What gear to use with the 402/th400??

[gregbr]

Hi All! On my restored 70 GMC (see pictures in signature) it came from the factory with the 402/th400 & 3.07 gears. Engine rebuilt, 30 over, Crane 270-2H (WG1104) cam, timing set at 10BTDC, factory intake & exhaust manifolds & Rochester, th400 stock rebuild with 350 torque converter, Eaton series 3 posi. While it drives great on the open road 55-60 (I always baby it), it seems sorta sluggish on take off. I'm thinking about a regear to a 3.42 to keep the same Eaton Posi.

Only driven to local shows and occasional trips around town. Would never drive it long distance or above 65, stays garaged, can't leave it out. What would you do & why?? Could I still be off on the timing and make a difference??



Just checked those pictures and I never loaded the ones showing it completed. Will fix that this weekend.

[JimKshortstep4x4]

My son's 72 has the 3:07 gear with a 402 in it and it has good power on acceleration. The cam is a mild one with the power curve in the 1,000-4,000 rpm range. What is the power band for your cam?

I would try bumping the timing up to 12-14 degrees tdc. Also, has the distributor been curved for the engine?

While a 3:42 gear will help the acceleration you may not need to go that route. If you do take the truck on a long trip the 3:42 gearing might turn into an irritation!

Jim

[special-K]

I`d try Jim`s suggestion,first.But,if still not happy,I think 3.42s will fill the bill.I`ve always been a grunt gears kinda guy,until more recently.I think 3.42s are a great compromise between 3.07s and 3.73s.My shortbed project has them because I hope to some day run a Gear Vendors o/d unit and it`s as tall a gear I`d run with o/d.In the mean time,or if the o/d never happens,the gears will be as hiway friendly as I need.Cruising 65 max. is fine with me and I`m not afraid of running the 3,000 rpm range.I prefer 2-lane hiways anyway.There was a time before o/d with a national speed limit of 70 mph.

[prostreetC-10]

One thing for sure is that a torque bulding BBC doesn't need the gears them little small blocks benefit from. My truck to the left has 3.73's and my old '68 BBC custom camper had a 3.54 setup. I wish I had the 3.54's. Still good for a trip on the highway but also deep enough for a nice hole shot on one of them there rice burners. I think you would be really please with a 3.42 set and I'd throw those 3.07's out in the field.

[Shyguy]

My son is running 3.42 gears behind a mild built 454 with a 3 speed and really likes the setup.

He won the burn-out contest in the local car show earlier this month.

The truck the setup is in is a '67 body on a probably 69 frame and suspension.

I wouldn't toss the 3.07s, may come in handy some day. Of course we seem to gather a lot of stuff that will come in handy some day.

Danny

[LUV2XCLR8]

My Blazer Build has a 342 and Factory Posi, but I'm running a 700R4.

[71 super]

Greg i replaced the 3.07 rear in my bb with 3.42 and it is still a little sluggish imo but everything i have had previously has been 3.73 or numerically higher- all the way to 4.56 so i have a little bias, but in retrospect I wish would have gone to 3.73.

[69sixpackbee]

I have 3.54's in my Dana 60 rear. Good compromise. Albeit, it is a 4-speed though.

[69GMCLonghorn]

I loathe people with HIGHWAY GEARS!!

[prostreetC-10]

Originally Posted by 69GMCLonghorn

I loathe people with HIGHWAY GEARS!!

What? It's not like they are at the plant installing them. We tend to get what comes in our trucks. I had an 80 Sliverado that I'm sure had 8.53's in it. I had about a top speed of 43!!

[gregbr]

Thanks for the input! I rebuilt the original dizzy, did not change anything on the springs & weights. I have not had it curved to the engine. My 67 4WD has a th350 & 3.73 and runs pretty good on 33" tires, That's why I thought the 3.42 would be better on the 402 2WD. I will adjust the timing this weekend and see what happens.

[Daves72'GMC]

I have a 402BB 30 over with turbo 400 and 3.54 dana 60 and an edelbrock cam a little bigger than stock and good on the highway and quick from the start and good milage.

[tim21391]

i got the 3.07 i think and its got plenty balls

[prostreetC-10]

Hey Tim....got any pics of your Mustang? I have a Fastback in my signature link below. Mine has a little 289 lawn motor engine.

[gregbr]

Tim, What cam & timing are you running? Engine stock or modiffied? Maybe I can adjust mine Sunday evening.

[tim21391]

i have an rv cam in mine.. not sure.. but i actually dont know what timing im at because im having trouble settin it i converted over to hei these past few days.. but when i had stock points in it had plenty of balls.. but it has a holley double pumper carb and an edelbrock performer intake

[69GMCLonghorn]

RE: the comment about highway gears, just having a little fun.

[special-K]

Originally Posted by 69GMCLonghorn

RE: the comment about highway gears, just having a little fun.

And,it sure was obvious to me coming from a Longhorn ma....I mean feller who must have the grunt gears.I love grunt gears,screaming on the hiway,or just goin`slower than the rest.

[prostreetC-10]

I missed one part of your posting. You say it seems sorta sluggish. You need more initial timing than 10 degrees. Most big blocks like the 16 to 18 degree mark. It will raise your idle so you will need to drop that down but you will probably pick up more low end drivablility......especially in these heavier rigs with those gears.

[gregbr]

Set timimg today at 14. Unable to road test due to yard work blocking the shed. Maybe Thurs....

[prostreetC-10]

Don't be afraid to throw a little more in it. If you run into detonation (pinging) then you have to back it off but I bet you could get away with 18.

[BossHogg69]

my '69 has 3.73's and when I was rolling the 454/TH400 combo it ran good at 60-65 on the highway. 70 got a little annoying. 3.42's would probably be a good compromise, but I like the way 3.73's come outta the hole!

[prostreetC-10]

I just finally looked at your pics and didn't realize that we were talking a 4wd. What size tires are on it? Tire diameter does come into play. I'm running 29's with a 3.73 setup. If you have 31/10.5 or taller, you can run deeper gear and get away with it. All the more reason to S-can those 3.07's

[gregbr]

Not sure what you saw, but the restored Big Block truck, the Gold colored one, is 2wd with 275-60-15 or 29" tires. The White truck is a 67 4wd with a 350/th350, 3.73 posi and 33" tires. The 4wd seems to have better take off then the BB.

['72customdeluxe]

you might try a 2000-2400 rpm stall converter first. It would help a good bit, yourcamshould need it, and it will help you still if you decide to get the 3.42's eventually