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Thread: Engine Builder questions

  1. #41

    Steve >> aka: GoDadGo
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    Quote Originally Posted by TexasAviator View Post
    Thanks for the replay all. I am not sure if it's a misfire, piston slap or some other anomaly. I am going to drive it some more and the car seems to like a lopey idle but I am going to go see my chassis dyno guy and have him look at it. It's in a narrow part of the band. The engine seems to be happy as can be 800 through 6000... any other ideas?

    What is your initial timing set at?
    Could your timing be a little too far advanced causing the initial ignition to make the piston slap at 700-750 RPM?
    My car idles well between 850-900 RPM and I'll hear some knocking and/or slapping when idling way down at or around 750 RPM.
    You're running a much higher compression than I am with similar cam specs so this may be the issue.
    Good Luck & Have A Very Merry Christmas Which Just Started Tonight!
    Last edited by GoDadGo; 12-25-2018 at 03:17 AM.

  2. #42
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    Maybe just a resonant vibration your engine makes at that rpm making something else vibrate?
    Mine makes one from about 2000-2050 or so, on the way up and down. I had a sound like singing tree frogs from under the hood, it was my air cleaner top. Tightened it a quarter turn and it went away, then started noticing it in the cowl area of the body.

  3. #43
    Brandon #9196 TexasAviator's Avatar
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    Quote Originally Posted by GoDadGo View Post
    What is your initial timing set at?
    Could your timing be a little too far advanced causing the initial ignition to make the piston slap at 700-750 RPM?
    My car idles well between 850-900 RPM and I'll hear some knocking and/or slapping when idling way down at or around 750 RPM.
    You're running a much higher compression than I am with similar cam specs so this may be the issue.
    Good Luck & Have A Very Merry Christmas Which Just Started Tonight!
    Sorry for the late reply, my grandmother passed away, Christmas and new years came and went...

    Anyway. It does it with timing settings anywhere in the 10-18 degree range, a little less in the higher range.

  4. #44
    Brandon #9196 TexasAviator's Avatar
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    I think there is a resonant freq somewhere causing the issue Murd. The car is running awesome but for some reason if I let the car idle low I just seem to feel a vibe on the engine, its very high frequency. It doesnt sound like piston slap or anything inherant with the 347 combo. I just wonder if its due to a very hard engine mounts from Prothane? The car sounds so good and rumbles at 650 -750 rpm but if there is something vibing the engine I dont want it to prematurely wear bearings or other parts with a resonant vibe.

  5. #45
    bobl's Avatar
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    Sorry about your Grandmother. I'd just idle it up to 800-850 and not worry about it. I found my car to be a bit easier to drive in stop and go traffic by idling faster. I can just ease the clutch out without adding any throttle and the IAC will take over.
    Mk IV Roadster, 347/516 HP, 8 stack injection, Astro Performance T5, 3-Link 3.55 gears, Purchased 08/2015, Graduated 02/2017

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  7. #46
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    Quote Originally Posted by TexasAviator View Post

    I think there is a resonant freq somewhere causing the issue <snip> I dont want it to prematurely wear bearings or other parts with a resonant vibe.

    It's supposed to do that.


    I use it for setting off car alarms in parking lots.


    Also known for loosening Miata lug nuts at red lights....

  8. #47
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    I would set idle at 8oo-900 rpm, drive it , enjoy.
    358 SBC, Winters QC, Sweet mfg, Coleman, AP Racing, ARS, TKO 600, Fuel Safe

  9. #48

    Steve >> aka: GoDadGo
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    Remember the combustion rate remains somewhat static; however, at lower RPM's you end up with more combustion occurring before your pistons hit Top Dead Center (TDC) than at a higher RPM's so if your initial timing remains unchanged (static at 750 RPM -thru- 1,000 RPM) then you've got a greater chance of Slapping Da-Pistons or rocking them in the bores at the lower the RPM range of this example.

    Basically your pistons are getting a lot more push back against the piston prior to it hitting TDC at 750 than ou do at 1,000 RPM.

    Here's a little Piston Velocity Calculator & Some Slapping Action you can look at for fun and/or insight:

    https://www.lmengines.com/piston-speed-calculator/

    https://mechanics.stackexchange.com/...is-piston-slap

    Hope This Helps & Sorry About Your Grandmother!

    Good Luck From The Dark Dart Side!
    Last edited by GoDadGo; 01-16-2019 at 09:44 PM.

  10. #49
    Brandon #9196 TexasAviator's Avatar
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    Thanks everyone,

    It was my last grandparent to pass. All the Patriarchs and Matriarchs of my past are with the Lord now. Its a unique time in my life, everything around you moves on and its hard to believe I am at an age of life that is midlife or approaching middle age. Not a bad thing just a new journey.

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