Justin Braune

Hello everyone,

Having built a Pat Musi 555ci BBC that will put about 700 ponies down. Working on the T56 Magnum and VanSteel setup and looking at an SST T56 Magnum.

I will NOT be tracking this, I won't be autocrossing it, its my fun weekend dad-mobile with the occasional ******* funtimes with the boys, but all around town/back streets and off track. I'd still l ike to be able to chirp 2nd and 3rd and get the throw back into the seat torque, but not looking to be doing the 1/4mi.
Thoughts on T56 Magnum 2.66 or 2.97??? And then for the rear suspension is it a 3.55 or a 4.11????

One recommendation is 2.66/3.55 combo, but on the SST speed calculator, seems like I'll top off pretty quickly???

Please help???

cagotzmann

Start with where is engine rpm is best without a lumpy feeling. This should be the best rpm for highway cursing speed. Then work towards what speed you like to cruise @.

eg engine RPM = 2000 RPM.

T56 Magnum close ratio
1st 2.66, 1.78, 1.30, 1.00, .80, .63 6th
or wide ratio
1st 2.97 2.10 1.46 1.00 .74 .50 2.90 6th

with close ratio T56
3.08 Rear axle @ 2000 RPM with close ratio = 83 MPH
3.36 Rear axle @ 2000 RPM with close ratio = 76 MPH
3.55 Rear axle @ 2000 RPM with close ratio = 72 MPH
3.70 Rear axle @ 2000 RPM with close ratio = 69 MPH
4.11 Rear axle @ 2000 RPM with close ratio = 62 MPH


Here a good choice is 3.7 or 3.55

with wide ratio T56
3.08 Rear axle @ 2000 RPM with close ratio = 104 MPH
3.36 Rear axle @ 2000 RPM with close ratio = 96 MPH
3.55 Rear axle @ 2000 RPM with close ratio = 91 MPH
3.70 Rear axle @ 2000 RPM with close ratio = 87 MPH
4.11 Rear axle @ 2000 RPM with close ratio = 78 MPH

Here a good choice is only 4.11

next determine where the best engine response range is from to max torque. eg 2400 rpm - 5500 RPM

with close ratio T56 1st gear
3.08 Rear axle @ 2400-5500 RPM with close ratio = 24 MPH - 54 MPH
3.36 Rear axle @ 2400-5500 RPM with close ratio = 22 MPH - 49 MPH
3.55 Rear axle @ 2400-5500 RPM with close ratio = 21 MPH - 47 MPH
3.70 Rear axle @ 2400-5500 RPM with close ratio = 20 MPH - 45 MPH
4.11 Rear axle @ 2400-5500 RPM with close ratio = 18 MPH - 41 MPH

Here I would say a 3.7 - 4.1 works for street driving where you can go WOT in 1st (max torque seat in the pants feeling) and still within street speed limits.

with wide ratio T56 1st gear
3.08 Rear axle @ 2400-5500 RPM with close ratio = 21 MPH - 48 MPH
3.36 Rear axle @ 2400-5500 RPM with close ratio = 19 MPH - 44 MPH
3.55 Rear axle @ 2400-5500 RPM with close ratio = 18 MPH - 42 MPH
3.70 Rear axle @ 2400-5500 RPM with close ratio = 17 MPH - 40 MPH
4.11 Rear axle @ 2400-5500 RPM with close ratio = 16 MPH - 36 MPH

Here I would say a 3.36 - 3.55 works for street driving where you can go WOT in 1st (max torque seat in the pants feeling) and still within street speed limits.

you get the idea but start with what your engine likes (RPM Range) low RPM is the important starting point. You want max engine response / torque without a lumpy feeling.

But a very good starting point is match the C6 Z06 or the C6 ZR1 specs. they have a 3.42 rear gear and the cars are great on the street and track. so the 3.36 or 3.55 is a good starting point.

Justin Braune

Question, when you're saying starting point c6 z06/zr1, what is the transmission on those? Close or Wide ratio or neither?

cagotzmann

I have a T56 Magnum Close ratio in my 78 with 3.08 Rear. Have a look at my install
https://www.corvetteforum.com/forums...-corvette.html

Also consider a twin disc clutch. Much lighter feel ( similar to a C6 ) but also will hold up to your engine better. To use a single disc with your engine will require a very heavy feeling clutch. The side effect of a twin disc is noise and chatter. There are no damping springs in a twin disc setup.

This is their specs, so neither. Z06 1-4 is a T56 Magnum close ratio + custom closer to the wide ratio.

Z06
Gear MM6 Ratio (:1)
1st: 2.66
2nd: 1.78
3rd: 1.3
4th: 1
5th: 0.74
6th: 0.5
Rev: 2.9


ZR1
Gear MH3 Ratio (:1)
1st: 2.29
2nd: 1.61
3rd: 1.21
4th: 1
5th: 0.82
6th: 0.68
Rev: 3.11

Both are 3.42 rear gear ratio's

gkull

The more realistic point is top speed calculated with your tire diameter and red line. With a .50 6th gear I would get a 4.56 or possibly a 4.30 rear end. With a .68 I would get a 3.90 or 4.11

I use a. 4.11 with a. 64 OD and a 2.89 first gear. It makes for easy take offs in town and traffic it still gives me 200 mph + in od with 28 inch slicks

4.11 really is nice. I've had it for 20 years the first 4 gears work just fine driving in city and od is for the highway when you are exceeding 60 70, 80 mph

555 ci will have the Tq to accelerate even in 6th

cv67

Is this it one of the Jegs crate engines, cam listed below?

Rear tire size, carb or EFI?
Youre gonna need a sticky tire. It will still haze them but they catch up quicker...street tires imo would be a waste and almost make the car unsafe.

https://www.jegs.com/i/Edelbrock/350/2264/10002/-1

gkull

My newest Vette came with this setup. It was so hopelessly over geared that it could go faster in 5th than 6th. I changed out the rear end to a 4.10 and made 6th usable. I only had an LS3 motor. the poster has a TQ monster 555, I like close together gears with less rpm drop between shifts. Big jumps cause a car to bog on a upshift unless you rev the motor up. There is no reason to gear a car for 240 mph in a .50 final over drive when because of air resistance it can only go 170.

I also like not having to slip the clutch to take off. My 2.89 X 4.11 = 11.8779 Is just perfect even with over 600 hp motors

firstgenaddict

A 3.55 with a 2.66 first would be quite easy to drive... my 3.55 with 2.52 first doesn't need to be slipped at all with a never removed BONE STOCK LT1.

jb78L-82

The more HP/Torque, the much less important is either gear.

With a 555 BBC and 700 Gross HP, you can pick any gear you want assuming you have good power off idle.

If the choice is 3.55 versus 4.11, definitely 3.55....a 4.11 gear with that HP will just spin the tires even easier than the 3.55 which even then will not hook up the tires. 2.52 first gear as well. Again, with that much power on the street with street tires will just roast the tires

Taijutsu

I would think that wide ratios for 1-4
Then close ratio for the ODs?
The jump to .50 is too much for most engines.

JMHO

jb78L-82

My 10 Z06 has a tremec 6060 with a rear 3.42 gear and:

Z06 1st gear - 2.66
Z06 2nd gear - 1.78
Z06 3rd gear - 1.30
Z06 4th gear - 1.00
Z06 5th gear - 0.74
Z06 6th gear - 0.50

I think these gears are perfect for the street with "only" 505 NET HP..............The C6Z06 can effortlessly accelerate in 6th gear on the highway above 65 MPH

Taijutsu

I was thinking of top speed rather than cruising!
The gaps between gears could be a limiting factor?

lionelhutz

Close ratio with 3.55 or 3.73 rear gears. With the power you have, a mid-high 4 rear gear would spin all the time. But, if you did hook the engine would rpm to redline so quickly that you'll be spending more time changing gears than accelerating.

Making the top priority for picking rear gears the top speed while using 6th gear OD is the WRONG approach for a street driven car. 6th gear is for highway cruising in a street going car. Heck, with 3.55 or 3.73 gears you could likely run somewhere in the 160-180mph range in 5th, so why would you need it geared for top speed in 6th???

Make sure the 1st gear operating range is what you want and that the 6th gear cruising rpm is what you want. Just run numbers like in the very first post.

jb78L-82

This explanation is correct about the OD utility on the street..OD's are NOT for acceleration on the street but for highway cruising. Aggressively excessive gearing in 1st or the rear end is a total waste for Big HP cars. Big torque multiplication was developed mainly for street cars to accommodate weak 4 cylinder engines in the 60's/70's, not for 5/6/700 HP BB V8's. I always chuckle when I read folks talking about Big V8's like a 350 + (yes a 350 is a BIG V8) needing 3.XX gear otherwise the engine will not accelerate the car...NONSENSE......

76strokervette

With that kind of power the stock differential isn't going to cut it. I would plan for some upgrades there too.

terrys6t8roadster

Real world my 6t8 SBC with 3.73 and .064 OD 26.5 dia tires engine rpm 7200 top speed in 4th was 145ish. Very streetable, now with a BBC that has 700 ponys 3.55s would be my choice. also stick with the .82OD, and why would you need another OD gear. T

cv67

Cubes will soak up some cam but thats still a fairly big hydraulic I wouldnt gear it too tall.
I always gear for the cam to work the way its supposed to the cruise rpm will be what its gonna be, not like lugging is good for it anyways.
You could get away wtih 3.55 but think youll be happier with a lower gear. The tires will catch up quicker and spin LESS than a taller gear.

Justin Braune

So I ended up just pulling the trigger on a full SST kit and went with 3.73 as the gear ratio. Now I'm working with VanSteel for the rear diff. Do you think I'll need a 12 bolt?

cv67

Id get the strongest thing you can afford, have they made any recommendations?

Justin Braune

Need a tire/wheel combo recommendation. Want something with a big fat rubber in the back, so thinking of keeping a 15" rim w/ some fat rubber, but open to suggestions!