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Thread: 7AFE Conrods, When Do They Bend or Break?

  1. #1
    Non qualified Domestic Engineer
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    Default 7AFE Conrods, When Do They Bend or Break?

    With a dead, ancient, SC12 in my AW11, I've been entertaining the idea of using a 7AFE engine to give me some extra capacity and just going for a small turbo (TD04) to try and retain some of the low RPM torque characteristics of the positive displacement supercharger equipped 4AGZE.

    From what I've found on the net, the 7A rods are considered to be pretty weak. Just as wide as the 4A rods, but not as thick as the 4A rods. They are also quite longer than the 4A rods.
    So does anybody have any genuine idea of the power limits of the 7A rods when the engine is leant on with some boost pressure?
    Obviously keeping knock under control is a prerequisite, and that will be straight forward enough, but rather than wreck a block and a crank with a flailing, broken rod, what would be a safe number to stay below?

    My target is a nice and tractable 160ishKW at the wheels. Believable on factory rods?
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    Junior Member Too Much Toyota dnegative's Avatar
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    Default Re: 7AFE Conrods, When Do They Bend or Break?

    Stock rods break before a 7AGE reaches the rev limit of a 4AGE.......... throw them away and buy aftermarket ones.

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    Default Re: 7AFE Conrods, When Do They Bend or Break?

    Do you have any information that is more specific about rod failure other than "Stock rods break before a 7AGE reaches the rev limit of a 4AGE.......... throw them away and buy aftermarket ones."?
    I'm not after revs, I'm after torque. That means I'm after combustion pressure(force).
    With a rev limit between 6500 and 7000rpm, can the rods take the force of enough combustion pressure to make my target 160RWKW?
    Quote Originally Posted by oldcorollas
    except for a very few exceptions
    "Don't worry what people think, they don't do it very often."

    Daily: Glorified Taxi (F6 Typhoon). Out Of Action: Twin-charged Adub. Ongoing Nightmare: Over re-engineered (not) Alfa Romeo 75.

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    Junior Member Too Much Toyota dnegative's Avatar
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    Default Re: 7AFE Conrods, When Do They Bend or Break?

    Nothing technical, no, but in my mind if they cant handle an extra 1000rpm reliably without failing catastrophically then I would think the chances of them withstanding over twice their "rated" power would be even slimmer.

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    Senior ****** Carport Converter Sam_Q's Avatar
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    Default Re: 7AFE Conrods, When Do They Bend or Break?

    it' revs not boost that kills these rods, they do seem to be stronger that what people give them credit for. I can't see any issues if you have a 7000 rpm rev limit

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    Default Re: 7AFE Conrods, When Do They Bend or Break?

    Hmmmmmmm, 2 conflicting opinions, I didn't expect that

    If I found out that the 7AFE used snap ring retained gudgeon pins, I'll just get a set of Spool rods and put standard pistons in there and go from there. If the pistons use press fit gudgeon pins, I might lower my target (rather than just pointlessly breaking an engine) and build a second bottom end with better rods and forged pistons.
    Maybe even Spool's stroker crank and an over bore and a bigger turbo and............

    In the mean time, I just need a decent little donk.
    Quote Originally Posted by oldcorollas
    except for a very few exceptions
    "Don't worry what people think, they don't do it very often."

    Daily: Glorified Taxi (F6 Typhoon). Out Of Action: Twin-charged Adub. Ongoing Nightmare: Over re-engineered (not) Alfa Romeo 75.

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    Bull now in china shop! Domestic Engineer NME308's Avatar
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    Default Re: 7AFE Conrods, When Do They Bend or Break?

    A very wise tuner once made the comment when asked about the strength of some component:
    'It will take a fair bit of horsepower and a little bit of abuse'.
    When asked about an uprated forged component the response was similar:
    'It will take a lot of horsepower and a little bit of abuse'!

    If you are looking for nice tractable power on the street then you will most likely not be intending to build a high rpm screamer so not much abuse from rpm...
    As long as you can creep up on the tune carefully or intend to leave it to a professional then you can limit the abuse on that angle too...
    I cannot imagine you have too much to loose from what you describe as your goal.

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  8. #8
    Junior Member Chief Engine Builder wiso's Avatar
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    Default Re: 7AFE Conrods, When Do They Bend or Break?

    from what I have worked out, as samQ says they aren't as weak as people think. everyone says they fail. but from what I have read it doesn't appear they fail from the shaft bending or breaking. it looks like bolts that join the cap onto the conrod stretch/shear/strip etc at higher rpms. because unlike 4age rods they do not have the bolt and nut holding them through, they just have a bolt from the cap into a tapped rod

    to get around the 160kw mark your gonna running up around the 17-18psi mark? I honestly don't know how it will take that so I can't help
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    Default Re: 7AFE Conrods, When Do They Bend or Break?

    Its not that expensive to uprate the rods. We have custom made rods to suit. At the end of the day if you throw a rod its gona cost you alot more to rebuild the engine. i would rather spend the extra to get it right and last you in the long run.
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    Toymods Events Secretary Too Much Toyota trdee's Avatar
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    Default Re: 7AFE Conrods, When Do They Bend or Break?

    As wiso said their weakness comes from the rod bolts, not the rods themselves. Boost "shouldnt" be anywhere near as much of an issue for the rod bolts as revs would be, but I dont think you will find anyone here that can give you a real quantifiable answer beyond "you shouldnt rev them past 7000rpm or badness happens"

    Basically you will have to try it and see.
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    Learner / modder / Backyard Mechanic PrettyCoolWagon's Avatar
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    Default Re: 7AFE Conrods, When Do They Bend or Break?

    What Sam Trdee Wiso and Bazda have said are all true as per the forum research I've done, common consensus - it's the bolts not the rods. Keep the rev limiter set to 7AFE standards and you'll be fine.
    I researched like crazy before starting a 7AGZE build.
    I bought forged rods through spool but would've happily bought through Bazda had I known he was selling them at the time, as the price difference is negligable and he's been good to deal with on other items and... also can give you first hand advice on such a build as well.
    Personally I'd say.. Forged rods is good insurance..

    If your determined not to fork out for forgies..
    There's a thread somewhere with comments from Billzilla (Bill Sherwood) saying they were using stock rods in races over in Asia somewhere to some fairly high revs IIRC they were shot peened though..

    I've heard they 'can' break but have never heard of any first hand accounts.., but I have seen first hand accounts (forum threads and photo's) of shearing the 6 flywheel bolts and also ripping out the entire centre of the flywheel. (Same guy over in the states both times)

    Press fit hardened steel dowels in the crank to flywheel are used to combat this.
    Edit - AND.. ARP flywheel bolts!
    Last edited by PrettyCoolWagon; 03-08-2012 at 08:41 PM.
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  12. #12
    Junior Member Grease Monkey
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    Default Re: 7AFE Conrods, When Do They Bend or Break?

    If the rod bolts are the issue, then you shouldn't have an issue running boost through it if you keep the rev limit down. Although its true that a decent set of rods is cheaper than a bottom end rebuild, if you're only going to get a used 7A motor thats not going to set you back too much. I'd probably leave it as stock, get the rest of your setup working on a mild boost level, and then save your money for the bits to wind up the revs and boost.

  13. #13
    Toymods Board Member Backyard Mechanic
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    Default Re: 7AFE Conrods, When Do They Bend or Break?

    The rods are similar to the blacktop 20v ones.

    Fine to swing to 8k or so

  14. #14
    Forum Sponsor Domestic Engineer Bazda's Avatar
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    Default Re: 7AFE Conrods, When Do They Bend or Break?

    There is more load on the rods compared to a 4age. Because of the increased stroke the angles are not really ideal.
    1988 Toyota Levin GTZ 10.88 @ 209.57 340kw
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    Senior ****** Carport Converter Sam_Q's Avatar
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    Default Re: 7AFE Conrods, When Do They Bend or Break?

    any know if it's true that the bolts pull out because their not a nut and bolt design?

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