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thread: 3VZ-FE Head Skimming & Compression Ratios

  1. #1
    wood_patrick's Avatar
    Sep 2010
    Llanelli, Wales, UK.
    321

    Icon14 3VZ-FE Head Skimming & Compression Ratios

    Taking a set of heads down for skimming next week, but not sure on how much I can safely take off without killing the engine...

    Read through a pile of stuff and I've seen people taking anywhere from 15 to 40 thou off but aren't sure how much I can or should push it really?

  2. #2
    cdwood2010's Avatar
    Sep 2010
    Earls Barton
    6,003
    You're not skimming then to raise compression, you're making sure they're not bent.

    Don't forget this is a non-interference engine, so you don't want to go too mad......

    Anyway get the block decked while you're at it.

    C.

  3. #3
    wood_patrick's Avatar
    Sep 2010
    Llanelli, Wales, UK.
    321
    Well Im getting them done to make sure they are straight but getting extra to increase compression... :-)

    Speaking with AdamH before, for every 0.1mm skim it will increase compression by 0.1

    So, a 0.5 skim will increase compression from 9.6 to 10.1 but a 40 thou skim is almost 1mm :-O which is a lot and I don't know whether that would stop it from being non-interference and/or whether it would cope... I would've thought it would with a tidy gasket set, but I can be a little optimistic.

  4. #4
    snowtigger's Avatar
    Sep 2007
    stockport
    5,475
    Speaking as one who's done the 10 thou and 1 mm route both just do the 10 or unroll there straight, then deck the block (iPhone put deck the bloke must be a fraudian slip) .

    I had no end of problems with 1 mm and oversize pistons last time I was going big bore and upping compression, you get fuel pinking and problems with pistons having to re pocket them so the valves clear and either timing belt/chain issues.

    Just sort the engine so it's straight flow the heads then whap a sc on there or turbo kit when available.

  5. #5
    wood_patrick's Avatar
    Sep 2010
    Llanelli, Wales, UK.
    321
    So how much clearance is there before it becomes an interference engine? (Rather than non-interference)

    I would've thought that if you're staying NA then getting to the point of having to re-pocket is counter-acting the benefit of skimming by enlarging the compression area? Or am I talking poop?

    Saying that I will probably stay within a safe margin of 10-15 thou if more is likely to be an issue.

  6. #6
    headcase's Avatar
    Sep 2010
    east sussex
    589
    You only pocket the pistons if you have a wild cam in . And a big overlap in cam timing say like a 285 degree cam :-)

  7. #7
    wood_patrick's Avatar
    Sep 2010
    Llanelli, Wales, UK.
    321
    Ah. Ok. Thanks. :-)

    I was originally thinking around 20 thou (~0.5mm) but really don't want to have to rebuild the engine if it goes bang within a week. :-/ However, reading up on the bi-metal gasket sets they are about 20-25 thou thinner than a standard head gasket so guessing that would (or *should*) be safe...?

    Might just get it done and suck it and see, at worst I've got spare heads so will just be bit of a pain really.

    Couldn't find much here or elsewhere on skimming so may aswell do a trial run. :-)

  8. #8
    cdwood2010's Avatar
    Sep 2010
    Earls Barton
    6,003
    Do it the old navy way.

    Assemble the engine with some plasticine on the top of a piston and rotate the cams by hand to see where the valves sit.

    Also you don't want to take any extra metal out of the combustion area unless you're looking at forced induction. Just focus on some porting and a bit of a sheen finish.

    C.

  9. #9
    snowtigger's Avatar
    Sep 2007
    stockport
    5,475
    Rock on Patrick I say see how far it goes you could be the first to find the point were the valves bounce off the pistons, as headcase has said on a standard engine none skimmed with high lift cams you some times have to machine the pistons for valve clearance, but skimming both head and block and fitting a thinner gasket IMHO is asking for trouble and 24 bent valves won't be fun you could always dry run it with some plasticine on the pistons to see if they get any were close to the valves.

    I've always gone to the bible of tuning na books like David vizards tuning the a series mini engine , or search na tuning done easy there's loads on the old engines like pinto cross flow cvh and cosworth and vauxhall xb xe series or American v8 tuning.

    Go for it I say and let's see what you can achieve blaze a trail for others to follow they said you couldn't twin turbo a 3vz and look at the bhp on that.

  10. #10

    Jun 2006
    Australia
    505
    Just to add to this, I had both my heads skimmed and block decked and using the thinner ferriday gaskets with no issues
    I always us 98 Ron fuel and never get any pinking even when lugging it, although I do also have ceramic coated pistons if that makes any difference
    Not sure how much was removed in the skim but it all adds up I suppose
    Engine pulls like a truck and I am sure this is in part due to higher compression

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