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Fiat 903cc compression ratio.

Discussion in 'Rear-Engine Fiats' started by ramona300, Aug 26, 2017.

  1. ramona300

    ramona300 Daily Driver

    My 903 in my coupe has the stock pistons with a compression ratio of about 9:1. They are the pistons with the little step, not flat tops. The engine is supercharged with about 9 psi of boost and burns E85 and is managed by a Microsquirt. In my other post you can see the problem I had recently with detonation. I have organized a set of rings to be manufactured but the specialist reckons I need to reduce the compression ratio to at least 8:1 by machining a dish into the piston crown. This I can do easily enough with the spare pistons I have. What I need to know is how much material do I machine off?
    I was thinking of just machining the pistons flat then making a dish and keeping the edge as large as possible. Thoughts?
  2. GregS

    GregS ProjectX

    I did have an 850 coupe back in the 80's, but don't have any experience with engine rebuilds on them. But I reckon you would be best off measuring/calculating the actual volumes and CR you have, then you can work out how much to take off. Don't forget to allow for skimming the head if it needs it, and the compressed thickness of a new head gasket. When I did my last X engine I upgraded to a racing style gasket, but I couldn't get one the same thickness as used previously, so it changed the CR a bit. Keeping the edge flat for high squish sounds like a good idea, but I don't know if it's as important with a boosted engine.
  3. Jeff Stich

    Jeff Stich True Classic

    Norco, CA
    The Abarth OT1000 & Abarth A112 engines have dish-topped style pistons that you can probably use as a "guide" to what works well. I'd start with machining your pistons flat to get rid of the higher-compression "step", then dishing them slightly from the center out to about 6mm or so just before reaching the outer edge, so there's the dish in the center & a flat "ring" around the perimeter.

    For reference, here's a used set of original OT1000 dished-top pistons, where you can sort of see the flat outer "ring" I mentioned (see 2 pistons on the right):


    And here's a birds-eye view as mounted in the block:


    You may want to first machine your pistons to a flat top, then re-calculate (measure) your compression ratio; you may find this is enough to lower that 9:1 down to what you're after. If you still need to go lower, machine just 1 piston with a very (very) shallow dish, then remeasure. Repeat as necessary, then machine the other 3 pistons to match. I have some OT & A112 pistons that I can (try to) measure the dish width/depth of, if you want? (Let me know) ;)

    Oh, & some various 850 compression ratio specs:

    "Standard" Sedan 843cc engine 100G.000 = 8.0:1
    "Super" Sedan 843cc engine 100G.002 = 8.8:1
    Sedan USA 817cc engine 100G3.002 = 8.9:1
    "Special" Sedan 843cc engine 100GB.000 = 9.3:1
    Coupe/Spider 843cc engine 100GC.000/100GS.000 = 9.3:1
    Sport USA 903cc engine 100GBC.040/100GBS.040 = 9.5:1

    **Note that some of the lower c/r's also result from a slightly different combustion chamber shape (depth) between the Sedan cylinder head & the Coupe/Spider/Special cylinder head (the Sedan chamber being deeper). This is why Fiat had 2 different chamber-depth measuring gauges amongst the various OEM 850 "service tools".

    The "Super" 843cc & USA 817cc Sedan engines got their compression bump mainly from the "step-top" type pistons, as they were otherwise a stock "Standard" Sedan engine (small valves, Sedan head & cam, single-barrel 30ICF carb). I've always figured that the .1:1 difference in their c/r's was likely due to the math of 843cc vs. 817cc measurements (rounding).
  4. ramona300

    ramona300 Daily Driver

    Thanks Greg. Phil from BOI is supplying the rings and he suggested the piston modification. He actually wants me to go to forged pistons but that's for the next engine!
  5. ramona300

    ramona300 Daily Driver

    Thanks Jeff. The OT1000 style pistons is what I had in mind. Just a shallow dish and keep the edge as large as possible.

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