I'm thinking of getting a new 12 bolt posi track rear end and would
like some info on the best gear ratio that would work for me. I want
it to be fast off the line, of course, but I would also like it to be
How much horse power? Rpm redline? Transmission and final gearing?
Why a 12 bolt? Have you done any research yet on the subject yet?
Camshaft lift and duration? Wheels and tires? Weight?
The best thing for you to do would be to call a differential company
and tell them what you want to do with your car.
Well... There are a lot of "I don't know"'s to those questions. First
of all, I haven't done a lot of research on the subject yet. I was
under the apparently false assumption that 12-bolt rear ends were posi
track, while 10-bolts were not. I have a 327 with, I think around
300hp. A three speed automatic trans (powerglide). Final gearing? I
don't know. Rpm redline? I don't know. Camshaft lift and duration? I
don't know. Weight? I don't know.
So obviously I have some more homework to do.
Thanks for replying to my question though.
Very long reply, sorry people.
Don't be alarmed that I'm about to pick your post apart. I just want
to help you get the right info together.
Everybody has to learn sometime.
Both 12 bolts and 10 bolts came with available posi or open. It came
down to what you ordered. More likely than not rebuilding a 10 bolt
(8.5") with a posi center section will handle anything you are likely
to throw at it, unless you plan on running over 500 hp with gobbs of
I had a 327 with a little over 300 hp. once. It was balanced, had a
steel crankshaft, 10:1 flat top hyper eutectic pistons, roller
rockers, a Comp Cams 292H bump stick, headers, full 2" dual exhaust,
etc. etc. You get the picture. It got all of 8 to 10 mpg when I was
babying it and 4 mpg when having fun, if that. The sound it made was
very lopey like it really didn't want to run and when you shut it off
it seemed to thank me. chsshhaa!
Does yours act similar? There are math equations that can get you into
the ballpark of the actual hp of a car.
The Powerglide is a 2 speed. If you have a 3 speed it is either a
TH350 or TH400, unless it has been upgraded to and OD trans.
With one wheel on the ground count how many times the drive shaft
rotates per one wheel revolution.
You need to know pretty much everything there is inside the engine to
determine this. Remember the 327 I wrote about earlier? I regularly
had it running deep into 7,000's every day I drove it. I'm sad that I
don't have it any more.
Your local scrap metal yard ,municipal landfill, or a truck stop may
be able to help you with the weight. For a small fee of course.
Sure, and good luck finding the information that you need.
This is not necessarily true. I have to disagree with you on this one. It
doesn't take just 500hp (and I'm assuming you mean RWHP not at the flywheel,
doesn't matter what you make at the flywheel, its what gets to the ground.).
Using some sticky tires and a hard launch can kill both the 10 and 12 bolts.
12 bolts are a little bit stronger however. And it doesn't take much torque
to destroy a 10 bolt. I've seen 10 bolts go with barely 300RWHP/300RWTQ.
Suspension (or lack of sometimes) can play a role in this disaster as well.
A good run on the dyno is a better way and more accurate way. Again you get
RWHP and RWTQ numbers which are much more telling in what the car can do.
The math can get you somewhere near the ballpark of peak hp and peak tq.
But what is more important is where and for how long you are making the
power. This will also assist in gearing the rear end and picking possible
future modifications. Also, you can get a better idea of ideal launch RPMs,
where you want your shift points to be RPM wise, stall speed on a converter.
Deep into the 7000's on a 327? What valve springs where you running?
This would be a good thing to know. Pull it out and see if any shops near
you have a cam dr. that can give you the LSA, lift and durration. Plus,
knowing this helps after/before the dyno so if you aren't happy with where
say your tq curve is. You can modify your choice of cam's accordingly.
Another good thing to know. Find someone with an auto scale and get this
number. Also good when picking your converter.
You can break any thing with the right amount of abuse. I stand by my
statement that a 10 bolt will easily take up to 500 hp. The axles may
or may not be up to the task depending on the torque numbers that the
engine is putting out. I'd worry more about the driveshaft than the
rear components though.
A dyno run is the best way to tell where your making power, no
arguments there. How many of us are lucky enough to have a chassis
dyno nearby though. I have a shop near me but it can get expensive,
Not believable? Its not a hard number to get to.
The heaviest I could find along with the strongest pushrods I could
get my hands on. I still bent a pushrod about 3 times a year. Valve
float would arrive at around 7,500 to 7,750 rpm. I don't remember the
spring rates, sorry.
I should have put in solid lifters but didn't. I think I could have
really gotten a good shot at destroying the motor if I did. As it is,
it is still living somewhere in Maryland.
The thing you need to understand is that GM made a lot of different
differentials with 10 bolt covers. Some were a lot stronger than others.
Generally speaking the corporate 10 bolts from the 70's were pretty damn
strong. The 10 bolt Chevy's from the 60's were so-so, and once you get into
the 80's forward they were marginally adequate for the stock applications.
As for the 327 turning 7k, no problem! 327's were known for being high
reving motors. Back in the 60's & 70's 327's that would turn 7500 were
fairly common. Many people would just take a stock 327 passenger car motor
and stick in the old 30-30 Duntov cam, and go for it.
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