stinky

As I do my rebuild/mod on my '79, I'm getting close to rebuilding the 3rd member. Since my car was originally an auto, the rear gearing is a 3:55 set. I'm installing the D&D Viper/GM 6-spd which has the following gear set: 2.66 1st, 1.78 2nd, 1.30 3rd, 1.00 4th, .74 5th and .50 6th. It is my intention to equip my baby with a 540ci BB with a target of at LEAST 600 lbs torque. I have modified the rear running gear; so will the 3:55 gearing be too short or do I need a taller gear? No drag racing at the track. This is for the street and will see a lot of hwy (hence, the 5th & 6th gears). So my fellow gearheads, what opinions can you offer up on this before I rebuild the rearend. Thanks in advance. Mike



[Modified by stinky, 11:59 PM 2/10/2004]

stinky

Anybody?

Fevre

My questions is how did you first post at 12.59 am then post at 11:50am? :confused:

I would think that to use 6th gear you would need the 3.55 or being going 100mph with a lower (<3.55). With a 3.55 at 70 you will be around 1700 rpm's in 6th.

stinky

Fevre...am vs. pm. I need to get this terminology down pat one day. Will the 3.55 gears suffice w/o lugging the engine at lower rpms or would i need to go to 3.73 or highter? I always get the tall gear/short gear mixed up. Like I said, I anticipate having alot of torque availible.

Gordonm

The question is where is the torque going to be. If you are running a cam with a lot of duration and it does not make good power until 2500 or above rpm and you have an over all ratio (.50 x 3.55) of 1.77 you will lug the motor. The motor will not be efficient or run good if you are cruising at 1700 rpm. It really matters where the motor is making power. With 540 cubes you should not have a problem with torque. I will be running a .68 overdrive with 3.70 gears but my cam makes good power from 2500 rpm and above. If I was going to cruise at 1700 it would be a dog. A lot depends on the motor combo.

Fevre

DOH, that makes sense, now.

What Gordom said, you should have enough torq to run at most any rpm but having it match your cruise rpm to your torq rpm range would be best. I would just give the 3.55's a try and go from there., no sense spending $ on changing it till you know for sure.

Glensgages

Here's a few things to consider...
Assuming your normal cruising speed (I see you drive highway miles) is 75 MPH, and you run 'typical' tires (approx 27.5" tall) on your C3,
a .5:1 6th gear with 3.55 gears will be approx 1600 RPM;
a .74:1 5th gear would be approx 2400 RPM.

Assuming you hit just 4800 RPM in 6th, "mathematically", you could hit approx 225 MPH; 4800 in 5th is approx 150 MPH.
Do you really NEED to reach those speeds?
:confused:

If this is 'JUST' a fun car to cruise-around in, why not throw as much gear at the Rat as you can? For instance, 4.56:1 gears produce these-type numbers:
75 MPH in 6th gear: 2050 RPM.......75 MPH in 5th gear: 3100 RPM
4800 RPM in 5th gear: 115 MPH.....4800 RPM in 6th gear: 175 MPH

600 ft/lb TQ, 2.66:1 1st Gear, and a 4.56:1 ring & pinion?

Can you say " tire smoke" ?
:yesnod:

mrvette

LIke you all said above, but these daze it's a rare engine/cam that doesn't have torque all over the place like nearly flat over the reasonable operating range....so I agree with keeping the 355s and if you need, change then...or make the simple expedient of driving in 5th instead of 6th...depending....
1700 rom vs 2400 is not all that much, really....shouldn't affect fuel economy much....not that with all those cubes and power you going to get any anyway...

with that power, I beleive for a street machine, you desperately need a DPFI setup, as if it's closed loop you can about double your fuel economy.....which is nice not having to stop every damn time you see a gas station....

GENE

norvalwilhelm

I ended up with going back to my 308's with my overdrive transmission. I only have .68 overdrive which gives my car a final ratio of about 2.2 or so in overdrive. I run 1300 at 50 , 1600 at 60 and 1900 at 70 . My motor can easily pull this and I like the combination. I have a 3.27 1st gear and with the 308's it is great out of the hole.
I started with 4.11's but found them too low, 1st barely got me started before I was pulling second.
355's with a .5 overdrive will give you 1.77 overdrive and that could be a little too low with the .5 overdrive. 373 would also work great with haveing less gearing in 1st in the transmission.

stinky

Thanks guys...things I needed to hear.

After reading the responses, I went to: http://gafba.com/calculators/ and used their Ring Gear Comparison calculator, came up with some interesting info. Thanks again. Mike


[Modified by stinky, 9:41 PM 2/11/2004]

jpatrick636

If you look at norvalwilhelm's gear set up, you will find that his first gear ratio is very close to 10:1. This is a good rule of thumb for a street car. So based on that, I would recomend that you go to a 3.73 which would give you 9.218. Anything higher will will just give you a lot of smoke and burnt rubber. (Unless you plan on some monster slicks.) Trust me, your new problem is going to be traction.

1971Ultra

This is very interesting topic and something I've been struggling with.

Currently, I am running 3.36 gears, so 3.36 x 3.27 = 10.98 call it 11:1, not 10 but close. (3.27 will be when I convert to the Tremec TKO).

I am contemplating on changing the gears to 3.73, so 3.73 x 3.27 = 12.2. Even farther from the rule of thumb of 10.

Then the rest of gears for the Tremec drop.

2nd = 1.98
3rd = 1.34
4th = 1.0
5th = .68

Excluding 1st gear, it would seem that 3.73 gears would be better served for the rest of the Tremec gears.

So 1st gear will basically be useless with either 3.36 or 3.73 rear gears but I would be better served with the 3.73 for all the other gears?

Also, what do you really use for 1st gear for anyways?

Comments appreciated.

Keep in mind this will not raced (OK maybe an occasional 1/4 mile) and I will do occasional long hauls.

Gordonm

As soon as the weather clears I will be able to answer your question. Ihave 3.70 gears with the Tremec trans also. There are a couple of others here running this same combo and are very satisfied with it. 1st gear will be a little short but the rest is very good. Should be fun.

stinky

Ultra,go to the web addy I posted and run a couple of comparisons thru the calculator, it'll give you a better idea what your rpm vs speed will be to see what you can live with.

stinky

Thanks Jpatrick I couldn't remember the sweet spot for launching but now you confirmed it was 10.00, that the # I needed to play with along with the rpm/speed calculations. Again, thanks everyone. :cheers:

ndirishr1

I put 4.11s in my '68 and it really woke it up and made it more fun to drive. A lot of rpms on the highway since I have a 4 speed. If you have overdrive gears that would bring it back down.

Larry3x2

Sad but true C3 Vette rear ends won't handle a true 600lb-ft through a 2.66 first gear with tires that "hook". My '69 Big block dynoed 529 lbft @2500 with a 2.52 first and 3.70's eats half shaft universals. Even with precision alignment etc. It is presently on my lift now for a fresh set of federal moguls. An engine of the size you propose needs a straight axle conversion. The post on powerband is correct. For street driving you don't need a 540, your heart does.
Larry

Larry

Larry

bbeck

I think you could get away with the gearing you have. Here's a website, so you can figure what kind of gear you wanna run to achieve the speed you want at the rpm's you wanna run.
http://www.car-videos.com/tools/speedrpm.asp