CWerner

I've got an RS500 due to be shipped in two weeks. It's planned to match up with my 3.08 rear end. I thought this would be a good combination. As I'm doing all of my prep work, It's become clear that the rear diff needs to be rebuilt.

So, now I have the opportunity to change my gearing to something else. I'm leaning towards 3.55. It will be matched up with an LS3 / 480 hp motor.

Thoughts???

jdk971

i have the exact set up as you do. i thought the 355 would be perfect. my rear builder put
paper and pencil to it and came up with 3.91. i was shocked. i am not a mechanic or builder so i went with his advise. i am very happy with the gears. cruise at 70 just at or
below 2k rpm. jim

CWerner

Jim,

Thanks for the opinion and glad to hear it from someone with my same setup. I'm working with a expert rear diff / corvette guy and he's telling me that I need to go with something at least 3.55. He's telling me that in 5th gear I will be running to low of an rpm for the 480 hot cam. Therefore, I need to get away from the 3.08.

My only concern is 1st gear. When you move up to 3.55, or even where you are, your first gear is so short. I'm expecting that the tires smoke even quicker with that gearing.

Do you even use first, or do you take off in 2nd most of the time?

Please give me some feedback about how each of the gears feel.

Chris

93Polo

I don't see a 3.08 gear being to low, depending on the tire diameter used.

The car should be able to cruise in top gear at below 2,000 even with the HOT cam if you are running efi. Cammed LS motors can often cruise at 1,600 rpm with efi. I ran stock 3.42s with a 6speed in my C5 with a cammed LS1 (228/230 at .050) and it cruised fine in 6th (.50) at 65mph. You may want to search the C6 tech area to see what gears they are using.

The ideal gearing depends on your goals, ETs, mix of hwy to surface roads, desrired gas mileage etc.

You can take a look at gear spreads with plenty of rpm calculators. I found this one with google which provides some nice detail.

http://www.gtsparkplugs.com/GearCalc.html

What size tires were used to turn 70mph at under 2k rpm?

wmf62

a LOT is going to depend on your engine's torque curve. my ZZ4 has a high flat torque curve and can easily carry my effective RER of 2.45 (3.36 rer), but my LT1 wasn't that happy. the ZZ4 has just about as much torque off-idle as the best the LT1 could put out at a MUCH higher rpm.

a 3.08 is going to be pretty 'iffy'...
Bill

jdk971

i am not a mechanic or know engines. i went with a engine builder, tuner and rear end builder that i trust. i am happy with the rs500. first gear is short, but not that short to
make me start in second. the dyno he used is 20% short of others. last run was 333hp plus 20% equals 399.6hp.

you have to take in your torque curve when determining the ratio.
jmho and what i have been told. jim

93Polo

3.07 gears with a RS500 1st gear will have roughly the same overall ratio as a M20 with 3.90s or a M21 with 4.56s. It also puts 1st between the overall 1st gear ratios on the 2 factory 6 speed options for C5s.

5th will be a 2.057 overall ratio vs the effective ratio of 6th gear in a M6 ratio C5 of 1.71. Sixth is a hwy cruising gear only, assuming a ~26” tire. 3.07 was the standard factory gear in majority of T5 5.0 Fox body Mustangs which had virtually the same ratios.

My High School best friend had a ’89 5.0 5spd with factory 2.73s which on street tires worked surprisingly well. We did the basic intake and exhaust, no internal mods. 3.55 geared cars with equal mods would often jump out a fender which he would often pull back when the 3.55 geared car went to 2nd. The big drop from 1st to 2nd most T5 variants/evolutions have, likely didn’t help the 3.55s keep traction. The 2.73 geared car was turning quite a bit of speed forthe 1-2 shift making the quickness of the shift less critical and less likely to spin the tires.

A LS3 has more than enough torque to pull 5th with 3.07s. Granted it may only be a hwy cruising gear but again it would be turning more RPM than M6 or M12 ratio 6speed C5s in 6th.

Is it ideal for what the owner wants?

It depends on how many hwy miles they turn and what their priority is for acceleration vs cruising RPM.

I like to take road trips, driving in Atlanta often means time on the hwy, and the car would make very few passes down the strip. So I like a 3.07 to 3.36 gear with a 5spd and an efi motor like a LS1 or bigger.

Many of the Fox body guys which take their car to the strip regularly like 3.73s or even 4.10s (if you don't run out of RPM before the end of the 1/4). They also run sticky tires. I also don't doubt they are a heckuva lot of to drive on the street even if they aren't as fast or getting better mileage.

A Dyno graph of a MAST LS3 in the same power range has 350+ft lbs at 2k rpm.

http://www.gmhightechperformance.com.../photo_03.html

Ideally every gear ratio would take into account peak hp & tq/power band and desired acceleration distance. Shorter rear gears can often mask bigger drops between gears, like the 1-2 in T5s or the 2-3 in C5 Z06 M12 ratios (been there).

CWerner

Damn,

Every time I think I've got the right information, new solid input (opinion supported by data) comes up and gives me the other angle.

So here's where I'm at. I want the priority to be gears 1-4. 5th will only be a cruising gear if used at all. In Houston, it's zero to 60 then a light or a freeway delay. Then it's back to cruising at 45 waiting to go 70, then back to 55.

93Polo's view tells me that 3.08 or 3.36 will better protect the big drop from 1st to 2nd that is there regardless..... by the way, I like to shift at about 3500 unless I'm driving with my wife...then it's 2500.

I've got both sides of the fence to wrestle with.

How about this....when Mike goes to rebuild it, we stay with the 3.08's if the gears aren't worn. If they are, then I have to make a decision. I think I'm really down to 3.36 or 3.55.

Crunch527

For what its worth:

Not exactly apples to apples but I have used three different gear setups, 3:55, 3:73 and 4:11s behind a TKO-600 (close ratio) 2.87, 1.89, 1.28, 1.0, 0.64, with a zz4 crate engine, a 500+hp 383 and currently a 425hp 383. All three of these engine have tons of low end torque. I also ran stock tires in all three setups.

Kiesler will tell you 3:73s are the best and even told me 4:11s would be a bit steep. Honestly, I didn't notice any difference between the 4:11s and the 3:73s. I didn't see any lack of drivability with respect to first gear with the 4:11s. Where I did see a marked improvement was when I went from 3:55s to 4:11s...huge improvement. The car was sluggish with the 3:55s...even with tons of low end torque, it just didn't feel right.

I am also looking at an LS3, 525 version w/ TKO600, with my current 3:73s and I am not changing gears.

I don't race but like to row through the gears but also have a good and comfortable cruising rpm. I don't take long trips so realistically, I spend most of my time zipping around town, and if I do get on the highway, I have a nice cruising gear (5th) to keep the RPMs in check and allow me to retain my hearing (side pipes).

I agree with jkd971, who has your exact setup and therefore, I think if you stay in the 3:73 - 3:90 range you will have the best of both worlds and be happy with how the car performs.

Frank

wmf62

i missed/forgot you already have a 3.08 set in the car; so your idea is the right one, just try it... you might like it...

you will be better prepared with your own seat-of-the-pants data to make a decision.

Bill

93Polo

If you can get the 3.08s running cheaply and think you maybe happy go for it.

Muncie ratios:
http://www.dandltransmission.com/muncieratio.html


I read up on all this stuff when I built up my C5 switching from a M6 ratio to M12 ratios. It can be confusing but ultimately it is whats fun to you.

M6 vs M12 debate:
http://forums.corvetteforum.com/c5-t...age-power.html

Again I'm not saying which is best, just to consider your options and goals. 3.73s if you're happy with the cruising RPM can be a heckuva lot of fun.

Also I have a friend with a '70 GTO running a Richmond 6speed single OD (.62) behind a LS3 with a factory LS7 cam and 3.73s. He was getting ~22mpg on a hwy road trip with 295/40 18s (little over 27" tire).

jdk971

try it with what you have. if you plan to push it a little, you will want
to upgrade your rear anyway. then you can make the decision of what
gear you want. talk to a couple of people that know this gear stuff. NOT ME. i would recommend a upgrade rear, but i know nothing about
gears. best of luck jim

CWerner

I really appreciate all these viewpoints.

Jim, as you first said, you have the exact setup I'm going with. That is the real SOP feel / Opinion info I'm looking for.

Do you feel the big drop between 1-2 as an issue? If not, then closer to 3.73 may be my answer. The rear end is coming out within 2 weeks regardless. I'm going to do something, but whatever I do I want to be done and hopefully not regret it. It sounds like your decision to follow your builders advise worked out well for you.

Here's a good question..... How easy is it to break your tires loose when your cruising about 30 mph? 40 MPH? 50 MPH? The reason I ask is I'm also trying to avoid having too much torque/gearing and I risk having an unsafe car. The C6's have the advantage of the electronic anti-slip controls.

wmf62

talk about spreads.... my MY6 behind my ZZ4 with a 3.36rer has a 1st to 2nd drop from 3.09 to 1.67..

not a problem if you have the torque.
Bill

devildog

Bill has the answer as usual...it is torque at cruising speed/rpm. The RStranny in my 61 with a 400SBC does fine with a 3:36 differential. My set-up makes 400 ft# torque at 2000 rpm. The car perform well at the 1800-1900 rpm at 70 mph. However, it just feels better at 2200 rpm so I just cruise at 80 mph. if i replace I will go to a 3:55.

In my Avanti which weights about same as Corvette I have 400 SBC that makes 330 ft# torque at 2000 rpm. It has a 0.67 overdrive. I just replaced the differential and went from a 3.23 to a 3.70 differential. i wish i would have chosen a 3.55.

The old rule of thumb is to have 10+ (3,37 tranny x 3.55 =12) so you have plenty of gear to launch. The cruise speed vs engine rpm is the question to answer. If you do not cruise at 70+ mph often, rear gear does not make much difference

You will love the RS tranny...just adds a whole new dimension to the car...launch and cruise. I can run comfortably with C5 and C6's all day in both the Avanti and 61 Corvette (as long as no tight turns).

Joe

devildog

Chris, I just saw you are in the Woodlands, I am near in the Louetea/249 area. A bunch of us gather Sunday mornings at Spring Cypress and Champion Forest. You are welcome to drive my cars and get a feel for the 3.36 and 3.70 in similar weight and hp cars.

Joe

We usually have 20-40 cars gather. We have several cars with 5 speed conversions that can give you ideas and rides. then you will have 20 more opinions.

CWerner

Joe

Thanks for the offer.

This weekend is out. Corvette Chevy Expo!!!

wmf62

my OD is .73, 80 is about 2450. i too like to keep above 2000 as that's where the ZZ4 starts it's flat torque curve of about 400, i usually drive in 3rd (1 to 1) except on the highway in order to keep the rev above 2K (although the engine will lug at 1200 in OD with little effort)

torque is your friend...

the first dyno chart is the GM chart for the ZZ4, the second chart is for the LT1 previously installed; notice where the torque is...

Bill

wmf62

and this is an actual chassis dyno chart for the ZZ4 installed in the car; add about 20% for drive train losses..
Bill

wmf62

video of dyno run

View My Video

Bill