Quote:
Originally Posted by curvedglass
cool. 
Thanks again for all the help.
I checked out your Impala build thread. Looks cool. My first car was a 67 Bel Air 2 door 250 PG. Solid but worn. Girlfriends Grandpas car. 34k original miles when I got it. Drove it from 1980-83. Would love to have it now of course. Would look good parked next to my 63. (Although my 61 Coupe DeVille looks good there too) |
Thanks, no problem.
Quote:
Originally Posted by joedoh
I am running a similar engine, a 120hp but only 140lb ft tq, with a 3.73 rear and 0.73 overdrive. my engine makes peak torque at 2900 rpm and with my current 31" rear tires I spend a lot of time in 4th, less in 5th. its interesting to note that a 3.73 in 5th would behave very similarly to a 3.08 in 4th (1:1)
I am installing a higher numerical rear end, 4.56 (4.30 would have been ideal but they are 3x the cost) to get my engine back in its peak torque band and allow me to use my 5th gear. it may seem like the best idea would be to just use 4th, right? after all, 4th with 3.73 will be similar to 5th with 4.30. except, like you have noted, there is a big rpm difference between 4th and 3rd, that would go away if I had the opportunity to have the same speed and rpm in 5th because 4th is much closer to 5th numerically.
but I dont think thats what your engine needs. aerodynamic load is exponential as speed increases, and these trucks are less aerodynamic than a brick. for me, I am reducing the gear to get closer to peak torque (at 2900 rpm, currently I am at 1800 or so in 5th), which will allow the truck to achieve its drag limited top speed (I am guessing around 90mph with my 140lb ft on a windless day and flat road).
for you, you are already well PAST your peak torque, which is in the teens, 1600 I think, so you should consider running a test with a TALLER tire to see if the additional torque around 1600rpm will let you have a higher top speed. telling a fat guy, used to walking, to run faster is the analogy here, you should instead see if he can pull more weight at a slower speed, and he probably can.
anyway, thats my stab at it. shorter tires may accomplish the same thing because of torque multiplication, but its not going to help mileage or engine longevity. my two pennies.
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The problem is that HP moves the truck, not torque. Example:
70MPH with 3.07 gears will be 2,668RPM. Going by the GM chart that Jay posted, net torque will be around 210FTLBS and net HP will be around 105HP.
70MPH with 3.73's (and a 745 rev per mile 235/70r15) will be 3,242RPM. Net torque is lower at 195FTLBS, but HP is up to 120.
With 3.07 gears you will have 3.07x210= 644.7FTLBS at the tire. With a 3.73 you will have 3.73x195= 727.4FTLBS.
A taller tire will further reduce HP and make it even weaker.
Quote:
Originally Posted by Keith Seymore
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X2. An OD trans and deeper rear gears would give you the best of both worlds. With a common 3.73 rear and the trans Keith mentioned, you'd have the 3.73 final drive ratio in 3rd (direct/1:1) for hills and a 2.72 final in the .73 OD 4th for cruising.