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02-09-2013, 11:24 AM
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CC Member
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Join Date: Aug 2012
Posts: 3
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Not Ranked
Backdraft build - Diff question
Hi guys
My backdraft cobra build is underway and playing around with some diff ratios.
Any advise out there??
Specs:
Engine = Roush 427R
Engine rated = 550hp, but dyna shows 500hp @ 6000rpm
Gearbox = Tremec TKO600
Gear Ratio:
1st = 2.87; 2nd = 1.89; 3rd = 1.28; 4th = 1; 5th = 0.64
Tyres = 315x18x35
I am going for the progressive slip diff (just to avoid some of that 360deg recoveries).
Have been looking at the 3.9 / 4.11 ratio but not sure what will kill me first: the noise or the power.
Last edited by henkdt; 02-09-2013 at 11:29 AM..
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02-09-2013, 11:42 AM
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CC Member
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Join Date: Nov 2011
Location: Austin,
Tx
Cobra Make, Engine: CSX6128 with Shelby 468FE
Posts: 63
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Not Ranked
Henkdt,
I would talk to Jay or Brian at Vintage but I am running the Coyote with a TKO 600 and 3.91 ratio.
I am pretty sure my previous BDR which had a 427R and TKO 600 was running a 3.63 ratio.
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02-09-2013, 11:48 AM
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CC Member
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Join Date: Aug 2012
Posts: 3
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thanks - how did she perform? power and fuel wise.
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02-09-2013, 12:00 PM
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CC Member
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Join Date: Nov 2011
Location: Austin,
Tx
Cobra Make, Engine: CSX6128 with Shelby 468FE
Posts: 63
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Not Ranked
It was a great combo. Loads of power and acceleration. Mpg was around 12 which was about what I expected.
I am no expert but I would have thought that the 427R with a 3.9 or a 4.1 diff maybe too much, particularly with all the torque that engine kicks out. The coyote is a lower torque, higher revving unit and the 3.9 suits it perfectly.
Andig
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02-09-2013, 12:13 PM
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CC Member
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Join Date: May 2009
Location: American Fork,
Ut
Cobra Make, Engine: 66 Cobra
Posts: 930
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Not Ranked
I was just working on a friends Shelby aluminum block 427 this morning for his Kirkham and we had this same conversation. With the .64 OD I would go with 3.50, but with the .82 OD 3.43. With the 3.9/4.11 gears first gear will be useless and and you are just going to spin the tires way to easy. So I vote for the 3.50s.
__________________
Wayne
"Everything is alive. If you get angry at a vehicle or the trans, it won't fix until you apologize and say you are sorry." "The vehicle always knows what it is doing and what the cause of it's bad feeling is. If you ask it humbly what the problem is, it will tell you. Then you and it will both be happy."
Gil Younger
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02-09-2013, 01:23 PM
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CC Member
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Join Date: Jun 2006
Cobra Make, Engine: Superformance, Roush 402R, Kentucky Cobra Club
Posts: 223
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Not Ranked
Plugging values into this tool and then building a spreadsheet (optional) will provide insights.
Second Strike Gearing Calculator
How you intend to use the car makes all the difference. |
02-09-2013, 03:13 PM
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Join Date: Mar 2012
Posts: 445
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With the 427 I would do the 3.46 unless you just like spinning tires in 1-3, and in that case, go with the 4.11.
If I had the Coyote I'd do the 3.91 or 4.11.
I have the 3.15 rear and spin too much in 1 and 2 during normal driving with an engine similar to the Roush 427R, but when the tires are heated up I start thinking that the 3.46 would be nice.
Last edited by itstock; 02-09-2013 at 03:17 PM..
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02-09-2013, 03:59 PM
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CC Member
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Join Date: Mar 2006
Cobra Make, Engine: BDR354/FMS392
Posts: 382
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My BDR has the same TKO-600. My rear came from BDR as a 3:46 open diff. I switched it out to a 3:73 with 40% lock up. It is way better.
Because the 1st gear is tall at 2.87, you can run alot more gear in the rear without winding 1st gear out at a low mph.
I find the real problem is the .64 OD. The jump from 4th to 5th is just way to great. I really wish I had the .82 OD.
I never noticed a difference in gas milage between the 3:46 and 3:73.
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02-10-2013, 02:22 AM
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CC Member
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Join Date: Aug 2012
Posts: 3
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Gentlemen - this has been awsome feedback. And you have confirmed my worries.
Chilibit - great calculator.
The 3.4 to 3.7 range seem to be the sweetspot.
Now I can worry about the colour - CHEERS!!
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02-10-2013, 09:38 AM
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CC Member
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Join Date: Jan 2008
Location: Nashville,
TN
Cobra Make, Engine: Backdraft, SBF 351w (463 CI)
Posts: 272
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Not Ranked
Why do u want the .64 over the .82? U will eventually want a LSD diff. 
Since u are going with a big V8. I would humbly suggest U go with a 3.46 and .82 OD (75MPH around 2750RPM). if U want a little more performance go with the 3.64 and stay with the .82 OD. (70MPH around 2750RPM).
Last edited by PLDRIVE; 02-10-2013 at 09:49 AM..
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02-10-2013, 10:03 AM
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CC Member
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Join Date: Jan 2009
Location: Jonesboro,
AR
Cobra Make, Engine: Backdraft 874 solid red Roush 427 small block 515 hp
Posts: 572
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3.46 is what I have. Unless you have drag radials, the lower gears are useless.
__________________
There were no atheists in the foxholes.
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02-10-2013, 02:10 PM
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CC Member
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Join Date: Dec 2011
Location: Voorhees,
NJ
Cobra Make, Engine: BDR #1230/B2 445CI SBF
Posts: 295
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I have drag radials and the 3:46 and it is still useless...I cant imagine a shorter rear end in the car provided we are talking 500hp plus. I was discussing a shorter gear with Brent due to the new engine we are building to help with "lugging" but ill just stay in 2nd through the neighborhood 
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Gary Bowman
Voorhees NJ/Key Largo Fla/Ventnor NJ
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02-10-2013, 02:12 PM
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CC Member
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Join Date: Dec 2011
Location: Voorhees,
NJ
Cobra Make, Engine: BDR #1230/B2 445CI SBF
Posts: 295
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Sorry "useless" referring to RPatton3's post about the lower gears...
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Gary Bowman
Voorhees NJ/Key Largo Fla/Ventnor NJ
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02-10-2013, 07:45 PM
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Join Date: Oct 2004
Location: Brunswick,
GA
Cobra Make, Engine: BDR 1311 428PI
Posts: 3,044
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I'm with those guys, 3.46 here, running street legal road race slicks and the lower gears of my TKO600RR are useless at anything above half throttle.
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02-11-2013, 06:56 AM
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CC Member
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Join Date: Dec 2000
Location: Broken Arrow. OK ( South Tulsa), USA,
OK
Cobra Make, Engine: 66 COBRA FE 427 /4SP. (HCS Coupe w/ 408 Stroker and TKO 600 -sold)
Posts: 5,595
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Just my point of view:
I have a 508 hp 408 stroker in a coupe with a TKO 600 with .82 5th gear and I have a 3.50 : 1 detroit locker. I will be replacing my gears with 3.73 : 1 for a little better overall performance. The current setup will spin the tires quiet well but coming out of a corner the increase in torque will help in the overall performance. I took the coupe to the track and found although the 3.50 gears work well a better combo would be going with 3:73 package. Granted, the 3:50 gears will yield better gas mileage but the 3:73 gears will improve track times. You can still cruise the freeways in 5th gear under 3000 RPM at 70+ mph. My redline is above 7000 RPM if I ever need those RPM's.
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02-11-2013, 07:09 AM
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Stolen Avitar
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Join Date: Oct 2004
Location: Brunswick,
GA
Cobra Make, Engine: BDR 1311 428PI
Posts: 3,044
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Not Ranked
Quote:
Originally Posted by Clois Harlan
Just my point of view:
My redline is above 7000 RPM if I ever need those RPM's.
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Clois makes a good point here as mine is a 462" FE that'll never see over 6k RPM and it makes a ton of torque down low. |
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