427/390 Auto with a 3.08 Final

[Duke W.]

Re: 427/390 Auto with a 3.08 Final

What is the as-built compression ratio? This is not the cranking compression pressure.

Was an OE replacement camshaft used or some aftermarket wonder camshaft?

There is absolutely no need for a higher stall speed converter with the OE cam. It will just murder fuel economy.

I'm beginning to think that you are another victim of an "engine builder".

Duke

[Chuck G.]

Re: 427/390 Auto with a 3.08 Final

Chuck's car does not run slower, his reaction time is slower...

Watch it, you young whippersnapper.

Chuck

[Chuck G.]

Re: 427/390 Auto with a 3.08 Final

Chuck,

Thanks for the information...However, I'm more enthralled that your wife pulled a 13.7!...Now isn't that the woman we ALL want as our mates in this madness we call "Corvette!" You be a lucky man, my friend...I have trouble getting my wife to go on a simple "Run" (although she always winds up having fun and meeting new people)

I'm off today to my mechanic and will really grill him on this...Funny, I've been asking a lot fo members if an auto trans can "Handle" a lower ration than 3.36. I'm told "No"...Your thoughts on that subject

Thanks so much,

Bob
(9944)

She regularly beat me, Bob. She would thrash me not only in the 69 Corvette, but also in the 88, 92, and 00 we used to run in Maryland.

She says she's a better driver. I say that I outweigh her by 100#.

Chuck

[William C.]

Re: 427/390 Auto with a 3.08 Final

And, looking at the distributor is not usually a part of a "complete rebuild"

[Gary R.]

Re: 427/390 Auto with a 3.08 Final

Bob,
Going to a 336 from 308 will not gain you much of anything except a rebuild cost for the diff. If it needs a rebuild then that's a different story but the BB cars had more then enough torque to run a 308 gear with either the TH400 or Muncie M-20 WR. I have rebuilt a lot of diffs for BB cars and the majority of them were 308 or 336. The change in rpm between the 308 & 336 is going to be minimal. I did the same swap with my SB Th400 72 and it really isn't any different.

As for the TH400 tran not working with a low gear, that is crazy. Those 400's were one of the best trans built and have been drag raced for years. If you decide on running a ratio over 336 numerially I would look at a higher stall as well. The old rule of keeping the stall just under the cruise rpm should be watched.Do you want to run down the highway at 70 turning about 3400 RPM? That's about where a 370 or 373 will be. 355's about 3100 rpm.

If it were me and the differential isn't leaking, chattering, has worn side yokes,pinion seal leaks, I would just drive and enjoy it.

I build a lot of custom diff for vettes that are drag raced or street/strip cars with 500-1000hp and at that level you need to address the 1/2 shafts, differential,and trailing arms as stock units will fail. You're not in that grouping your 2.5" 1/2 shafts will hold up fine. What a lot of guys don't realize is the 2.5 GM shaft has a .134 wall tube. There are new shafts being sold today with only 083 & 095 walls. For street use they are ok but I wouldn't want to load them in some of the cars I build IRS for because they will twist for sure.

[Bob S.]

Re: 427/390 Auto with a 3.08 Final

Bob,
Going to a 336 from 308 will not gain you much of anything except a rebuild cost for the diff. If it needs a rebuild then that's a different story but the BB cars had more then enough torque to run a 308 gear with either the TH400 or Muncie M-20 WR. I have rebuilt a lot of diffs for BB cars and the majority of them were 308 or 336. The change in rpm between the 308 & 336 is going to be minimal. I did the same swap with my SB Th400 72 and it really isn't any different.

As for the TH400 tran not working with a low gear, that is crazy. Those 400's were one of the best trans built and have been drag raced for years. If you decide on running a ratio over 336 numerially I would look at a higher stall as well. The old rule of keeping the stall just under the cruise rpm should be watched.Do you want to run down the highway at 70 turning about 3400 RPM? That's about where a 370 or 373 will be. 355's about 3100 rpm.

If it were me and the differential isn't leaking, chattering, has worn side yokes,pinion seal leaks, I would just drive and enjoy it.

I build a lot of custom diff for vettes that are drag raced or street/strip cars with 500-1000hp and at that level you need to address the 1/2 shafts, differential,and trailing arms as stock units will fail. You're not in that grouping your 2.5" 1/2 shafts will hold up fine. What a lot of guys don't realize is the 2.5 GM shaft has a .134 wall tube. There are new shafts being sold today with only 083 & 095 walls. For street use they are ok but I wouldn't want to load them in some of the cars I build IRS for because they will twist for sure.

Gary,

Thanks for the education.....I don't know why my guy is telling me that anything lower than a 3.36 will be tough on the trans....Right nows I'm on the freeway at around 2700 rpm (70 MPH)....It is a sweet ride with all that power, and lots of room on the tach, but I like a nice low end acceleration every now and again....

We did talk about a Stall Converter...so, I'll know on Monday what he finds.

Thanks again!

Bob
(9944)

[Bob S.]

Re: 427/390 Auto with a 3.08 Final

Watch it, you young whippersnapper.

Chuck

Ha!!! I'm collecting Social Security! Who's the "Whippersnapper?"

[Bob S.]

Re: 427/390 Auto with a 3.08 Final

Bob,
Going to a 336 from 308 will not gain you much of anything except a rebuild cost for the diff. If it needs a rebuild then that's a different story but the BB cars had more then enough torque to run a 308 gear with either the TH400 or Muncie M-20 WR. I have rebuilt a lot of diffs for BB cars and the majority of them were 308 or 336. The change in rpm between the 308 & 336 is going to be minimal. I did the same swap with my SB Th400 72 and it really isn't any different.

As for the TH400 tran not working with a low gear, that is crazy. Those 400's were one of the best trans built and have been drag raced for years. If you decide on running a ratio over 336 numerially I would look at a higher stall as well. The old rule of keeping the stall just under the cruise rpm should be watched.Do you want to run down the highway at 70 turning about 3400 RPM? That's about where a 370 or 373 will be. 355's about 3100 rpm.

If it were me and the differential isn't leaking, chattering, has worn side yokes,pinion seal leaks, I would just drive and enjoy it.

I build a lot of custom diff for vettes that are drag raced or street/strip cars with 500-1000hp and at that level you need to address the 1/2 shafts, differential,and trailing arms as stock units will fail. You're not in that grouping your 2.5" 1/2 shafts will hold up fine. What a lot of guys don't realize is the 2.5 GM shaft has a .134 wall tube. There are new shafts being sold today with only 083 & 095 walls. For street use they are ok but I wouldn't want to load them in some of the cars I build IRS for because they will twist for sure.

Hi Again Gary,

I'm hearing a lot about leaving the 3.08's in and moving to a Stall/Troque Converter for low end acceleration. Also, a lot about "Mapping" Can you shed any light on these two suggestions?

Thanks,

Bob
(9944)
P.S. I won't be racing...the car is a fine cruiser, but, as mentioned in earlier posts...I just love to punch it from a rolling start every now and again, and with all that power, it doesn't seem to have the "Kick" I expected....

[Bob S.]

Re: 427/390 Auto with a 3.08 Final

What is the as-built compression ratio? This is not the cranking compression pressure.

Was an OE replacement camshaft used or some aftermarket wonder camshaft?

There is absolutely no need for a higher stall speed converter with the OE cam. It will just murder fuel economy.

I'm beginning to think that you are another victim of an "engine builder".

Duke

Engine is completely stock. The Compression is 10.25:1 The cam in the engine is stock.

[Duke W.]

Re: 427/390 Auto with a 3.08 Final

"My mechanic is someone I trust implicitly, but can be very opinionated."

Okay, you say the above and say your engine is not different from the OE configuration. If that is the case, there is something wrong, and it's not the gears and not the OE torque converter. It could be something simple like a malfunctioning centrifugal advance or the carburetor butterflies are not fully opening. Basic checks like these should be done before you go any further.

We've given you some suggestions that you should look into. I'd also advise you to get a second opinion including a chassis dyno test before you go back to your existing mechanic.

Duke

[Bob S.]

Re: 427/390 Auto with a 3.08 Final

"My mechanic is someone I trust implicitly, but can be very opinionated."

Okay, you say the above and say your engine is not different from the OE configuration. If that is the case, there is something wrong, and it's not the gears and not the OE torque converter. It could be something simple like a malfunctioning centrifugal advance or the carburetor butterflies are not fully opening. Basic checks like these should be done before you go any further.

We've given you some suggestions that you should look into. I'd also advise you to get a second opinion including a chassis dyno test before you go back to your existing mechanic.

Duke

Duke,

I truly appreciate your information...it's been more than helpful, as is all info I've received. I'm concerned you think there is a "Problem" I'm just looking for more acceleration out of the hole. I'm thinking what I'm getting is what the 427/390/3.08 is designed to give. That said, I just thought a lower final drive would give me what I was looking for, but I keep running into opposition...especially from my mechanic.

Thanks,

Bob

[Patrick H.]

Re: 427/390 Auto with a 3.08 Final

Duke,

I'm thinking what I'm getting is what the 427/390/3.08 is designed to give.

Thanks,

Bob

I don't think so, and neither do most of the others commenting on this thread.
I've seen stock base motor 350s give plenty of "punch" on the low end (and so did Duke in his example above), so if your car doesn't do that then look to the engine as has been mentioned already.
Carb and distributor are the first two places I'd look - but I'm just reiterating what has been said.

Patrick

[Bob S.]

Re: 427/390 Auto with a 3.08 Final

I don't think so, and neither do most of the others commenting on this thread.
I've seen stock base motor 350s give plenty of "punch" on the low end (and so did Duke in his example above), so if your car doesn't do that then look to the engine as has been mentioned already.
Carb and distributor are the first two places I'd look - but I'm just reiterating what has been said.

Patrick

Thanks, Patrick....

I appreciate your input. My problem is, I'm getting advice along your line of thinking, and Duke's. Then there are those on this forum that are all for lowering the final drive. I hope you can appreciate my confusion.

Bob
(9944)

[Patrick H.]

Re: 427/390 Auto with a 3.08 Final

Thanks, Patrick....

I appreciate your input. My problem is, I'm getting advice along your line of thinking, and Duke's. Then there are those on this forum that are all for lowering the final drive. I hope you can appreciate my confusion.

Bob
(9944)

Rear end work = $$$$
Engine tuning including carb and distributor = $ to $$

Any questions?

Oh, and if the carb and distributor aren't working well now, they won't work any better at the higher RPM's you'll turn with a 3.70 gear.
Make sense?

Patrick

[James G.]

Re: 427/390 Auto with a 3.08 Final

I'm thinking of dropping my final drive from 3.08 to a 3.36 for a bit more low end acceleration. I'm told that's about as low as I can go since my car is an automatic, and it wouldn't be able to handle anything lower. Any info on this?

Also, would the switch from the 3.08's to 3.36 show an appreciable difference on acceleration?

No question about it, my 68 is a fine freeway cruiser with loads of passing power, but I've always loved that occasional "Eyeballs to the back of the head, thing" from a roll....(Just every know and then, mind you

Thanks,

Bob
(9944)

My 67 air 427/390 POWERGLIDE car had a 3:55 gear in it when we did a body off frame restoration in 1992. The tank sticker showed 3:08. I thought the 3:55 would be fun, so I left it. The car ran very strong and would burn the tires off. On the freeway however, it ran 3000RPM at 68 mph and I always wished ''would you please shift to high gear?''

Gas mileage was also 9-10 mpg. You could order 3:36, but nothing hiher numerically. Dukes answer covers it well. If you want to go racing, buy a race car.