franz italy

Dear all,

I suppose this topic has been already discussed but I try to open it again because I need some suggestions (please keep it as simple as possible) but I'm not a technician/mechanic.

So, the situation is this: C3 with 350 c.i. engine (engine has 8 years and 4000 miles, carburetor, theoreticlaly around 300 hp) with TH400 (3L80) transmission and a final gear ratio crown/pinion that I don't know yet but I hope to discover next days. Tires are orignal size.

At 80 mph car runs at 3500 rpm... too much for my feeling. So my question is: how is the soution, considering $$$ too? I can't afford a 5 manual gerabox conversion (unfortunately). Being in Italy... you know we love manual shifting.

So... once I discover the differential ratio, i could look for a new crown/pinion configuration? I sow some options from some big C3 sellers in USA, around 1000 US $, that with shipping and UE customs and installation will become over 2000.

Some other guys told me about a OVERDRIVE, but I barely know what it is and have no idea of prices and shipping and so on.

Any chance to modify the TH400 or to buy a 4 speed automatic transmission? hoping the price is not astronomical here in europe?

thanks for the help!!

MelWff

you can buy in the US a 200r4 or 700r4 automatic, I have no idea what is available in Italy.
I have a 3:70 rear end and at 70 mph do 3500 so my guess is you have a 3:55.
The th400 was not called a 3l80 until 1987 so best not to call it a 3l80 to avoid confusion.
An overdrive is a thought but not too many people do it.
Your best bet for Europe with it's high cruising speeds is a 3:08 rear end swap.

franz italy

thanks so much Mel ! I was just browsing Bowtie website... interesting. Didn't understand the differences between 200r4 and 700r4 from their website, but OK I'm on the right direction.

so you would 1st step go on 3:08 with th400 and or/later to think for 3:08 + overdrive?

sorry for all my questions!

TCracingCA

Where is your power band?

The best real World scenario to cruise freeways and have performance on tap and potential mileage would be to design your ideal and favorite freeway cruise speed to be at the lower envelope of where you are in your power band!

What do you have, an engine power band and gearing of
Examples only:
2500-5800
3500-7000
4000-7500
1500-5500
?????

Some smart building can widen the power band. More gears means that you have a better ability to hit your best rpm mark at a specific speed!

The13Bats

i think more bang for the buck would come with the trans with overdrive swap,

I knew a guy with a 700r4 in a old monte, it had a mild 454 and gears in the 2s yet still could melt the tires and cruise at 70 with low rpm...im sure there were down sides i dont know about.

mrvette

I have a good used 308 ratio ring/pinion set in my desk drawer, no rust....they fit the earlier iron diffy up to 77 I think it was 78 they went to the aluminum bat wing differential.....

if interested, PM me.....

DUB

Knowing that you are on the right track to getting what you need...I guess it has a lot to do with what you find out from Bowtie Overdrives in regards to what rear end gear you should use to get what you are looking for in regards to performance and also rpm's at 80 MPH.

I also understand the cost factor when gettign parts from the United States and getting them shipped to you....which is why making sure that if you do change something and then take it further in the future...you are not having to take a step back and do something differently again.

DUB

71VetteLover

As you only drive it 500 miles a year I would leave it alone.

The13Bats

Isnt that the truth,
I missed the 500 part,

I recall a thread where some cat did the math how long it would take for an ob trans swap to pay for itself,

Basically if mpg is the reason its not worth it,

Also if half your drive time isnt highway its not really worth it either,

In my case i want to go from auto to gear box, just an old muncie 4 speed, in many ways its downgrading but i just want a stick in my old sports car,

71VetteLover

Just disconnect your tachometer so you won't know how fast it's spinning.

Torqued Off

Another option....slow down. Do you enjoy the car more going that fast?

Sunracer

One thing to consider-I believe adding a 4spd overdrive automatic transmission will improve the value of the car. Changing to a 3.08 rear diff will not change the value of the car.

Something to consider as you consider the cost of a rear gear change versus a change to a better transmission

franz italy

Hehehe I think I didn't explain myself... 500 miles/year was the average habit of previous owner... If I had a decent gas mileage I could make weekends and some little holidays with my wife and our c3 😍 . Last weekend we did 350 miles...

cooper9811

Given that information, I would look towards a 700r4 or 200r4 transmission vs the TH400.

I would prefer a trans swap over a rear end swap to maintain decent acceleration at takeoff.

When I bought my 79, I had a 700r4 with a 3.55:1 reaer end, and it was a mice combination of low-ger power at takeoff and crusiing on the highway. You should see somewhere arouynd 2000RPM at 70 if I recall (might want to check that with a calculator).

If money is not an issue, you might want to look into a manual transmission like a Tremec - there are complete kits available from vendors like SST, all parts required are included.

franz italy

unfortunately money is an issue LOL :-)
I hope today to discover my rear end ratio.... thanks for all guys... I'll keep you updated!

7T1vette

You have either a 3.55 or 3.70 differential ratio. Either one is a VERY GOOD match for matching with an overdrive auto transmission (either 700-R4 or 200-4R; these are different transmissions, but either will do a good job if upgraded appropriately for your engine's output power/torque).

These O/D transmissions have a final output gear ratio of .67:1.0, so your engine rpm's will be 2/3 of what they presently run (2300 at 70mph). Also, you will maintain (or even improve) low-speed or standing start take-offs because of low gear ratio differences from the THM-400 in your car.

Engine will be happier at highway speeds, and you wallet will retain more money as the O/D trans is MUCH more economical.

But, only swapping differential gears to a 3.08 ratio will lower engine rpm at highway speeds to about 2700 at 70 mph.

c3_dk

Hi Mr. Italy :- )

Now you own a Corvette, don't make it into a 1996 Mercedes 300D :- )
If you want to lower your RPM, then buy a TH2004R or TH700

Or just drive it, as it is.....

Just my 2 cents :- )

franz italy

OT: Hehehehe for that I have a russian Uaz with a 240D mercedes engine... can't reach the 70 mph. Maybe it's why they lost cold war! Anyway she is indestructible.


Regarding the C3, I really love to cruise at low rpm... here in Italy 2.0 litre is a HUGE engine and I can't think I haver to drive at higher rpm than them at same speed with a 5.7 litres!

c3_dk

The only way for me is, min. 3.70 rear and 5/6 speed manuel, or TH2004R/TH700

My 69 convertible has 3.70 rear, right now I'm replacing my M21 transmission with a 5 speed T5WC.
My 69 Coupe is SBC 427, TH2004R and 3.70 rear.

c3_dk

BTW I love your UAZ :- )