What ratio is the rear end

[eedeutsch]

I know Backdraft uses BMW series 3 rear ends. What is the ratio ? I have a
450 hp Iconic Custom 427 supplied from Speed Fanatix. The car starts to
lug at 1700 rpm. Is this normal? The car is new one I received in May.

[twin turbo]

I'm not familiar with the Backdrafts, but it's easy to figure your approx rearend gearing by jacking up the rearend of your car and count how many turns your yoke/driveshaft makes for each revolution of your tire. Cheers.

[Cashburn]

Should be a tag on the rear cover of the diff.

[Ron61]

If the tag is missing Isky used to make a small round calculator out of cardboard that had two wheels, one on each side. You could set your RPM and speed with the height of your tires and it would tell you what your gear ratio was. In fact it would tell you everything about your engine and gears. I have a couple that I have use for years and they are as accurate as any computer made today. I don't know if they still even make them, but I called several years ago and they sent me 3 new ones as I had just about worn my original one out and it was hard to read the numbers on it as some of them are quite small. It was easier than the turning the wheel one turn and counting the driveline revolutions, though I did that several times also. Any of the above methods works but for the driveline method to be accurate you need to know the range of gears that came in that years rear end. If the tag is still there that is the best.

Ron

[MaSnaka]

If the tag is there I would suggest you at least verify that the tag is correct and nobody before you swapped gears. I had an 8" with a tag that said 411LS when in actuality it was a 3.80. I never gave it much thought until I swapped rear ends.

John

[GLS-BDR226]

Not to be a wise guy, but couldn't you call and ask? They had the records for my build which was back in 2005.

[427 COBRAJET]

I have a new 2013 BDR # 1246 WITH A Roush 427 R MOTOR 558 HP IT also lugs at 1,700 try keeping it over 2,000 it runs better at 2,200.Rear end gearing is 3:47. Gary

[Trueoo7]

Originally Posted by 427 COBRAJET

I have a new 2013 BDR # 1246 WITH A Roush 427 R MOTOR 558 HP IT also lugs at 1,700 try keeping it over 2,000 it runs better at 2,200.Rear end gearing is 3:47. Gary

If its lugging, thats not the gears you're feeling, thats the cam.

[tpatodd]

I am not sure what my ratio is but I can tell you mine doesn't like anything less than 2000 rpm. Chugs along snapping my neck. I always just figured it was the nut behind the wheel.

[GBowman]

I was going to say the same thing...i need to stay in 1st in the neighborhood

[toddl]

Gary, have you been brave enough to get out of 2nd yet?

[cbreez]

To answer the question, many, many of these cars (like mine did) came with the stock 3.08 or so bmw rear gear...it's very tall and your car could very easily lug unless you keep it in lower gears. It's really too tall for these cars. That being said, you will never use the 5th gear (OD) unless you are tracking down the interstate.

[Cashburn]

The most common rear ratio in production over the years has been the 3.46. The early years the 3.15 was pretty common. Today we see a healthy mix of 3.73 and 3.46, though a 3.15 will come through on occasion. In those cases we often swap it at our shop to match it to the engine and transmission better AND we select the OD of the TKO600 based on this as well.

We had a big block years ago that was fantastic with the 3.15s though.

If you setup the 3 properly when building your car you will use all the gears.