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Old 07-07-2016, 11:17 AM   #31 (permalink)
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All three of those options would allow for an easily accessible wider range of gearing. But any one of them would increase the time and expense of the project considerably, and also add a significant amount of extra weight and parasitic drag.
The inline overdrive you only have to do once. The quick change rear end requires occasional 'adjustment' but I doubt it adds weight or parasitic drag. It's swap meet season right now.

If you swap the whole axle (twice!), bleeding the brakes would get to be a drag.

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Old 07-07-2016, 12:35 PM   #32 (permalink)
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Yeah, I've thought long and hard about other options, too--such as a Gear Vendors overdrive, a 4WD transfer case with the "transfer" part cut off
Thanks, hadn't see the Gear Vendors unit -reading their site and it seems pretty convincing. It's pricey but could be taken to your next car or sold after you're done with it. If you did a 4wd transfer case mod it would whine when in low, I think a lot of them are speed limited too (?).
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Old 07-07-2016, 12:37 PM   #33 (permalink)
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Originally Posted by M_a_t_t View Post
I have a 95 Astro and after reading your story I was looking at specs on your car (stock) and we have, or had, the same wieght, transmission (4L60E), engine, almost, I have 4.3 v6 yours had a 4.3 v8 option, and same towing rating. I wanted to just be able to get 30 mpg highway. I think with our vehicles being fairly similar and you getting 43-46 mpg I should be able to hit 30 with mine, despite it being less aerodynamic. Im planning on doing aeromods though. I would like to do a manual swap but that'd be a long ways away.
Matt
You're right--mechanically, our vehicles are very similar.

The best highway mileage I got before the manual swap and kill switch was about 32 MPG. That's with the stock (fairly high) 2.56 ratio, a more aerodynamic body from the factory, and aero mods.

If you want 30 MPG highway in your Astro without a manual swap, you'll almost certainly need to change your gearing and do as many aero mods as you can. That might get you there...

But a manual swap and kill switch will definitely put you over 30 MPG.
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Old 07-07-2016, 12:39 PM   #34 (permalink)
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Originally Posted by freebeard View Post
The inline overdrive you only have to do once. The quick change rear end requires occasional 'adjustment' but I doubt it adds weight or parasitic drag. It's swap meet season right now.
Quick change rearends are cool, but even if I got one for $500 it would be at least $1500-2000 total by the time I figured out brakes, mounting, axles/hubs, modifying the gas tank under the car to fit, etc.

Then I would be worried about long-term durability (especially towing), and the high parasitic losses of a QC rear end are well documented in the racing community. They eat more HP than a 9" Ford rear, which is already less efficient than the GM diffs.

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Originally Posted by freebeard View Post
If you swap the whole axle (twice!), bleeding the brakes would get to be a drag.
I'm planning to put quick disconnects on the brake lines, so I won't have to bleed them each time. I will also make an easily removable bracket for the parking brake cables, so they won't have to be readjusted either.

I'm figuring that with both of those things in place, the axle swap will only take me about an hour or an hour and a half. That doesn't sound like a lot of time to me, and if I schedule all my towing projects together so that I don't have to change it very often, it shouldn't be too bothersome. But I may change my mind after doing it a few times...
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Old 07-07-2016, 12:55 PM   #35 (permalink)
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How much ratio change do you think is needed? Perhaps swapping on a set of smaller diameter wheels and tires will do the trick. It might look funny but then again the skirts might hide it. Put air shocks or air bags on the rear to level it out again- helps with the load, aero, and looks.

When I first got my '59 running after it sat for more than 30 years, I borrowed the far smaller than stock wheels and tires from my Corvair for the drive home and until another set was bought. IIRC they were 13% smaller than stock, or than what I run now which is quite a bit oversize. The difference in available starting torque and responsiveness from just that alone was astonishing!

P.S. I quickly played with a tire size calculator; 15" tires are available that would make the effective rear axle ratio 2.74 vs 2.14; 14" are too if they clear your brakes.
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Old 07-08-2016, 03:31 AM   #36 (permalink)
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Originally Posted by funkhoss View Post
I actually have a 4.3L L99 V8 (with only 40,000 miles) sitting in my shed right now. It would bolt/plug in without any modifications.

I don't think I'm going to swap it, though. If I didn't use the kill switch, it might make a difference, since it uses less fuel at idle. But I never idle, and I can keep the LT1 in roughly the same BSFC range as the L99 by just shifting sooner and keeping the RPM lower.
A 5.3L running on 4 cylinder (~2.6+L?) might be a bigger jump. I'm guessing your car maybe requires ~25hp at 55mph, and your LT1 makes ~80hp at 1k rpm, which puts you at ~30% load.

Corvette C7 LT1 vs Our LS3 dyno graphs - Camaro5 Chevy Camaro Forum / Camaro ZL1, SS and V6 Forums - Camaro5.com

Going to ~60% load might make low speed cruising a lot more efficient if you can tolerate how the engine runs with AFM on.

http://www.tulatech.com/uploads/7/5/...13-01-0359.pdf
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Old 07-09-2016, 10:52 AM   #37 (permalink)
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Your getting close to the mpg range that I got with my Honda Civic VX (on the freeway)! This is amazing.
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Old 07-09-2016, 02:55 PM   #38 (permalink)
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What about a two-speed rearend?

Weren't those available O.E. from GM in the '60's?
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Old 07-09-2016, 02:58 PM   #39 (permalink)
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An old hot rodder told me about street racing waaaaaay back when with his '62 or '63 Impala and it had a two-speed rear axle.

Could have been pulling my leg...
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Old 07-09-2016, 03:03 PM   #40 (permalink)
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Absolutely amazing work, by the way, OP.

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