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MG MGB GT V8 Factory Originals Technical - best rear end ratio
|I'm in the process of putting together a Dana 44 axle for my '67 V8 project, and I'm wondering what experience others have had with different ratios. I gather that the general consensus is to move away from the 3.90 stock ratio but the 3.07 ratio seems rather long for the purposes of quick off the line acceleration. The Dana can be had with 3.07, 3.23, 3.30, 3.54, & 3.73 ring and pinion sets. I'll be using a BW WCT5 with a 2.95 first gear and .63 OD ratio. My slightly massaged Buick 215 should make at least 185HP and is set for a wide powerband favoring 1500 to 4500 RPM. Thoughts please. cj|
|Chris Jones (TX)|
|Several sites on the web let you plug in all your trans ratios, rear end ratios, and tire sizes to match rpm's in different gears. It's a nice tool for selecting what you want. My A is going with the same trans, a 3.90 with 15" tires(24" tire diameter). Quick first gear acceleration and will turn about 2400 @70 mph in 5th which was OK for my taste.|
|I am going for 3.54. This will give me 60mph at around 2,400 in 5th with standard diameter tyres.|
|Chris--I am running a 3.08 in my GM posi rear end on my 5.0L Ford conversion with a T5 WC --1st ratio of 3.25 and find it just fine to my taste. I do have more HP than you would so that may be a factor in getting off the line.All the best.|
|Oh boy what a can of worms!!|
Gearing I have found more and more is VERY personal issue.
My car last year was stock rear with a mustang drivetrain (WC T5) and stock tires. I found 1st gear absolutely useless except for maybe 1st gear idling in very heavy stop-go traffic. THe few time I went to the track I was launching in 2nd gear and having to use 5th (Major racing no-no) down track as I ran out of motor in 4th gear. I have one of those calculator spreadsheets on my website. I decided it would be best to try and match a stock mustangs rpm/mph. Many mustang people gear down to get better off the line, but also remember they have a good 500-1000lbs more weight to get rolling. The V8 is a torque monster right off idle as compared to a 4cyl or even 6cyl. Torque is what gets mass moving, so given our lighter weight and the torque of the V8 we can be a little more "highway friendly" in our gearing and still gave a quick car of the line. IN THEORY...
So my optimum gearing calculated to be 3.08 however I just got a rear end that has 2.73 installed and I do not have a set of 3.08's for it so I will likely take the extra drop in RPM and see how the 2.73's do..
Let me now if you use MS Excel. I have a quick and dirty spread sheet I worked up for the same purpose. I can email it to you.
I would look at two factors in choosing a rear end ratio. Look at the top speed in 1st gear. If it is real low (speed that is, not ratio) you may not be happy doing a lot of in town driving. Also look at the rpmís at 70 in 5th gear. V8's donít sound happy at sustained high rpmís. About 2,000 to 2,200 rpmís at 70 will give a easy freeway drive. With a 2.95 first gear you can look at 3.23 to 3.54 ratio's. The low fifth gear (.63) would indicate 3.30 to 3.73 as a ratio. With a spreadsheet program you can put in each gear ratio for the transmission and the tire size. Then just alter the rear end ratio and print out the results.
download the 4.5 version.
It has many many cars preloaded including MGB! It's great to play around with to compare cars also. You can play around with different paramters and determine optimum settings.
|If anyone is interested, I uploaded an Excel spreadsheet which can calculate speeds with transmission and differential gearing adjusting for tire size at:|
Easy to use. Enter your gearbox ratios, differential ratio and tire diameter. Then see the results as mph/1000 rpm, speed at xxxx rpm and rpm required to achieve a cruise speed.
I want 3.31 diff for my set-up but hard to find.
|I have multiple versions of mine for various excel versions on the website.|
|I have a 3.23 (8 3/4" chrysler) It works very well with the T-5 .73 5th. First gear is not to low or high, at 70mph rpms are 2350 in 5th. this is with 215/60-14 tires.|
|Here is what has worked for me, all with WC T-5, .63 5th:|
GT with 4.2 Rover, 195/65/15 tires, 3.55 Ford 8", 70 @ 2000rpm 5th.
Roadster, stock 215, same tires, 3.70 MGC, 70 @ 2500 rpm 5th.
GT with 5.2 Buick 300, 215/60/15 rear tires, 70 @ 2000 rpm 5th.
All these cars intended for extensive driving at freeway speeds- 60-80 mph with 1/4 mile speed a secondary consideration. Don't get me wrong, all were/are fast off the line, but only the 300 GT will really push you back in the seat.
The roadster with a stock 215 & 3.70 MGC rear was the real suprise. It has proven to be an excellent combination. I would not have used the 3.70 rear except it dropped in my lap for $100.00 & needed no mods to make it work. I would not recommend it for an engine with much over 200 hp, as 1st gear would be useless. You would shift much too quickly into 2nd.
I'm running 195/60/14 tires, 3.40 Ford 8" non WC T-5 with .63 fifth. It's a great highway cruiser turning a little under 2200 RPM (2175 according to my spreadsheet) in 5th a 70 MPH. Traveling west on I70 into the higher elevations a .73 fifth would be a a little better. Traveling east towards Nebraska or Kansas the 3.40 and .63 fifth is ideal. One of our friends in the local club is running a 3.5 Rover with a SD1 and 3.90 Diff and I believe he can run between Denver and Glenwood Springs mostly in 5th gear.
I think when you drove my car a couple of years ago in Glenwood Springs I was probably still running 175R/14s. The 195 were a real improvement.
Chris, I hope you finish you car in time to drive it to Grand Rapids in August.
|Ok, to add to this, if you have a lot of high end BHP such as a full race motor, you will need to gear differently then a stock low bhp motor. Basically, if your engine has its peak bhp at 6500 rpm, then you not want a 3.08 rear axle like the guy who has a car tat peaks at 4500 rpm.|
So, besides for driving habits and personal preference, you will also need to look at what is correct for your engine torque peak/BHP peak as well as what you want- top end, cruise or best 0 to 60 mph. In general for the V6 that we use has higher torque peaks and holds up at high rpm, so we use 3.4 to 3.7 with LSD- depending on the gearbox and BHP of the motor.
|BMC Brian McCullough|
|Thanks for all the great replies! Despite the wide variety of options presented, I'm still at a loss to pick the best ratio. I guess it's a matter of what feels good and that's what I was really trying to determine. I drove a few V8 projects in Cleveland a few years ago and they all had great throttle reponse. Can anyone who has driven a number of ratios, please tell me what the best for "on ramp" response as well as 60-80 MPH passing. I'm looking for the ratio that makes the most of 11-12 lbs/per BHP. Gee! what a great hobby! cj|
|Chris Jones (TX)|
GREAT POINT!! The greater the torque the more you can get away with a gear like a 3.08. The high RPM also is key. Many pushrod V8's won;t want to rev over 6k in stock form, but they have loads of torque off idle, so a lower gear like a 3.08 or 2.73 works very well. A V6 does not have the torque off the line tso a 3.08 would lug the motor down and make the car feel and start very slow, but they rev higher often, so using a 3.55 or so gearing can let the motor get up into it's power band faster and keep on going..
In the majority of 2.8 to 3.4L V6 GM conversions, I reccomend 3.42 ratio with the standard 140-200 BHP and up to 220 foot pounds of Tq. That is with a standard size tyre and wheel or tire size not terribly far from stock (not to many are). This ratio is best with the Camaro V6 T5 which has a 1st gear of 3.76 and a 5th of 0.72....
The car which has a Camaro V8 T5 and any of the V6 engines listed or not gets the 3.73 ratio. Why would a person hardly change ratios from the stock MG B setup??? Well, the 2.95 1st gear and overdrive 5th of 0.68 (some say 0.63) to 1 make it possible to run close ratio gearing compared to the V6 T5 AND the MG B Stock box.
Also, the majority of people know that the rear axle change is not just for gear ratios, but if they work it right, also for bigger brakes and a quality limited slip differential without the additional cost of the super expensive Quaffe unit.
Most of the V6 engines Love High RPM. I rev my 3.4L to somewhere well beyond the 6,000 rpm tach I have in that vehicle and the engine has never been altered.
From your original postings:
185bhp out of a stock BOP/R 215 ci...
I assume REDLINE of 5,000 to 5,500 rpm Max for the BOP/R
Assumed Tq would be around 210..
I would personally say 3.23. or maybe even as far as 3.42, but that sounds wrong comapred to what most bop/r owners have done.
A 3.07/3.08 is ideal for most BOP/R guys (??????).
Again, For a higher revving V6 (redline somewhere between 6000 and 7000+) (185 bhp and 210 tq) with the same Camaro T5 ratios, as listed above, that would lead me to a 3.73 ratio.
|BMC Brian McCullough|
3.07,.. my factory v8 came as standard with a 3.07 ratio, and it suits the car in its current 190bhp ish guise well. 100mph is delivered at approx 3,300 rpm which makes 70-80mph cruising really relaxing. There has always been enough torque for a good hard standing start without the motor bogging down, in first she will spin up quite easily but still give a long hard pull and the ability to pull ahead at the strip as the opposition ease off to change gear but you still have a few mph to go!. So i would say that even if you do a few 1/4 mile events then 3.07 or 3.27 with larger dia tyres is the way to go.
|I like 3.55 with our cars weight and size it sould give great performance.|
This thread was discussed between 05/03/2004 and 14/03/2004
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