Differential change

[hoppy]

I am thinking of changing the 3.54's in my Era 377 to 3.31's. Any suggestions for a rear end builder in Socal?

[joedeats]

I think I would need more info to make an educated guess on the gears. what tune is your engine in? does it have a wild or mild cam, and what are the cams specs. What trans are you running? the gears in a C6 for instance are alot different than most 4 or 5 speeds. How do you use your car? daily drives, drag racing, etc. Lets just say it uses a stock scj cam, and most of the rest of the tune is close to stock as well 750 carb or less etc, it has a C6 and you use it for shows and daily rides. You wont really notice it to much difference if your going from 3.55s to 3.31s but if your going from 4.10 or bigger it will be a HUGE difference. The 428 is a big engine with good torque with a smaller engine it would be very noticable but with your current set up not much, but your rpm would go down which is nice for freeway stuff. Your top speed would theoreticaly go up but the cobra has the aerodynamics of a 40 year plus year old car so it wouldnt be to much different. Bottom line you wont notice it at all I bet. I went from 3.55s to 3.91s in my daily driver Super Bee running a near stock non orginal 440 and I didnt really feel much difference at all.

[Rick Parker]

Try Currie in Anehiem.

[hoppy]

joedeats:
Have 428 SCJ with 10.75 CR
Comp Cams FB 282S-B
Gross lift 0.571/0.571
Duration 0.05 236/236
High Riser with 750 DP
MSD-6
W/R Toploader w/big in and out

Can't put the power on the ground now and highway cruise 70-80mph is 4,000+ rpm and noisy for 100 mile trips to Montecito. I am expecting that I have enough torque to still be a street light bandit and get a little relief at cruise by dropping down to 3.31.
What do you think?

good to know everything OK with your penis

Rick: Thanks for the referral.

[NeedAntiVenom]

Terry,

Wide Ratio:

3.54 @ 80 = 3670 RPM
3.31 @ 80 = 3420 RPM
3.07 @ 80 = 3170 RPM (first is a little long)

TKO:

3.54 @ 80 = 2300 RPM

TKO-II:

3.07 @ 80 = 2630 RPM

Richmond:

2.88 @ 80 = 2970 RPM

You'll drop by 250 RPM going to a 3.31, a TKO, TKO-II or Richmond might work better. TKO will get you what you want without doing a diff swap, but the jump from 4th to 5th is fairly big.

Hope this helps.

[hoppy]

Something is not computing, and maybe its my speedo gear.
At indicated 80 mph I am turning +/- 4000/4100
Rear tires are Yoko 295/50/15, I think about 26.5 Dia.

I have been told that with 3.31's I should be turning about 31-3200 rpm @ 75-80 mph

I don't want to give up the top loader, just slow the engine down a bit at cruise speed

[MetaJacko]

Hi,

I'm currently running a 428, Toploader 3.31 on 295/50R15 Cooper Cobras and get about 3100rpm at 75mph

On my calcs you should be doing about 3350 - 3400rpm for your dif at 75

I'd guess ether your speedo or rev counter is a bit out or your 295/50R15 tyres are quite a lot smaller than mine (unlikly as they would have to be LOTS smaller!!)

A quick way I found of checking your speedo to within 5% is use a daily driver and drive a known route recording the distance then use your cob on the same route. It will not be brilliantly accurate but at least its in the same ball park as the real distance.

Hope that helps

Paul

[DonC]

Terry:
Here's a web site with an easy to use plug-in form that you can use for comparisons.
http://www.368s.com/tools_rpm.phtml
DonC

[Anthony]

With 3.31's you won't notice much difference. Change to 3.07's, first gear will be more usable, and your cruising rpms will obviously be better.

I have the w/r toploader with the 3.07 rear. I think it's perfect.