Another gear ratio question

[GABoy005]

I am about to start a build for a buddy of mine on a '71. We are tentatively planning to run a 347 (approx. 400hp and 400lb-ft) with an AX-15 and stock D20 behind it. This truck will likely see 90-95% street use. We are planning to use 31x10.50R15 or similar sized 16" or 17" wheel tire (ie. similar rolling radius). I have not disassembled the axles yet, but I am assuming they have 3.50 gear sets in them.

Would you guys recommend stepping up the gear ratio to 3.89 or 4.11? The stock gear set will put the engine at about 2150rpm @ 70MPH. 3.89 or 4.11 gears will increase this to about 2400 and 2550rpm respectively. I don't want the 0.78 OD in the AX-15 to cause any lower power driving conditions on the highway.

Thoughts?

On another note, is this amount of power for street use and these sized tires overkill or just plain terrifying to drive? All my broncos have stock 302s in them. Can't imagine what 400ft-lbs of torque must feel like on this short wheelbase.

Thanks, Rees

 

[blubuckaroo]

That's a lot of power for sure!
All that won't matter though when selecting a comfortable gear ratio for your 90% street use.
To make a gear ratio approval, I also need to know about the cam in your motor.
Do you have the cam card?

 

[GABoy005]

Not yet. I am meeting with the engine builder next week. What questions or request should I be making when I talk engine details with him? And how would this affect your gear ratio decision?

 

[Broncobowsher]

Torque and HP being both at 400, puts that RPM at 5252. Simply put you are making a higher RPM engine and you need to keep the RPM up. You are building a mustang that can roll down the road at 1200 RPM. Expect to find a good freeway speed to be in the low 2000 RPM range.

Pull one of the online RPM gear ratio calculators, plug in your tire size and overdrive ratio. Set the RPM at 2000 @ 60 MPH. Presto, you have a good starting point for what gearing to be looking at.

 

[GABoy005]

Thanks, we haven't nailed down the engine yet. I plan to do that next week, but this certainly helps. I will likely try to get the motor setup for a little more low end power. I will post again when I have some more specifics.