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Old 09-17-2013, 06:10 PM
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Help picking rear gear ratio

Hey guys I need help picking out a gear that fits my project
its a 72 chevy nova weighs about 3000lbs
has a gm 12 bolt rear with a spool
SBC 355 afr heads, edelbrock roller cam .539 lift 234 in/238ex dur at 50
full roller valvetrain single plane intake manifold, Professional products 750cfm throttle body efi, GT45 turbo at 14PSI, th350 trans with a 3000RPM stall tq converter, 33 inch tall tire ....
right now I have 4.56 gears In it and It feels like every time I get into the powerband the car runs out of gear the car doesn't get into boost until around 3000-3200 plus the normal sluggish highway speeds are a killer help me out please
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Old 09-17-2013, 06:17 PM
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Originally Posted by Pupsvette76 View Post
Hey guys I need help picking out a gear that fits my project
its a 72 chevy nova weighs about 3000lbs
has a gm 12 bolt rear with a spool
SBC 355 afr heads, edelbrock roller cam .539 lift 234 in/238ex dur at 50
full roller valvetrain single plane intake manifold, Professional products 750cfm throttle body efi, GT45 turbo at 14PSI, th350 trans with a 3000RPM stall tq converter, 33 inch tall tire ....
right now I have 4.56 gears In it and It feels like every time I get into the powerband the car runs out of gear the car doesn't get into boost until around 3000-3200 plus the normal sluggish highway speeds are a killer help me out please
A set of 3.08's or 3.27 ish would work much much better.....turbo needs to see load, and with a highway gear its going to load and pull hard right to your likely 6500rpm shift point....the converter needs to be very tight cause when the turbo sees that kind of load its going to blow right through it. A tight 2500 stall and 327ish gear is going to make this a terrifying ride lol.
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Old 09-17-2013, 06:22 PM
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A GT45 that isn't seeing boost till 3000+rpm? That seems odd in itself.....
And I apologize, I didn't see your 33" tire size, I used to 28"(275DR)
A set of 355's would likely get the job done.....ap72 will likely chime is soon with some good input.
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Old 09-17-2013, 06:22 PM
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yeah and it is very scary as is lol im a little confused about gear ratio some older hotrodders have used terms like numerically higher and things like that and im lost is there a easy way to understand them...
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Old 09-17-2013, 06:26 PM
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I would be almost willing to bet its already blowing through the converter. This thing should be making 600+ HP and shouldn't ever feel "sluggish" even when starting to spool.
Who's the converter by?
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Old 09-17-2013, 06:34 PM
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its a b&m holeshot converter it lights the tires up at anything more than 1/4 throttle and then when my mph finally get up then I run out of gear and acceleration dies off
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Old 09-17-2013, 06:34 PM
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yeah and it is very scary as is lol im a little confused about gear ratio some older hotrodders have used terms like numerically higher and things like that and im lost is there a easy way to understand them...
If you find it scary now grab some diapers if you get the gear and converter right.
Is it tuned on a dyno or just laptop on the street?
Gears are straightforward.
Right now at 60mph your seeing 2800rpm and
Switching to a 355 will give you just over 2100 rpm at the same speed.
Assuming your turbo is sized right you should be just getting into boost at that rpm.
A tight 2500 converter will make a huge difference.
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Old 09-17-2013, 06:40 PM
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it is self adaptive through a laptop interface very cool yeah I would say im seeing more like 3200 at 60 miles an hour but its hard to say with no speedo
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Old 09-17-2013, 06:40 PM
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its a b&m holeshot converter it lights the tires up at anything more than 1/4 throttle and then when my mph finally get up then I run out of gear and acceleration dies off
Def a converter issue, an off the shelf holeshot, even a nitrous holeshot will not even come close to living behind what your doing. In suspect that's why it feels like its running out of steam so early.
Call precision and talk to them about your combo.
The only reason that converter you have now isn't in a million pieces is the relatively light weigh vehicle and the steep gear, if it had less gear, and a chance to hook you will kiss that trans goodbye.
Has it seen a track yet?
What your MPH and rpm through the lights?
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Old 09-17-2013, 06:49 PM
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it is self adaptive through a laptop interface very cool yeah I would say im seeing more like 3200 at 60 miles an hour but its hard to say with no speedo
Sounds like slippage to me.....the math says 2800@60 with a 33" tire.....
I would look at switching to a 355gear and calling precision and getting something made that suits your combo better. The off the shelf B&M WILL come apart and cost you a transmission. Your motor is a bullet that's half fired lol....a gear and converter change will shock you.
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Old 09-17-2013, 06:51 PM
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it has not been to the track yet I just finished getting the car together why would the converter fail? What more is there to a converter than the stall(another subject im not too well versed in)
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Old 09-17-2013, 07:10 PM
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it has not been to the track yet I just finished getting the car together why would the converter fail? What more is there to a converter than the stall(another subject im not too well versed in)
Converters are designed around vehicle weight, intended usage, HP, torque, and powerband.
Your off the shelf b&m is just fine for a hot street car, but is designed around a baseline, 350hp SBC, 3200# vehicle weight. It has a certain amount of slip designed into it based on that baseline. So a little more or little less power will make it "stall" a little higher or a little lower, not an big deal. But now your trying to put 6-700hp and = amounts of torque through the same converter designed around that 350hp SBC.
Its going to slip bad and create ungodly heat in the trans as well.
A "tight" converter will allow it to meet your powerband without slipping or "blowing through"
Once its locked it will let the Turbo load and just keep putting power down instead of slippage.
I'm not great at explaiing this so I apologize lol, but right now your not even close to putting down the power a motor like this is likely making.
Normally I'm a "budget" kind of guy, and love off the shelf Hughes converters, but with the money you have in this you may as well make the call and do it right
You won't be dissapointed
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Old 09-17-2013, 08:29 PM
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def sounds like the ticket! so you think the gears and the converter are both to blame. Man cars are money pits it never ends never ordered a custom converter before what kind of info do I need in hand before I call I don't wanna sound like a dope when I get on the phone. I was really crossing my fingers that the guys on here would say the gears were probably the cause because its easier and cheaper then replacing both
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Old 09-17-2013, 09:23 PM
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def sounds like the ticket! so you think the gears and the converter are both to blame. Man cars are money pits it never ends never ordered a custom converter before what kind of info do I need in hand before I call I don't wanna sound like a dope when I get on the phone. I was really crossing my fingers that the guys on here would say the gears were probably the cause because its easier and cheaper then replacing both
Actually give these guys a shout
Performance Torque Converters - Performance Torque Converters
I had them and precision mixed up, (not that there is anything wrong with precision) ptc does big HP 10.5 outlaw stuff and are very familiar with high HP turbo cars, they actually have a form on there site to help choose the right piece.
They are also priced suprisingy reasonable for a custom converter.
Yah they are money pits for sure, but your this far into it and no sense using an off the shelf until that will likely come apart sooner rather then later and cost you a trans rebuild as well as a new converter. The beauty of a Turbo motor is your converter will almost act like Dr jekle and Mr Hyde, in low or no boost situations it will not have enough HP to reach its rated stall speed, so it will leave lights like a stock unit, tool around town and not build unwanted heat, but when the boost comes in and it sees a bunch of power it will stall where its built to stall. You can actually get away with a "tight" 3000-3500 stall and a set of 355 gears happily. It will cruise on the highway out of boost with minimal converter slippage and get decent mileage and be docile. You mash the gas tho and it will come alive like you wouldn't believe.
Give them a shout, they are more then help full and will not make you feel silly at all.
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Old 09-18-2013, 09:45 AM
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A few things:

Turbo cars react better with more 'street friendly' rear gear ratios. 3.42 or 3.55 would be a lot better. You REALLY need a dedicated converter for your application. Off-the-shelf is NOT gonna cut it here.
PTC, as mentioned before; but Freakshow if you're really budget conscious would be a very good alternative.
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