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Thread: Throttle Blade troubles

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    Administrator ucfbrett's Avatar
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    Default Throttle Blade troubles

    OK, seeking to avoid trouble here.

    I keep hearing about Miatas breaking throttle blade shafts. I initially heard it was for breathed on engines that build more power and spin higher than stock. Then I see Joey Jordan's post that his Spec Miata broke one, too.

    Is there a preventive fix that's legal in Spec Miata? Is it as simple as epoxying the bolts that hold the butterfly to the shaft?

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    Senior Member robburgoon's Avatar
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    No fixes that are SM legal. You could epoxy them, but then I could protest you for improved flow.
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    Sir flink
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    Epoxy (or punch) them anyway - if one of those things comes out it could nuke the entire engine.

    That's if you're very unlucky - usually it will just rattle around in the cylinder then get shot out the exhaust, hopefully in the direction of a competitor. I blew four piston rings out the tailpipe before one of them managed to cause a mashed rod bearing.

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    Senior Member bellwilliam's Avatar
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    Brad: there is no fix. We have tried ~8 variations... and failed each time.

    I don't have the exact number, but about 50% of them goes kaboom.

    Don't know the exact reason why they break. guessed wrong every time so far.

    it is probably that damn nitrous we've been running
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    I wanna go fast! thepass's Avatar
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    The 1.6's don't seem to have this issue.. just sayin'
    Ryan Passey
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    "The sport dedicating to the spirits of time attacking motoring" -Emilio

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    http://www.trackhq.com/Banners/yellowsitesponsor.gif emilio700's Avatar
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    Quote Originally Posted by thepass View Post
    The 1.6's don't seem to have this issue.. just sayin'
    They do. You just haven't broken one yet.

    From about 6 years ago Throttle shaft breakage - MX-5 Miata Forum
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    But obviously I just dont get it. -fatbillybob

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    Quote Originally Posted by ucfbrett View Post
    OK, seeking to avoid trouble here.

    I keep hearing about Miatas breaking throttle blade shafts. I initially heard it was for breathed on engines that build more power and spin higher than stock. Then I see Joey Jordan's post that his Spec Miata broke one, too.

    Is there a preventive fix that's legal in Spec Miata? Is it as simple as epoxying the bolts that hold the butterfly to the shaft?
    What we learned is that they have significant wear over the years and they brake when you get off the gas quickly to shift etc.... there is a set screw near the linkage make sure their is no gap at all in that area, that will help prevent it. We have broken 3 all 1.6's 2 in our STL more modified car and this is the first one in spec miata also a 1.6.

    I may come watch you guys slog it our in the rain and I will show you this weekend.

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    http://www.trackhq.com/Banners/yellowsitesponsor.gif emilio700's Avatar
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    Typical breakage. It's engine vibration and a little shaft torque that breaks them. Throttle pedal stops to prevent shaft torque did not affect the breakage rate.

    imgp9333.jpg

    The JDM NB's have a 7500rpm fuel cut (USDM is 7000). They also have this mass damper to reduce vibration. We presume it's to help prevent shaft breakage. The only common denominator we have seen WRT to shaft breakage is time spent at or above 7000rpm.


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    We have only had it happen on Decel during shifts... fast shift off the gas hard and the mechanism slams and breaks........ Their is too much play in the return spring....their is a set screw ( I think it's called the throttle idle stop) that needs to be adjusted to not allow any gap between the screw and the mechanism when it is in the nornal not reving position. The thought is that it over rotates past where it should and brakes the shaft.... Every throttle body I have looked at since our issue has had a gap. If you make sure their is no gap it lessens the likely hood of this happening, especially in A SPEC MIATA which you only revs to 7100- 7200 If you rev it higher 7500 + 100% agree with emilio, but my suggestion is check that gap.... Lewis at Rush explained to us what was happening. Hope it helps.


    1.6-throttle-body.jpg

    I put this video togehter so everyone can see the latest 2 throttle body breakages we have had in 2 different 1.6 miata's.

    Throttle Body Breakage 1 6 Spec Miata and 1 6 STL Miata - YouTube
    Last edited by Joe jordan; 02-28-2014 at 11:00 PM.

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    If this is really prevalent and the result is a blown motor 50% of the time I would:
    1) find out the metallurgy of a stock TB rod
    2) make a hardened rod of better material
    3) make the CNC file open source for anyone to make on say E-machine.com
    4) petition your sanctioning body to allow the modification (it seems that everyone could benefit)

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    Senior Member bellwilliam's Avatar
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    Quote Originally Posted by fatbillybob View Post
    If this is really prevalent and the result is a blown motor 50% of the time I would:
    1) find out the metallurgy of a stock TB rod
    2) make a hardened rod of better material
    3) make the CNC file open source for anyone to make on say E-machine.com
    4) petition your sanctioning body to allow the modification (it seems that everyone could benefit)
    You work on Ferrari engines. We work on $800 engines. Different perspective
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    Quote Originally Posted by fatbillybob View Post
    If this is really prevalent and the result is a blown motor 50% of the time I would:
    1) find out the metallurgy of a stock TB rod
    2) make a hardened rod of better material
    3) make the CNC file open source for anyone to make on say E-machine.com
    4) petition your sanctioning body to allow the modification (it seems that everyone could benefit)
    Thanks for your thoughts. We will come up with something!

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    http://www.trackhq.com/Banners/yellowsitesponsor.gif emilio700's Avatar
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    Quote Originally Posted by Joe jordan View Post
    We have only had it happen on Decel during shifts... fast shift off the gas hard and the mechanism slams and breaks........ Their is too much play in the return spring....their is a set screw ( I think it's called the throttle idle stop) that needs to be adjusted to not allow any gap between the screw and the mechanism when it is in the nornal not reving position. The thought is that it over rotates past where it should and brakes the shaft.... Every throttle body I have looked at since our issue has had a gap. If you make sure their is no gap it lessens the likely hood of this happening, especially in A SPEC MIATA which you only revs to 7100- 7200 If you rev it higher 7500 + 100% agree with emilio, but my suggestion is check that gap.... Lewis at Rush explained to us what was happening. Hope it helps.


    I put this video togehter so everyone can see the latest 2 throttle body breakages we have had in 2 different 1.6 miata's.
    It's not the stop. We have seen plenty break at sustained full throttle, not when shifting. It's vibration on a brittle material. You can address this 3 ways:

    -Change resonant frequency of shaft. We do this by adding epoxy to the shaft. This also contains the bits if it does still break. So far this has been the most reliable fix
    -Change material and /or increase diameter. I had a friend in AZ make a few for us this way British Car Service | Tucson Repair, Restoration and Cars for Sale Ask for Ryal
    - Reduce engine vibration amplitude. Good luck with that.
    - Swap an entire new TB in. We asked Dave Hsu of Skunk2 for one back in 2009. In 2012 it hit the market but it is also imperfect. Typically needing some fine tuning and loctite to make bombproof.
    Last edited by emilio700; 03-01-2014 at 05:12 PM.
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    Senior Member robburgoon's Avatar
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    Sounds like epoxy is the best bet for SM folks that aren't afraid of a weenie protest

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    In the picture the shaft broke and the screw head. Why are both breaking? Which first? Why does locktite help prevent shaft breakage? If the shaft breaks due to screw coming loose increasing forces on marginal shaft that would be like lugnuts coming loose braking wheel studs and hogging out the lugholes in the wheel. What happens if you change the stress concentrations on both parts by doing something like chamferring the shaft holes and loctiting in a finer pitch higher quality screw? Is someone here an engineer who can do some FEA or knows the science of plain holes vs. countersunk holes?

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    http://www.trackhq.com/Banners/yellowsitesponsor.gif emilio700's Avatar
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    Quote Originally Posted by fatbillybob View Post
    In the picture the shaft broke and the screw head. Why are both breaking? Which first? Why does locktite help prevent shaft breakage? If the shaft breaks due to screw coming loose increasing forces on marginal shaft that would be like lugnuts coming loose braking wheel studs and hogging out the lugholes in the wheel. What happens if you change the stress concentrations on both parts by doing something like chamferring the shaft holes and loctiting in a finer pitch higher quality screw? Is someone here an engineer who can do some FEA or knows the science of plain holes vs. countersunk holes?
    We have seen at least one failure with a broken shaft (always in the same spot/mode) with an intact screw in the #4 intake runner. This indicates the shaft breaks first and most times, pops the head of the screw. Sadly, I have a pretty large sample set.
    My rough estimate, failed shafts has cost our team about $20k in engines over the last 7 years. Not to mention lost races, entry fees, etc.
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    Senior Member fatbillybob's Avatar
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    Quote Originally Posted by emilio700 View Post
    We have seen at least one failure with a broken shaft (always in the same spot/mode) with an intact screw in the #4 intake runner. This indicates the shaft breaks first and most times, pops the head of the screw. Sadly, I have a pretty large sample set.
    My rough estimate, failed shafts has cost our team about $20k in engines over the last 7 years. Not to mention lost races, entry fees, etc.
    OK that's good. Perceived failure point is the shaft. So if chamferring the shaft hole does not improve the stress profile on that part of the shaft what about welding up the shaft in the flat cutout and grinding it down round like the shaft so it can you got more mass of metal and it can still be inserted in the throttle body. Then make a half-moon shaped washer contoured to the shaft to lock the plate and screw in place against the shaft. The end result would be the same with more shaft mass to prevent breakage and better stress profile on all the parts.

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    Quote Originally Posted by fatbillybob View Post
    OK that's good. Perceived failure point is the shaft. So if chamferring the shaft hole does not improve the stress profile on that part of the shaft what about welding up the shaft in the flat cutout and grinding it down round like the shaft so it can you got more mass of metal and it can still be inserted in the throttle body. Then make a half-moon shaped washer contoured to the shaft to lock the plate and screw in place against the shaft. The end result would be the same with more shaft mass to prevent breakage and better stress profile on all the parts.
    This is WAY over my head . . .
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    http://www.trackhq.com/Banners/yellowsitesponsor.gif emilio700's Avatar
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    Quote Originally Posted by fatbillybob View Post
    OK that's good. Perceived failure point is the shaft. So if chamferring the shaft hole does not improve the stress profile on that part of the shaft what about welding up the shaft in the flat cutout and grinding it down round like the shaft so it can you got more mass of metal and it can still be inserted in the throttle body. Then make a half-moon shaped washer contoured to the shaft to lock the plate and screw in place against the shaft. The end result would be the same with more shaft mass to prevent breakage and better stress profile on all the parts.
    A chamfer would probably help but I'd be hesitant to remove more material from the shaft. Shaft doesn't need to be 3x stiffer. Just stiff enough so it doesn't resonate and the material at the stress riser stays well below fatigue limits for that metal. So your idea of brazing (welding is too hot) could work. I brazed one that lasted forever. Not sure where that TB ended up. A lot of what I do here is looking for fixes that anyone can do, not just some exotic or expensive machined part. We're currently testing a particular 3M aerospace epoxy that has specific properties that
    could allow it to stay bonded and flexible.

    I feel for the STL and Spec Miata drivers killing good engines when that screw head hits the piston and powerless to do anything about it within the rules.
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    Senior Member bikeindirt's Avatar
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    My thought was not about adding strength... but mostly changing the the resonance.

    I laid some wet carbon over the shaft and onto the butterfly (hard to describe) but looks like a ~1.5" circle of carbon fiber in the middle of the ~3" butterfly... Covers the screw heads.

    Not that I stress mine that hard... but never had a failure. Recently had one rebuilt with a "oversized" butterfly/shaft.

    BID

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