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Old 04-08-2013, 01:09 AM
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1968 firebird , highway RPM way to high

Hi Guys and Gals,

So i have a 1968 firebird with a BOP TH350 it, i'm planning on throwing a 200r4 in it in a few months because the RPM on the freeway is so high. The th350 transmission is used, no idea how many miles. The rear diff is a gm 12bolt it was rebuilt before i bought it but no idea what the gears it has in it.

so at 56mph i am taching 3500 rpm and i have 28 inch tall tires, i used several gear calculators and everything says that equates to 5.21 gears. is it possible that my gearing in the diff can really be that high or maybe my transmission is not shifting into second gear??

thanks

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Old 04-08-2013, 04:58 AM
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5.21 seems pretty steep......If I were you I would take the rear cover off the rear end and count the gears....Its not that big a job and costs nothing. At least then you know where you stand.
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Old 04-08-2013, 05:15 AM
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Quote:
Originally Posted by starin View Post
Hi Guys and Gals,

So i have a 1968 firebird with a BOP TH350 it, i'm planning on throwing a 200r4 in it in a few months because the RPM on the freeway is so high. The th350 transmission is used, no idea how many miles. The rear diff is a gm 12bolt it was rebuilt before i bought it but no idea what the gears it has in it.

so at 56mph i am taching 3500 rpm and i have 28 inch tall tires, i used several gear calculators and everything says that equates to 5.21 gears. is it possible that my gearing in the diff can really be that high or maybe my transmission is not shifting into second gear??

thanks
Your math is right, but is the tach?

If the trans is in second, the gear ratio would be a more realistic ~3.5:1. But like poncho said, checking the gears yourself is a good idea.

I'd also be checking the shift linkage to be sure the trans is actually shifting into high gear.
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Old 04-08-2013, 06:22 AM
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If you have 5.? gears you should know?When I had 5.13s I was at 3700 rpm on the hiway. At 7,000 rpm I was a little over 100MPH.top speed was 115. with 5.? gears you will feel all the shifts and certainly feel slippage
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Old 04-08-2013, 07:30 AM
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Originally Posted by vinniekq2 View Post
If you have 5.? gears you should know?When I had 5.13s I was at 3700 rpm on the hiway. At 7,000 rpm I was a little over 100MPH.top speed was 115. with 5.? gears you will feel all the shifts and certainly feel slippage
X2 on everybody's advice, I had 4.88 in my 67 Firebird and at 60 MPH the RPM's where around 3,500. Top speed a wopping 107 MPH and that's what it hit going throw the traps at the end of the quarter mile.....once, I will not speak about the other times....LOL.

Count the teeth in your ring and pinion...do the math that way, then you know for sure. If your not sure about the transmission, try shifting it manually...see if your RPM's drop from 1st, to 2nd, to drive. If the RPM's don't drop from 2nd to drive, you don't have a 5.21 rear dif.

Ray
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Old 04-08-2013, 07:50 AM
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I don't know of any 5-plus ratio ring gear/pinions that are made for a 3-series carrier (4.88 is about the steepest for a 3-series) so you might look up the casting number to see if the housing is a 4- or 3-series.

Still best to count the teeth- that way you can see if it's a posi, the condition of the teeth can be seen, etc.

A word of caution: Be sure to use a W I D E pan to catch the diff lube! It stinks to high hell, and if you get it on the floor of the shop or driveway (let alone on you or your clothes), you'll regret it.
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Old 04-08-2013, 08:06 AM
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Originally Posted by cobalt327 View Post
I don't know of any 5-plus ratio ring gear/pinions that are made for a 3-series carrier (4.88 is about the steepest for a 3-series) so you might look up the casting number to see if the housing is a 4- or 3-series.

Still best to count the teeth- that way you can see if it's a posi, the condition of the teeth can be seen, etc.

A word of caution: Be sure to use a W I D E pan to catch the diff lube! It stinks to high hell, and if you get it on the floor of the shop or driveway (let alone on you or your clothes), you'll regret it.
I might be wrong but in a 12 bolt diff, I think you could get into the 5's and even 6's...The 10 bolt was the one that the 4.88 was the steepest...like I said I could be wrong but, something just jogged my memory.

Oh, and if you spill gear oil on your clothes and your married and you bring it home to your wife to wash...in most states that's grounds for divorce.

Ray
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Old 04-08-2013, 08:14 AM
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Originally Posted by 69 widetrack View Post
I might be wrong but in a 12 bolt diff, I think you could get into the 5's and even 6's...The 10 bolt was the one that the 4.88 was the steepest...like I said I could be wrong but, something just jogged my memory.

Oh, and if you spill gear oil on your clothes and your married and you bring it home to your wife to wash...in most states that's grounds for divorce.

Ray
I believe you're right. What I was talking about was using a 5-or-steeper gear in a 3-series carrier. They make thicker gear sets (up to 4.88) to use in the 3-series housings, but I don't think there are any in the range that the OP thinks his ratio might be. I suppose a spacer could still be used, though.
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Old 04-08-2013, 08:27 AM
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Okay, gotcha, thanks again...damn I'm on a learning role.

Ray
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Old 04-08-2013, 09:06 AM
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A 12 bolt housing is a 12 bolt housing. The 2,3 and 4 series comes from the location of the ring gear flange on the differential. A 2 series differential has the flange offset to the left more than a 3 series and a 3 series is offset to the left more than a 4 series. This is to make up for the difference in the diameter of the pinion gear head for 2.56 to 6.xx gears. The car 12 bolt uses a 2 series differential for 2.56 and 2.73 gears, a 3 series for 3.07 to 3.73 gears and a 4 series differential for 3.90 to 6.xx gears. There are special thick 3.90, 4.10, 4.56 and 4.88 gears that are made to fit the 3 series and there is a special thin 3.73 that is made to fit the 4 series.
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Old 04-08-2013, 09:11 AM
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you really d ont have to pull the cvoer to get a reasonable est of the rear ratio. put the car on jackstands trans in netral and spin the tire 360 while conting the driveshaft revoltions .that s mch easier . do yo hear the rpm drop 2x?
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Old 04-08-2013, 11:22 AM
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thanks for the help guys, when i first got the car last year it was a shell and i dropped the motor and trans in there when i was still living with my folks at there house. now i rent a town house and i'm not really allowed to work on it. i'll try the technique of spinning the tire and seeing how many times the driveshaft spins in one rotation.

i feel the trans shift but im not sure how many times, i know it shifts once but i can't feel the second time. the rpm on the highways definitely sucks
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Old 04-08-2013, 11:38 AM
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do you have a tach? you should see the drop in rpm
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Old 04-08-2013, 11:43 AM
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i'll have to check, so reaching highway speed from a dead stop how many rpm drops should there be? one drop from 1 to 2nd and another drop from 2nd to 3rd right?
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Old 04-08-2013, 11:45 AM
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yes you are correct
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