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Thread: 7AGE (16v or 20v)

  1. #21
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    A standard 7afe bottom end will last as long as you keep to a sensible rev limit (I stick to 6000 in my 7AGE, has never looked like failing)

  2. #22
    Senior Member shelldrake's Avatar
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    Any more info on this build?

    Does anyone know the piston to valve clearance of using a 20v head with standard cams? If one was to build a bullet proof 7a bottom end, would you use forged 7a pistons? Or forged 20v pistons?

    Are the pins and pin heights the same?

    Has anyone worked out comp ratios with those combinations?

    Thinking about the possibilties...

  3. #23

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    never had clearance issues with my sump too cross member either.
    i did figure out copression ratios but with the 16v head.
    can't rember what they were though, will see if i can't dig up my old info.
    i know that the stock 7a pistions bring the compression down too around 8.5:1 from memory, just right for turbocharging.
    but be warned, they are not very strong.
    rods are weak due too the factr they are fitted with rod bolts, rather than studs and nuts, and where the thread is cut into the rods, there is a thin bit with a sharp edge.
    having said that, ive never actually heard of them breaking.
    i did alot of measureing and found it would be very difficult too fit a 7a crank and rods into the 4a block.
    and even if you could, con rod angle becomes unacceptable , whcih further loads already weak conrods.
    the crank is a pretty sturdy unit though, but not nearly as good as a 4age one.
    if i were too go this path again, id save up my dollars and buy some better rods, and do some serious crank work too enable it too rev like a 4age safley.

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    Unortunately nothing has really been happening with the engine, I have been concentrating on bodywork and suspension. I need to get a differen alternator, and 20v alternator brackets. I havent really been looking around, but I don't think they will come cheaply.

    I was planning on using 20v pistons, low compression ones though.

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    Senior Member shelldrake's Avatar
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    <div class='quotetop'>QUOTE (starni_boy @ Jun 11 2008, 08:07 AM) </div>Mmmmk... If I was to go this path, I would be aiming for 11:1-11.5:1, But I'm not sure if this is achievable with stock silvertop pistons. If anyone has the figures to save me the trouble, it would be great....

    Crank work = $$$

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    I am aiming for stock 20v silvertop compression, maybe 0.5 above that. Stock compression is 10:1 for a silvertop, so might go up to 10.5:1 CR.

    Looking at the ACL 8.5:1 forgies, which should give me a decent ratio. If not, I will skim the head and put a TRD HG on.

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    Veteran slydar's Avatar
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    i have the bracket you want.

    also, are you building the engine NA? if so the extra compression should be welcomed. 7a/4a's are a short rod ratio engine, they are very knock resistant.
    78's

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    Senior Member shelldrake's Avatar
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    <div class='quotetop'>QUOTE (slydar @ Jun 13 2008, 07:40 AM) </div>So does anyone know approx comp ratios for building one with stock 20v pistons (bt or st), (standard or trd head gasket)??

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    well the cr increases a little bit with the extra displacement. stock st are 10.5 and bt is 11. there was some discussion on it in the ae86 only section. i think it was about half a point extra. basically a bonus the way i see it.
    78's

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    7a block with a silvertop head and pistons: 11.5:1
    7a block with a blacktop head and pistons: 12.1:1

    7a block with a blacktop head and silver pistons: 10.9:1 (have to double check my calculations on this one though)

    I planning on doing an article next that has a big chart for various combinations of engine parts with the reseulting theoretical compression ratio.
    30kw club

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