SA Super FE rear end gear w/ cr toploader ?

[nkotb]

Hi all,

I've been pondering the pro's & cons of a SA's Super FE with a close ratio toploader, & but I am still waffling between a 3.08 and a 3.31 rear diff gear... Any sage advice? (Rear tires will be 295/50/15's & some highway driving)

Thanks,
David

[Aggressor]

The 3.31 is a good highway gear. If you are leaning towards the 3.08, I would couple it to a wide ratio toploader to keep your bottom-end grunt. With a wide ratio toploader you could even consider a 2.88, if fuel economy is an issue, and still have the effective ratios of a 3.31 with a close ratio toploader.
I have a 403 FE with dual 4 bbl, non-progessive carb linkage(straight pull), close ratio toploader, 295/50/15's, and 3.31 powerlock. On a long haul trip on interstates I average close to 20 MPG.
- Geary

[nkotb]

Geary,

Thanks for your reply. The following thoughts come to mind... Gas mileage is not my real priority per se', but rather running the engine @ the higher rpm range that 70-80 mph would produce from a 3.31 vs a 3.08, for extended periods of time. It's a net drop of about 212 rpm in fourth to go with the 3.08. I think I want to stay with the CR, mainly because of the tall 1st & second, & the lesser rpm drop between shifts. I just didn't know if the 3.08 would sap the 1st & 2nd gear accelleration too much considering I'm making 500+ ft/lbs of torque.... make sense? Do you think your highway rpm's are a bit high or just right?

Other thoughts?

Thanks ya'll,
David

[Aggressor]

David,

While it's a bit noisy, I like the road feel, sound, and the power under the heel.

Here's some rpm to mph data points.

2100 rpm = 50 mph
2300 rpm = 55 mph
2500 rpm = 60 mph
2700 rpm = 65 mph
2900 rpm = 70 mph
3100 rpm = 75 mph
3300 rpm = 80 mph

As you can see, there is a 5 mph increase per 200 rpm increase. I initially considered using a wide ratio/2.88 combination for lower cruising rpm but opted for more kick in the pants acceleration in top gear. Also driving the car is almost like driving an automatic. Start off, drop it in forth, and putter around town at 25 MPH. Lug it down to fifteen MPH, put your right foot down smoothly and it will walk away with no fuss. Contact Poorboy (Don Pigott) or the Weavers. They have experience with taller rear ratios.
-Geary

[nkotb]

looks like a 3.08 might be boggy in parking lot situations...

[Tony Radford]

That's interesting. I've got a top loader and I thought a 3.31 back gear. I'm tacking 4K at 80 MPH.

[nkotb]

Hi Tony...

What size tire are you running? sounds more like a 4.+ rear... if my math is right...

here's my calculations for a 295/50/15 tire & a 3.31 from this website.
http://www.368s.com/tools_rpm.phtml

Engine Redline
1st Gear Ratio 2.32
2nd Gear Ratio 1.69
3rd Gear Ratio 1.29
4th Gear Ratio 1.0
Final Drive Ratio 3.31
Tire Size 295/50/15
Engine RPM at Given Speeds

Gear Ratio RPM at MPH 10 20 30 40 50 60 70 80
4 1.000 418 836 1,254 1,672 2,090 2,508 2,926 3,344

[Naumoff]

Tony,
Your speedo could be off.
My speedo is reading 6 mph slower than I am really going.
Found out with a hand held GPS.

I have a 3.31 and a CR toploader.Same tire as Geary. Happy with the highway and around town. The parking lot speed is a little fast in first at idle so I am on and off the clutch pedal a bit. But I can live with that.

Geary,
At what speed are you getting 20MPG.
When I went to Carlisle at 75mph :shift: I was averaging about 11mpg.
We do have different engines though.

[eliminator]

Out of 10 cars unless someone went to the old charts that Ford had to compute the speedometer gears, 9 are incorrect. I had Dan Williams compute mine, tire size, gear ratio, ect. It's correct, there are many, many combinations of the right gear that goes on the tailshaft inside the toploader and the cable gear. All are color coded. The tachometers I have seen are off as much as 500 to 600 RPM so don't count on what you see as a true indication. The computed rpms and speeds are in the ball park, the others are not. True speed could easily be provided and checked with a GPS, they don't lie.

As for the gear, FE motors don't like to lug around under 2,000 RPM, and depending on the stroke, 390, 428, and 427 some even like 2400/2500 and up. My 427 being one that likes 2400 and up, I am considering a change from the 3:31 to a 3:54. For my driving, no Interstates, back roads, and no trips, the 3:31 is a little on the tall side. I coulnt even start ot think about a 3:08.

[nkotb]

Good info fellas! 3.31 is looking like a good choice.

Thanks again,

David

[Aggressor]

Tony, I know it's not a popular item with the FE community but I installed a vacuum advance which could explain part of the mileage. Bill Parham does not recommend it but under light load (interstate cruising, big horsepower, light weight vehicle) there is no reason not to take advantage of another 4-7 degrees of advance. You will get the argument that it you don't want your timing to be varying that much and it should be locked down. My response is that the vacuum advance is only in during partial throttle/light load conditions. When you punch it the vacuum goes away and your back on your basic 34-38 degree setup. From personal experience I am enjoying a smooth running street car that can show it's Mr.Hyde side with a sharp movement of loud pedal. Another item which may contribute to mileage is the Rhodes variable intensity lifters. My cam is not fully in below 3200 RPM. With lift and duration reduced I experience a very good bottom end response and high cruising vacuum. Another contributor is the pair of Edelbrock 600's that I'm running. A dual Holley setup would definately reduce my mileage.(Maybe pickup a few more horses?) Normal cruising speeds for me are 55-70 MPH range.
-Geary

[pgermond]

I'm no mathematician by any means, but I know what works for me and my driving - around town, back roads, some highway (<150 mi one-way).

Good luck with your choice......

[Naumoff]

Thanks Geary,
I was curious in how you were squeezing that Milege out of a 2x4 FE.
What will you do when you get your twin Paxtons.:D

[Aggressor]

Tony, Due to financial considerations($9000+) the Paxton jump is maybe a year down the road. Then it is a brand new ball game. The FE has a Crower blower cam in place. MSD ignition with pressure variable retard must be installed. Vacuum advance goes away. Pressure variable fuel delivery must be engineered. The Edelbrocks will be replaced with force fed Holleys.(6 lbs. of pressure down the throat) Establishing my new baseline and sorting it all out will be "interesting".

-Geary

[Naumoff]

Make good notes as I will( and I think a few others) will be very interested in your project when the time comes.

I think when you are done you should be able to squeeze 6MPG and a set of rear tires per tank of gas.:cool:

6#. What is your compression ratio?

[Aggressor]

Tony, Compression was kept to 9.25-9.50 to 1 because of the future blowers. Only 6 pounds of boost.

Tires? , I thought that when you first got your Cobra - you smoked them one time- got it out of your system - and then settled back to cruise for the rest of your life at 10 mph under the legal limit. Ya know -Like your Grandmother driving her full sized Oldsmobile land yacht.
Hey! Looking back maybe I got it all wrong??????
-Geary

[Naumoff]

Geary,
I am kinda doing it backwards.
I have never been one to be hard on my equipment.
6000 miles and the same set of tires.
Besides no one wants to race me around here. That Southern Automotives engine is a real talker. I think it scares people.
I have to admit it is also very fast. So they're not many daily drivers around here that can touch my Cobra.


Leaving for England today. We will be back 30th July. Talk to you when I get back.