Gear ratio

[CHIEFER69]

Hey guys just wondering what you guys are running in your mustangs! I currently have a 3.25 gear ratio and wondering if that would of been enough to spin those tires...I have the 351 Cleveland CJ... I took it to the shop because I feel like its not performing like it should and they recommended changing the gear settings but I was just thinking the timing and carb needed a bit of tuning.... Its has a 750 cfm edelbrock in there now... Im really curious what you guys run.  The shop said it would cost 3k to change the gears

 

[mjlan]

That seems like a lot of $$ for a gear swap. Heck, you can get a ring and pinion set for $250!

A change to 3.50 would get the car away from a stop with more gusto at the expense of highway cruise rpm.

A stock 4V CJ engine is pretty peppy, are you confident in its health? 

 

[c9zx]

If they quoted you $3K find another shop. Chuck

 

[Bentworker]

The only way you should spend $3,000 on a gear swap is if you are installing a limited slip, really fancy (expensive) carrier, chromoly axles, nice gears etc...

A gear change alone should be 1/3rd that cost at the most.
 

You could buy a brand new carrier with bearings, gears and limited slip from Strange for about $1600-1800 and put it in yourself.

 

[CHIEFER69]

The motor has been worked on throughout the years according to the first mechanic.. It has the 750 cfm edelbrock carb, long tube hooker headers, blue thunder intake manifold, msd coil, msd 8.5 mm spark plug wires and autolite sparkplugs that run hotter...  The motor had new rods put in before and it passed the compression test.. So Im comfused I figure 3.25 should be enough to launch decently I assume.. Im think it still needs tuned right and check on the timing but the last mechanic said He couldnt do anymore.. Im thinking the guy just didnt know or wanted to work on it

 

[jpaz]

I had my gears changed from 3.25 to 3.50 and I had a true track Posi unit installed for about 1400. That was about 3-4 years ago, but I can’t imagine that it would be 3k now like you got quoted. Sounds like you said, the mechanic doesn’t want to deal with it. 

 

[Don C]

Did you get the compression numbers?

One of the problems with engines that have been worked on previously is that you don't know what they did or tried to do. Why new rods? Rods are generally reused unless it's a performance build or they were damaged. Did the P.O. put in a different camshaft and do it correctly? Did they also put in new pistons, and if so flat topped, dished, or domed?

They did make good selections on the intake and carburetor, getting rid of the original 4300D carburetor, but other than that who knows?

With those unknowns you just won't know what your engine is capable of.

$3K for a gear change and adding posi-traction is on the ridiculous side. As others said, time to find a mechanic that wants to work on your car.

 

[Stanglover]

Here's what I have in my 71 M code; 4 speed top loader. 3.25:1 now with Traction-Loc (used to be a one-wheeler-peeler) rear axle. Stock intake with a Holley 670 street avenger carb. Distributor is a factory motorcraft with altered limit slot (L10) and Pertronix Ignitor II module and matching coil and Autolite 25 plugs. Timing is set at 14 deg. initial, L10 = 20 deg. crank for a total of 34 mechanical plus about 4 deg. vacuum on top. The carb is basically stock other than larger squirter #35 and primary plates drilled for idle a/f ratio. And yes I can spin the tires with ease, not that I want to as tires here cost too much. I chose not to swap the gear ratio, but kinda wish I'd gone with 3.50:1.

As for the rip off artist who wants 3 grand, he must have a mortgage way too big or a mistress on the side. Run, Forest, run.

 

[Jeff73Mach1]

The Edelbrock carb may be part of the issue.  An improper stall range on a torque converter can dog up a motor's performance too.

3K is a joke  A rear center section with posi built and ready to drop in can be bought for under a grand all day long.  Install is under 3 hours work if the mechanic is old and slow.

 

[CHIEFER69]

DonC There was a broke rod and when they replaced it they left the broken piece in the i take and bent another rod.so I had to replace another one.. I added the blue thunder intake and the carb was already on there....  Im thinking the Edelbrock carb might be giving me the problems

 

[mjlan]

When you say 'broke rod' and 'intake' are you referring to a valve train push rod that would operate a valve? That's a lot less severe of an issue to breaking a connecting rod...

You have an aftermarket intake and headers- what about the cam? If the cam is still the stock piece then its possible that these aftermarket parts are not the best parts for your particular combination.

As others have alluded to, engine mods are a double-edged sword: parts have to be matched with each other. If multiple people have had their hands in your engine then who knows what parts are in there, and deciding what to do next will be hit or miss without pulling everything apart to see what is what. You can't root cause the problem if you don't know what the compression ratio is (to Don's point about what pistons are in it?)...

Digging into what your primary complaint is- does the engine pull fine but not accelerate or is it that the engine doesn't make enough low-end power? Automatic or manual transmission? Where are you located?

 

[B229218]

push rod OR conn. rod??  :classic_blink:  Learn to work on own old car, old car mechanics cheap+honest+good =hen`s teeth.

As for rearend,  try quick performance in mid west, NOT CURRIE ENTERPRISES!!









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Gear Ratio


  Gear Ratio  3.00  3.25  3.40  3.50  3.70  3.80  3.89  4.11  4.22  4.30  4.56  4.63 [+$85.00] 4.71  4.86  5.00  5.14  5.29  5.38 [+$85.00] 5.43  5.50  5.67  5.83  6.00  6.14 [+$85.00] 6.20  6.33  6.43 [+$85.00] 6.50   5.29 LW [+$40.00] 5.43 LW [+$40.00] 5.67 LW [+$40.00] 5.83 LW [+$40.00] 6.00 LW [+$40.00] 6.20 LW [+$40.00] 6.33 LW [+$40.00] 6.50 LW [+$40.00] 6.66 LW [+$85.00] 6.83 LW [+$85.00] 7.00 LW [+$85.00] 6.66 AX [+$149.00] 6.83 AX [+$149.00] 7.00 AX [+$149.00] 7.16 AX [+$149.00] 7.33 AX [+$149.00]  


 









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• [SIZE=10pt]Fully Assembled, Ready to Bolt in[/SIZE]
  
• [SIZE=10pt]Reconditioned, Stock case, yoke & pinion support (not intended for over 400 HP)[/SIZE]
  
• [SIZE=10pt]New Timken USA bearings[/SIZE]
  
• [SIZE=10pt]New Motive Performance Ring & Pinion[/SIZE]
  
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Customer Revi

 

[CHIEFER69]

Lol my mistake.. i should of mentioned it. They were push rods... I never got the compression ratio stats.. He just was listening too it and said that it sounded healthy.. Im located in KC.... As far as I know it has the stock high lift cam... It accelerates pretty good on the highway.. Just the initial press on the gas starting it takes off its boggs down....Like pretty much telling me wth you trying to do too me lol Take off just doesnt have enough spunk

 

[Stanglover]

To me, this sounds very much like what I was experiencing. Bearing in mind my motor is an M code, but with 10:1 dished pistons and a Melling MTF 2 cam, slightly higher lift then stock, producing 17"Hg vacuum at hot idle. I narrowed it all down to two things, timing and carb. To me and I really don't know why, the Edlebrock carb just looks wrong. It looks like it's mounted sideways, weird eh! Some like them some don't. I have never used one and doubt I ever will. I think there are better carbs out there suited to 351C's. Going back to mine, the Holley Street Avenger 86670 with vacuum secondary's and electric choke is finally pretty damn close to perfect for this motor. It has factory jets and power valve (6.5) but it was very rich at idle and smelled of gas. I mentioned before that the primary plate were drilled. I took the car to to tuning specialist who drilled a 3/32" hole in each plate, front and center. This allowed the transfer slots to be closed down to "square" and allowed the mixture screws to be functional again. The only other 'mod' was to increase the "squirter" from a stock #31 to a #35. This alone eliminated the bogging it was experiencing when I hit the loud pedal. I did try changing accelerator pump cams, but ended up back at stock. If I were to change carbs again, it would be between the Quick Fuel HR735 cfm or a Summit  equivalent.

Timing I found, again on my motor, needed to be 14 deg. initial with 20 deg. on the crank plus about 4 deg. vacuum on top. The distributor is a stock Motorcraft/Autolite, but with a modified limit slot (L10)  to produce the 20 deg. on the crank, 34 deg. total mechanical. The cam spring I used are 1 stock heavy and 1 Mr.Gasket 925D. I'm not going into all the fine details right now, but that should give an idea.  It also has the Pertronix Ignitor II and coil. Most 71-73 stock dist. were set with a L15 limit slot or 30 deg. on the crank, but only had 6 deg. initial. This was for emissions I believe, not good for performance.

Your engine requirements may be very different depending on what's in the motor, but I think carb and timing are your main issues, but could be other factors. It's frustrating at times for sure, but do one thing at a time.

 

[Spike Morelli]

I have ben running a 3.50:1 Trac-Loc in mine for 30 years now. My car, as bought, had a 3.00:1 open rear end. I changed the rear because I knew my purpose was to not only drive the car everyday, but also tow my boat(s). The lower gears give me more grunt to pull with, and the Trac-Loc helps on wet ramps, and when I may have to pull way over on the shoulder of a highway with the trailer, putting the right wheel in the dirt.

Now that I've explained WHY I changed the rear gears, I will tell you that, Yes, the highway rpm is more elevated than before. However, being that I am an "old man" of 65 years old, I learned to drive in an era when 3.73s, 4.11s, 4.56s, and 4.88s were common in hot rods and musclecars, and we all drove confidently, listening to our engines sing. My rpm is about 3500-3700 at 60/70 mph with the 14" rims, and it's really not annoying to me, a good compromise for everyday street and highway use. I also haven't made the mistake of installing too loud of an exhaust system, which has a lot of bearing on what rpm you feel comfortable with. Just stock manifolds and restorer exhaust pipe system, with Turbo mufflers replacing the stock cans .

You may want to go for a ride in someone's car equipped with 3.50s to help decide.

 

[rackerm]

I assume you have a 4spd and not a 3spd manual or auto. I went with 3.50 gears with an Eaton Limited slip on my 73 351C with a C6 Auto (3spd), 27 inch tires, and a Edelbrock Performer intake and a 4300A 600 cfm big bock square bore, otherwise stock. I liked the response off the line, but the gears were not great for highway driving. I was running about 3,200 rpm at 65 mph. So I decided to swap the 3.50 for 3.00, and as you would expect it is much better on the highway (about 2,600 at 65 mph), but off the line not too great.

In my opinion, with a 4spd you have more latitude and 3.50s may be a good compromise for both off line and distance highway driving. With a 3spd, I would not go any higher than 3.00 (maybe 3.25) if you want to drive highway distances. 

 

[Hemikiller]

Hey guys just wondering what you guys are running in your mustangs! I currently have a 3.25 gear ratio and wondering if that would of been enough to spin those tires...I have the 351 Cleveland CJ... I took it to the shop because I feel like its not performing like it should and they recommended changing the gear settings but I was just thinking the timing and carb needed a bit of tuning.... Its has a 750 cfm edelbrock in there now... Im really curious what you guys run.  The shop said it would cost 3k to change the gears

$3K was the "let's see how dumb you are" price. 

Unless you'r running some ridiculously tall tire, the 3.25 should be fine with a near-stock 351CJ. Before dropping a bunch of money, perform some basic tune-up type work to make sure the engine is running as it should. If you can't spin a tire with a 351C, something is off. Maybe skip the local wrench shops and try a dyno tune session on a chassis dyno. 

 

[Spike Morelli]

I assume you have a 4spd and not a 3spd manual or auto. I went with 3.50 gears with an Eaton Limited slip on my 73 351C with a C6 Auto (3spd), 27 inch tires, and a Edelbrock Performer intake and a 4300A 600 cfm big bock square bore, otherwise stock. I liked the response off the line, but the gears were not great for highway driving. I was running about 3,200 rpm at 65 mph. So I decided to swap the 3.50 for 3.00, and as you would expect it is much better on the highway (about 2,600 at 65 mph), but off the line not too great.

In my opinion, with a 4spd you have more latitude and 3.50s may be a good compromise for both off line and distance highway driving. With a 3spd, I would not go any higher than 3.00 (maybe 3.25) if you want to drive highway distances. 

Nope, I have an FMX automatic, and a mild, almost stock "H" code ( has a 4v cam)....the original engine, under the hood complete with 2 barrel. You may be thinking that's just crazy, but the darned thing just runs too good, really. I DO have another engine in the garage, which I've been aquiring components for, to someday install in the car. My choices for the no.2 engine include one of Crower's "torque monster" cams, one of their mildest but better events than a factory 2 or 4 barrel cam, worked up Aussie 302c heads with dished pistons to match, an Edelbrock prototype 351c-2v runner intake ( rare pre-production item new in the box), a 600 vac secondary 4160 holley, and various oiling mods, con rod mods, etc. The major focus will be towards brutal low to mid range power and strength/reliability........exactly where I drive, as do most of us, whether anyone really wants to admit it or not. When I do this I will remove the original engine complete, save it,  and swap in the new engine. As smooth and powerfull as I believe the new engine will be, I still maintain that the "H" code in my car currently is suprisingly strong as is.  

 

[Spike Morelli]

I tow drag boats with my Mach, and it's never let me down, 351c "H" code and all....