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By famdoc3 - 10 Years Ago
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My 57 T'bird has an .80 over 312 with an Isky 300 series cam, G heads ports cleaned up hardened seats, and stainless valves. Borg/Warner super T10 with a special wide 2.64 first gear and a 2.73 rear. I'm running an original 2x4 manifold with twin throttle body fuel injection. It wants to run quite rich to run smooth. I'm getting 10-11 mpg. What are you guys seeing from your y-blocks in your cars? It looks and sounds great. The car makes seemingly plenty of power but it is expensive to run. MIKE
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By Joe 5bird7 - 10 Years Ago
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I have a 57 bird with original 312 (D code), the T-85 3 spd overdrive, and original rear end (don't know gear ratio). The overdrive is probably the biggest factor but on trips with mostly freeway driving I get mileage in the low 20's (21, 22 mpg). Not sure what it gets in stop and go driving, never checked it. It runs very well, and has plenty of power.
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By miker - 10 Years Ago
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Well, I'm running a Blue Thunder with a single EFI, and doing a lot better than 10mpg. With a bit of freeway driving, I get close to 20. I'm no expert on dual quad manifolds, or EFI conversions. As I remember, the short lived EFI duals were on 84 or so Vette's and Camaro's, using two barrel throttle bodies. Since you're still running a "wet" manifold, and the car want to run rich, I'd guess you don't have enough air velocity to keep the fuel in suspension. Most dual quad EFI's I see are 400 or 500+ CID motors, big cams, and no thought to mileage. Have you welded a bung in the exhaust and looked at the emission side? Do you have an oxygen sensor in the system that feed back to the computer? I'd guess you're washing the cylinder walls with a lot of gas that isn't burning. Even though the injectors atomized the fuel well, they can't stop it from dropping out of suspension in the manifold runners.
If the 300 Isky is what I'm guessing it is, the old RPM300, I ran that cam years ago. In a .060 312, at 11.25:1, .7 O/D and 427 rear, even that 'bird did more like 14mpg with a 650 Holley.
I think the stock ratio with the o/d in 57 was about 4:11. Think, not know, being the important part.
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By NoShortcuts - 10 Years Ago
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Joe 5bird7 (8/9/2014)
I have a 57 bird with original 312 (D code), the T-85 3 spd overdrive, and original rear end (don't know gear ratio).
[quote] miker (8/9/2014)
I think the stock ratio with the o/d in 57 was about 4:11. Think, not know, being the important part.
A '57 'Bird Specification Manual prepared by the Ford Car Body Engineering Department, Ford Division lists the rear axle ratios as- standard transmission....... 3.56:1 (both the 292 and the 312 engines using two different 3 speed trans.) - overdrive transmission....... 3.70:1 (312 engines only using the T-85 overdrive trans.) - automatic transmission..... 3.10:1 (312 engines only using the Fordomatic trans.)
Two '57 'Birds with overdrive that I parted years ago had 3.70s in them. |
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By charliemccraney - 10 Years Ago
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My truck averages over 16 hwy with overdrive and 13 without overdrive, at about 70mph with an engine which is not terribly different from yours.
I think your rear gear is waaaaaaay to tall. People seem to thing that tall gears guarantee good mileage but you can go too tall which makes the vehicle very lazy and brings the mileage down. I'm going to be trying a 4.11, from a 3.70 in part because I think my mileage will increase with the shorter gears because of the power band of my engine - the other part is I think it will be faster.
There are two isky 300 cams in their catalog, the RPM300 and the F300. Looking at the specs for both of those, I would not expect either to have a smooth idle though the RPM should be the milder of the two. You didn't say where it needs to be rich to run smooth but if you mean idle, I just don't think it's going to happen when it is tuned correctly. They are more performance oriented than stock and will have at least a slightly lopey, performance sound. Further, if it is either of these cams, I am sure that your rear gear is too tall. 3.55-3.90 is recommended for the RPM and 3.70-4.11 for the F.
So my thoughts are that it is geared wrong and tuned incorrectly.
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By speedpro56 - 10 Years Ago
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A 3.90 gear with a 70% overdrive will give you a 273 final gear on the highway for good gas mileage. So with your 2.73 you should be OK. Your first gear in the trans 2.64 is underwhelming for take off thou. The yblocks seem to like 36 degrees all in and usually idles around 12 degrees average, checked with the vacuum plugged. If you can smell gas then you're too rich and make sure they're no vacuum leaks. My 56 crown Victoria has a 3.90 gear with a T85 overdrive poked and stroked 312 and pulls great in overdrive when cruising and my final gear is 2.73.
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By paul2748 - 10 Years Ago
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I just made a 2200 mile trip with my 56 Bird (312, C4, stock rear, 3.40??) Overall average was 17. Lots of highway travel, 65 MPH, r7uns about 2800 rpm. I got a 57 dizzy with a pertronix, stock carb modified for the 57 dizzy.
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By Hoosier Hurricane - 10 Years Ago
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My F engined auto trans '57 Bird probably gets about the same mileage as yours around town, I say "probably" because I love the car and don't check the mileage because it doesn't matter. The difference between 12 and 18 mpg doesn't amount to enough money to concern me. If your Isky cam has a lot of overlap, you are diluting the intake charge with exhaust gases at low rpm and need a little extra fuel at that point.
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By famdoc3 - 10 Years Ago
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Appreciate all of the responses. You are all somewhat right. If I used my T'bird like most sane people do Iwould dump my 273's in a heartbeat but I use it most frequently for a 70 mile round trip to work mostly done at 80 miles per hour. It just gets comfortable and hums along. A great highway cruiser. I used to have trouble launching with those gears but with the fuel injection it has low end torque like it never had before. May try to lean it up a little and yes it does close loop with an O2 sensor. Using a very tunable Accel gen 7 computer. MIKE
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By miker - 10 Years Ago
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An Accel gen 7 is what's in mine. Since I still have the OEM dizzy (Load o matic), I've got a Ford magnetic module in it and the computer runs the ignition curves, one for WOT, one for cruise. My throttle body is the old stand alone Retrotek/proformance/boss EFI , with the sensors in place, but the electronic replace by the Accel. I'd be interested to know what throttle bodies you're running. Figuring out a way to have a fake E code is on my list.
80 on the road sounds plenty sane to me.
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By slumlord444 - 10 Years Ago
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My '57 Bird with the factory dual quads, 3 speed, 3.89 rear end, Crane cam, got around 17 on an economy run once. With the stock engine and dual quads it would get around 9 around town with my foot in it.
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By famdoc3 - 10 Years Ago
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I'm using the common g.m. Throttle bodies from their v 6s and v8s with their smallest injectors f
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By miker - 10 Years Ago
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I sent a PM.
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By charliemccraney - 10 Years Ago
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The gearing could be tested pretty easily. Have you done a mileage check cruising in 3rd gear? That would be approximately equivalent to having a 3.70 in the rear.
If mileage increases, then you want to change the gears. If it stays the same, then you may want to change the gears to make it more enjoyable in the lower gears. If it drops, then 2.73 may be ok but 3.70 is definitely too short.
After considering what Gary said, I do agree that 2.73 should do alright with that motor on the highway. My total ratio in overdrive is 2.33 (3.70 x .63). But it does not hurt to confirm with a simple test. It could still be too tall for city economy which could be dragging your average down. My city mileage is 11-12.
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