69 427/390 HP THD400 Speed in Gears

[Don W.]

I have the vette in the subject line, just purchased. The Protecto plate shows a 3.08 rear. The THD 400 seems to shift from 1st to 2nd at about 10 mph. Traveling down the freeway at 60 to 65 mph shows about 3500 rpm. If I'm going about 45 mph and manually shift from 3rd to 2nd, I see about a 700 rpm change. This does not seem correct to me; the 69 427/435 4sp I had about 33 years ago would tool down the freeway in top gear at 65 to 70 mph in the 2600 rpm range. I think that had 3.08s too, but maybe they were 2.73s.

I'm going to have a full service done on rear tomorrow and will ask the mechanic to count the teeth and confirm a 3.08 rear.

If the previous owner changed the rear, how many components do I need to change and how difficult is the task?

I'm also going to get the THD 400 checked out.

I've posted a few other questions on the board and I greatly appreciate everyone's input.

It's a pleasure to be back as an NCRS member, this time with a vehicle that I can restore.

Thanks,

Don

[Verle R.]

Re: 69 427/390 HP THD400 Speed in Gears

Don,

What you describe sounds more like a 4.11 rear gear.

If you have standard tires and a 3.08 you should be running about 2500 or so RPM at 65.

How many parts do you have to change? It depends on what was changed before. If they installed a four series carrier you will have to change the carrier, ring gear and pinion at a minimum. Check all bearings, clutches, stub axles, etc. Change all seals.

Verle

[William C.]

Re: 69 427/390 HP THD400 Speed in Gears

#.08 should be in the area of 26 miles per 1000 rpm. Shift point on the 1-2 shift is dependent on engine load, so if it consistently shifts 1-2 at 10mph there is a problem, if increasing the throttle opening raises the shift point it could well be normal. Are your speedo and tach reading accurately?