Change Rear End Gear Ratio

[mark69sportsroof 31]

Hey Everyone,

I'm looking for expert advice on logic of changing my 69's rear end gears.  I currently have a Ford 9" posi with 3:50 gears,  28 spline axles.  I'm looking at changing over to 31 spline axles with 3:70 or 3:89 gears.  Looking to increase horse power and torque.  Tranny is a WC T-5.  Thanks  Mark 

[barnett468 418]

12 minutes ago, mark69sportsroof said:

Hey Everyone,

I'm looking for expert advice on logic of changing my 69's rear end gears.  I currently have a Ford 9" posi with 3:50 gears,  28 spline axles.  I'm looking at changing over to 31 spline axles with 3:70 or 3:89 gears.  Looking to increase horse power and torque.  Tranny is a WC T-5.  Thanks  Mark 

so exactly what is your question?

changing the gear ratio will not have any appreciable affect on horsepower per se.

.

 

 

 

 

[mark69sportsroof 31]

Barnett468,  Sorry about my poorly written question.  Would there by any  increase in hp or torque vs parts cost.  Mark

[Guillaume69 150]

Zero effect on hp and torque. Much like when changing gears with your tranny. Or your bicycle.

It’ll change the way your current HP/torque is transmitted to the rear wheels though. The higher (or shorter) the ratio, the quicker the rear wheels will spin for a given rpm.

Drag strip oriented cars have high rear end ratio (4.30 for SCJ drag pack, for example). Quick acceleration, poor gas mileage. It can also result in reduced top end speed (i.e you hit the rpm red line sooner). That’s why drag engines are usually prepared to sustain higher rpm ranges.

“Cruisers” have lower (longer) ratio. Not as quick when taking off at a traffic light, but smooth highway cruising and “reasonable” gas mileage (lower rpm for a given speed). Also, with enough hp, you can get a higher top end speed. I can go up to 140 mph with my car (351w, toploader long ratio, 3.00 rear end).

3.50 is somewhere in the middle, obviously. What use of your car are you thinking about? Drag strip or road trips?

[barnett468 418]

3 hours ago, mark69sportsroof said:

Barnett468,  Sorry about my poorly written question.  Would there by any  increase in hp or torque vs parts cost.  Mark

Well, the engine itself will loose less torque if you install numerically higher gears because it will not be working as hard to move the vehicle, but I don't think you are asking the right question. It is much better if you simply post in detail what you have and what you are wanting to achieve etc.

3.89 gears will make 1st gear in that trans nearly useless, so under normal driving conditions you will be shifting from 1st into 2nd at around 10 mph, lol.

 

[Mike65 475]

In my last Mustang which was a 68 GT Coupe I had installed 3.50 gears with a C-4 auto & was happy with the combo, in my current Mustang 69 Coupe I had installed 3.50 gears but also installing a trc-loc & this Mustang has a WC T-5. The Mustang is not drive-able yet so I cannot comment on how I like it. My car will be a weekend cruiser, so I did not want to go any higher with the gear ratio.

[mark69sportsroof 31]

Thanks for all the advance.  I have a professional built FE 390 with stroker to  445 ci.  It has 11:1 compression.  It has a Quickfuel 750 carb with mechanical secondaries.  It has custom 2" headers with 3" exhaust from the header to the mufflers and 2 1/2" from muffler to tips.  It has chassis dyno just over 400 hp and near 500 torque.  The car is a cruiser and show car.  I'm always looking to tweak more power out of it. 

I thought the higher gear ratio would increase the horsepower.  Thanks Mark

 

[1969_Mach1 333]

Like Barnett mentioned, rear axle gear ratios have no effect on the horsepower and torque the motor is making.  I think there is just some confusion on what you are asking.  Do you want the car to accelerate quicker?  If that's what you are getting at then yes, a numerically higher ratio will achieve that.  I'm not familiar with your trans.  But going by Barnett's comment that the 3.89:1 rear axle ratio would make first gear in your trans useless, I would think the only other option is the 3.70:1 ratio.

[barnett468 418]

36 minutes ago, mark69sportsroof said:

Thanks for all the advance.  I have a professional built FE 390 with stroker to  445 ci.  It has 11:1 compression.  It has a Quickfuel 750 carb with mechanical secondaries.  It has custom 2" headers with 3" exhaust from the header to the mufflers and 2 1/2" from muffler to tips.  It has chassis dyno just over 400 hp and near 500 torque.  The car is a cruiser and show car.  I'm always looking to tweak more power out of it. 

I thought the higher gear ratio would increase the horsepower.  Thanks Mark

 

these 2 1/2" pipes may be reducing the power in the upper mid and top end range.

"2 1/2" from muffler to tips"

 

what heads are on it?

what intake?

what cam?

is the idle fairly smooth or is it rough like it has a big cam?

how far will it rev at max rpm?

what mufflers do you have?

why does it have 11:1 compression?

do you have a cross over tube in the exhaust?

from your specs you should easily be roasting the tire (or tires) and not needing to ever use 1st gear under normal driving conditions.

 

 

 

 

[mark69sportsroof 31]

Barnett468,  here is what I got.

Heads   -  BBM Gen 5.1 Alum FE heads

Intake -  Blue Thunder dual plane Alum 4V

Cam - Erson FE Hyd cam Hi Flow 3H 240* .551* 108

Idle  -  smoother then rough  

max RPM - 6200

mufflers - Dr Gas spin pro 4 trap mufflers

compression - told the builder I wanted max street able horsepower and torque engine 

 x pipe - Dr Gas 3" X pipe

hanks Mark

[barnett468 418]

27 minutes ago, mark69sportsroof said:

Barnett468,  here is what I got.

Heads   -  BBM Gen 5.1 Alum FE heads

Intake -  Blue Thunder dual plane Alum 4V

Cam - Erson FE Hyd cam Hi Flow 3H 240* .551* 108 

Idle  -  smoother then rough  

max RPM - 6200

mufflers - Dr Gas spin pro 4 trap mufflers

compression - told the builder I wanted max street able horsepower and torque engine 

 x pipe - Dr Gas 3" X pipe

hanks Mark

"told the builder I wanted max street able horsepower and torque engine"

ok, these 2 things are not exactly synonymous. It's a bit complicated. You are getting the torque mainly from the engine size and you are getting the hp from a fairly high revving cam, but the cam is costing you some low end performance.

How far do you typically rev the engine when you are getting on it hard?

How often do you get on it hard?

Are you willing to give up some peak rpm for more bottom end power?

 

Is this the cam you have?

FE Ford 352 - 428 cid Hydraulic Cam & Lifter Kit - 240°/240° @.050 .551 lift

Runs strong 4000-75000 RPM. Needs lower gears, 4 barrel, headers and compression for maximum performance. Rough idle.

• Fits FE Ford 352-360-390-406-410-427-428 cid engines
• Grind - Hi-Flow IIIH
• E240621
• RPM Range 3800-6800
• Duration @.050 Lift 240/240 deg int/exh
• Valve Lift .551/.551 int/exh
• Lobe Separation 108 deg
• Advance 0 deg
• Advertised duration 316/316 deg int/exh
• Lift based on stock 1.73-1 rocker ratio
• Use valve spring 3050
• Complete with Lifters and Assembly Lube
• Use High Zinc content oil for protection from lobe failure

 

 

 

 

 

 

[SM69Mach 68]

On 11/16/2018 at 9:51 PM, mark69sportsroof said:

Hey Everyone,

I'm looking for expert advice on logic of changing my 69's rear end gears.  I currently have a Ford 9" posi with 3:50 gears,  28 spline axles.  I'm looking at changing over to 31 spline axles with 3:70 or 3:89 gears.  Looking to increase horse power and torque.  Tranny is a WC T-5.  Thanks  Mark 

 

On 11/17/2018 at 10:00 PM, mark69sportsroof said:

Thanks for all the advance.  I have a professional built FE 390 with stroker to  445 ci.  It has 11:1 compression.  It has a Quickfuel 750 carb with mechanical secondaries.  It has custom 2" headers with 3" exhaust from the header to the mufflers and 2 1/2" from muffler to tips.  It has chassis dyno just over 400 hp and near 500 torque.  The car is a cruiser and show car.  I'm always looking to tweak more power out of it. 

I thought the higher gear ratio would increase the horsepower.  Thanks Mark

 

This is the part that caught my attention.  Unless something is done to that WC T-5, I would be careful with that transmission.  They typically are not built to hand that kind of torque unless they are modified.  

I too have a 445 FE, but opted for the TKO 600, just to be on the safe side.  

 

[barnett468 418]

Yes good point on the trans, however,  It will be fine as long as you dont  dump the clutch or speed shift it 

[SM69Mach 68]

2 hours ago, barnett468 said:

Yes good point on the trans, however,  It will be fine as long as you dont  dump the clutch or speed shift it 

Where's the fun in that!  HAHA

[1969_Mach1 333]

25 minutes ago, SM69Mach said:

Where's the fun in that!  HAHA

If the goal is to tweak more power out of it, it doesn't sound like the OP plans to drive it too easy.  So plan on a future trans replacement.  I was wondering why such a big CID motor with 11:1 CR is only making 400 hp?  That's a bitt out of my wheel house.  Barnett, do you have any thoughts on that?  Or is that about right for that motor?

[barnett468 418]

8 hours ago, SM69Mach said:

Where's the fun in that!  HAHA

yes, i know but i was only saying that the t5 "can" be used, lol.

[barnett468 418]

8 hours ago, 1969_Mach1 said:

If the goal is to tweak more power out of it, it doesn't sound like the OP plans to drive it too easy.  So plan on a future trans replacement.  I was wondering why such a big CID motor with 11:1 CR is only making 400 hp?  That's a bitt out of my wheel house.  Barnett, do you have any thoughts on that?  Or is that about right for that motor?

He mentioned he had it dynoed on a chassis dyno, so that can eat around 40 to 90 hp, and a mustang dyno will provide a much lower reading than any other type of chassis dyno, plus i have seen dyno figures vary by maybe 30 hp on the same engine on the same day when dynoed at 2 different facilities, so i take dyno readings with a grain (or sometimes a pound) of salt except when using the same dyno to compare changes etc.

 

.

 

[mark69sportsroof 31]

1969_Mach1,  It has 525 hp at the flywheel with 420 at tires.  The Dyno guy shut it down at 5200 rpm due drive train vibrations.  The RPM has a max of 6200.  The drive train vibration has since been fixed.  Also a Mile High elevation also affects it.  Mark     

[1969_Mach1 333]

31 minutes ago, mark69sportsroof said:

1969_Mach1,  It has 525 hp at the flywheel with 420 at tires.  The Dyno guy shut it down at 5200 rpm due drive train vibrations.  The RPM has a max of 6200.  The drive train vibration has since been fixed.  Also a Mile High elevation also affects it.  Mark     

At the motor that sounds more like it.

[mark69sportsroof 31]

Barnett468,  I do have the E 240621 cam.  Your numbers listed are pretty much the same.  I'm not to mechanically incline  so a lot of the numbers I don't understand.   It sounds like the gear change won't help me gain more power.  Good to know.  Always learning and looking to fine tune the performance.  Thanks I always look forward to your reponses along with rest of the gang here.  Mark

[barnett468 418]

25 minutes ago, mark69sportsroof said:

Barnett468,  I do have the E 240621 cam.  Your numbers listed are pretty much the same.  I'm not to mechanically incline  so a lot of the numbers I don't understand.   It sounds like the gear change won't help me gain more power.  Good to know.  Always learning and looking to fine tune the performance.  Thanks I always look forward to your reponses along with rest of the gang here.  Mark

ok but you didn't answer my questions so i can't really help you anymore without that info.

[mark69sportsroof 31]

Barnett468, sorry again I got caught up in all the cam numbers and forgot to answer the three questions.

When I get on it I hit around 5000 rpm up to 6000 rpm.  My Petronix 3 is set a 6300

I drive it on the weekends about 3-5 miles minimum and get on it 1-2 times

I am open to give some peak rpm for more bottom end power.  Just not sure what that curtails.

Thanks Mark

 

 

 

[barnett468 418]

20 minutes ago, mark69sportsroof said:

Barnett468, sorry again I got caught up in all the cam numbers and forgot to answer the three questions.

When I get on it I hit around 5000 rpm up to 6000 rpm.  My Petronix 3 is set a 6300

I drive it on the weekends about 3-5 miles minimum and get on it 1-2 times

I am open to give some peak rpm for more bottom end power.  Just not sure what that curtails.

Thanks Mark

ok thanks. the good news is that your goal is achievable to some degree, unfortunately, it requires a cam change at the very least, and because your compression is so high (i would verify this first), you would either need to remove the heads and have the chambers enlarged if that is possible, or install different pistons and then get the engine rebalanced, OR, call chris straub and explain what you have and what you want and he will custom design a cam that will work, however, it probably won't work as well as it would if you first reduced your compression. i can suggest a few cams that will do what you want but they will also increase your compression and then your engine will pink itself to death, and i'm actually surprised it isn't pinging some now which makes me question the amount of compression it really has.

https://straubtechnologies.com/

 

also, if you remove the heads, you can gain more hp by having some mild port work done to them.

 

.

 

 

[Mach1 Driver 560]

17 hours ago, barnett468 said:

 your engine will pink itself to death, and i'm actually surprised it isn't pinging some now which makes me question the amount of compression it really has.

 

Oh- you spell like me. I thought you were using a term unfamiliar to me... or are you?  :)

[barnett468 418]

2 hours ago, Mach1 Driver said:

Oh- you spell like me. I thought you were using a term unfamiliar to me... or are you?  :)

i've been on the ozzie forum too much, lol.