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  #1  
Old 12-06-2016, 02:25 PM
Yadkin Yadkin is offline
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Default Adding overdrive

I'm at the consideration stage of another modification for my '64. The most popular OD addition seems to be a product by Gear Vendors that bolts in place of my existing tailshaft. http://www.gearvendors.com/hrford3s.html

With my existing 225/70/15 tires and 3.00 differential I'm turning:
2200 RPM at 60 mph
2400 RPM at 65
2600 RPM at 70.

The Gear Vendors unit has a .714 ratio so this would be reduced. The third column is my estimated torque based on OE specs:
1600 RPM at 60 ; 300#-ft
1700 RPM at 65; 325 #-ft
1800 RPM at 70; 350 #-ft

My daily driver, a '12 Jeep Grand Cherokee, I figure weighs about 400# more than the TBird. For comparison:
1700 RPM at 60 ; 260#-ft
1800 RPM at 65; 260 #-ft
2000 RPM at 70; 260 #-ft

So this should work well. I'd like to know what others have done.
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  #2  
Old 12-06-2016, 09:01 PM
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1,800-RPM sounds wonderful at 70-MPH but where does that fall in the torque range of your cam???

If you're at the lower end of the spectrum, you're not taking advantage of peak torque. I would want to see my engine running around 2,500 (or more) at 70-MPH. You are leaving the good stuff at speeds you rarely use. - Dave
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Old 12-07-2016, 11:13 AM
OX1 OX1 is offline
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Quote:
Originally Posted by simplyconnected View Post
1,800-RPM sounds wonderful at 70-MPH but where does that fall in the torque range of your cam???

If you're at the lower end of the spectrum, you're not taking advantage of peak torque. I would want to see my engine running around 2,500 (or more) at 70-MPH. You are leaving the good stuff at speeds you rarely use. - Dave
More important than that, what is your terrain, elevation, etc...

I can get away with a stock 2bbl 302 (with 4bbl added), 33 tires, T-18 4 speed
stock 3.5 gear (15%ish overdrive compared to stock gears/tires), and 60 MPH on flat ground @ sea level
on my early bronco (absolute brick aerodynamically, even compared to SB), but the guy in
the Tenesee Mnts tells me I'm nuts for running this setup.

We have that disagreement all the time on message boards, since everyone's
terrain/expectations are highly different.
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Old 12-07-2016, 12:07 PM
pbf777 pbf777 is offline
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Speaking in very general terms of use, one would not require, or seemingly benefit greatly, with the expense and installation of an overdrive unit, in the scenario as you have outlined.

The considerations you present are: 1964 T-Bird, although not a light-weight, it also is not the worst automobile aerodynamically (@60-70 M.P.H.), therefore not relatively high loads to push at these speeds.

Also, the 3.00:1 axle ratio & tire size resulting in the R.P.M. values your list when coupled to the O.D. unit may be a bit slow, depending on the speed in which you travel of course. At the relatively low load presented at cruising speeds the peak torque values are not so relevant, but if the engine R.P.M.s fall too low, or as we sometimes term "under-the-cam" then inefficiencies within the induction & exhaust may overcome the savings in R.P.M. reduction.

Generally, the overdrive units are utilized when one is applying more axle gear multiplication then you present; which may also be a consideration here. Keep in mind, your particular driving style and environment has as much to due with the appropriate component choice as the math. Scott.

Last edited by pbf777 : 12-07-2016 at 01:23 PM.
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Old 12-07-2016, 12:52 PM
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Quote:
Originally Posted by OX1 View Post
More important than that, what is your terrain, elevation, etc...
Exactly! That's why you want to cruise in that torque curve, especially in mountains. We call that the 'service' your engine is built for.

Steve has already experienced driving outside the torque curve and it compelled him to change his cam grind. Now it's transmission ratios.

This is typical, everyone wants big power but when they get it at high RPMs they miss having torque at low end and fuel efficiency. You can't have it both ways. Edelbrock heads have intake valves so large the heads won't fit a 352 because they hit the cylinder walls. But a low RPM mill creates much more low end torque with small valves, like pickup trucks come with.

I guess the bottom line is this:
Know what service you really want BEFORE the rebuild, then the driveline can easily be matched. What are the choices? Compression ratios, cam grind, heads, torque converter, transmission ratios, rear end gears and tires. All these factors play important roles in the type of service your car will live in. All the rest; carb or EFI, cam timing, exhaust system, distributor curve, shift kit, etc., are all 'tuning' aids.

Car manufacturers are experts. They hire the finest driveline engineers and they have over 100 years of experience. OEMs supply the best combination packages for all types of street service. Before you depart from stock, consult a builder who knows the correct service combination or you will end up with very expensive mis-matched parts that don't work well together. Not everyone races so a 'racing team' shop may NOT offer good advice for engine efficiency and longevity in a daily driver. - Dave
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Old 12-08-2016, 06:45 PM
Yadkin Yadkin is offline
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My cam grind is for low RPM grunt. Although it's a roller, I assumed the torque values at RPM from a factory motor.

When I drive this car on 421 from Wilkesboro up to the Blue Ridge Parkway, there is a long, steep climb, much of it 8 to 10% grade. The car is very happy in 3rd gear at the posted speed limit and above. In my Jeep, 4 speed plus OD, it typically downshifts twice, into third. With the TBird, I'd expect to drop out of OD into 3rd.

My desire for an OD is for flat and rolling terrain at 65 and above.
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Old 12-09-2016, 09:46 AM
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X2 on the OD.

Irregardless of whether or not the engine would be in the correct power band etc I make a 125 mile interstate trip in mine often. The speed limit is 70 so most of the time I'm traveling about 80 (ish) to keep up with traffic.

Got to be turning 3000+ rpm.

If I could drop that RPM in half and only come in and out of OD for the occasional hill etc that would be exceptional. I'd never get my return on investment with gas mileage but couldn't beat it for interior noise reduction and just a better feel that you're not running the engine at higher revs. As a kid we made many trips to Fla in our Tbird and it lasted just fine so know they are capable of the higher revs for extended periods - just like the lower revs of a modern OD engine I guess.

Steve - I haven't researched - how is the OD engaged - vac module? electric switch? cable?
Eric
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Old 12-10-2016, 11:09 PM
Yadkin Yadkin is offline
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The type I'm looking at is electric shift with automatic clutch, and has about a 28% reduction ratio. The manufacturer claims that it can be used concurrent with all of the transmission gears, basically making a 3 speed into a six speed. I think that's a bit impractical for a driver. My intent would be to shift into OD once I got up to speed, just like I drive my 05 Mustang.

Online reviews of the unit confirm that this is the practical use. Up shifts are supposedly good, but downshifts are reported as "clunky". The manufacturer claims shifts can be done at full power. I'm a bit concerned with operation.

No question that OD saves engine wear. Modern car engines last a lot longer than the old cars without OD. The manufacturer claims up to a 28% increase in fuel mileage and I sincerely doubt that.
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Old 12-11-2016, 01:27 AM
Thunderherd Thunderherd is offline
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There's a couple of things I'd check on that before I went that route. Although it sounds really good. I had an Airstream motor home with the GearVenders OD unit on it. Yes, it performed really well. Really lowered the rpm's when engaged. But. When not engaged in OD it was a 15% under drive. Might not be a problem for you, depending on your rearend gearing, could be a benefit. But mainly, and I consider this a safety issue, if you left it engaged in OD, when parked, and put the transmission in park, there was no lockup. What I mean is, place the trans in park and the motor home would roll off if you didn't engage the parking/emergency brake. And you have to be moving to change the gears in the Gear Vender unit. You can't just come to a stop and then flip your switch to change gears. For example, to shift up, leave your foot on the accelerator pedal, slightly under load, flip the switch, and GV unit doesn't change gears till you lift your foot off of gas pedal (releaving load on drive train) and then give it gas again. Same process down shifting. This may sound complicated, but it's really not after a time or two doing it. But you gotta remember to shift out of OD before parking. PS. There was a few times the unit wouldn't shift into OD because of electrical issues. No big deal there. But when OD was working, gas mileage was considerably better. Like from about 8 mpg without, to about 12 mpg or a little better with. 30 foot motor home with a 454 off brand motor in it. (454 Chevy, lol).
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Old 12-11-2016, 01:12 PM
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Quote:
Originally Posted by Yadkin View Post
The type I'm looking at is electric shift with automatic clutch, and has about a 28% reduction ratio. The manufacturer claims that it can be used concurrent with all of the transmission gears, basically making a 3 speed into a six speed. I think that's a bit impractical for a driver. My intent would be to shift into OD once I got up to speed, just like I drive my 05 Mustang.

Online reviews of the unit confirm that this is the practical use. Up shifts are supposedly good, but downshifts are reported as "clunky". The manufacturer claims shifts can be done at full power. I'm a bit concerned with operation.

No question that OD saves engine wear. Modern car engines last a lot longer than the old cars without OD. The manufacturer claims up to a 28% increase in fuel mileage and I sincerely doubt that.
There's a guy I know here in NZ that runs one of those over drives (Hone?) behind the TH400 in his '49 Chev coupe, the motor is a pretty stout big block Chev - stout enough for high 9's in the 1/4.
He also gets reasonable gas mileage too.
And I have not heard of him having any issues with the o.d.
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