Which engine spec to rebuild to?

Discussion in 'Mech Tech' started by Pathfinder, May 28, 2015.

  1. Halfway then - 8.5:1 ? Or maybe I'll leave it, I've almost decided I'll keep this one as a thrasher and build a big sensible "touring" engine as well. :D
    holmsen likes this.
  2. You can run street engines at a higher comp than that...

    As Holmsen says, that data is over 20yo and fuel composition has changed a lot... Normal unleaded is now 10% ethanol, and that will lower the running temperature of the engine... It'll almost be like running water injection on a turbo engine....
  3. @zed Your engine(s), your choice.

    @zed @Paul Weeding @holmsen I'm not technical at all, i was asked to find out the CR of my engine so asked the engine builder who knows his s h i t . He doesn't go over 7.9:1. - I passed on his comments as I thought it would be helpful.

    John Maher Racing has produced a Guide to performance engines he has lots of info on compression ratio unleaded fuel etc. It says he doesn't recommend going much over 8.0:1. @zed have a read of the JMR CD I sent you, it has loads of info that may be helpful when building your next engine.

    I'll post a few JMR bits below about compression ratio and unleaded fuel
  4. I hope JMR don't mind me posting this, his £12 CD is well worth it for people thnking of modifying their engine, COMPRESSION RATIO
    The most common cause of engine failure (apart
    from incorrect assembly or poor parts quality) is
    using too high a compression ratio.
    Most people don't
    even know what compression ratio their engine is

    The other main consideration is the octane
    rating of the fuel. If your cylinder heads have the
    appropriate seats and valves for unleaded fuel, you
    have to be certain that the octane rating of the fuel
    you use can support the compression ratio of the
    All my street engines can run on unleaded
    The higher the octane rating of the fuel, the more
    resistant it is to detonation. The higher the compression ratio, the greater the heat level generated in the combustion chamber. If this heat level gets to a
    critical point, it will ignite the incoming air/fuel
    mixture before the spark plug ignites. The explosion
    will try to push the piston back down the cylinder
    while it's still on it's way up. This is an extremely
    efficient way of destroying parts. It's commonly referred to as 'detonation'
    Many factors will determine what the C.R. of a
    particular engine should be set at.
    For starters, an air
    cooled engine will usually have a lower compression
    ratio than it's water cooled counterpart. Combustion
    chamber temperatures in an air cooled engine are
    higher than those in a water cooled engine, so attempting to run a C.R. of 10:1 on a mildly tuned
    street engine because your friend's factory Ford does,
    is not a good idea.

    Higher C.R.s can only be run if a fuel with a suitably
    higher octane rating is available.
    In all out drag race
    engines, C.R.s of up to 15:1 are used but they are run
    on super high octane racing fuels (114 to 117 octane)
    and are only run for a couple of minutes at a time;
    while they are on full throttle, under load for a matter
    of seconds.
    When building a stock engine, I set the C.R. at 7:1.
    This takes a little more work, but gives the customer
    an engine that runs cooler and will last longer.Engines using modified heads, performance cams,
    improved carburetion etc. breathe better than stock
    and will help the heads to run cooler. 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.
    zed likes this.
  5. @paulcalf
    He-he - I am listening Paul, you know I like your engine and you know I want to be able to cruise faster or I wouldn't keep asking you to bug your builder. I'm trying to get in the right frame of mind for a rebuild. I want a cam with less duration too - this one doesn't really let the engine run properly below 3000 rpm. I see yours has 11 degs less than mine, that's a big difference.
    paulcalf likes this.
  6. And so it goes on. Calculated v dynamic CR. @paulcalf
    Your 8:1 figure is dynamic and can be more or less the same as calculated with a shorter duration cam (like yours). The longer the duration, the more lowering effect the cam has on the dynamic CR, so it's possible my dynamic CR is nearer yours than I thought. I'll see if I can find a calculator later.

    Then just when you thought you have it cracked, your engine's volumetric efficiency (or lack of in my case probably) further reduces the CR as the pots don't completely fill. Bugger!
    paulcalf likes this.
  7. That is John Maher saying 8:1.
    The bloke who built mine said 7.9:1

    I haven't got a clue about any of the above or on engine building full stop, hence why I got someone to build mine.

    Just sharing my experiences and anything I've seen that may help those who actually know what they are doing.

    Remember It was only last September, that I first changed my own oil and filter and adjusted my valves after 17 years of bay window ownership!
    Weasel likes this.

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