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Thread: Increasing 3.9 Compression Ratio

  1. #11
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    I have a set of heads from my old 3.5 with the 3 rows of bolts and a set that came with the 3.9 with the 2 rows of bolts.

    I thought I had read somewhere that the later 2 row type heads, like I got with the 3.9, flow better than the 3 row heads.

    Is this correct, I am wondering which set to send off to be machined.

  2. #12
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    Yes ,correct, the later heads flow better.
    1. You've worked out the chamber size differences, re; tin vs composite gasket.
    2. 3.5 heads have 38mm inlets, 3.9 have 40mm.
    3. later inlet valves have 'waisted,' stems, also, slight change in inlet ports.

    On the Bench flow machine at Lloyds engineering, later 3.9 heads flow better by 5%.(flow, not HP)
    Want more?
    Spend a day with die cutter and remove the Primary bottle neck with these heads, ad a further 7% flow.
    That being, open up the valve throat area (seat inserts) from 34mm to 36mm. Blend in the port.
    Triple cut the seats at machine shop.
    On the Dyno at Lloyds, between 20 and 30 horsepower gain with no other mods.
    All charts and info from Lloyds in UK.

  3. #13
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    I have ended up buying a set of 9.35:1 pistons, and I will use the tin head gasket. This should get me very close to 10:1, which I was aiming for. I didn't want to take that much material out of the head then have issues with the valve train. Not the cheapest option but the best I think.

  4. #14
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    Quote Originally Posted by Richard Kaye View Post
    I have ended up buying a set of 9.35:1 pistons, and I will use the tin head gasket. This should get me very close to 10:1, which I was aiming for. I didn't want to take that much material out of the head then have issues with the valve train. Not the cheapest option but the best I think.
    Which is exactly what I did many years ago 🙂
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  5. #15
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    My 9.35:1 pistons have arrived (they are very shiny), the rings are already installed on the pistons. I have also ordered a die grinder and cartridge sanding rolls, some 200mm long feeler gauges to measure the piston clearance and bought an air compressor. I will have a go at cleaning up all the casting marks in the ports myself.

    I am not going to remove the motor until I have all the bits I have ordered, but need to get a valve spring removal tool. The one I am looking at says its suitable for valve spring retainers of 25mm and 30mm diameter. So, has anyone got a head with springs in or a retainer to measure the diameter of it for me?

    Hopefully all the parts will be here in the next week or so, then I can remove the motor and get started on it. It will be interesting to see if cleaning up the ports and the new high compression pistons make much of a difference.

  6. #16
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    if you're going to clean the ports consider enlarging them.

    any gain in port enlargement creates gain in performance , by now someone on this forum could tell you gains to be made.

    when I was "hotting up holdens" a port and polish was the first thing you'd do. sunday arvo job.

    2nd cheap performance gain was headers.

    Just flowing the head(s) would give you about 10-15% better power or fuel economy.

  7. #17
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    If you have the engine out also get the block decked and possibly line bored (if that is a problem.) Also have it balanced.

    I had a 3.5 in Saudi which was bent from heat cycles and needed a line bore, and my 3.9 leaked water from the ends of the the heads even though they were shaved. I put it down to block deck warp.

    This is especially true if you use tin gaskets as both head and block must be perfect for them to seal.

    It's not cheap doing what you are doing , if you want it to be right and last.

    Regards PhilipA
    PS from my experience the 3.9 heads and later 4.0 heads are in some ways worse at gas flow than the late 3.5 heads. I bought new 4.6 heads for my 3.9 and they were worse than the 3.9 heads in the exhaust port area as if the dies had worn . There was quite a ledge in the exhaust port which took quite a bit of work to smooth. Most gains are from the exhaust port area , while the main gain of the 3.9 over the late 3.5 was just the waisted inlet valves.

  8. #18
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    Quote Originally Posted by Richard Kaye View Post
    My 9.35:1 pistons have arrived (they are very shiny), the rings are already installed on the pistons. I have also ordered a die grinder and cartridge sanding rolls, some 200mm long feeler gauges to measure the piston clearance and bought an air compressor. I will have a go at cleaning up all the casting marks in the ports myself.

    I am not going to remove the motor until I have all the bits I have ordered, but need to get a valve spring removal tool. The one I am looking at says its suitable for valve spring retainers of 25mm and 30mm diameter. So, has anyone got a head with springs in or a retainer to measure the diameter of it for me?

    Hopefully all the parts will be here in the next week or so, then I can remove the motor and get started on it. It will be interesting to see if cleaning up the ports and the new high compression pistons make much of a difference.
    G`day 31.7mm , 10 bolt heads have larger ports the valves are the same size 3.5 and 3.9 as far as your 88 is concerned it is only the early 3.5s that have smaller valves, 3.9 tin is thicker than 3.5 tin , 3.9 10 bolt , head bolts are torque to yield best practice not to reuse , 3.9 14 bolt heads use same as 3.5 and are reusable , your 88 3.5 bolts can be used .
    The exhaust valves in your 3.5 should have the same stem wasting as your 3.9 but they will be flat across the valve face so less ccs , 3.9 will have a concave . I have these valves from a 3.5 in 10 bolt heads on a crossbolt 4.0 so know they work the engine is in an 85 using v belts .
    The heads have measurin points on each corner that tell what std is and if they have been machined .
    I used to use tin gaskets but found i had to have the heads off more often than i liked though it was higher comp by removing material from the head faces , enough that the inlet wouldn`t go on std but ther is a formula to work out how much to take off the inlet face on the heads so a std inlet fits and if you match port the heads i`d suggest you also do the inlet manifold . I d also suggest you look into setting the timing using a vacuum gauge .

    Cheers
    PLR or peter r elsewhere

  9. #19
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    To get to 10:1 compression ratio I am considering taking 30 thou off the top of the block so I can use the composite gaskets. To do the calculation I need to know the volume of the dish in the 3.9 9.35:1 pistons, does anyone have that figure. I have assumed for my calculations that it is 18cc.

  10. #20
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    Quote Originally Posted by Richard Kaye View Post
    To get to 10:1 compression ratio I am considering taking 30 thou off the top of the block so I can use the composite gaskets. To do the calculation I need to know the volume of the dish in the 3.9 9.35:1 pistons, does anyone have that figure. I have assumed for my calculations that it is 18cc.
    G`day , calculations are fine but actual measurement may differ .

    Haven`t measured a 3.9 but a 4.0 with 30 off would have the piston 10 above the deck . different mechanicals but something to look at .

    Rover type composite after crush around 2 thou on surafce and 6 on fire ring so running thickness is less .

    An 8.37 4.0 is 17.5cc
    PLR or peter r elsewhere

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