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MG MGB GT V8 Factory Originals Technical - Rear end: open or locker/posi?

I was visiting with the guy i'm getting my transmission from... he runs a mustang shop. Somewhere in the conversation he mentioned that he'd recommend an open rear end rather than a locker unit. rationale was all of the Cobra kits he's worked on were open & he had a customer with a 302 conversion into a Miata who started out with a locker but switched it to an open because it'd fish tail too much -- especially on hard curves & in the rain...

Those cars (cobra kits & the Miata) are both pretty light like our MG's, so it would seem like what makes sense for them would make sense for us... what do you think?

Rob
rficalora

There are lockers and then there are lockers.

A true locked rear end such as a Detroit Locker, or welding the spyder gears is only good for straight line driving- drag racing- and will probably get you killed in the rain.

Factory style posidrive rears are standard on many high performance cars such as the Cobra Mustangs, HiPo Cameros, and many pick up trucks & rear wheel drive SUV's. These are not fully locked until a high rate of acelleration is acheived. They use clutches, cones, or tapered gears to allow them to lock & unlock via the amount of hammer you have down, & whether 1 wheel is slipping. A read of any hot rod or performance magazine will give you several adds to read. Quaffie is the brand of choice for a stock MGB rear- it may be the only choice. I use an Auburn unit in my 8" Ford rear, & D & D offers this as an option in their S-10 rear.

I had tried another brand, but it clunked when it locked, & was pretty harsh. The Auburn is absolutely quiet and seamless as it locks & unlocks. It is great on the strip as well as on the track or autocross.
Jim Stuart


Open differentials are cheap, safe, and easy to drive.

When you wrote "locker" I'm assuming you meant "Detroit Locker", which is a very different animal in design, performance, and price from popular limited-slip differentials, such as Ford's Traction-Lok (which many of us are using.)

In most MGB/V8 applications, a limited-slip differential will make it possible to come out of slow corners significantly quicker. If you're buying and narrowing a junk-yard axle anyway, the price premium for limited-slip isn't great.

Personally, my MGB has a Traction-Lok (in an 8.8 Ford axle from a Mustang), and I don't find the car fish-tails too much.
Curtis

If you want a "posi-type" differential, consider a Detroit Truetrac (see http://www.detroitlocker.com/images/DTT-Handout-04.pdf). It seems to be a Quaife-type, but for larger American rears.

One advantage that I can see is that you can limp home on a smaller-sized spare tire without burning up the limited-slip clutches.

Wayne
Wayne Pearson

Among the "posi-type" differentials (not lockers), there are two distinctly different types: Clutch type and gear type.

Coming out of a corner with the hammer down on a clutch type, the clutches engage, effectively locking the differential, causing the same type of problems that you'd have if you were using a true locker differential, which is a serious amount of under steer. With both rear wheels locked together, they will both turn at the same speed. With both wheels turning at the same speed, the car has a tendency to go in a straight line - not good for turning.

With the gear type, torque is appled equally to each rear wheel, but they are not locked together, so the outside wheel can turn faster than the inside wheel, avoiding the excessive understeer of the clutch type.
Dan Masters

Actually, with a clutch-type differential, the clutches are always engaged as they are applied by spring pressure. The only time the clutches step in is when something happens to make one wheel spin faster than the other, as in a turn. The clutches fight this behavior, wanting both wheels to go the same speed. If one wheel wants to spin faster than the other, it must first overpower the clutch pack (like going around a turn normally with full traction at each tire). But if you hit the throttle and break one tire loose, you are likely to break the other loose as well because you don't have traction with the first tire.

The only reason I mention this at all is that throttle pressure has nothing to do with locking up a clutch-type differential.

Wayne
Wayne Pearson

Again, excellent info... I appreciate you guys helping me along...

Sounds like i do want a gear type posi then. I'm getting the rear end casing from a guy recommended to me by the shop i visited yesterday... he goes around to the local yards & pulls the good ones & then sells them to the local performance shops. Assuming I can go with a used chunk, is there a brand or type I should tell him other than just saying I want a good gear type posi?

Also, on a related topic, I'm also working out what ratio I want & realized I don't know what some ideal RPMs are... I have the formulas to figure out what the RPM will be at various speeds & have the gear ratios to figure out what speed I'll be in each gear at various shift points, but what I don't know is what a good 70mph cruising RPM is -- e.g., what will give good fuel economy but still have reasonable acceleration left. Similarly, what RPM should I be after as a target shift point RPM?

Motor will be 302 about as follows, standard bore, CAM like Edelbrock 2221 [.52 lift/Duration at 050 inch Lift: 227 int./234 exh.]; Weiand Stealth or Edelbrock Performer RPM intake, and 650CFM Edelbrock carb.

rficalora

My limited knowledge is that the gear-type unit is preferable to a clutch-type, but also significantly more expensive. Which is why I'm sticking with the ford traction-lock unit for now. A Detroit Truetrac gear-type unit for my 8.8 is just shy of $500. What type of rear end did you decide to use?
Ryan Reis

Aftermarket limited-slip differentials are in the $4-500 range. I don't think there are any gear-type as OEM (American).

My opinion: Whatever rear you choose, if it has a "posi", try it out and see what you think. You can always take out the differential and sell it, and replace it with an aftermarket unit.

Another opinion: I think the cam you're picking is not going to have enough torque to cruise and get good economy. I'm assuming you want to cruise at 70 mph at 2-2500 rpms in OD. It all depends on what you want to do with the car and what compromises you are willing to make.

Wayne
Wayne Pearson

There are also locking units that replace the stock spider gears in a stock rear end such as the Powertrax units and the older Detroit Locker units. Talk about an easy install! You also have to be aware that they clunk very loud as you turn corners and the gears release. Is that corrected in the newer units?

Jeff Schlemmer

clutch pack difs. such as the Ford Trac-Loc are ok , but i believe road racers and others that do not drag race swear by the Torsen dif. ck it on Google and see its advantages. it is similar to the Quaife? type but costs less. Ford Racing has it for around $500 retail,dealer net is less. I am pleased with the Trac-Loc in my 8.8. Used the alternate clutch friction disc order rather than the factory sequence which added 1 friction and deleted 1 steel disc. hooks up great with the 3.55 R/E RPM is just right for cruising with the 205-50-16 tires & wheels.
BTY, anyone narrowing the Mustang 8.8 with drums, DO NO OFF-SET the center carrier, leave it centered. it places the driveshaft too close to the tunnel on the right side. even with the same OD. shaft (thick walled .020) it is close.
Hope to see you'll in Townsend! Merry Xmas Faster
kelly stevenson

Rob, I used the MG diff in my 302 car but with the 3.307 ratio. With the Ford T-5 and the .675 5th, it turns 2500 RPM at 80 MPH with 195-65-15 tires. This seems just about right to me - not too short or long. It revs strongly past 6000 RPM with the standard HO cam and AFR heads so I would be tempted to spend money on heads before the cam. I have a Quaife which you wouldn't know was there cornering but will spin both wheels in a straight line. Only problem is they are super expensive - about $900 USD (out of the UK). But it does work well if you are using the standard casing. Otherwise the relatively cheap LSD's for the Ford axle should work fine. Good luck, Bob.
Bob Elwin

Wayne (and others)...

RE: Another opinion: I think the cam you're picking is not going to have enough torque to cruise and get good economy.

Can you help me understand that -- e.g. what's the relationship between CAM torque & fuel economy at highway speeds? Till you said that I would have thought it was just the opposite... the more torque the cam produced the lower the fuel economy???
rficalora

Quite true, but the rpm range where the torque occurs is important. The 2221 cam you mention has a range of 1500-6500 rpms, compared to a stock cam of, say (I'm guessing), 800-1,000 rpms to 4,800-5,000 rpms. The stock cam will produce more torque at your cruise rpms than the aftermarket cam, which means that you will need more throttle than the stocker to make the required torque to maintain cruise rpms.

Wayne
Wayne Pearson

This thread was discussed between 22/12/2005 and 23/12/2005

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