Porsche 911 CDI timing tweaks transform engine performance

Hundreds of Classic Retrofit CDI+ ignition units are now being used on Porsche 911s all over the world. One recent adopter is Jonny in Minneapolis, who has fitted a CDI+ unit to his 1978 Porsche 911 SC 3.2-litre short stroke with MFI backdate project.

Jonny recently started driving his car, breaking in the engine, transmission and suspension upgrades. After the engine was run in, Jonny noted that, at 7,000 rpm the car seem to hit a soft rev limiter.

Jonny Hart replied in the project build thread on Pelican, confirming that the stock (soft) rev limiter on CDI+ was set to 7,000 rpm. This takes out 10° in advance so drivers feel a dip in power to encourage them to lift off the throttle. The cyclic ignition cut hard limit is set to 7,250rpm: this cuts four sparks out of five for a ‘rolling burn’, to prevent fuel wash and backfiring.

Our CDI ignition is a mappable unit so, to release a bit more power on his new engine, the owner decided to tweak the timing on his CDI unit.

“I've been running a conservative advance curve: no advance from base idle timing (10 degrees) up to 2,000 rpm, then 22 degrees advance coming in at 3,000 rpm. In other words, 10 degrees at idle and 32 at higher rpm. It was easy to check and verify. I recently decided to add some polish to it.

"I was getting the occasional bit of kickback on startup so pulled some timing (-4 degrees) below 800 rpm for easy cranking. At all reasonable idle speeds from 800 to 1200 rpm, it's a flat line at 0 degrees, then rising to max advance of 22 degrees at 3,000 rpm, with a few degrees extra until higher RPM, when the cam comes in and cylinder pressures are even higher, so I drop to 20 degrees advance.

"I set the base timing at 12 degrees. With the relatively low compression (9.5) and the cooling brought by the fine MFI atomisation and the rich mixtures again keeping things under control, I think that 32 degrees at 7,000 rpm is a good choice. I also bumped up the rev limiter a bit.

"The tweaks made for quite a tuneup, let me say! Everything about the motor became better. I didn't think that was even possible. Starts easier, idles better, and has a low- to mid-range punch that's just nuts! It also uses those extra revs up top. I might bump the limit a little more as it's still pulling hard when the fun stops.

"All in all, my car already felt pretty good, but this fine tuning was a huge positive change. Love that CDI+!"