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Old 11-08-2005, 02:13 AM
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Rear End Gears???

I am currently exploring ideas for a 1984 firebird that I am going to build as a street car. I plan on having a 407 in it and would like to see at least 500hp. I plan on around a 2500-6000 rpm range. I will have an automatic transmission either a turbo 350 or a th400. My confusion comes in at the rear gears and stall converters. by reading a few posts on here and talking to a few buddies of mine I was under the impression that for a street car, a lower gear ratio around 3.55 or so would be a good idea then I found that 2600-2800 stall would also be a good idea for the common street car.
After seeking some professional advice through e-mails, I was told that to more effectively transfer hp to the rear wheels, a 4.10 rear gear with a 3500 stall would be better. I was thinking this was more for a drag type application.
If I was wanting a car for the street/strip, cruising weekend car that would rartely go on the highway would the better idea be a 3.55 with 2600-2800 stall, a 4.10 with a 3500 stall or something in between? I appreciate all help! Thanks!

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Old 11-08-2005, 07:01 AM
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If you plan on putting 500 Hp through a stock 7.5 rear end its not going to last long time. If it hooks up too man times it will shred teeth off the ring gear. Thats from experience. You can add a girdle to the differential but thats just putting a bandaid on it.

Your stall converter is going to be pretty dependent on your engine combo and its designed rpm/power range. Do you have any specs on your engine combo.... cam specs,heads,compression ratio,intake???

IMO.... 3.73 gear and 3000 stall would be a happy medium but then again it depends on your engine combo.

give some specs on that 407
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Old 11-08-2005, 07:26 PM
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Definetly the 4.10's,,they'll put a wider smile on your face. don't know about the converter though, shifting gears puts a wider smile on my face.
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Old 11-08-2005, 08:29 PM
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You don't need steep gears like 4.10's for that 407, especially if you want it to be streetable. A more moderate gear like that 3.73 would be the best of both worlds.
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Old 11-09-2005, 11:36 PM
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Quote:
Originally Posted by bigbadbowtie
give some specs on that 407
Cam: .372" lift, .558" lift with 1.6 rockers, 112 degrees of seperation
Heads: 67cc combustion, 215cc intake, 78cc exhaust
looking for 10 or 11 to 1 compression. Fully carburated with edelbrock Super Victor intake.

I hope that helps man. I really appreciate y our comments.


As for the other two comments, Im starting to lean more towards a 3.73...4.10 might be too much for a street car. One of my buddies with a turbo 302 stang had 4.11 gears and didn't like cruising at 4 grand and 55mph but Im sure there are ways to adjust that too. Im still confused. Thanks to all!
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Old 11-10-2005, 09:40 AM
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You won't make 500 hp at 6000 with a 406.

You'll need 13 or better CR. and need to turn it 7200.

3.73 are a good choice with a TH400 and 3000 stall.

Oh...a trans cooler is mandatory.
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Old 11-10-2005, 10:10 AM
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Quote:
67cc combustion, 215cc intake, 78cc exhaust
Won't be driving that on the street. I just built a 406, 458 HP, 460 lbs torque, on pump gas, and it wasn't easy. It cost me $6700 US. For another $2000, I could have purchased 500 HP on pump gas.

I used 5.7 Scat, forged, cap screw, I beam rods. I had to shave 4 of the rods to clear a .520 roller cam. For a .588 cam you will require H beam rods. H beam 5.65 rods cost a fortune. A 5.7 H beam is cheaper but still a couple hundred more than an I beam.

1.6 rockers will cause you geometry problems. Comp sells retro roller cams with 5.2 roller rockers and retro roller pushrods that are a matched set.

A 407 with 64cc heads, 5.65 rods, flat tops, will be 11.6 compression. Not on pump gas. There goes your street driving.

The 5.7 rod will give you a 409 at 11.7 compression with a 64 cc head. 10.8 compression with a 70 cc head.

A 160 cc runner is going to give you streetability as well as better low end response. Especially with 2800 stall. To get a 220 runner to work with that cam. You should have at least 4000 stall. There goes your street driving.

By going to an AFR 170cc head and a .600 roller you may achieve 500 HP but you are going to beat the valve springs out of the engine within a ten thousand miles. With 1.6 rockers you'll probably kill them within 5000 miles.

Then you have to keep this thing cool. You will require an aluminum rad.
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Last edited by bracketeer; 11-10-2005 at 10:48 AM.
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Old 11-10-2005, 02:07 PM
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Quote:
Originally Posted by bracketeer
A 407 with 64cc heads, 5.65 rods, flat tops, will be 11.6 compression. Not on pump gas. There goes your street driving.
Would the compression only be worse if the rods were 6.000 H beams? Because those were the kind that I was going to use on it. The original engine is a 350, with a 4.00" stroke from the crank and 6.000" H beam rods and a lil bit of boreing it was going to end up as a 407.

Is my entire project not possible? If not it was a good imagination I guess? Thanks guys
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Old 11-10-2005, 08:21 PM
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Phoenix7, for gears 3.70 - 3.73 with a 26 or 28 inch tire, to be streetable use a TCI Breakaway Torque has approx. 2200-2400 stall and works for 265 to 280 duration cams, and DO NOT forget a tranmission cooler. I suggest not to go any larger on the torgue in street traffic, we all know that heat is a transmissions' enemy. Hope this helps!
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