Over Drive Anyone?

[SpiderJim]

Just reading about folks who switch rear axels and such to reduce RPM. Anyone installed one of those aftermarket overdrive units?. The perfect product would be an electronically switched OD that could be stuck inline with the drive shaft (or similar) to drop those HWY RPMS. Come on Brady you like to build stuff. I would love to try something like that. No messing with the normal rations until flicking that switch. Think I will investigate this more. Any input or ideas?

My 81 Fi Spider is great around the city or for short trips but hate the RPM on the HWY. I want to cruise the country but know it would drive me mad. I have a C5 vette convertible that cruises at 75MPH @ 1800 RPM getting 29 MPG. Unfortunately is useless in the city due to being to low at the spoiler and hits every bump. So the Spider OD would be the cats meow and that way I can sell the vette.

[spiderrey]

interesting

[azruss]

I went the other way. I put a 71 rear end on an 80. Top end, i dont need. wanted something with a little more hop. These motors are designed for RPMs, i'm afraid you would kill it trying to get below 2k at highway speed. I had a Volvo with overdrive. it actually got better fuel economy without the overdrive because the motor was in its sweet spot.

[So Cal Mark]

I love the sound of cruising at 4k, and that's right in the middle of the torque peak with my setup. Big ol' V8s are far different than these motors and it's impossible to compare the performance of the two

[BenjamminsFiat]

This topic has been broached a lot it seems. I know where I stand, I'd like to have my particular car geared higher so that I can cruise at lower RPM's on the highway. I can absolutely understand Mark's viewpoint, just don't have the same preference. I'll wind out each gear to 6,000 when I'm playing around, but want to cruise when I'm on the highway. If I need more power, I'd just downshift to fourth. When I had my old 911, I'd downshift all the way to 3rd if I really wanted to scoot. 3rd would take you up to 100MPH if you really rung it out.
BoxDogRacing (I think that's his screen name) did it and LOVED it. I guess it's too early to see if it affects his reliability, but I would expect it would be pretty reliable since it's an "original" part.

Azruss: I'm really surprised at your experience with the Volvo. I wouldn't have expected that, but I'm a novice at most of these topics so I tend to get surprised a lot, lol

I'm definitely in the camp that says: Do whatever you want to do to your car. I love modifications that make them more enjoyable to drive for each owner's preference. I'm going to try to install the 95-amp alternator this weekend... have been waiting to do that since I heard of the upgrade.

I'm not sure how much help I could be but I'm definitely interested and subscribing to this topic. Great idea.

[Zippy]

I'm with Mark on this. I love the sound of the high revving engine. Usually the only time I shift to fifth gear is if I'm on a long drive on the freeway. If I'm just running around in traffic on I-5 I keep it in fourth just so I have a little more grunt. I like to pass the slowpokes with a little gusto if you know what I mean.

[SpiderJim]

I'm with Mark on this. I love the sound of the high revving engine. Usually the only time I shift to fifth gear is if I'm on a long drive on the freeway. If I'm just running around in traffic on I-5 I keep it in fourth just so I have a little more grunt. I like to pass the slowpokes with a little gusto if you know what I mean.

Wow! I am surprised that you can get out of first on I-5..... Always bumper to bumper when I visit Laugh. I understand and appreciate the sound of a mellow exhaust but would still rather have lower RPM when on a long trip of hwy driving.

[Colonialgirl]

I had a '69 Alfa Spider out in CA ; Bought it new. The 5th gear was an over drive. Interesting thing was that in 5th gear the MPH = 2x RPM. So, at 3500 RPM you were sitting on exactly 70 MPH, 4000RPM=80 Mph. 7500RPM=150MPH. No, I never got caught, I did drive it just with the Tach for 2 years. Loved that car, but when I moved across country in '79, I had to sell it.
I do like my '76 Fiat Spider, nice little car, but I've got lots of work to do before it hits the road again. I do think an OD would be handy here in Florida with nothing by flat roads in most areas.

[vandor]

Those electrically actuated overdrives are rather bulky, it would be difficult to find space for it behind the tranny.
Fifth gear is already an overdrive (ratio under 1:1), so what we really need is a long overdrive. I found one in Europe, but they are very rate, took me 2 years and it still needed all kinds of mods to fit.

[baltobernie]

I saw your posts on this modification, and it did indeed look like a lot of work to get the taller gears to fit. I sent e-mails and photos to every "custom automotive gear manufacturer" that I could find here in the states, and couldn't get anybody interested. I still think it's a viable product. What we need is somebody with a laser measuring thingy who can create a CAD of the existing gearset. Looking at the exploded view in the manuals and the gears my buddy loaned me, it appears that both 5th and reverse need to be changed. Is that true? What is the new tooth count of 5th? What MPH are you now running at, say, 3000 in fourth and fifth?

[TX82FIAT]

Seems like a lot of work and modification. The gear ratios vary based on year. I think most spiders have a .881 5th gear. Some of the older spiders owners have changed the rear end from 3.9:1 to a 4.3:1 for better highway RPM's. I would look into cars like the 131 and see what differential they offered that may work. I know 131 Abarth had final drive options available, ranging between 6.6:1 and 3.9:1. With that said, one might have to sacrafice lower gear RPM gains for higher end RPM loss? Not sure how it all would work out. Not a mod I would want to be one of the first to do.. but would consider... the speed limit in Texas is 75 all over the place with a lot of open road between cities.

[vandor]

> I sent e-mails and photos to every "custom automotive gear manufacturer" that I could find here in the states, and couldn't >get anybody interested.

It would be insanely expensive to manufacture those gears. There are gears with similar dimensions that could machined and welded onto the Fiat gears, but it would be expensive (much less than having them made, tho!). A friend of mine with a machine shop has this project on a back burner, maybe one of these days something will come out of it.

> it appears that both 5th and reverse need to be changed.

The countergear is a one piece unit with both 5th and reverse gears on it.

> What is the new tooth count of 5th?

If I remembered that... the new ratio of 5th gear is 0.80, so it can be calculated.

>What MPH are you now running at, say, 3000 in fourth and fifth?[/quote]

I *think* it is a true 70 mph at 3500 rpm in 5th. Which would mean most Spider speedos would show ~75 mph.
Since it is a 0.80 ratio versus the stock 0.88, it is 10% longer than stock, so it lowers rpm by 10%.

[vandor]

> The gear ratios vary based on year. I think most spiders have a .881 5th gear.

The only ratio that changed was 5th, 1970 and earlier had 0.91, after that it was 0.88.
I know the shop manual says some years have a longer 5th, but in reality I have never seen that ratio in any
transmission.

> Some of the older spiders owners have changed the rear end from 3.9:1 to a 4.3:1 for better highway RPM's.

I have, but I wanted even lower cruising rpm.

> I would look into cars like the 131 and see what differential they offered that may work.

The 131 diff is not a direct swap, you'd have to swap the gears. The 1979-on Bravas had the same 3.58 ratio as the automatic Spiders, so that is an option for some, but I wanted to retain more performance by not going lower than 3.9.
Also, the late style rear end weighs 20 lbs more, so I would not want one in my early Spider.

[fiatfactory]

.

[So Cal Mark]

or just drive at 4500rpm, what's the big deal? I do it constantly