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Thread: compression ratio

  1. #1
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    compression ratio

    hey guys can any one help me work out what my compression ratio should be? I have a stroked sr20det the pistons are forged but regular size the crank makes the engine a 2.2 it also has a 2mm hks head gasket

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    I <3 BBS LM Actual_Ben_Taylor's Avatar
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    Depends on the pistons, could be anything from 8:1 to 9:1

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    I found out that the pistons are 8.5:1 and the crank is 92 mm stroke surely the crank has an effect on compression?

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    If the piston crowns protrude to stock deck height and with a standard thickness HG, the extra 6mm of stroke will add around 7% to the CR (over a stock crank, as there is more volume squeezed into the same space). But assuming the crank, rods and pistons were bought as a kit, the stated CR for the kit should be correct as all of this will be taken into account.

    *just noticed you have a 2mm head gasket, this will pretty much exactly offset the extra CR gained from the stroke*
    Last edited by hooky; 12-09-2017 at 20:29.

  5. #5
    Guest zeppelin101's Avatar
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    When you say "the pistons are 8.5:1" - under what assumptions? You would need to know the head volume, swept volume and head gasket volume the designer assumed when putting them together. In Small Block Chevy land the pistons are labelled by the dish or dome cc to calculate CR, it seems to be something unique to Japanese tuner engines where the CR is already specified with nothing attached to it

    If it's 8.5:1 for an otherwise stock engine (i.e. stock heads, stock HG thickness / diameter, stock bore / stroke) then the piston is 11.2 cc dish.

    Swept volume with 92mm crank and 86mm pistons is 534.4cc.

    Standard head volume is ~46.1cc (you really ought to measure though as there is some variation with them being a cast part).

    A 2mm head gasket (presume 87mm bore) is 11.9cc.

    Have you measured / can you measure the deck clearance when the piston is at TDC? i.e. how far the piston sits down the bore when the piston is at TDC (or if its above!). We'll assume it's a 0 deck block for now.

    Compression ratio therefore is 8.72:1.

    If there is a 0.5mm deck (i.e. piston is 0.5mm down the bore when at TDC which I think is the intended stock position) then this would be 8.41:1.

    Ultimately you need to know the actual cc of the piston, head and deck clearance to calculate it properly.
    Last edited by zeppelin101; 13-09-2017 at 06:27.

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    thank you guys for the info
    Mr zeppelin 101 no assumptions I found the paperwork for the kit and it said 8.5:1 it has forged wiesco pistons, also has eagle connecting rods, the stroked crank and 2 mm gasket. As for the head the one on the engine is stock but I have built a new head to go on with uprated cams springs retainers rocker arms ras new hg etc so I cant measure the deck clearence atm but when I fit the new head on I will

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    I think what he means is those pistons are 8.5:1 if you have a stock motor but if you put a 2mm head gasket on its no longer 8.5:1 it's lower

    Pistons can't have a compression ratio that comes from the whole build

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    Double post

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