Fun to drive rear axle change from 3.08 to ??

[Patrick H.]

Re: Fun to drive rear axle change from 3.08 to ??

yes, matching drive train with the rear axle modified from 3.08 to 4.11. The 4.11 was not correct and didn't match the build sheet so it when back to 3.08

If you own it, putting the 4.11 back in is then the most cost-effective option.
Fun up to 60mph, and then the high revving engine will bother you, at least for those used to today's 1500rpm at 70 mph.

[Don W.]

Re: Fun to drive rear axle change from 3.08 to ??

I'm getting it dyno tuned on Thursday and I'll see if it runs and performs any better. It only produces a little over 200 HP at the rear wheels...Don

[Patrick H.]

Re: Fun to drive rear axle change from 3.08 to ??

I'm getting it dyno tuned on Thursday and I'll see if it runs and performs any better. It only produces a little over 200 HP at the rear wheels...Don

390 gross in 1969.

The engine is similar enough to the 1972 454 that you could likely think 270-300 net horsepower for this engine.
Figure 20% loss with automatic transmission, and you have 216-240 hp at the rear wheels. With distributor tuning and jetting you might get right were you're supposed to be.

Patrick

[Peter M.]

Re: Fun to drive rear axle change from 3.08 to ??

Don,
I agree with Richard. The 3.08s should be a very good street gear. Hopefully the tune will give you what you want.
Kind regards.

[Don W.]

Re: Fun to drive rear axle change from 3.08 to ??

thank you all for the replies and info. I'll post the dyno test results on Thursday evening or Friday of this week...Don

[Joe R.]

Re: Fun to drive rear axle change from 3.08 to ??

Apparently some of you internet gurus have NEVER driven a real Corvette on the street.

I totally agree with Patrick and at least go with a 3.70. Most fun for your buck on the street!

3.08's are for AC cars with touring in mind!!!!!

Live a little !!!!!!

JR

[Don W.]

Re: Fun to drive rear axle change from 3.08 to ??

The 427's with THM-400 auto trans were all equipped with 3.08s in the rear and there were no option gear ratios available for that configuration

[Gary R.]

Re: Fun to drive rear axle change from 3.08 to ??

There will be little change in RPM or feel going to a 336, a little more with 355's, for more punch go to 373 over 370. If you to a 373 you will need about a 2400 stall converter.

when I rebuilt my TH400 I looked at a steeper 1st gear but kept the stock one. I built it for st/strip and around town it's normal but nail it and it shifts hard. Changed from a 308 to 336 and really there was not much to speak about, not worth the cost for just that in my opinion. I have a 2000 stall converter and it's ok too.

If you have a 411 that would be the best "bang" for street driving because you have it but again look at the converter. We ran just about every ratio at one time, mostly with 4 speeds but they are 1:1 finals so cruising is the same. The 411 got old fast. My buddy ran a 456 and loved it but it was dead over 60mph in 4th.

As Ed mentioned and knows, going to a used diff or rebuilding one is involved and not all the same. Many used diff's are worn or broken so expecting to find one to bolt in and go may be a crap shoot unless you know the person you buy it from. I was at a swap meet on sunday and a fellow had a diff and box from a 64. After a quick look they were not from a 64, but later C3 years. I have seen used diffs with broken posi cases sold as good used. I have had in "rebuilt" diffs that were outright misrepresentations with crack posi's, incorrect pattern setup, and lash in the 020 range - this came from a well know vette rebuilder. A stock diff if build correct will hold up to your power level and more. With an automatic and street tires there will be less shock loading so depending on how heavy your right foot is will go a long way. I have driven 600hp vettes without breaking anything but I wasn't out slamming gears, my son can convert a stock vette diff into instant junk in very short order.

I agree with the others, look at tuning the BB. I have been in sold that were slugs, slow shifting, gas guzzlers but once tuned were a new animal altogether. Old fashioned carb, intake, cam and exhaust made big improvements.

[Don W.]

Re: Fun to drive rear axle change from 3.08 to ??

Here are dyno results from a 8/21/14 on a mustang dyno at a shop in Auburn, WA:
Base 427, 390 HP, auto trans
Max Power: 201.9 HP @ 4300 RPM
Max Torque: 272.1 Ft-lbs @ 3650 RPM
HP @ 50 MPH: 15.00
Weight:3400.0
Test RPM stopped at 4,300 RPM because the shop wanted no liability for an engine they did not perform the build on.
Don

[Ara G.]

Re: Fun to drive rear axle change from 3.08 to ??

Do you ever drive it on the highway? If not, the 3.70 will give you a lot more punch around town.

If you ever take it on trips I suspect you'll want to stick with the 3.55, as anything lower will get you a lot more rpms at today's 70mph speeds.

Couldnt' have been said better

[Patrick H.]

Re: Fun to drive rear axle change from 3.08 to ??

Test RPM stopped at 4,300 RPM because the shop wanted no liability for an engine they did not perform the build on.
Don

I would suspect that there is a fair amount more power left behind because they stopped the test early.
With a run to 5500-6000 and some tuning at the right shop, you'll hit the expected targets.

If possible, ask them to start collecting data as low as 1500rpm. It doesn't cost any extra, but you can learn a lot.

[Don W.]

Re: Fun to drive rear axle change from 3.08 to ??

Here are my dyno tune results from this mornings run:
Peak power at ~4100 RPM and flat line after that
260HP @ 4200 RPM
328 ft-lb torque @ 3600 RPM

[Duke W.]

Re: Fun to drive rear axle change from 3.08 to ??

Mustang dyno results are 10 to 15 percent than Dynojets, which are the most common, so it's difficult to compare your data with what's out there.

A base engine/TH400/3.08 axle in good working order will light the tires from a dead stop, especially if you hold the brake, floor the throttle and release the brake. A 427/390 should do that by just punching the throttle from a dead stop without holding the brake.

Since you're already beyond the limit of starting traction, what's the point of a shorter axle?

Duke

[Duke W.]

Re: Fun to drive rear axle change from 3.08 to ??

If possible, ask them to start collecting data as low as 1500rpm. It doesn't cost any extra, but you can learn a lot.

Tough to do with an automatic. But do at least one pull in first from idle speed. The engine will immediately jump to the converter stall speed of about 2000, and the data probably won't be meaningful until about 2500.

Duke

[Patrick T.]

Re: Fun to drive rear axle change from 3.08 to ??

Do you ever drive it on the highway? If not, the 3.70 will give you a lot more punch around town.

If you ever take it on trips I suspect you'll want to stick with the 3.55, as anything lower will get you a lot more rpms at today's 70mph speeds.

That's what I did. I had a 4:11 in the '57 and it was no good for me at highway speeds, the rev's were just too high. I recently changed the rear end to a 3:55 and that was just right for driving on the highway and around town. JMO.