Rear Gear Recommendations

[69Rcode_Mach1]

Hey guys my car is in the shop to have some paintwork done from that little accident i went crazy over. All the sheetmetal was saved and car just had paint damage. Anyways i figured i might want to blow some bucks while its there and put in some rear gears. They can get me gears with positraction for like 200 bucks and then 100-120 bucks to install? What do you guys think? I don't have the tools to install the gears myself so a shop will have to do this one. I don't really want to drive all the way back there and i would like something to play with until i get my raceheads and roller rockers(which may be a while). My question is what do you guys think i should get look at my sig for help? I was thinking 3.55's or 3.73's. This car is a daily driver but it doesn't see much freeway driving. My current gearing is 2.75 no posi. Also i wanted to know if i will really feel some difference in my seat of the pants torque from doing this and if the posi will make some good difference with my traction issues? Thanks guys Im hoping these really compliment my new set of exhaust since ive only driven it one day with the new exhaust on and then off to the shop.

 

[fastback brian]

with the comp cam and afr's go with the 3.70-3.89 and a looser converter for street driving. 4.11's and converter for more street strip.

 

[69Rcode_Mach1]

yeah its gonna be a street car so i want it to be driveable. Is there a big difference in mileage between the 3.73's and 3.89's. Also im not going to be keeping the auto. I will be doing a t5 swap instead of the converter once im done with my engine. Also what do you guys think about the other questions? Thanks

 

[fastback brian]

3.89's will work great all around with your planned engine combo and t-5 swap. And for sure a posi, best to get the stronger four pin posi for the best traction.

 

[69Rcode_Mach1]

i guess ill just get 3.93's in my car. Will the posi make a big difference in traction or little? Also how will the power feel going from the 2.75's. My car needs a wow factor(due to my elevation my car doesn't pull too hard, it pulls hard upto 3500 rpms and its over so i might as well pull as hard as i can till cam is in) will i feel a good difference in low end jump. I wanna get nailed to that seat even harder. Im guessing the 2.75's were really limiting.

 

[fastback brian]

3.93's? You will notice the posi even more with the gears. better to spin both tire than just one. And even a bigger kick when the t-5 goes in. 2.75's are good for salt flat race cars not street cars.

 

[69Rcode_Mach1]

cool so im gonna really feel a kick than before? I hope it satisfies my disease till i can get the race heads.

 

[69Rcode_Mach1]

Also will i be able to drive my c6 with these till then? It won't kill my motor? I know mileage will go down somewhat which is okay since i won't be driving as much when school starts and winter rolls around. May be a good 6-9 months till t5 is in.

 

[69Rcode_Mach1]

Anyone........Anyone........

 

[one2gamble]

it will just rev higher

 

[mrfomoco]

I'm running 4.57's with my T-5 and love it. My car rarely sees the highway, but i can cruise at 2800 - 3000 rpms on the highway and 2000 rpm's around town when I'm just puttin around.

 

[69Rcode_Mach1]

cool, what kind of power difference will i expect going from 2.75's to 3.93's? Will i notice a big difference?

 

[VictorII]

I run a 3.40 Tractionlock rear with a 3.35 first geared T5 (with a built high revving 302). That thing leaps but still runs at under 2,000 RPM at 65 MPH in fifth (.68 fifth). Sure does surprise some of the big block cars that aren't optimized well. Depending on the power your 351 is going to have, the T5 might not be strong enough. Have you thought about a Tremec? The magic number is somewhere between 300 and 325 ft/lbs of torque. My T5 is out of a '93 GT so it will handle much more than the early T5s that took around 260 ft/lbs. If you are built, you might not be happy with what happens during a high RPM shift. By the way, it cost me $400.00 for a Tractionlock rear chunk for an 8". Unless they are doing you a favor, that seems kinda cheap unless it is an open rear end.

For comparison, my setup breaks the tires loose in each gear, including fifth. Have to roll out at about 3,000 and then nail it or it will spin until the tires are gone. The body is a '65 convert. Your Mach is heavier I think, but I know way more about the '65s than the Machs so I'm not sure.

You'll want posi for sure. Otherwise, you'll burn that one tire to pieces. Also, you will definately notice a seat of the pants increase. The engine will be in its power band a lot sooner and shift into each higher gear more quickly. Another cheap thing to consider is to recurve the distributor to match the new gears and engine power band. A kit costs about $25.00 and it will take about half an afternoon to install and adjust.

 

[69Rcode_Mach1]

cool thanks victor i should clarify im getting a g-force t-5. explain more to me about recurving the distributor. What does it do? And is it hard i don't know anything about iginition and timing yet.

 

[VictorII]

cool thanks victor i should clarify im getting a g-force t-5. explain more to me about recurving the distributor. What does it do? And is it hard i don't know anything about iginition and timing yet.

Basically, recurving refers to modifying when the timing comes in and how much total (vacuum and mechanical) you have. It does make a difference as the curve is slow from the factory. You want it to come in faster than stock but not so fast that it causes the engine to ping. One a 351 I don't know how much total you want, but I have it set at 38 degrees on my 302. It isn't hard to setup, just time consuming. You can also have a speed shop do it for you for under $100.00. As I mentioned, the kit is around $25.00 (Crane) and if you like doing stuff like that it is fun.

First, you take the point mounting plate out of the dizzy and put in lighter springs to get the mechanical timin in earlier. Usually, just one lighter spring in place of the heavy spring that is probably in there now. Leave the lighter one alone at first. Then you replace the vacuum advance module with an adjustable unit and then tune it so that the transition is smooth. When you nail the gas pedal you lose vacuum anyway, so mechanical is important. You will probably also advance timing at idle. Renting or borrowing a dial back timing light is essential in my experience so you can see total timing of each system, mechanical and vacuum and then both together at the same time as you view a tach. There is a dial on the back of the light that you turn to get the stobe back to TDC. Read the dial and you know your advance. It might be a good idea to pick up a book about engine building on small block fords to learn more or check the Crane web site. The 351 is covered in there quite well.

 

[Max Power]

I went from 2.75 to 3.50 in my 69 fastback last year and the difference is pronounced.

If your motor is running out of steam early now, this gear change will make that even more evident.

You dollar figures are definitely lowball. For quality stuff you will be spending a lot more.

 

[69Rcode_Mach1]

Hey guys thanks for all your help. I checked and for some reason the guy mixed up the prices for regular gears and posi. They suggested i do a limited slip and that would cost a total of $800.00 with install!!!! I don't have that kind of cash and my afr's are more important right now than gears ill do gears last probably. I know that posi is always locked up whereas limited slip has a clutch. Is a full out posi cheaper and whats the disadvantage. I know that my tires will screech while taking a turn is that bad for the car?? Any suggestions to help me make a decision for the future will be appreciated. What would a posi cost are they cheaper than limited slip?

 

[Ozsum2]

Hey guys thanks for all your help. I checked and for some reason the guy mixed up the prices for regular gears and posi. They suggested i do a limited slip and that would cost a total of $800.00 with install!!!! I don't have that kind of cash and my afr's are more important right now than gears ill do gears last probably. I know that posi is always locked up whereas limited slip has a clutch. Is a full out posi cheaper and whats the disadvantage. I know that my tires will screech while taking a turn is that bad for the car?? Any suggestions to help me make a decision for the future will be appreciated. What would a posi cost are they cheaper than limited slip?

I will be hard on the rearend, bearings and tires. Best use is for straight away driving.

 

[nosaj122081]

Posi is the GM term for a limited slip differential. Any limited slip diff+gears+setup will run in the 800ish range. Spools are locked all the time; I would never run one on a street car because of it, hard on things.

Also, I think building the motor first and leaving the gears for last would be a BAD idea, with a crap gear and a big head-ed, big cam'd motor, the car will feel like a brick.

 

[69Rcode_Mach1]

im doing all the build at once i have cam and intake with exhaust installed. I want to get my heads and then put on the rear gears before and finally put on the rest of the build. This way ill save some mileage till then and money until i get g force. I have to do the gears because im leaving the stock stall until i get t5 so with 2.75's my cam probably won't even run. i will probably do the 3.73's that seems perfect for cruising and power.