903cc (100GBC) distributor advance curve

E

ErnieD

Low Mileage
Hi, does anyone know where I can find the distributor advance curve for the 903cc (100GBC) engine.
Ive got a “123 ignition“ distributor and I’m setting up the curve.
I know that the static advance is 10 degrees and centrifugal advance is 28 degrees
but it would be good to know the steps in between and at what RPM the total advance should be.
I hope someone can help. Thanks
 
1677610593865.png
 
Thanks vwchemist, very much appreciated. Can I ask if these curves ok for the 100GBC engine
and if the figures are engine RPM and advance.
Many thanks again. Cheers, Ernie
 
These are out of the 1968 Fiat 850 shop manual, I'm not sure which year GBC came out. According to the text the plots are degrees of advance vs engine RPM.
 
Hey Ernie. As far as i know the dizzies fit all 3 series, 817, 843 and 903.
As to the advance curve, that would depend on if you gave the middle finger to the man and ripped out all the "pollution control" crap. Thats if you ever had that stuff in Oz. I would want the optimal curve for my set up, knowing that sub optimal might cost, you, say 1/2 hp on these tiny engines. Send a PM to Steve at FiatFactory. He's a regular on this board and located in Oz like yourself.
 
dnudelman said:
Hey Ernie. As far as i know the dizzies fit all 3 series, 817, 843 and 903.
As to the advance curve, that would depend on if you gave the middle finger to the man and ripped out all the "pollution control" crap. Thats if you ever had that stuff in Oz. I would want the optimal curve for my set up, knowing that sub optimal might cost, you, say 1/2 hp on these tiny engines. Send a PM to Steve at FiatFactory. He's a regular on this board and located in Oz like yourself.
Click to expand...
As I recall the shaft lengths are different between the 817/843 and the 903, this can be dealt with going backwards with a spacer on the lower deck height engines but not the other direction obviously.
 
kmead said:
As I recall the shaft lengths are different between the 817/843 and the 903, this can be dealt with going backwards with a spacer on the lower deck height engines but not the other direction obviously.
Click to expand...
All 850 distributor shafts are the same length* from the base of the distributor on down. It's the oil pump/distributor intermediary drive shafts that are different lengths between the 817/843cc engines & the 903cc engine - the 903cc one being slightly longer to make up for the slightly taller block height.

*Excluding distributors used on the Moretti Sportiva 850 & Siata Spring 850, which have an extension on the bottom of the distributor shaft (necessary due to the mechanical tachometer-drive assembly between the distributor body & the engine block).
 
Jeff Stich said:
All 850 distributor shafts are the same length* from the base of the distributor on down. It's the oil pump/distributor intermediary drive shafts that are different lengths between the 817/843cc engines & the 903cc engine - the 903cc one being slightly longer to make up for the slightly taller block height.

*Excluding distributors used on the Moretti Sportiva 850 & Siata Spring 850, which have an extension on the bottom of the distributor shaft (necessary due to the mechanical tachometer-drive assembly between the distributor body & the engine block).
Click to expand...
Hmmm. I know someone bridged a spacer on a 903 to 843. Perhaps they are as lacking in knowledge as I…

Thanks Jeff.
 
kmead said:
Hmmm. I know someone bridged a spacer on a 903 to 843. Perhaps they are as lacking in knowledge as I…
Click to expand...
The only case I can think of a spacer being used is if you're using a 903 oil pump/distributor intermediary drive shaft in an 817/843 block.
 
Info as shown in the official supplement to the factory 850 Shop Manual pertaining to USA-model 850's with the 100GBC 903cc engine:

IMG_20230301_212239615_HDR~2.jpg IMG_20230301_212253607_HDR~2.jpg
 
This is my spark table. This is total timing not just advance. Use the figures 97 MAP and under will give you a good start for an NA engine. I use E85 exclusively but 98 would be fine on an NA engine.
The other method is to use a timing map generator. Just fill in all the squares accurately.
http://www.useasydocs.com/theory/spktable.htm You may need to manually adjust the figures in the idle area once you get to tuning. Just remember that all the hp is in the timing!!!!
Penrose timing..jpg
 
ramona300 said:
This is my spark table. This is total timing not just advance. Use the figures 97 MAP and under will give you a good start for an NA engine. I use E85 exclusively but 98 would be fine on an NA engine.
The other method is to use a timing map generator. Just fill in all the squares accurately.
http://www.useasydocs.com/theory/spktable.htm You may need to manually adjust the figures in the idle area once you get to tuning. Just remember that all the hp is in the timing!!!!View attachment 71057
Click to expand...
Thank you ramona300, absolutely fantastic.
 
You must log in or register to reply here.
Back
Top