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Discussion Starter · #1 ·
I have a '69 Malibu with the following:

1984 GMC 461 with stock compression
Stock peanut port heads
Isky 270 hydraulic flat tappet cam, .510/.510, 216/216, 114
Recurved GM HEI
High-rise intake
Edelbrock 1806 650 cfm AVS carb
TH-400 with 1800 stall
Headers
3" exhaust, 2 1/2" over diff, 3" tail pipes
12-bolt 2.73 open
27" rear tires

This is a daily driver which is why I have 2.73 gears. It still has plenty of SOTP oomf. I'm going to go with a posi, which isn't available with new 2.73's. The next step up numerically is 3.07, which would also work fine as the car use to have a 9" rear end with 3.00's.

I've been reading posts about how some guys have found no decrease in mpg, and in some cases a slight increase, when they went to a numerically higher gear. They attribute it to the mildly steeper gears bringing the engine up to its cruise rpm, making it more efficient. This cam's rpm range is 2000-6000. Presently, the engine turns about 2250 prms at 65 mph. With 3.07's, it would turn about 2550. With 2.73's, the car is getting about 14-15 mpg on the highway.

Where is the cruise rpm for a cam like this? Should I go as far as a 3.23? (about 2700 rpm's at 65 mph)

Thanks for any advice.
 

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3.07 should be good with you engine combo. The most I'd go is 3:55's.
 

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Discussion Starter · #4 ·
I'm leaning towards 3.07's, but If I would go to 3.23's for that extra SOTP power, would it decrease my mileage that much, or would I be in the cam's "sweet spot" without hurting my mileage? Has anyone tried this?
 

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I don't think you'd notice much difference between 3.07 and 3.23, afterall its only a 5% difference.
 

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Discussion Starter · #6 ·
How about the SOTP difference between 3.07's and 3.23? SOTP is impossible to quantify, but I'd be willing to sacrifice ~5% of mileage for a noticeable boost in power. Anyone done the switch from 3.07's to 3.23's?
 

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Maybe a little, you'd have to have one sensitive seat in your pants to be able to tell though. :D
 

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Discussion Starter · #8 ·
Thanks, Webfoot. I'll probably go with the 3.07's. In a blind taste test, I'd probably never know the difference. It'll be nice to have posi, though. With the peg-leg, I've got to ease the pedal down or she'll just spin for a good 3 or 4 seconds. I'm sure it'll be much faster off the line with just the increased traction.
 

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I think you'll notice a snappier throttle just going from 2.73's to 3.07's. I have 3.08's in my Camaro with a posi and 4 speed and power is excellent. It dosent pull rpm's like my Beamont with 3.73's but I had to go with a TKO600 to get the highway rpms down. I still haven't fiqured out what SOTP means!
 

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I only have limited mileage on the new combo, but going from a 3.08 to a 4.10 was an incredible SOTP improvement...doesn't hurt that it's a posi now also.

I realize mine is more like a 30 % increase in gear, but I'm sure you'd notice a real kick in the butt going from 2.73's to 3.23's, which is about an 18% increase in gear I think.:thumbsup:

SOTP...seat of the pants
 
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