Bp3060ct or BP3472ct in 79-Ghia

[Hans]

Hi there,
Looking into a crate engine.
I want to keep the C4 and 2.47 rear axle.
Do you agree the 347 is a better match?
I don't care for speed over 100 miles, just about driveability for daily driving.
Thanks.
See the graphs please.

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[mudgepondexpress]

With that gear set, you will want the engine with the most bottom end torque values. the more cubic inches (347) the more power you will make at lower RPMs.

That 2.47 is really, really high. You could go to a 2.79 or close and gain a lot more driveability, and the acceleration would improve a ton.

Kenny

[Hans]

Thanks Kenny,
I like the 2.47 because it gives 2700rpm at the max speed limit. And it's the original axle.
But with 2.79 gearing it might be fine too.
That would result in 3050 rpm @ same 75 miles/ 120 km/h.
The graphs show that the torque of the 347 is higher exept at high rpm's (that I will almost never reach).
Hans.

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[mudgepondexpress]

Looks like you are running TRX tires from the picture. If you upsized the tires you might get a lot of that cruise RPM back.

Even the GT sized 225/60/15 could help.

[Hans]

I entered the values in a tire calculator.
Yes, that cuts the difference in half.
Thanks.
I have a set of turbine wheels also, intended for daily driving. The TRXes are reserved for special occasions.


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[FoxChassis]

That 7.5", or worse 6.75", rear will not last long behind that 347, although the C4 may brunt some of the torque damage. You would do well to change that rear out for an 8.8". Lowest ratio is 2.73:1.

[Hans]

It's a 7.5''. Thanks, noted.

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[xctasy]

There are 8.8 axles out there. There was a 2.47 axle ratio fom the 4.9 litre sx in the F150 from 84 to 86 as a base model standard with floorshift manual.

See

http://vb.foureyedpride.com/showthre...30#post1888430

The moon shot gearing was used only to get Corporate Average Fuel Economy based on the highway 55 mph figure upwards. Going from 2.73 to 2.47 adds about 1.5 mpg to the highway figure at 62 mph. The 1980 Big Fox Tbird and Panther option AOD is like going from a set of 3.27s to a 2.05 axle. It saves 3 mpg and a 351 with an AOD and an effective top gear of an 1.95 axle ratio is like going back to a 255. You save 100 cubic inches of 55 mph fuel consumption. The Police 351s used the same amount of fuel as the 255 option.

Dont Fear the Gear. You wont loose your mind and end up like Don Quixote de la Mancha and charge headlong into a Windmill...

I have the Snook fuel economy calculation program used by the French and Australians on the Total Oil Economy runs. It very accurate. This is what Ford used to define gearing when Congress inaugurated the CAFE average Fleet fuel economy. Over drive SROD, T5 , A4LDs and AODs went a step further, preserving common 2.73 to 3.73 axle ratios.

The 78-81 7.5 " 2.26 and 2.47 and the other 8.7 and 9 inch axle 2.50s and the rare 8" 2.36 were only used from 1969 to around 1986.... generally to make great EPA window stickers and convince people to try a 255, 300 or 302 or 351M that came with it. The 1969 250 six cylinder Mustang started it all with a 8 or 9 inch axle ring and pinion with the 2.36 or 2.50 gears. AMC, GM and Chrysler Corp did the same thing, but Ford was first. It dates back to the Columbo Poor Mans Overdrive concept of making.a wider ratio spread with either a second axle ratio or just a really shallow 2.26 to 2.50 axle and looser converter 3 speed auto.GMs THM 200 and the switch pitch DynaDrive were other examples.

A 347 will love a 2.47 axle.

[xctasy]

With that gear set, you will want the engine with the most bottom end torque values. the more cubic inches (347) the more power you will make at lower RPMs.

That 2.47 is really, really high. You could go to a 2.79 or close and gain a lot more driveability, and the acceleration would improve a ton.

Kenny

Dont forget that the 255 4.2 liter used a 2.26 axle for 1980 to 81 only, the lock up clutch C4 called the C5 pulled the ratios back to 2.73 for 82. The 78-79 302 5.0 used the 2.47.

Even the 3.3 SROD had a 2.49 effective top gear (3.08 x 0.81).

The T4 four speed 3.3 used a 2.47 axle too.

Even with a bigger cam, 2.47 is the one to use for a 347

[Hans]

Thanks for all the numbers.
The 2.47 is close to the 5 speed XMSNs with 3.73 rears that sum up to 2.61 in fifth.
We don't have mountains around here.
And it's not hard to spin a tire at the traffic light with this gear already with the 100 ponies at the tires (dynoed it once).
The hunt for an 8.8 starts.


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[FoxChassis]

You are going to want an 8.8 from a 1986-1998 V8 Mustang or 1986 V8 Capri