Changed from 3.91 to 3.23 gears - Track Results

[Jims451]

Like the subject header says, I changed my 3.91:1 gears to 3.23:1 gears and took the car to the track last night. I ran a best of 12.845 ET at a bit over 107 MPH in the heat (88 degrees), so I know the gear swap slowed me down less than 0.3 second from my best run at Bandimere (a 12.56 ET).
I think the true difference is probbly closer to 0.2 second.
On the highway at 55-60 MPH, with the 10" dynamic converter my engine speed is at about 3,000 RPM, but when I let off the gas the engine goes to about 2,700 RPM so I'm getting a bit of converter slippage on the highway using these gears.
I think for my setup, 3.55:1 gears would probbly be what I need for a good compromise between highway driving and racing at the track.

[DartMan360]

WOW 3,000 RPM at 55. I have 4.56 and at 69 im pushing 3,800. How tall are your tires. just something else you might want to consider is going to shorter tires. this will rasie your grear. I have 26 and estimate im pushing a 4.70 gear

[451Mopar]

I have the 26"x11.5" M/T street ET tires for the track and 255x60x15" (about 27") for the street. Also, I think my spedometer may be off to the slow side because at 55 MPH on my speedometer I'm passing most other cars on the road
On my calculator, it says 2700 RPM should be about 65 MPH, this is the speed limit of the road I was on even though my speedometer is probbly reading slow. Anyhow at this speed (65 MPH?) my RPMs are about 3,000 RPM (estimate about 11% slippage.)
With 3.55:1 gears @ 65 MPH and 3,000 RPM the slippage would be 1% so the 3.55:1 gears look pretty good (or the 3.91's with TALL tires?)

[mopars1]

I am sorry 451, you lost me somewhere. You are going to change gears and that will change you converter slippage? I have never heard of such a thing! Could you explain that to me? Maybe I am impaired in some way. Am I reading the post wrong?

[tim]

not to but in or anything but i have 73 barracuda 340 auto and i just swapped from 3.55,s to 3.91,s. My rpm has jumped about 4 to 500 rpm at certain speeds with 235/60/r14,s bf goodrich.The car had original 3.23,s. My wife drove the truck and we used walkie talkies. Heres the results.At 40 mph on my speedo is 50 kmh at 1700 rpm, at 60 miles an hour on my speedo is 80 kmh at 2650 rpm,at 70 mph on my speedo is 90 kmh at 3000 rpm, at 80 mph on my speedo is 100 kmh at 3500 rpm, at 85mph is 110 kmh at 3800 rpm. So as you can see I have lost some highway cruising as before with 3.55,s it was cruising at 110 kmh at about 3400 rpm. I have not gone to track yet but I soon will as I will also had 15 by 26 hoosier slicks on the back, because i had a severe traction problem before the new gears and the slicks.It is much more snappier now though so it should improve my et.s I,m gonna keep the gears in for the year and try them out and if I go on a long trip I will swap back to 3.55,s as they seem to be a very good all around gear. Tim Walton

[Jims451]

mopars1, I'm not really changing the amount of converter slippage. The problem is the stall speed of the converter is around 3,000+ RPM.
Currently with my 3.23:1 gears, if I drive at 65 MPH the engine turns about 3,000 rpm to maintain that speed. If the converter was not slipping (or if I let off the gas) RPM will be about 2700 RPM with my current gear/tire size combination.

With the 3.55:1 gears, at 65 MPH the engine RPM (with no converter slippage) should be about 3,000 RPM which is closer to my converters stall speed at the HP level I need to overcome aerodynamic drag and friction at that speed, IE partial throttle.

Let me add that converter stall speed is not absolute, it varies depending on the engines torque output and the load on the transmission.

[duster360]

i'm sure everone knows this but just in case, be sure to have a trans cooler because if you do alot of cruising with the converter slipping your going to make ALOT of heat and possibly burn your trans fluid which can lead to complete failure, i'm sure you have a good trans cooler but if you don't get one. like i said i'm sure everyone knows this already but just in case someone doesent it might help them

[bwlizard]

Thanks 360 SPELL CHECKER

[70Challenger R/T S.E. 383]

Your speedo probably is off since you didnt mention changing your speedometer gear after the gear swap.
C-ya

[Dustbuster]

you only lost .2 on the 1/4 going from 3.91s to 3.23's?

somethings gotta be wrong! thats unreal....3.23's are like medicore performance with some fuel economy....3.91's are like your tachin at 4800 at 65!! (not really but you know what i mean..)

oh well gear ratios are an enigma (dun dun dun)

i know that if you didn't change your speedo cable (like challenger said) your speedomter will not show the proper speed...its based on what gear your in, and rear axle ratio and how many RPMS.... it a big old ugly nasty equation that a simple little toothed gear does for us so we can look down and not get pulled over for doing 120 in a 20!!!

[451Mopar]

Yep, the 451 stroker is a Torque monster! I'm thinking of someday going to a larger cam to pick up some more HP on the top end. With my 10" Dynamic Converter (I love this converter with my setup) and the engines torque, I have a hard time keeping the tires from spinning, even with stickey tires.

[bk340dust]

tim instead of swapping gears for long trips go to a 15" wheel and taller tires like a 27-28" tire on the back. It's a lot less work. Going from a 25" tire to a 28" tire will reduce your motor rpms by about 300 rpms

[SmokeDog]

451Mopar, Just goes to reafirm how correct the choice of power plants was. For the general beterment of my fellow Mopar brothers. This is proof posative,Mopars do make more torque. If it was a chevey,it would have lost a full second. SMOKEDOG Know more mister nice guy,down on you'r knees wench.

[dschumers]

I changed to 3.23's from 3.91's in my Slant car and it slowed down exactly 2 tenths. MPH styed the same. Much nicer to drive on the highway now too.

[Mach Manny]

i'm going from a 2.71 8 1/4 to a 3.23 spooled 8 3/4. I dont have a tach right now, so i have no clue what RPM i'm pulling on the freeway, but let me tell you 3rd doing about 70 it's UP there.

i'm actually not looking for top end, only torque so after i setup my rear end i can work on that 451 also. I only have a spread bore on a single plane, so i just want mad torque and trottle response, and easy revs. I'm not a big fan of 'flying', espeically in my baby.

Anyone guess waht i'm going to gain going from 2.71. I really oughta get a tach and timeslip records!