rear diff ratios??

hinds57 · #1 (**permalink**) 12-04-2004, 01:52 PM

Hey guys!

Wondering if any of you could help explain the benefits of these 3 gear ratio options? 3.54:1 , 3.31:1 , 3.73:1. I'm trying to decide which is best. I would like my FE to run at a comfortable low rpm at say 60-70 mph, but still be appropriate for some city traffic. Any input would help.

Thanks in advance, hinds

CSCHWER · #2 (**permalink**) 12-04-2004, 02:03 PM

With the 3.54 you will probably be turning about 3200 rpm at 70mph. The others will be about 200 rpm more and less. This is with a 1:1 4th ratio

Chet

Ron61 · #3 (**permalink**) 12-04-2004, 02:28 PM

Hinds,

Are you running a 4 speed or 5 speed? I run 3:27 gears with a 5 speed and a .6 split, but if I had it to do over again I would go with the .8 split. In 5th at 70 I tach 1900 RPM and in any other gear I like the 3:27s over the 3:91s in my other Cobra. These are light cars with not a lot of weight on the rear end and 1st gear with the 3:27s is almost useless with my tires. I do nearly all of my street driving in 3rd and 4th, and around town 2nd is great. And with a big block, you will have more torque and power than I have. So the lower gears will be harder to get traction with and if I were you and wasn't going to drag race, I would definately stay above the 3:73s.

Ron

tony martin · #4 (**permalink**) 12-04-2004, 02:34 PM

HI THERE,

GO TO THE SITE BELOW WHERE U WILL FIND ALL THE CALCS. U NEED.

REGARDS

http://www.smokemup.com/auto_math/index.php

ENTDOC · #5 (**permalink**) 12-04-2004, 02:50 PM

most would suggest a 3.31 or 3.07 with a big block, especially with a 4 speed. chuck

Roger Vincent · #6 (**permalink**) 12-04-2004, 03:01 PM

Quote:

Originally posted by hinds57

Hey guys!

Wondering if any of you could help explain the benefits of these 3 gear ratio options? 3.54:1 , 3.31:1 , 3.73:1. I'm trying to decide which is best. I would like my FE to run at a comfortable low rpm at say 60-70 mph, but still be appropriate for some city traffic. Any input would help.

Thanks in advance, hinds

This site might also be of help to you: http://www.secondstrike.com/Technical/GearCalc.asp

Excaliber · #7 (**permalink**) 12-04-2004, 05:41 PM

3:31 here, but you also have to calculate the transmission gears to get it "right".

I have a close ratio top loader, not the wide ratio. There is a BIG difference between the two in 1st gear ratio.

I shift into second at about 65 mph, thats not so much the rear gear as it is the combination of rear/trans gear.

Cruising rpm for me was about 2800 (4th. 1 to 1) but the motor like 3200 or higher better. SO, I'm changing cam profile to match my expected cruise rpm.

...I guess it's not a simple question.

If I had my choice I'd go with 3.31 and a wide ratio trans. If a 5 speed I'd go 3.5 or lower and PAY ATTENTION to what Ron said. The RIGHT 5th gear ratio is a MUST have!

Chaplin · #8 (**permalink**) 12-04-2004, 05:58 PM

I have 3.31s with TKO II (.8 5th gear) and think it is about perfect. RPM is right around 26-2700 at 70-75, which is pretty good for highway cruising. But it really depends on what cam you are running, because the cam dictates in what RPM your engine will run efficiently. Once you have that figured out, match your gear ratio accordingly.

Ernesto is right about the trans gears- if you wanted to get a 5sp, I would get the TKO 600 which has the same .8 5th gear as the TKO II, but 1st gear is in the 600 is more usable than the TKO II (which is still not too bad- I think 1st is good to about 40-45 mph?)- can't remember the exact ratios of each off hand at the moment though.

decooney · #9 (**permalink**) 12-04-2004, 08:04 PM

Its an answer that depends on other inputs too. It sort of depends on what spec motor/cam you use, what trans you'll use, how tall your rear tires will be, and what type of driving you will be doing.

e.g. If you build a mild FE, than you might not even be happy with 3.31s for freeway driving, and 3.54s or lower will be worse. Bigger cams in the FEs do great with 3.31s on the freeway, however if you do a lot of freeway driving, then a 5-speed is going to be a must if you choose spin the motor less. The more I've tried different combinations, the more I've learned how each change can make a noticeable difference.

Do you have an idea of what cam spec, trans type, tire size, and driving distances you'll be looking at for your ERA?

Excaliber · #10 (**permalink**) 12-05-2004, 01:20 AM

I know to many times with my past cars in general the gearing makes a huge difference in how the car "feels".

You learn to live with whatever you end up with,,,,, but it's always in the back of your mind. I "wish" I had a lower/higher/different gear combo. The motor just aint "right" unless I'm doing a certain speed, etc.

Gearing and cam selection I'm afraid is something most of us just don't spend enough time on. Once the build is done, it's extremely difficult to change it. You just "learn to live with it",.

slithering · #11 (**permalink**) 12-05-2004, 05:46 AM

Food for thought, most big blocks( especially 427's) don't like to run at low rpm's, below 2k. I'm running 350's, I like the quickness of the wind-up.

Chris

Anthony · #12 (**permalink**) 12-05-2004, 06:11 AM

The choice of the transmission you're using will detrermine the significance of the different rear ratio's, including ones you didn't list.

vettestr · #13 (**permalink**) 12-05-2004, 07:17 AM

Here is a link to software that will show 1/4 mile ET and times with speed for each gear and all the items are variable. Meaning you can repeat this run with 28 or 28.5 tire and a 3.55 or a 3.73 ratio rear or a different 1st gear ratio = etc... They charge 5 bucks for 30 days of use and as many times in that 30 days as you can input so DO YOUR homework as heard so many times. I use this all the time and find it is reasonably accurate with real world outcomes. Know you do not want to drag car but this is great info for dialing in . http://www.smokemup.com/auto_math/edragstrip.php

RICK LAKE · #14 (**permalink**) 12-05-2004, 08:12 AM

hinds57 Hope you get the ERA car. Now for the fun. You have the motor. How much power?

Let's say 450hp, what trans are you going with? auto,4speed, 5speed, or 6 speed? Once you figure out your trans the rear ratio will take care of itself. You will have a Jag rear. Your ratios 2.88,3.07,3.31,3.54,3.73,3.91,4.09. I run a 3.31 with a 360hp motor, runs 12's in the 1/4 idles at 700 rpm with a glass of water on it and runs on reg gas. What are you going to do with the car? Cruise,roadrace, car shows, or police magnet??

Try and give somemore info and the CC club will give you the best drive train ideas for the car. My thoughts are 5speed with 307s no overdrive. 6speed with 3.31. For your motor build a tork monster. 4speed 3.07 wide ratio trans.Let's get some motor numbers.

Rick Lake

Excaliber · #15 (**permalink**) 12-05-2004, 10:05 AM

Five speed with 5th being 1 to 1 and a 3.07 rear gear?

Very interesting, I think you might be onto something. That would give you an adequate low 1st gear compared to the high rear gear. But the final gear ratio, due to the 3.07 would make cruising rpm delightful.

Of course cruising rpm is more about cam selection than any other single factor.

Now WHAT trans has a 5 speed with no overdrive? BMW is the only one I know off. And of course an ERA almost demands a big block FE.

Turk · #16 (**permalink**) 12-05-2004, 10:54 AM

Am I the only one here with a 3.42 rear end?

Excaliber · #17 (**permalink**) 12-05-2004, 10:56 AM

With my close ratio (and the high 1st gear) I wish I had a 3.42!

RICK LAKE · #18 (**permalink**) 12-05-2004, 12:53 PM

Excaliber Nash or Richmand sell a 5 speed that has a 1-1 5 gear. Jegs and Summit sell it. I have a G-Force that has a 1-1 5th gear. Alot depends on the Tork and HP band of the car and what you are going to do with the car. If you have a car that doesn't come on the cam until 3500 rpm this motor is not going to like to lug around and 2200 rpm. A high gear for this car would be better and a little more drivable. 3.54 or 3.73, however the down side of low gas mileage and headphones for the sidepipe. The motor I have comes on cam at 1800 rpm and with the wide ratio 5 spd or 6 spd, the 3.31 are perfect and help the motor whine up quicker. Down side is at 5400 rpm the motor is all done with power. I have a 6 grand chip in my MSD6AL. It saves the motor in many ways. Al my trans have GM input splines for the clutch because of easier to get a GM clutch than find a Ford clutch when racing. You many want to look into an Alum 20 lbs flywheel, with a 2600 car you don't need 40 lbs for a car this lite. The motor will rev quicker also. I went 3 seconds faster this year at R&G, same course only difference was flywheel. Just a thought. Rick Lake

Ant · #19 (**permalink**) 12-05-2004, 02:16 PM

I intend on running a 3.27 rear end on 26.5" rubber, with motor running at around 2500-2800rpm cruising, not sure what camshaft yet, priority is to have as close to 600bhp on mostly pumpgas 427 as possible!

Transmissions that I am aware of with 1.1 Fifth gears are:

Richmond 5 speed
Jerico 5 speed and any race trans