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Choice of compression ratio AGU

Metal Nov 15, 2020

  1. Metal

    Metal New Member

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    Hello,
    with compression ratio are you running you engines?
    I have A3 1.8t AGU with stock CR 9.5:1 which i am currently rebuilding as one of the rods have bend.
    Currently i have chinese hybrid T03/T04e turbo at 1.2 bar getting 325 bhp and i plan to use it until it broke.
    My ecu is 3.8.2 and i don't plan to use aftermarket ECU.
    The idea for the next turbo is GT2871R or G25-550.
    1. I have to select Pistons and i'm in doubt what CR to select? 8.5:1, 9.25 or 9.5?
    2. What size of injectors i would need for GT2871R or G25-550? Currently i have 440cc
    3. What size of intercooler would i need for GT2871R or G25-550?
     
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  3. S3AMJ

    S3AMJ OEM+

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    I would recommend the following bud...

    Others can chip in.

    You will be better off with an aftermarket ECU - Ignitron (better protection, easier to set up, map and monitor)

    Compression wise 9:25.1 or 8:5

    You’ll want 1000cc Bosch injectors @4 bar, gives you plenty of head room.

    Toyosport type B Intercooler or similar, don’t go Airtec

    Go G25-550, don’t bother with the GT28 that turbo is kind of obsolete now.

    Hope that helps.


    Sent from my iPhone using Tapatalk
     
    Last edited: Nov 15, 2020
  4. <tuffty/>

    <tuffty/> Badger 5 Edition...Its all about the flow... Staff Member Moderator

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    Don't bother with 8.5:1... they will make the engine too unresponsive off boost... 9.25:1 is fine

    GT28 isn't in the same league as the G25 turbo so no real comparison there... biggest issue you will have is tuning it at all on M3.8.2... you will no doubt be using a MAF which is pretty pointless... not even sure the injector drivers are that good so more modern injectors may be problematic

    At the very least you should look at DTA/Emerald for that level of tune... although the better option is to convert to wideband/ME7/DBW operation and use an Ignitron

    <tuffty/>
     
  5. Metal

    Metal New Member

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    Thank you for your help.
    JE Pistons 9.25 are here and the block is at the machine shop. I will use standard Elring head gasket which by my calculations will lower the compression to 9.14.
    I am changing my mind and i plan to migrate to 7.5 and use Ignitron. I have watched all of the videos in youtube about it. It's awesome!

    The question is how hard it is to migrate to 7.5? I need to remove the dashboard and i understand i need the following parts.
    1. Engine loom
    2. DBW trotthle body
    3. Ignitron
    4. Engine loom
    5. lambda loom (I think it's separate)
    6. Lambda sensors
    7. Throttle pedal
    8. MAF
    9. Knock sensors
    10. Coilpacks


    I don't need:
    1. ECU 7.5

    Is that all?
     
  6. <tuffty/>

    <tuffty/> Badger 5 Edition...Its all about the flow... Staff Member Moderator

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    Not a conversion I have personally done so cannot comment from experience... you might need the dash from a later car ME7 based car (or ME7.5 preferably)...

    You will only need the one (wideband) lambda... you can also add VVT and EGT (both std on ME7.5 S3's)... VVT will improve spool of bigger framed turbos and EGT is a good thing to have for protection

    Bin off the MAF... no need to use that ever with Ignitron... speed density is where its at with that... you will need a suitable MAP sensor though and needs to be fitted into the inlet manifold to work for speed density... some peeps weld a boss on but I used the centre PCV outlet on the std inlet to feed the MAP sensor... I think there are pics in my thread on that

    The stated CR of the JE's should take the head gasket and head chamber volume into account so 9.25:1 should be 9.25:1... I would use lower CR than that realistically though as there is no real need to... I ran 9.25:1, std head gasket, ARP head bolts (which you should consider or ARP studs) at 32psi/620hp... the only thing that let me down was the H beam rods which aren't rated to 700hp like IE tell you lol

    <tuffty/>
     
  7. Metal

    Metal New Member

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    I have read your topic, it is really unfortunate that this rods failed...Also a lot of good information there

    I've made a lot of research and for standard stroke pistons
    1. JE list CR's with head gasket 0.9mm
    2. Carillo CP list CR's with head gasket 1.22mm
    3. Wiseco list CR's with head gasket 1.67mm

    Also i have:
    1. Manual tensioner kit + gates blue belt
    2. ARP head stud kit
    3. Pro Race I-beam rods
     
  8. desertstorm

    desertstorm Moderator Staff Member Moderator VCDS Map User

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    The problem with tuning engines is usually the result is only as good as the weakest part. In your case sticking with the 3.8.2 ECU would not be good at all. Going with an Ignitron will mean you will get a lot more out of the car. The base wizard maps work very well and get the car up and running really quick. With the excellent logging facilities you can tune the car yourself or provide a lot of information to a remote tuner.
    I was running 9.5:1 Wossners before one failed and wiped the engine out. Rebuilt the engine with the JE 9.25 pistons using Hurricane I beam rods which are the same as the PRE rods you have. Very happy with it .I am able to run a little more timing with the JE pistons due to the lower CR. The engine is still perky off boost which is what you lose going 8.5:1 CR.
     
  9. Metal

    Metal New Member

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    Thanks guys. I will change the ECU with an aftermarket one. Currently i have the following issue. I machine bored the cylinder to 82.07mm, my pistons are 81.97mm, per JE instruction i need 10mm/4 thou/ piston to wall clearance.
    The second compression rings are with a gap of 0.6mm and one is 0.65mm. The spec is 0.45-0.5mm.
    I didn't cut them at all. I am wondering if leaving them like this would be fine? Anyone with similar experience?
     
  10. desertstorm

    desertstorm Moderator Staff Member Moderator VCDS Map User

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    Ring end gap is determined by what you intend to do with the engine. The more aggressive performace you want the bigger the gap, add nitrous and you end up with a bigger gap still. I gapped my rings at 20 thou top ring and 22 thou second ring . Which is 0.5mm and 0.55mm .
    Having a slightly larger gap than a smaller gap is preferable. I have seen a few builds that run quite a bit more gap on the second ring than the top ring.
    The reasoning behind that is the ring seals by pressure differential between the top and underside. This is why there is a groove machined in the piston between the top and second ring. It increases the volume of space the blowby has to occupy so reduces pressure. Having a larger gap on the second ring any pressure to bleed away more easily.
     

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