Agg build, pb head

Discussion in '8-valve' started by Adam8v, Sep 4, 2021.

  1. Adam8v New Member

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    Hello chaps been away for a while with family life, so the Mk2 has been shelved for a while.
    Anyway back to the job in hand..
    I have an agg block, with the intention of getting my pb head ported, polished and bigger exhaust valves.
    Can I just slap this together and run it on the standard digi set up and still get good power gains?
    Also I was looking into problems with compression ratios with different cams, is this something that will need addressing?
    Skimming the head etc.
    I was under the impression it was bolt head and block together and off you go.
    Could it really be that simple!?
    I've seen a few threads about lowering compression ratios to accommodate different cams, so that's thrown me a bit.
    Also can anyone suggest a decent cam, I am looking to go quite lumpy, as its not a daily driver.
    Lastly, is there any benefit to using the agg injectors over the pb injectors, just curious on that one.
    And also would a higher pressure fuel regulator be a good Move?
    Any replies would be much appreciated
    Cheers
    Adam
     
  2. Tristan

    Tristan Paid Member Paid Member

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    I built a similar engine. AGG bottom end and pb head.
    Stock AGG pistons drop the compression a bit. You need higher comp ones to get the benefits of the conversion, especially if you fit a lumpier cam.

    Mines got a tsr cam and head, 4 branch, slightly skimmed pistons, 4 branch, lightweight flywheel.... And... It's ok. Tbh not the big improvement I was expecting. But it's still a nice road engine.
    Doing it again, I'd fit forged pistons and the Omex I have going spare.









    Or a 16v [:D]
     
  3. mr hillclimber Club GTI Supporter and Sponsor

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    The head and block will bolt together, but if you want more performance there's a bit more to it.

    The std basic digi set up is ok to a point if set up properly (there are some good guides here for that).
    However, it won't tolerate really lumpy cams...much over 276 degrees...and still keep a decent idle and nice road characteristics. In fact 276 is lumpy but drivable.

    The complete opposite is needed though in terms of c.r.. you need more of it not less for a performance cam, but the exact cam dictates the need to get the best out of it.

    As for the head, there is zero need to use a bigger exhaust valve unless there's a bigger inlet valve used, and they're no longer available in std fitment stem sizes currently unless a big batch is commissioned.

    Std valves are more than good enough for 150hp if the porting is right (NO polishing!) in road trim...I've seen 180 to 200 in race trim on std size exhaust valve size...as you won't see over 150 without a more radical set up that the digi won't cope with.

    Std PB injectors are ok at the power level and spec your looking at but yes an adjustable/higher pressure reg will help.

    For a nice road use engine around 145hp with strong torque and a cam around 270-272 would be a good set up.
     
  4. rubjonny

    rubjonny Administrator Staff Member Admin

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    main thing you'll need apart from whats already been mentioned is a 2e distributor, or see if you can swap the PB dizzy trigger wheel into the AGG dizzy. plus remove the 2 locking pins from the base of the AGG dizzy so you can adjust it when time comes to set ignition timing

    edit: or you can get a dizzy adaptor ring from TSR so the old PB dizzy will fit, need to swap the 2.0 dizzy gear over
     
  5. Tristan

    Tristan Paid Member Paid Member

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    As usual, John above is spot on.
    The adapter ring is a relatively straightforward item to get turned up on s lathe if you know anyone with a machine shop.
     
  6. Adam8v New Member

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    Guys,
    Thank you so much for your help and advice,
    I'm re-living my youth with this Mk2 as I had one when I was 17 and loved it.

    I'm an engineer by trade and rebuilt various engines, PD diesels being one of them,
    So I'm quite happy to have a crack at all of it apart from the machining of pistons and anything that requires tight tolerances.

    Mr Hillclimber thank you for the detailed advice, very much appreciate, could you suggest anyone that would take on the work of altering the C.R, for a particular cam?
    I'm not going for anything competition or silly power, just something more punchy than the PB, that keeps up with modern traffic.
    ABF is a nice idea, but I've always been a fan of the 8v and like the more original look.

    Rubjonny.
    Thank you, I've read a lot of your posts and pointed me in the right direction for many things over the years!.
    I'm going to use the 1.8 dizzy with an adapter plate, just to save sourcing one from a 2E.
     
  7. Tristan

    Tristan Paid Member Paid Member

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    Sorry, I don't have dimensions, but they're easily measured. The top "collar" is as thin as it can be, around 1.0mm iirc, and the 1.8 dizzy worked 110% for me, for many thousands of miles, including several track days.
     

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  8. rubjonny

    rubjonny Administrator Staff Member Admin

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    surely working at 110% would be bad, too many sparks per rotation :p
     
  9. Tristan

    Tristan Paid Member Paid Member

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    Haha clever! :p
     
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  10. NateS2

    NateS2 Paid Member Paid Member

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    Not really related to this build but I thought it was interesting what @mr hillclimber said about idle on lumpy cams. I'm still slowly working on Digifant development and I wonder if fixing idle may be a somewhat similar affair. Digi has fast and slow idle control loops, fast being controlled by the timing processor and slow being controlled by the fuel processor and ISV. The ISV is really only meant to get engine into the vaguely right area and adjust for very high load changes. So I wonder if the fluctuations caused by big cams are large enough to get the ISV adjusting, but its too slow and just causes the oscillations to get worse.

    If that is what’s happening it would be solvable but adjusting the fast idle control loop for the bigger cams I think.
     
  11. Tristan

    Tristan Paid Member Paid Member

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    I suppose it depends on what you call a "big" cam, realistically if you start running a 300 or similar, you'd be far better off without Digifant.
     
  12. mr hillclimber Club GTI Supporter and Sponsor

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    Good call Adam, a MK2 8v does a great job as a 'perky' road car, no need at all for a 16v for that use.
    My old 2ltr 8v road car with 150hp was as lively as you'd ever need, and an ABF needed working very hard to stay in touch.

    I can help you with the C.R needed for any given cam, it's timing, and application. There are a few ways to achieve the same end goal, some being slightly more efficient than others, but it depends on the work you fancy putting into it.
    Same with the head work. I've flowbench developed these over many years now, and it's not just bigger holes that do the job, and certainly not shiney port finishes. Walk away from anyone trying to sell you a head on that bases.
     
  13. mr hillclimber Club GTI Supporter and Sponsor

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    There could certainly be something in your thinking, and some adjustments could help. But in general, bigger overlap cams and shared plenum type manifolds don't mix well due to back pulsing and charge robbing...the more radical the cam, the harder it is to control.
    In addition to that, big cam duration & overlap isn't as desirable or even needed on mild type road engines anyway for lots of reasons not relating to fuel control.
    Could be worth looking at in mild applications, but you'll tie yourself in knots trying to get a plenum fed engine to run smooth with a big cam. It will be like trying to answer a question nobody is asking.
     
  14. mr hillclimber Club GTI Supporter and Sponsor

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    Far better off Tris binning digi or kjet after 280 degrees. In overall terms, bigger cams just compress the power band higher with plenums still in use. You can end up with a similar peak number but needing more revs to achieve it, and loose low to midrange performance to get there. Far better off overall with the right cam for the application and use than trying to drive around a compromise, and a disappointing one at that.

    Totally different ball game on open induction where race engines with 320 degrees and more can idle fairly smooth and drive surprisingly well lower down.
     

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