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Thread: 488's or 513's

  1. #1
    Senior Member RACING JUNKIE
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    488's or 513's

    Ok guys, it's time to change out the gears in the rear.
    I broke a dana 60> footbraking. Gear was in car when I got it three years ago, not knowing how long it was in there before.
    I think it served it's purpose and it was time anyway, no hard feelings.

    It had 557's I only run 1/8 tracks and a short 1/8 mile track at that.


    The old gear had me trapping @ 7800 which is alittle more than I'm comfortable with running a BBC so I'm wanting to tone it down a bit.

    With good traction my sixty foots are always in the high 120's - MPH >109 - 112
    Best ET was a 560 (last weekend)

    Not concerned with going faster, I just don't want to slow it down much if any.
    I had several guys with similar cars/combos tell me they switched from 513's to 488's and seen no difference in ET's

    I have 32x14.5 slicks - 1.76 glide - 5800 rpm converter in a fairly light 69 camaro

    So with that being said, what would you go with:

    488 or 513


    Thx in advance for any input, Cp
    ***IN GOD WE TRUST***

  2. #2
    Senior Member RACING JUNKIE DirkaDirka's Avatar
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    If the way I am thinking is correct but I think you wont see any difference with going with the 5:13's. The only diff might be the RPM's dropping. If I am wrong would somebody correct me.
    Its all fun and games till someone loses a nut!

    http://www.racingjunk.com/profile/400466

  3. #3
    Senior Member RACING JUNKIE
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    I run a 32x14 Goodyear with a 8" converter and a glide ..With 5.13 gears I am just under 7000 at the stripe in the eighth mile. 60' 1.31

    540 BBC in my Nova which weighs around 3200 with me in it.

  4. #4
    Senior Member RACING JUNKIE
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    I would prob go with 513, i think the role out on those 32x14.5 is something like 103, and that would kill a little gear. 488 would lower trap rpm, but may hurt short times, and yours look good now, would'nt want to hurt that. jmo

  5. #5
    Senior Member RACING JUNKIE
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    /?

    513!
    Later
    G 8)

  6. #6
    Senior Member EXPERT BUILDER
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    id look at your converter! math says u should be @ 6550 rpms with that 112 mph! so if u are at 7800 rpms thats like 20% slippage!!!!!
    good luck

  7. #7
    Senior Member RACING JUNKIE
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    What math? give me your formula please.


    Those numbers are a tad confusing, its a short 1/8 by approx 60 feet.

    Thx Cp
    ***IN GOD WE TRUST***

  8. #8
    Senior Member MASTER BUILDER
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    It doesn't matter whether it is a shorter track or not, as all you do is plug in the RPM's and what the MPH is at that RPM. In your case using the formula from Wallace Racing (http://www.wallaceracing.com/calcmph.php) with the 5.57 gear , 32 tall slicks, and 7800 rpm the MPH should be 133.37 without any slippage or tire growth. Now if you keep adjusting the RPM's down until you get the MPH that you are actually running, the number will be 6500RPM that corresponds to 111 MPH. That shows that you have 1300 rpm's of slippage. This other calculator from Wallace (http://www.wallaceracing.com/converter-slip.php) shows that the converter slippage is 17% assuming a rollout of 102" for your slicks.

    To give you an idea, my car has the 16x33 slicks, with 4.56, 1.80 powerglide, weighs 2700lbs and my 60fts are 1.238 leaving at 4000 and I run the 1/8th at 5.64 @ 121 mph at 6200 rpm's at the finish and that is only about 8% slippage. If I was to make a gear change and was only going to be running 1/8th mile, I would go to the 5.13 which would move the RPM's to around 7000 at the finish.

    Bill

  9. #9
    Senior Member RACING JUNKIE
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    Thx Bill,
    I appreciate your reply as I do all, only way I'm going to learn anything about anything is to ask questions, sorry if I sounded sarcastic to anyone.

    The rollout on my phoenix slicks are 103, I remember seeing it on the tires when I bought them, double checked and both were dead on 103.

    I doubt that an extra 1/2 would change those figures much, just throwing that out there.

    I'll admit my converter is in noway fresh (2 seasons) I'm sure it has some slippage.

    What would be the worst case scenario or the point at which I need to get the converter addressed?


    Over 15% over 20% I have no idea.

    The car runs very consistent as it is and yanks the front wheels and totes them a good bit. The tranny fluid has always looked red as it did when I put it in no burnt smell either.

    My main concern is the rpms at the stripe.

    Thx, Cp
    ***IN GOD WE TRUST***

  10. #10
    Senior Member MASTER BUILDER
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    Charles,
    The difference on the roll out of the slicks will move yours on up to 18% slippage. The slippage isn't always about how old the converter is, but how efficient it is. Age can make them inefficient, but they can be built that way also.

    I feel that a good number is 300-400 rpms of slippage and mine is at about 450rpms, so I would definitely be looking at any converter that has over 10% and much over 500rpms of slippage. This is horsepower lost that is not moving the car.

    If you were able to have your converter rebuilt to where it only had about 10% slippage, which would be about 500rpms then that would put you at about 7000 at the stripe. What rpms' does your engine fall to on the shift as this is the effective stall of your converter? Mine drops to about 5800rpms on the shift into high gear.

    Bill


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