Topic: Over-boring a 903 Fiat engine block

[batman]

Would it by worth the effort., has anyone tried------- and how about fitting larger valves in a 903 head, surly somewhere around 90bhp is achievable with more torque to suit the normal road conditions we drive  today  ( without reving the thing to 7000rpm+every time I need to go to the supermarket)   thanks    Dave.

[Paul vander Heyden]

Dave,

It is possible to overbore the 903 block to 67.2mm, but ONLY if you off-set bore the block to match the A112 70HP bore spacing configuration.

BTW - Getting 90 brake horsepower, at the flywheel, is no inconsequential matter.  Remember that the standard A112 1049cc motor was rated at 70 HP at around 7000 RPM.  What you are after is getting a 23% increase in horsepower over that.  For a small, natutally aspirated motor that is pretty substantial and would equate to 1.43 HP per cubic inch. (for a small block 350CI Chev. that would be 500 HP)  From personal experience many racers with compressions of 12:1, and higher, MIGHT get 90-95 HP at 8000 RPM.  This means special gasoline, better breathing, larger carburetor, AND more than 7000 RPM.   This then introduces a whole new range of cost/reliability issues.

Just remember, as you go up in horsepower, the cost of each additional horsepower is exponential.  Yes you can get 110 HP from a 1080cc A112 motor, but not without going to extraordinary measures.

Paul Vanderheijden
www.scuderiattopolino.com

[batman]

Paul, is there a Fiat engine with a lager displacement that would fit without to many modifications,giving more torque with less revs with not a  dissimilar weight , I've built ,tuned and raced Alfas for over the last 40years,--- but the last time I owned a rear engined 600 Fiat was in 1975, so I'm hoping to have some fun this summer.  Thanks  Dave.

[Pantdino]

For hillclimb cars in Europe some people install the Fiat 128 / X 1/9 SOHC engine.
See the first car shown in my upload to youtube:

http://www.youtube.com/watch?v=nfN-5iEgmeo

I don't know what all is involved in doing this installation-- it looked to me that you have to make some significant alterations in the firewall.
Jim

[viotti600]

It is possible to overbore the 903 block to 67.2mm, but ONLY if you off-set bore the block to match the A112 70HP bore spacing configuration.

 It should also be noted that the best (only?) way to get the correct offset needed for this bore size in the 903 block (without making the cylinder wall thickness/spacing too thin) is to start with an engine block that has the standard 65mm bore.

-JS.

[Paul vander Heyden]

Dave,

Sorry to take so long in answering.  Been out of the loop for a bit.

There is not easy "bolt-up" solution, other than the Autobianchi 1050 block.  Yes, Fiat 128 and even 124 twin-cam motors have been used.  The 600 D gearbox is up to the task, but ready-made adapter plates are hard to come by.  You pretty much have to make one yourself from a thick piece of Aluminum plate.  Of course all of these motors are heavier, particularly the twin cam variety. 

Needless to say there is the " Law of Unintended Consequences" that comes into play, as now there are other areas (brakes, cooling etc) that are impacted.  Then again there are those that like to tinker, yours truly included.

Good luck

Paul Vanderheijden
Scuderia Topolino