Need Help with Gearing

priobe · #1 (**permalink**) 07-13-2004, 11:25 AM

Hello,
I am building a cobra EM with a 427 SO and toploader. If I am not mistaken the Tranny is a close ratio and the rear end is a 8.8 diff. with 3:73 gears.
I have been getting mixed feeling about changing the rear gear.

A friend is telling me to use 3:23 gears.

Does anyone have a simular setup? Or can help me with the gearing.
I really dont want to pull the body up to change the gear, if it is undrivable.

I want to be able to enjoy my cobra

Thanks in advance!

lewismc · #2 (**permalink**) 07-13-2004, 11:52 AM

I have an EM I built a few years back. My EM has a corvette differential with a 3:73 rear, 5 speed Richmond transmission powered by a 351C. I have no problems with this set up.

My car has no problem launching with these low gears and I'm not turning more than 3500 rpms on the highway. The top speed is ~150mph without an overdrive gear. Which by the way is pretty damn fast!

With the torque from your 427 you could certianly put taller gears in your rear diff. without it effecting your low speed performance. But I don't think you really have to. Your car should take off like a rocket.

Good luck,
Lew

priobe · #3 (**permalink**) 07-13-2004, 12:55 PM

lewismc,
Thanks for your reply. I am a little concerned about the 4 speed trans and that gear. I see that you have a 5 speed do you think there will be much of a difference?

SCOBRAC · #4 (**permalink**) 07-13-2004, 01:00 PM

I wouldn't go with 3.73's enless you are not doing much long distance or are are mostly building it for 0-60 1/8th and 1/4 mile times.

I am using 3.54's and they are pretty low for the freeway. I'm at 3200 at 71 mph. But they will get you into the 11's depending on power and still give you a top speed of 155 mph.

I am considering 3.31's

priobe · #5 (**permalink**) 07-13-2004, 01:32 PM

SCOBRAC

Well the motor was Dynoed at 551hp.

Are you also running a toploader 4 speed?

rdorman · #6 (**permalink**) 07-13-2004, 01:53 PM

If it where me, I would go three twenty three!
Rick

priobe · #7 (**permalink**) 07-13-2004, 01:59 PM

rdorman,
Thats what a friend of mine was recommending. Do you think the 3:73 is too much.
If I purchase new gears. I want to make sure that I purchase the right gear.
What do you think the speeds would be?

MidOHasp · #8 (**permalink**) 07-13-2004, 02:02 PM

priobe...

give me an email address and I will give you a gearing calculator I wrote that has some popular transmission choices already programmed in. All you have to know is tire dimensions.....

Clois Harlan · #9 (**permalink**) 07-13-2004, 02:07 PM

Ditto with what Michael said...I too am considering 3.31 gears.
I can spin my tires off with the 3.54 gears if I want.

You are going to have plenty of torque and rpm. If I were running a small block and a T-5 tranny I would definitly be using 3.73 gears.

Clois

priobe · #10 (**permalink**) 07-13-2004, 02:26 PM

MidOHasp,
Thanks
email address is
priobe@bellsouth.net

Is the program pretty acurrate?

MidOHasp · #11 (**permalink**) 07-13-2004, 03:02 PM

Priobe, you have mail.

100% accurate but does not account for underinflated tires or human user error.

Very simple calculations...gears and revolutions per minute to not lie unless you have a slipping clutch.

Just make sure you enter your info properly and it will be accurate. I promise. I have cross referenced other gearing calcs, etc...

rdorman · #12 (**permalink**) 07-14-2004, 06:45 AM

You left out some important information for a best recommendation. Namely, primary use. Is this a cruiser, open track car, drag car, etc. What size tires will you be using?

I said 3.23 for a variety of reasons. With mountains of torque and power in a light vehicle there is no need for additional torque multiplication. With a four speed and one to one fourth gear you will probably find 3.73 a bit much for freeway cruising (3600 RPM with 275/60/15 at 80, 3100 at 70). That with a motor that will generally not rev as high as a small block, etc, etc.

On top of that, after years of reading this very same question, those with similar setups seem to prefer the 3.23.

Hope that helps!

Rick

Ron61 · #13 (**permalink**) 07-14-2004, 07:10 AM

Priobe,

I have had 3:27 gears in my car since I built it in 1996 and I don't even have the BB. But I do have a 5 speed and with the .6 split I tac 1900 RPM at 70 on the highway. Even with the locker rear end, first and second gear are almost worthless when coming out of a turn as it just breaks loose. Your BB with the high torque would be even worse. At 60 MPH in 4th I tac 2525 RPM which has been verified by radar, so I am not turning the engine over real hard. If you are going to drag race, then lower gears definately. The only thing I would change in mine if I could keep it would be the split in the tranny. I would most likely go to the .8 split.

Ron

priobe · #14 (**permalink**) 07-14-2004, 08:39 AM

Thanks everyone for the feed back.
I think it will best to go with a 3:23 gears.I would love to enjoy the car as well as wipe the streets clean with it.

This sounds like it would be a all around gear.

My concern was also with motor noise.. at cruising speeds. But I guess there is noway around that.

How bad is the cruising noise. I heard some people recommend a set of ear plugs..

lewismc · #15 (**permalink**) 07-14-2004, 09:48 AM

What noise? Did you say something? Huh? Could you please speak up.

The noise is the best part!!!

priobe · #16 (**permalink**) 07-14-2004, 10:13 AM

Exactly,
I love the noise. I just want to always hear it

Ron61 · #17 (**permalink**) 07-14-2004, 10:25 AM

Priobe,

I do wear ear plugs on long drives. All so when around town as the noise from all the car alarms going off is defening.

On short drives or cruises I don't wear the ear plugs, but on a longer drive that noise gets to me and I can't hear for 5 minutes after getting out of the car. And mine is quite compared to some of themaround here.

Ron

bmcmillan · #18 (**permalink**) 07-23-2004, 07:41 PM

I would have used 2.73's if the Ford Motorsports gears were available in that ratio. 2,500 RPM's is turning too fast in a BB 2,500 lb. Cobra on the highway. You need to be thinking about 1,800 RPM.

If the suspension is set up correctly, you have a serious highway cruiser with the higher gears.

For track, the FE is such a stump puller that you will actually turn better times if you don't rev it as much.

Definitely the 3.23's.

Brian

CC5S · #19 (**permalink**) 07-23-2004, 08:31 PM

Look into the Tremec TKO 600 or TKO 600 (road race) 5-Speed tranny and go with 3.73 rear gears. New TKO can handle 600 ft/lb torque which is nice.

TKO 600 5th gear = .64

TKO 600 (road race) 5th gear = .82

If you do not race and want to cruse, go with TKO 600. If you race or want higher RPM/noise go with TKO 600 (road race).

Big plus for the TKO 600's is that it has 8 different shifter locations, based on where you want it the shifter can pop up about most good locations for your Cobra

Good luck.