cdi box & ignition coil questions for 2.7L MFI 911

[stylomylo]

I've been having a lot of hot/cold/warm starting (and stalling) issues on my car and my mechanic decided to replace the stock ignition coil with a japanese one by-passing the cdi box. it seems to be working fine now (fingers crossed). obviously the car is not true to it's original form now, but can someone tell me how this will change the characteristic of the engine and if there are any long term (damaging) effects on the engine?

and if it indeed a cdi or ignition coil issue, should i replace it with a stock unit (are they still available - my engine is a 1974 2.7L MFI) or should i replace it with a after market type such as MDS?

many thanks in advance

[T77911S]

there are as many opions on MSD vs BOSCH as there are people on this board, maybe more, some may go both ways. i went with the MSD because i could put in a CD and coil for about $300 and you can buy it locally, nice if it ever goes bad.

[Walter_Middie]

There's no damaging effects to your engine - you just are not getting the performance you should. The CD ignition provides a much better spark, especially at high RPM where your current setup cannot recharge itself very fast, but the CD can.

You don't mention what kind of car your engine is in. If this is an all original car and engine, I'd definitely have your original Bosch CD box rebuilt, or if you are feeling like spending a small fortune, buy a new one. If this is not an original car and engine combination, you can get an aftermarket CD like the MSD much cheaper. But I'd definitely get some kind of CD ignition eventually - and make sure you get the correct coil if it is mounted upside down, like the high vibration type.

[stylomylo]

thanks walter for your quick response.

my car is a 1974 911S with 2.7L MFI engine. the engine is 'almost' in original condition but because the starting issue is driving me mad and all i want is for the car to start easily. i'm not fussed about keeping it original. i only want a VERY reliable start without spending a fortune (ustand i have to rob a bank to buy an original new CDI box?), hence my question about the MSD set up.

btw, where can i buy a new CDI box?
if i were to go for the MSD system, which are the correct parts i should buy?

thanks

[T77911S]

MSD- 6AL and blaster coil. you can get the MSD with or without the soft rev limter function.
there is a bit more involved in installing the MSD. it is quite a bit larger than the bosch. i mounted mine above the relay panel, but had to relocate a relay, just laid it over and mounted it, no big deal.
BOSCH- you can have it "rebuilt", well repaired.

[Walter_Middie]

Disconnect the negative terminal from the battery. To allow better access to the ignition unit on the left side (drive side) of the engine compartment, remove the black plastic cover from the ignition rack, the air pump, air filter housing, and the two warm air heater hoses. Additionally I removed the distributor cap and moved the spark plug wires to get them out of the way.

There are only 3 wires going to the original CDI unit, two white wires and a red one.
One white wire goes to the + coil (or transformer) terminal
The other white wire goes to the distributor points and the tach
The red wire is +12 volts switched through the ignition switch on the dashboard.

Each of the white wires is wrapped with a grounded shield that is connected to the CDI unit case at one end, and the + coil white wire ground shield runs to the – coil terminal and is grounded on one of the posts that the coil mounts on, the other white wire ground shield is grounded on the engine near the distributor. All these wires can be removed as a complete harness, unplugging from the coil, unplugging from the distributor, unplugging from the CDI unit, and unplugging a white plastic connector for the +12 volts switched through the ignition and the tach signal – just follow the red wire from the CDI unit to the white plastic connector. The red wire from the CDI unit also contains a small canister about an inch in diameter and two inches long. I believe my manual refers to this as an RPM sensor for the tach. Remove it along with the rest of the harness. Additionally remove all ground wire attachments that this wiring harness is attached to. Removing all these wires really cleaned up the clutter in my engine compartment.

Remove the three allen bolts that mount the CDI unit to the ignition rack, and remove the old CDI unit. Remove the two nuts that mount the coil to the fan shroud. Remove the old coil from the coil mount and install the new coil the same way the old coil was mounted (be sure to get a coil that can be mounted upside down – the oil filled ones don’t like to hang upside down). When mounting the new MSD unit, I used a ¾ piece of plywood to move it out away from the inner fender wall that was interfering with one corner of the new MSD unit. The plywood must be trimmed to fit the upper rear corner and to clear one of the mounting bolts for the ignition rack. I mounted the plywood using the original CDI allen bolts, and used the screws supplied with the MSD unit to mount the MSD box to the plywood.

The wire connections required for the new CDI unit consists of:
ground
+12 volts on all the time
+12 volts switched through the ignition switch, distributor points
+ coil
– coil

The ground wire can be connected to one of the bolts that mount the ignition rack, and the coil and points can be connected using the wires and connectors that are already attached to the new MSD. Remember the white connector left in the engine compartment from the old unit that consists of +12 volts switched from the ignition switch and the tach sender? The switched + 12 volts for the new MSD unit can be connected to the red wire side of the white connector. The tach can be connected by running a wire directly from the new MSD unit tach output, to the black and purple stripped wire side of the white connector. I connected the +12 volts on all the time to one of the posts on the engine compartment fuse box that had +12 volts on all the time. That’s all there is to it. Now fire it up and enjoy.

[blacklotus99]

Just out of interest can I ask why you would have an MFI type motor in what would have been a CIS car? You say the engine is mostly original, if so where did it come from?

[ischmitz]

There are several discussions and opinions out here about the different ignition solutions. The stock Bosch CDI system seems to be favored by most because it is reliable and works fine.

The aftermarket solutions (Permatune, MDS) are favored by some because they can be had cheaper than an original Bosch CDI box. Often people with a failed Bosch box have gone the aftermarket route. Then there are promisses of multiple sparks, hotter sparks, etc. that claim performance gains. I consider this smart marketing for the most part for what it is worth.

Personally, I would always favor the stock setup, especially since the Bosch CDI design is very reliable and rugged. And if a box fails it can be rebuild. Some of the aftermarket products are encapsulated in resign. If they fail they become an instant paper weight.

But there are no adverse effects by using an aftermarket CDI setup. One potential issue can arise due to improper or poor wiring. The MSD boxes do not come plug&play, they require cutting and splicing into the stock harness.

Cheers,
Ingo

[Joe Bob]

Some aftermarket boxes also require tach adapters and other issues crop up, I'm with Ingo...stick with the Bosch.

[RSTarga]

I would suggest that your starting problems are more likely to be caused by your MFI not your CD box. If you have never had the MFI pump rebuilt it is probably time. When running correctly nothing is sweeter.

[stylomylo]

Originally Posted by blacklotus99

Just out of interest can I ask why you would have an MFI type motor in what would have been a CIS car? You say the engine is mostly original, if so where did it come from?

I live in asia and my 1974 911S is a euro spec car, hence 2.7L MFI

[stylomylo]

Originally Posted by ischmitz

Personally, I would always favor the stock setup, especially since the Bosch CDI design is very reliable and rugged. And if a box fails it can be rebuild. Some of the aftermarket products are encapsulated in resign. If they fail they become an instant paper weight.

Ingo

where can i buy a new bosch CDI for my 1974 911S 2.7L MFI euro spec car ?

[Joe Bob]

New? Why?

Get a rebuilt one.....

[stylomylo]

Originally Posted by mikez

New? Why?

Get a rebuilt one.....

where can i buy a rebuilt one ?

[ischmitz]

I sent you a PM

Cheers,
Ingo

[76S]

ischmitz,

This is 76S, okay I'm asking you about rebuilding Bosch CDI Boxes and Porsche Motronic DMEs

I have a Bosch CDI Box that needs to be tested and possible rebuilt, please advise.

Regards,

[ischmitz]

Hi 76S, I sent you a PM, please check your messages. Sorry for not responding sooner. I was out of town.

Ingo

[T77911S]

hi, i have 245/45/16's on the rear. the inside edge on both tires is wearing, kinda bad. its not long before the cords will show i am sure. i did a check on the toe this weekind and it looks like it is a tad less than 1/4 inch toe in. so my toe out idea is shot. it does not look like i have much neg camber, but would a little neg camber make just the edge wear. the rest of the tire is pretty even, the ouside is not excessively less worn. i do have a lot of noise from the rear, but i understand the yok's are noisy anyway.

[BReyes]

Had them many miles? For mine, it is normal to wear inside. I prefer it as driving preference. Once the tread gets low enough I notice it. Getting good life out of tires is possible knowing inner wear in the rear is inevitable is secondary. Some tires comes with shaved thread depth and still can get great life ultimately wearing inners first.

Regards,