These transmissions however were superseded in 2007 with ones having slightly different gear ratios and a few improvements to counter noise complaints, resulting in a surplus of unused F40’s. Sometime in 2008 these transmissions started appearing for sale through a number of online sites for approximately $400 - $500, an incredible deal considering they were brand new, and worth between $4000 - $5000 in 2007. I bought mine for a grand total of $666 including shipping, duties and taxes.
The next couple posts will be about how I modified a manual transmission to fit to the Northstar engine. Northstar engines were only ever mated to automatic transmissions from the factory. Of the few people that have installed Northstars into Fieros, some have transplanted the whole Cadillac power plant into their engine bays, but a sports car with a big heavy automatic simply didn't appeal to me.
The engine will be installed in a transverse configuration simply because there isn't enough room to install it longitudinally. That meant I needed to find a stout transaxle. Luckily a couple years ago I stumbled across the perfect manual transmission conversion for my Northstar. In 2006, Pontiac offered a new high-powered version of the G6 sport sedan called the GTP. An optional six-speed manual transmission (fabricated by GME in Germany) known as the F40 came with the following gear ratios: 3.77 / 2.04 / 1.32 / 0.95 / 0.76 / 0.62 / Rev 3.54 with an axle ratio of 3.55. This gave a final drive ratio (in top gear) of 2.51:1. The torque capacity was officially rated at 295 lbft (or 400 Newton-meters, hence the F40 designation).
The nice thing about the F40 and the Cadillac Northstar is that they have nearly compatible bell housing configurations. The major differences are the starter locations, a slight interference with the Northstar’s rear mounted water log (coolant manifold), and one bolt hole that doesn’t line up with the rest. What follows are the modifications needed to get the engine to mate with the F40.
First, I made a template of the engine's bell housing flange to see where it would interfere when placed on the transmission. I marked a circular pattern where the starter protruded and cut out a triangular notch where there was interference with the water log on the left of the template.
Once the hole was cut out of the template for the starter, I marked where the transmission webs would need to be notched to clear the starter:
The next step was what I thought would be the easy part: cutting out the notch. Ha! That aluminium is hard! I used a high speed burr on my die grinder but it galled up regularly with aluminium:
Here is the nicely finished notch once I was satisfied the profile was right and after I cleaned the edges with a 2” sanding drum:
Next on the “to-do” list was to cut the triangular wedge out of the bell housing to clear the water manifold at the back of the engine. I simply traced the outline from the template I made earlier onto the bell housing. Then, to make a nicer radius at the corner where the two cuts meet, I first drilled a hole. I used a cutoff wheel on my die grinder to make the cuts and used a file and smoothed out the freshly cut surfaces:
The last two modifications involved bolt holes A and B shown in this photo:
Hole A is simply an unthreaded hole with an alignment dowel pressed into it, so it couldn’t be used as-is to secure the transmission. And hole B didn't line up with corresponding hole in the transmission bell housing.
Then I tapped the engine block where the old dowel was...
... and screwed the new threaded alignment dowel into the block. That fixed one problem.
To address hole B, I fabricated an offset adapter to pick up the misaligned bolt holes. Here it is after welding it up:
And here it is installed. The upper bolt threads into the engine block and the lower one into the transmission:
The F40 weighs 123.5 lbs wet, according to the internet, so it was awkward but manageable to install by myself:
Of course the transmission is only mocked up since it still needs a flywheel and clutch assembly.