Marelli to Bosch Conversion (Pics)

[Fishcop]

I just thought I'd post some pictures of my distributor which has been rebuilt from a 1975 911 to 911T/E spec.

As a result of having endless problems with my old Marelli distributor and through the advice of this board, I decided to replace the dizzy with a Bosch. Problem is they are hard to find and expensive. John Walker (Workshops) suggested I machine down a later model distributor to fit the earlier engine. So I did.

Basically I stripped the Bosch down and had it machined to exactly match the profile of the Marelli and added a new O ring. This immediately worked in my car, but I decided to go all out.

Next stop was a Crane XR700 optical ignition set up... big improvement, possibly the single biggest. I also added a new rotor button, cap and Top Gun spiral wires. Now I'd cured most starting problems including a persistant "hot start" problem.

Finally on the advice of some friends I sent the distributor away to be "professionally" rebuilt and re-graphed for 2002 fuel. I used Performance Ignitions (Melbourne, Australia). The rebuilt the whole unit, replaced the springs, re-graphed it to 10 degrees BTDC and got rid of the ugly vacuum retard diaphram left over from the '74 engine with a nice blanking plate.

Am I happy? Oh yes!

Distributor $100
Rotor button (6500rpm) $80
Cap $30
Leads $120
Crane XR700 $300
Crane Coil $100
Rebuild/re-graph $150

For a total of $880, I have a perfect electronic ignition for my previously Marelli/points ignition and the driveability/performance is amazing!

Oh, by the way, that's Aussie Half-Dollars to you... $440 USD (a new Bosch distributor works out to be about $1200 Aussie from Pelican) so I've gotten away with this cheap I reckon.

[CamB]

Haha I dodged that bullet by spending {amount cencored} on a new engine for the '69

Good write-up John!

Cam

[Phoenix]

Fishcop:

Have you noticed any improved performane in fuel milage?

I've been considering a similar setup since coverting from CIS to PMOs. The car runs very rich (pitch black exhaust tip) and fuel milage went down from 25 to 12. I've taken it to 2 mechanics who say the PMOs are adjusted just fine.

I'm now thinking it may be the ignition and things might improve if I switched from points to electronic.

[Joeaksa]

John,

You have far too much time to play with 911's on your hands! Now I bet you are going to have orders for the same mod coming in from around the world!

Beautiful mod and nice write up! Everytime I turn around you are doing some mod (H1's) that really works nice.

Whats the next project???

Joe

[RoninLB]

I don't get it!! Fish says "Big improvement"..and so do others who use the optical and Pertronix..am I missing something?? I don't understand the "Big improvement" over properly adjusted good points...I'm still using points so I wonder about this stuff, I'm not as smart as I look





Fish..good thread

Ph....I will have PMOs..and I already replaced the stock dizz w/ 2.7RS dizzy. which PMO used to recommend. I think before they started selling some dizzy stuff..so Pelican has the 2.7RS dizzy/brand new for itty bitty $$$ ...................Ron

[Jim T]

I found a pretty big improvement when I put the Crane optical ignition in my last 914.

I think you are right, in theory it shouldn't make a difference over very fresh, spot on points installed in a brand new distributor. But, very fresh, spot on points are often exist in the land of theory only.

One tangible example in my 914 was that with points, it would almost always take 2 tries to start. Even with fresh points, even after adjusting a few times. But after putting the Crane in, it fired the first time, every time.

Also, points are affected by worn distributor bushings. The optical unit is not (within reason).

Also, points start to wear, changing dwell and timing, the second you start the car.

To me, that was the biggest advantage of the optical ignition. On my 914s, I put it up there as one of the top, must-have mods for a daily driver. It will normally improve the way an old engine (with worn distributor, etc) runs. It is a total "set it and forget it" thing - always in perfect tune. For $100, its a god-send on an old, points-driven daily driver car, IMO.

[CamB]

Easy answer to that Ron (I think)

- John's 911 doesn't have CDI
- Tim's 914 didn't either

And a pertronix or (especially) a Crane should help those cars 'cause there is more voltage for the coil.

At least that is how I understand the difference between them and points and condenser.

[RoninLB]

Originally posted by CamB
Easy answer to that Ron (I think)

- John's 911 doesn't have CDI
- Tim's 914 didn't either

And a pertronix or (especially) a Crane should help those cars 'cause there is more voltage for the coil.

At least that is how I understand the difference between them and points and condenser.

Cam..if so, then I can understand it..but I still don't get it..unless the pitting on non-CDI ign.points is the issue...what da you think guys.........Ron

[Fishcop]

Thanks for the good feedback.

Pheonix, I can't honestly say I've detected a much of a fuel improvement. But I drive my car hard when it comes out of the garage, so there is never a mode of casual/touring to evaluate. Last time I took the time to calculate mileage it came in at about 17mpg.

Joe, although I live in a pretty large country town (by Australian standards) of 130,000; it is bloody hard to find exactly what you want stock off the shelf. We are in cattle country up here, so unless in is Ford/GMH/Toyota I have to do everything myself. Next project is to bend up some sway bars out of scrap metal! No not really... but sway bars are next

Ron, I'm baseing my opinion on the difference between the old Marelli/points set up. The Marelli hadn't had a new rotor or cap for approximately 17 years, so jumping from this unsatisfactory set up to accurate dwell-timing/powerful spark and serviceability has kind of excited me But I'm sure you are right about a well tuned Bosch set up with new points/cap and rotor. The really noticable thing for me was the instant start-up hot or cold. I'd been suffering quite badly with hot-start issues and this seems to have been a remedy.

Note: This is a JWW inspired idea, so I am very grateful to him and others who regularly inspire me to take on these little projects and who have saved me from plungeing head long into expensive mechanic bills (eh Cam ) . My wife hates you all

[Two Oh]

Originally posted by RoninLB
I don't get it!! Fish says "Big improvement"..and so do others who use the optical and Pertronix..am I missing something?? I don't understand the "Big improvement" over properly adjusted good points...I'm still using points so I wonder about this stuff, I'm not as smart as I look





Fish..good thread

Ph....I will have PMOs..and I already replaced the stock dizz w/ 2.7RS dizzy. which PMO used to recommend. I think before they started selling some dizzy stuff..so Pelican has the 2.7RS dizzy/brand new for itty bitty $$$ ...................Ron

+++++++++++++++++++++++++++++++++++++
Ron, The points are constantly in decline. The best performance is available whenever you put a new set of points in. As soon as you start the engine, the contact points on the breaker will start to oxidize, the performance of the switch---and the entire ignition system ---will steadily decline from this point onward. If you keep replacing the points and you keep checking to make certain that the play in the distributor shaft is not excessive, a breaker-point distributor will do a really good "average" job.

The Crane system and the Pertronix Ignitor are the best thing to happen to old distributors since sliced bread.


Fishcop, Excellent!! You should post this over at the early 911 board I'm sure they would appreciate your talent.

[Fishcop]

Two Oh, there's an early 911 board? Where do I go to have a look at that?

Cheers

[Two Oh]

John, I hope these lead you there.
Best Regards 2.0

http://www.early911s.com

http://www.early911sregistry.org/Links.html

http://www.911t.org

Just checked them. All three will lead you to early 911 sites. Navigate around and you will find the message boards/forums.

[Neilk]

John,

There is the Early S registry board. Go here and click on the link for the board. I would give you the direct link, but the URL looks a little strange.

So what exactly was machined. I didn't quiet understand it. Did you have to machine it so that it would fit in the hole?

[Fishcop]

Thanks for the links 2.0, but being just a T, I didn't think I'd be welcome

Neilk, the shaft of the distributor must be machined. There is a difference in the hole diameter between the early engine cases and the later. It's about 2mm, also there is a difference between the O-ring locations (but it is not critical). So if you were to take the route I have and use a 2.7 sourced distributor, you need to give the old Marelli to the machine shop so they can match the Bosch to it. The O-ring groove must also be deepened and you'll need a new smaller O-ring as a result.

Cheers