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  • 1 Post By DanEC

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  #1 (permalink)  
Old 12-23-2020, 02:14 PM
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Cobra Make, Engine: 289 ERA
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Default Rear Diff

I have ERA3001 which is I believe their first 289 street car. It has a 289 w/a Galaxie T10 4-speed and what I think is a 3.xx Jag rear. My issue is that 1st gear is difficult to start out in as it's very high. The first time I drove it I thought I was in 3rd. My question is will installing a 3.5x or lower rear alleviate much of this issue or do I need to look at the gearbox instead? I believe the current ratio is a 3.07 or 3.31.

Any help would be greatly appreciated. Thanks!
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Old 12-23-2020, 02:30 PM
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From your comments I would suspect a 373 would be good for you.
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Old 12-23-2020, 02:48 PM
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Can you measure your rear tyre rolling diameter?

Can you verify what diff ratio you have now?

We need to know more before giving a recommendation on gear ratio selection.

I would have a Richmond 5 speed or Tremec TKZ 5 speed with 3.73s.

Gary
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Old 12-23-2020, 03:25 PM
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Quote:
Originally Posted by Stables Auto View Post
I have ERA3001 which is I believe their first 289 street car. It has a 289 w/a Galaxie T10 4-speed and what I think is a 3.xx Jag rear. My issue is that 1st gear is difficult to start out in as it's very high. The first time I drove it I thought I was in 3rd. My question is will installing a 3.5x or lower rear alleviate much of this issue or do I need to look at the gearbox instead? I believe the current ratio is a 3.07 or 3.31.

Any help would be greatly appreciated. Thanks!

If your car is lazy in first gear with a 3.07 or a 3.31 first gear, the problem is not your transmission, the problem is your engine's low speed torque. Something is wrong with the tune or with the build. You should be able to light up the tires at virtually any engine speed above idle with a transmission first gear like either of those — irrespective of your rear end ratio.


Ed


p.s. I don't believe any T-10 had a 3.07 or a 3.31 first gear so I am going to believe you are speaking to a ring and pinion. Commercially available T-10 first gear ratios would look like 3.42, 2.88, 2.64 or 2.43. None of those first gears would have any problem with a 3.07 or a 3.31 rear end gear ratio. This brings me back to tuning and/or build.

There was also a 2.20 (or thereabouts) close ratio gear set. If you managed to get a 2.20 first gear and a 3.07 ring and pinion with an aluminum flywheel you might have difficulty. The short way home is to put in a steel wheel.
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Last edited by eschaider; 12-24-2020 at 02:22 PM.. Reason: Spelling and Grammar
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Old 12-24-2020, 12:40 PM
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Something around 3.5 would be a good compromise, I believe my fathers original 289 Cobra had a 3.5 gear. If you have a lightweight flywheel it can make starts from a stop difficult with a tall rear end gear.
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Old 12-24-2020, 01:26 PM
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Default T5 trans

Hi,
I would go to a 1990 mustang T5 trans with 3.35 1st gear ratio. Your trans probably has a 2.24 to 2.64 1st gear ratio and is way too high for a small block and you would have overdrive if you wanted to cruise 75 mph down the hwy.
Perry
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Old 12-24-2020, 03:14 PM
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Are you sure you weren't actually in 3rd? When I first got my ERA running I started out a bunch of times in 3rd until I got the feel of the reversed lever shifter down. However a 289 car has a more conventional shifter handle. Haven't done it in a long time now.
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Old 12-25-2020, 05:45 AM
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I’m guessing you have a close ratio unit, 2.36 first gear. Found this out- Ford used a 2.73 first gear in the Falcon, Comet and Mustang models, and a 2.36 first gear in their larger cars and the Shelbys

Well, if you’re satisfied how 4th gear is set up, you could switch to a wide ratio toploader, 2.78 first.

Like stated above, similarly, if you determine your t10 is a close ratio, you could swap out to a wide ratio unit.

Also like stated above, you could switch out the tranny for a 5 speed, ?t5/tremec, which has a lower 1st gear, and now overdrive.

Finally, if you are ok with the engine running a little higher in 4th, you could swap steeper gears in the rear.

And the option of swapping both the tranny and rear gears, which obviously would be the most expensive.
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Last edited by Anthony; 12-25-2020 at 06:29 AM..
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Old 12-25-2020, 07:33 AM
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I don't have any details beyond the original basic kit invoice. The car was built by Chuck Gutke of Cobra Restorers (Kennesaw, GA). He passed away several years ago. I believe that his wife Sandra continues the business, so there's a small chance that she has more information.
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Old 12-25-2020, 08:26 AM
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Quote:
Originally Posted by Stables Auto View Post
I have ERA3001 which is I believe their first 289 street car. It has a 289 w/a Galaxie T10 4-speed and what I think is a 3.xx Jag rear. My issue is that 1st gear is difficult to start out in as it's very high. The first time I drove it I thought I was in 3rd. My question is will installing a 3.5x or lower rear alleviate much of this issue or do I need to look at the gearbox instead? I believe the current ratio is a 3.07 or 3.31.

Any help would be greatly appreciated. Thanks!
I'm betting you are starting in 3rd gear.

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Old 12-25-2020, 09:07 AM
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Me too. Try starting off down in second and see if that's easier. If it is, then you're definitely starting off in third.
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