Clewett XDI Ignition question

[craigmakes3]

Anyone install a Clewett XDI ignition upgrade to their 911sc? Looking for your impressions - performance, gas mileage, etc.

Thanks

[Gordo2]

I went with the XDI in my SC - but it was part of an overall engine rebuild on a 3.2L that I swapped into my car.




As such, I can't really speak to any performance changes that would be noticed if simply swapping for the stock ignition system on a unmodified 3.0L engine.

Here's a link to part of the installation - the wiring doesn't really need to be as complicated as I made it, but I wanted to "simplify" my wiring harness configuration:

Engine Harness / Multi-pin Connector for Carb Conversion '83 SC

As far as performance goes on my engine - I love it. Its easy to adjust and makes you realize how important it is to get the timing optimized if you want to optimize a carb set up.

Meanwhile, if were rebuilding an engine and keeping the CIS - I would also stick with the stock CDI/distributor ignition system. It's a good setup for the engine when everything's working properly.

Gordo

[fred cook]

When I rebuilt the engine in my SC I added the XDi system in the form of twin plug. The engine got a major rebuild, going from a stock 3.0 to a 3.3 Short Stroke with twin plug Carrera heads. I did keep a modified version of the CIS injection that uses the early large port air box and runners with the hardware from a later small port system. The bottom line is that it starts easily and runs well. I haven't made an effort to check fuel mileage as the odometer is not working. However, it doesn't seem to be much different than when the engine was stock. I have a/c on my car and to make the XDi work with the a/c I used the kit for a 3.2 Carrera that comes with a double pulley. In order to make that work, I had to swap the rear engine mount to one from a Carrera and modify it just a bit. To mount the coils, I made a "shelf" out of steel angle and mounted it in the area between the engine and the rear firewall. The control box is mounted inside the car on the vertical surface just below the passenger side rear seat. That way, if needed, I can tweak the timing while sitting in the drivers seat. One thing that I really like about the system is that I no longer need to buy distributor caps and rotors!

[CBRacerX]

This seems like a good place to drop this question. I've got a freshly rebuilt stock long block SC (small ports) with 46IDA3C's, SSI and Dansk Sport Muffler. Also have an XDi (single plug) unit that I'm working to setup the timing properly. Only use 93 octane gas. As all of you XDi owners know, this is a combination of Initial, 3K and 8K dials. These are all additive, so total timing at 8K is Initial + 3K +/- 8K.

What did you SC single plug owners end up with?

[Gordo2]

Dyno runs showed my engine performed best at 33° total advance.

Seems to get off the line best with ~ 14° initial.

Gordo

[CBRacerX]

Originally Posted by Gordo2

Dyno runs showed my engine performed best at 33° total advance.

Seems to get off the line best with ~ 14° initial.

Gordo

Thanks Gordo! Was that 33 at 3K? How did you set the 3K and 8k knobs?

[Gordo2]

Dyno showed advance setting where the engine made it's best power from ~3.5k RPM up to redline - which turned out to be 33°.

I adjusted my initial advance based on feel - how's it run coming off the line.

According to the Xdi manual, the initial knob controls advance from 400 RPM's through 1k RPMs. I found 12° - 14° gave it the smoothest / strongest acceleration.

The 3k knob controls the advance from 1k RPM's to 3k. Since my best total advance turned out to be 33° - I added 19° (14° + 19° = 33°...).

I didn't try, or feel the need to reduce (retard) or increase the advance at higher RPMs; the dyno showed I actually began to lose power as I advanced the total beyond 33°, and also showed the power decreased as I went with lower 3K knob settings (I did a few dyno runs, adjusting by ~ 3-4°; bracketing the sweet spot).

I think most people use the 8k knob to back off of advance slightly at high RPM's (to avoid pre-igntion). I'm not running high CR pistons and my measured CR came in somewhat lower than the advertised piston CR - therefore I didn't see a need or benefit to backing off on the advance.

Keep in mind - the XDi settings are somewhat unique to the engine, and largely related to the cam profile (I believe). My cams (GT2-102 with a "carb friendly profile" according to Camgrinder), and engine responded to the 33° total advance.

Prior to putting it on the dyno, I was running around 34-36° total and was surprised to find that I gained power by backing it down to 33°.

Gordo

[CBRacerX]

Originally Posted by Gordo2

Dyno showed advance setting where the engine made it's best power from ~3.5k RPM up to redline - which turned out to be 33°.

I adjusted my initial advance based on feel - how's it run coming off the line.

According to the Xdi manual, the initial knob controls advance from 400 RPM's through 1k RPMs. I found 12° - 14° gave it the smoothest / strongest acceleration.

The 3k knob controls the advance from 1k RPM's to 3k. Since my best total advance turned out to be 33° - I added 19° (14° + 19° = 33°...).

I didn't try, or feel the need to reduce (retard) or increase the advance at higher RPMs; the dyno showed I actually began to lose power as I advanced the total beyond 33°, and also showed the power decreased as I went with lower 3K knob settings (I did a few dyno runs, adjusting by ~ 3-4°; bracketing the sweet spot).

I think most people use the 8k knob to back off of advance slightly at high RPM's (to avoid pre-igntion). I'm not running high CR pistons and my measured CR came in somewhat lower than the advertised piston CR - therefore I didn't see a need or benefit to backing off on the advance.

Keep in mind - the XDi settings are somewhat unique to the engine, and largely related to the cam profile (I believe). My cams (GT2-102 with a "carb friendly profile" according to Camgrinder), and engine responded to the 33° total advance.

Prior to putting it on the dyno, I was running around 34-36° total and was surprised to find that I gained power by backing it down to 33°.

Gordo

Thanks for the detailed response. I completely understand that your engine is not using the stock SC cam that I'm working with. So I'd be interested to hear from any XDi users who might be viewing this thread about where they set their timing.