IS IT WORTH CHANGIN' THE REAR RATIO ??

[Rainer]

I like better ET's -- who not http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif - here are some facts:
2400 pounds roadster
355hp sbc
th350
converter ?? - can hold the car up to 1900 rpm
2,56:1 rear ratio (locked rear end)
29" bias ply tires or 28" hurst slicks
I run 0-60mph in 4.9 sec on bias ply - ET time I don't know exactly - it must be around 13.00 I think

this is the stuff I have - but I wonder if it's worth changin ring and pinion to 3,50:1 and how much faster it would be - or what would you do?

[DRD57]

I doubt it would run 13.00's with 2.56 gears.

3.50's would help the car launch much better and would get you closer to that number if not even quicker. 3.50 is an all around good gear for street and strip with the power train combination and weight that you have. The fuel mileage may not be that much different. Sometimes those really tall gears can lug the engine which can hurt the mileage. When I changes from 3.00's to 3.50's in my Model A coupe the mileage remained the same.

[burger]

Rainer,

Try plugging your combination into CarTest... it's not the real world so don't treat what it says as fact, but you'll get an idea of what to expect. Download the free v4.5 at the bottom of the page.


http://www.cartestsoftware.com/cartest4.5/

[Rainer]

wow working on that 1 hour and doesn't worke the way I like - comp dummy http://www.jalopyjournal.com/ubbthreads/images/graemlins/mad.gif

[38Chevy454]

wow, 2.56 is a tall gear, especially with your 29 inch tall tires. You will definitely have better acceleration with lower gears, at least to the point you have traction problems. I think a 3.5-3.7 range would be good choice. It depends on how much high speed driving you want to do, then keep the gear more like 3.5 range. The determining factor will become traction, as more gear (higher number) will give you increased torque on the tires. Bias ply are not known for exceptional traction. However, I think you will not get into traction problems until you get into the 4.0 and higher number gear ratios. Tire spin means not accelerating. Good for show, bad for acceleration.

Is your engine a 355 cubic inch? Or 355 horsepower? If you have a 355 cubic inch, then you have more low-end torque than if you have a (for example) 283 making 355 horsepower. The amount of low-end torque you have is really the determining factor of what gear ratio is best for your traction. The stall speed of the torque converter also plays a role, it becomes a balancing act. From your decription it seems you have a slightly higher than stock, but not much converter.

[draggin'GTO]

Swapping from a 2.56 to a 3.50 will likely cause you to start having some traction problems due to the increased torque multiplication of the new lower gearing. If all you're looking for is acceleration then maybe the 3.50s would be the ticket, but again traction will likely become an issue.

If the engine has a mild stock-type cam the engine might not be able to take full advantage of the lower gearing either. If you do a lot of open-road driving I would say change to a 3.23 gear at the lowest, that is plenty of gear to accelerate a 2400 pound rod with 350 horses and still stay highway 'friendly'.

My own '64 Tempest daily driver (3800# w/o driver) has a mild 350 HP 8:1 compression 455 under the hood, TH400 trans and a 2.56 one-legger rear. Launching softly (2.350 sec. 60' time) so I don't blow the tire (235/45-17) off the rim it still runs 13.80s at 102 MPH in the 1/4 mile using only 1st and 2nd gear. I swapped to a 2.93 gear and the car didn't go any quicker and actually slowed by about a tenth, the mild stock Tri-Power cam works best with the taller gearing in this application.

I see no reason why your rod with the close to the same power and 1400# less weight can't be running at least mid-13s with the 2.56 gears. http://www.jalopyjournal.com/ubbthreads/images/graemlins/wink.gif

----- Bart -----

[Morrisman]

Looks like you are geared for about 180mph flat out. You could drop that very comfortably, have a snappier car, and probably get better mileage by letting your engine spin a bit easier at cruising speeds. I ran 3.70 with 26 inch tyres and always wanted another gear to shift to, so 3.20 (do they make 'em?) or 3.50 with 29" tyres sounds a good guesstimate to me. Paul

[Dragrace66]

Rainer's engine is a GM Goodwrench ZZ4 (350cui with 355HP)with 750cfm Carb

[burger]

Rainer,

I spent a couple of minutes playing around with CarTest, and I'd recommend a gear ratio around a 3.40... Here's the accumulated data:

Gear Ratio: 2.56
0-60: 4.4
1/4 Mile: 12.9
MPG: 18.6
Launch RPM: 700

Gear Ratio: 3.40
0-60: 3.6
1/4 Mile: 12.4
MPG: 15.1
Launch RPM: 700

Gear Ratio: 3.65
0-60: 3.6
1/4 Mile: 12.4
MPG: 14.4
Launch RPM: 700

Gear Ratio: 4.11
0-60: 3.8
1/4 Mile: 12.2
MPG: 13.0
Launch RPM: 1000



Ed

PS- I threw in the MPG coz I know gas is expensive over there.

[Paul2748]

in my 48 Ford with a warmed over 302 I changed from a 3.00 to a 3.55. Very little difference in mileage, but a great difference off the line. Seems to be a good street gear.

[Rainer]

edmurder thanks for checking that for me!!
Gear Ratio: 2.56
0-60: 4.4
1/4 Mile: 12.9
MPG: 18.6
Launch RPM: 700

Gear Ratio: 3.40
0-60: 3.6
1/4 Mile: 12.4
MPG: 15.1
Launch RPM: 700

the hop between this ratio's is great - 0.8 sec on 0-to 60mph - sound realy good to me - I'm surpriesed that it would be that much on such a light car

[Hot Rod To Hell]

Why exactly would the 3.40 accelerate the car faster from 0-60, but slower from 60 to the buck-o-change trap speed in the quarter??? (you picked up .8 from 0-60, but only .5 in the quarter!?!)