3.58 Gear Ratio, pre '79

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aj81spider
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Re: 3.58 Gear Ratio, pre '79

Postby aj81spider » Sat Mar 17, 2012 3:04 pm

I can't comment on your pictures. However I have a 4.3-1 and I'm not a fan. I'm shifting from first to second about halfway through the intersection and I'm over 4000 RPM at 70 mph. If I could convert to the 3.58 I would in a second.
A.J.

1974 Fiat 124 Spider
2006 Corvette
1981 Spider 2000 (sold 2013 - never should have sold that car)

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maytag
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Re: 3.58 Gear Ratio, pre '79

Postby maytag » Sat Mar 17, 2012 5:22 pm

....
Last edited by maytag on Tue Mar 27, 2012 12:23 am, edited 1 time in total.
I'm no Boy-Racer..... but if I can't take every on-ramp at TWICE the posted limit.... I'm a total failure!

spiderrey
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Re: 3.58 Gear Ratio, pre '79

Postby spiderrey » Sat Mar 17, 2012 7:26 pm

76 didnt have an automatic. I think if you have a 4.3 rear end and you rotate the tire once, the drive shaft will rotate 4.3 times. Or it might be the other way around.

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maytag
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Re: 3.58 Gear Ratio, pre '79

Postby maytag » Sat Mar 17, 2012 10:11 pm

.....
Last edited by maytag on Tue Mar 27, 2012 12:23 am, edited 1 time in total.
I'm no Boy-Racer..... but if I can't take every on-ramp at TWICE the posted limit.... I'm a total failure!

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So Cal Mark
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Re: 3.58 Gear Ratio, pre '79

Postby So Cal Mark » Sat Mar 17, 2012 10:22 pm

I love the 4.30 ratio, I wouldn't have anything else
Mark Allison
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maytag
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Re: 3.58 Gear Ratio, pre '79

Postby maytag » Sat Mar 17, 2012 11:22 pm

.....
Last edited by maytag on Tue Mar 27, 2012 12:23 am, edited 1 time in total.
I'm no Boy-Racer..... but if I can't take every on-ramp at TWICE the posted limit.... I'm a total failure!

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124JOE
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Re: 3.58 Gear Ratio, pre '79

Postby 124JOE » Sun Mar 18, 2012 10:36 am

yes ive had the 4.3 up to 100mph bearly
it was screeming 5,500 on the tac
and wouldnt go any faster

these are made for good take off not top end speed
when you do everything correct people arent sure youve done anything at all (futurama)
ul1joe@yahoo.com 124joe@gmail.com

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Re: 3.58 Gear Ratio, pre '79

Postby FiatMac » Sun Mar 18, 2012 3:34 pm

maytag wrote:so, I'm not a math guy. My brain shuts down. So somebody help me here:

chocked one wheel. turned the input at the diff 10 times. here's what I get.

the one currently IN the car (that my gut says was for a '76 Automatic) gets 4.5 turns of the wheel for every 10 turns of the driveshaft.

the one I have that came from the '78 gets 5.25 turns at the wheel for every 10 turns at the driveshaft.

What does that make the ratios? whatever it is, the one currently installed is the lower-geared, and I've been happy with it. So I'll probably refurb it and keep it. I'll need to find a panhard bar, as the current one is 'dun'. (I might toy with making an adjustable-length version out of it)

Anyone know the procedure for replacing bearings & seals at the axle for the non-removable-cover type? I mean, what holds-in the axles? Do I have to pull the pumpkin?


Here’s the math.

The gears in a differential are arranged such that the average speed of the two wheels multiplied by the gear ratio is a constant. In equation form this would be:

Differential Input Turns = Gear Ratio(W1 + W2)/2

Where Wl and Wr are the revolutions of the Left and Right wheel respectively.

So if the rear end ratio is 3.58:1 and we are traveling in a straight line with both wheels turning at the same speed W, then for each 10 turns of the input we can solve the equation by substitution:

10 = 3.58(W + W)/2 = 3.58(W)

Or

W = 2.79 turns

If we are trying to determine the rear end ratio by chocking(holding stationary) one wheel while counting the turns made by the free wheel with 10 turns of the input shaft, then the equation becomes,

10 = 3.58(W + 0)/2 = 3.58W/2

Or,

W = 5.59 turns

Below is a list of this calculation for each of the available ratios.

Rear Ratio /Free Wheel Turns per 10 turn input
4.3 / 4.65
4.1 / 4.88
3.9 / 5.13
3.58 / 5.59


Maytag,
Not sure how accurate you were with your count, but the 4.5 count for the ‘76 is very close to the count above for a 4.30 rear ratio. The 5.25 that you got for the ’78 is closest to the count above for the 3.90 rear ratio. You can get better differentiation between the numbers if you use a 20 turn input count.
Stan McConnell
Retired Mechanical Engineer
Salisbury, North Carolina
82 2000 Spider (driving)
78 124 Spider on the rotisserie
76 124 Spider parts car or possible Lemons racer
83 parts car

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maytag
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Re: 3.58 Gear Ratio, pre '79

Postby maytag » Sun Mar 18, 2012 3:42 pm

.....
Last edited by maytag on Tue Mar 27, 2012 12:22 am, edited 1 time in total.
I'm no Boy-Racer..... but if I can't take every on-ramp at TWICE the posted limit.... I'm a total failure!

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So Cal Mark
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Re: 3.58 Gear Ratio, pre '79

Postby So Cal Mark » Sun Mar 18, 2012 3:42 pm

if your motor tops out at 5500 rpm you have more problems than rear gear ratio
Mark Allison
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Headers, ignitions, wheels, cams, flywheels
starsmark@hotmail.com 909-981-3566

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DocGraphics
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Re: 3.58 Gear Ratio, pre '79

Postby DocGraphics » Sun Mar 18, 2012 4:11 pm

Fiatmac

Thank you, I will be using this to figure mine out too!
Don Raugust
1978 Fiat 124 Spider "Fiona" - Burgundy/Tan
2011 pics: http://s918.photobucket.com/albums/ad22 ... 0Pictures/
2012 pics: http://s918.photobucket.com/albums/ad22 ... cs%202012/

ljayr

Re: 3.58 Gear Ratio, pre '79

Postby ljayr » Mon Mar 19, 2012 3:42 pm

The '76 is a 4.3.
The '78 is a 3.58 from an auto or a 3.90 from a 5 speed. To verify, remove the rear cover and check the ring gear. The gear ratio will be stamped on the side of the ring gear.
As far as the panhard bar mounting goes, I dont know. You would have to check with someone that has been around these cars for a much longer time than I have.
100 MPH with a 4.3, doubtful. 3.90 or 3.58 possible.

Jason

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Re: 3.58 Gear Ratio, pre '79

Postby boogiedude » Tue Mar 20, 2012 2:56 am

124JOE wrote:yes ive had the 4.3 up to 100mph bearly
it was screeming 5,500 on the tac
and wouldnt go any faster

these are made for good take off not top end speed


I've got a 4.3 in my car and I've taken it up over 6grand in fifth. I didn't hit 100. I don't think your speedo is accurate, I know mine is up into the 70's(I've checked it on speed signs :wink: )

spiderrey
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Re: 3.58 Gear Ratio, pre '79

Postby spiderrey » Tue Mar 20, 2012 9:25 am

With stock gears and engine my old car topped out at 108 mph.

vandor
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Re: 3.58 Gear Ratio, pre '79

Postby vandor » Wed Mar 21, 2012 12:40 am

maytag wrote:In my 76, I found this:
Image


I assume you know that the panhard rod is seriously bent and needs replacement?
Csaba
'71 124 Spider, much modified
'17 124 Abarth, silver
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