crank. That leaves a lot of potential variance and change due to slop throughout the whole drive system, timing belt stretch, and dimensional changes due to temperature changes of various things.