Austin 850 performance conversion

[abri]

Thanks, I've had the gasket for a few years. I've had the plan of doing this conversion for a long time but other priorities have always got in the way.

Anyway, back to the build...

So to illustrate the problem I had with the carb choke levers clearing the bulkhead cross member, below are two photos taken from more or less the same angle. The top one shows the carbs on their original Sprite manifold test fitted to a head on a bench.


The second one shows the same carbs and the same head but with the Downton manifold fitted. Notice the angle of the measuring tape and the distance between it and the bottom of the float bowl.


I test fitted this in the engine bay and the choke lever arms cleared the bulkhead cross member - only just!! But I was very happy with this because it meant I could actually use the manifold. And I was chuffed at having a 60s tuning company part on my car

So with the carbs rebuilt and fitted to the manifold, everything was test fitted to the loose head to make sure it goes together well


This exhaust manifold is from an MG1100 I believe. As far as I know they were the same as the Cooper 3-1 manifolds in terms of the diameter of the pipes. The Cooper S manifold that looks very similar has thicker diameter pipes (correct me if I have that wrong). The downpipe is certainly thicker on the S manifold. On this one you can see the weld where I had a downpipe welded on in the same shape as the Cooper manifold. This pipe is 38mm diameter on mine. 

The next challenge was what to do for the air filter system. I know most people fit the wire mesh "pancake" filters to these carbs. That's what came on the Sprite that was fitted with these carbs as far as I know. However, Vizard suggests (and other experienced people I spoke to agree) that the pancake filters are not the greatest when it comes to flow. Vizard found that K&N filters flow better than just about anything and continue to perform even when covered in dirt. My aim was performance but also proper filtering since my car is used as a daily and gets its fair share of dust and sand blown around the engine bay. So I had my mind set on a K&N filter as the ideal. I could fit two round filters or one oval one to cover both carbs. The issue was SPACE!

This setup from Vizard's book stuck in my mind and gave me the idea for mine



So I went to Masterparts and after looking at quite a few options I ended up with this



The next task was to make up a filter box to fit this to the carbs. Luckily a friend with access to CAD tools, a supply of aluminium and people who can cut and bend it came to the rescue. Before that though, he assisted me with ideas, drawings and measurements and was kind enough to have a template drawn up and cut out which I could test fit and measure for any adjustments before the final version was made (thanks Shawn!!)
 
Here is the template being test fitted: 


That was very close but with the filter in place I could see that it was not going to work without some afjustments



With the filter in place, the back plate would have to move a bit upward hold the filter in a position where it would fit. The measurements were taken, the adjustments made and the final version looked like this:


The two plates are bent to keep the filter in place at the top and bottom. Four stainless steel bolts hold the two plates together tightly on the filter. The bolts are positioned to keep the filter from moving out of position. 

The filter setup as it is, is not ideal for flow because the angle into the carb throats is basically 90 degrees. SU carbs like a ram tube that creates a venturi/funneling effect into the throat - that's why BMC/Leyland put elbows on them between the air filter box and the carb throttle body. Vizard also conclusively shows in his book flow bench results of different ram tube shapes and how much better the carbs perform with them. 


On my to-do list for this project then is having some stub stacks/short ram tubes made up that can fit inside my filter box without being too close to the cover plate. 

More updates to follow when I've uploaded more recent pics to Flickr