Big motor build

Discussion in 'Modified Shizzle' started by daisy1, Apr 17, 2019.

  1. So started dry fitting my new motor together today and need to fit spacers to get the deck height right. Need a total of approx 5.8mm need to check accurately tomos and wondered if it’s best to fit spacers under the barrels to do most of the work and fit copper shims in the heads get it accurate? Doing it this way should get me a better cylinder head seal right?!

    Pics just for fun!!

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  2. Is this the 2.3l? I’d fit cylinder spacers and no head gaskets, but I only needed circa 2.2mm.
  3. With nothing in the cylinder a proud by 2.6mm at top dead centre. So to get the compression right I need a total of 5.6mm


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    Geordie likes this.
  4. 3mm deck height?
  5. [​IMG]

    According to cb performance website

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  6. Or thinking about increasing the chambers in the heads maybe?!

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  7. Deck should be 1.0mm to 1.5mm to have a decent squish, with 3.2mm there will be no squish at all and engine efficiency will be very poor. The problem is your combustion chambers are very small. Just as a comparison my 2316cc type 4 has 96mm bore, 80mm stroke, 61cc combustion chamber and 1.55mm deck, 9.0:1 CR
  8. Yeah might open up the chambers to 60/65cc and get the deck height down

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  9. It would also decrease the engine width; with 5.8mm cylinder spacers there would be a lot of other fit problems - tin, exhaust, pushrods and tubes etc.
    vinnyboy, F_Pantos and daisy1 like this.
  10. Another question, are the compression ratio calculators on the web accurate? Think I’m gona set me deck height to 0.4mm and open my chambers up to 69cc. Don’t want to cut my chambers and find it was the wrong decision

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  11. I’ve not seen one that’s not accurate but you could compare several to confirm. I use this one

    You’ve gone from one extreme to the other for deck. 0.4mm is tight; I’d go for 1.0mm as a minimum (0.040” in old money). You seem to be aiming at 9.1:1 CR so 1.0mm deck would be 65cc combustion chambers. 1.5mm deck is probably more achievable, 62cc chambers – but I don’t know how much you can take out of type 1 heads.

    What cam are you using?
    Last edited: Apr 18, 2019
  12. Yeah, learning as I go mate. After a morning reading up I’m aiming for 1mm deck and open the chambers out to 66cc. This should give me my 9.1 and is achievable with standard shins.

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  13. I edited my earlier post.

    You seem to be aiming at 9.1:1 CR so 1.0mm deck would be 65cc combustion chambers. 1.5mm deck is probably more achievable, 62cc chambers – but I don’t know how much you can take out of type 1 heads.

    What cam are you using?
  14. They’re cb performance 044 heads so should be ok. Going to do that first and get all 4 pots matching the shim the barrels to suit based on the cc’s I get.

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    77 Westy likes this.
  15. You’re learning on a 2332cc type 1 for a bus, brave man. :)
  16. Well learning how to build a non stick motor......what’s the worst that can happen ;)

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    77 Westy likes this.
  17. Out of interest anyone ever dished the pistons? If so how much?

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  18. There are some guys in Ireland who were playing with negative deck height to get a better squish for a turbo motor.
  19. There is a thread on here about compression ratios for buses. That seems too high

    Ill try and find the thread

    This is what John Maher said
    Depending on
    the specification, a modified street engine can safely
    run a higher C.R. than stock but I certainly don't
    recommend going much higher than 8.0:1 for an
    engine that is intended for daily use.
    If you’re tempted to run high C.R. to get that extra bit
    of horsepower, the life and reliability of the engine
    will be shortened. Engine temperatures will soar, the
    heads will crack and the cylinders will distort, creating case pressure and oil leaks. Some people will fit external oil coolers to make them feel more comfortable about the oil temperature, but they're not addressing the root of the problem: excessive combustion chamber temperature.
    C.R. could be reduced by increasing the deck height
    with cylinder spacers (the way most other people do
    it, if they bother to check!) but I prefer to keep the
    deck height on a street engine as close as possible to
    0.060".Running extra deck height will reduce the C.R. (put
    some figures in to the equation and find out) but will
    not give as efficient a burn in the combustion chamber as a close deck height / dished piston combination. As the piston approaches TDC, it compresses
    the air/fuel mixture. The flat top of the piston comes
    within 0.060" of the flat area of the combustion
    chamber (even closer when things have warmed up
    and expanded), referred to as the 'squish area'. This
    forces the air/fuel mixture into the combustion area
    of the head. With a dished piston you still have the
    flat area around the perimeter of the piston to force
    the mixture towards the centre of the combustion
    chamber. In an engine where the C.R. has been
    lowered by merely running extra deck height, pockets of unburnt air/fuel mixture can remain on the
    outskirts of the piston, resulting in less force on the
    piston crown, therefore lees power at the flywheel.
    On larger capacity engines, it isn't possible to achieve
    the desired C.R. by dishing the pistons alone (about
    0.070" is as deep as you can go without any sacrifice
    in reliability). E.g. a 2007cc engine (78mm crank,
    90.5mm pistons), with 0.060" deck, 51cc heads and a
    0.070" x 70mm dish in the piston will have a C.R of
    8.5:1. To extract maximum performance, deck must
    remain at 0.060". If we want to lower the C.R. to
    8.0:1, we need more volume in the combustion chamber. Great care must be taken not to remove material from areas that will affect the flow performance of
    the head. If the chambers are modified to give a
    volume of 56ccs, this will give a C.R. of 8.0:1. On an
    engine equipped with 40mm x 35.5mm valves, Engle 120 cam and a pair of 44mm or 45 mm carbs, this C.R. will give excellent performance, with no detonation or cooling problems. Providing the components are of good quality and the correct assembly
    techniques have been followed, there is no reason why a combination like this shouldn't last 80 to
    100,000 miles.

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