What do you think of this setup? sb400/700r4/.308

[Hazieview]

I now have a sb400/350th/.308 I am thinking abou replacing the transmisson with a 700r4 because my th350 is giving me problems. I put it in gear and the truck doesn't move. Sometimes it takes it a wall to actually go in gear so I think it has probably been hammered too much. I figured that with the sb400 I'll have plenty of torque like always but I have a quiter and more effiecient ride with the 700r4 because I'll be running lower rpms when cruising down the highway. Right now I can smoke that one rear tire no problem but it's getting to be less fun these days. I'd rather just make an everyday driver that still sounds good and will get down when I need/want it too. Figured it would be less strain on my engine as well. However I just hope I dont destroy the trani or rearend with the kind of torque that sb400 can make. I'm planning on beefing it up a little but right now it just has a performer intake and a small rv cam. Sooooo.....

What do you guys think?

[Blazer Eagle]

well the 700r4 actually has a lower first gear so you could smoke your tires a little better, however it does have the 4th over drive gear which does run you in lower rpms

[Hazieview]

Sounds like what I'm looking for... Smoke 'em when you feel the need and still get down the highway a little more comfortable and affordable than the 350th..... ?

[krue]

My truck with the stock 350 a 2004-r and 3.07 gears seems like it is a dog on the hiway in o/d. I'm looking for a 3.73 rear gear to get the hwy rpm back up to about 2k which is where my 350 likes to hang out.

[Hazieview]

So would the sb400/700r4/.308 be a dog too? I figured as long as you had a kick down you shouldn't have any problems....?

[Pyrotechnic]

Depends on tire size. With a 28" or larger tire and an older flat tappet cam motor, forget it. You need at least 3.42's. It's going to want to shift out of OD constantly. A 400 MIGHT get away with it, but it's still a less than ideal setup. You might even get worse gas mileage because the engine isn't in it's sweet spot.

With a newer roller cam, fuel injected motor with modern heads and high compression, it can work, but that's a whole different ball game.

[krue]

My 92 with TBI will run a lot lower RPM at hiway speeds without much trouble, but my 70 is a dog. It actually worked better with the th350 in it but I wanted the 4 speed to get a lower first gear more than having the o/d. You're probably better off with a stout th350 and the rear you have.

[66truckin]

as was previously said you need at least a 3.42 gear to run that 700-r4. You would have no problem doing 150mph with that 3.08 though

[fast75]

some things to consider before doing this swap. the cost of rebuilding a 700r4 is almost double that of rebuilding a 350 or 400. the 308 gearing you speak of isn't going to cut it with a 700, it'll fall on its face. i have two buddies who tried 700's and both have since took them out. if you decide on the 700, go with at least 342 gearing, 373 would be better and still good on the hwy. just my two cents............divide the number of teeth on the ring gear by the number of teeth on the pinion to figure your gear ratio...............

[Hazieview]

I use to have a .373 posi rearend. The truck would leave HARD!!! Now it doesn't leave as hard but will still smoke the hell out of that one tire. However I top out at like 110mph.....

So what you all are saying i'll be better off keeping the th350 and rebuilding it?

[b454rat]

I had a 90 GMC that I built years ago. Had a real nice 406 in it. Was originally 350/700/3.08s. It wasn't tearable with the 350, but after I put the 400 in it, thing was a monster. It never down shifted on the highway, converter never unlocked either. I live in NYS, and at the time was in the military and stationed in DC. On I81, there are some hills, and never had a problem.

My current truck has the same combo, up to 3rd it's not bad, but was in OD it is a dog. A 454 will cure that....

I was gonna put my 400 into it, but have the 454, and get a cummins, so dont' have a use for a small motor anymore....

[CAMAROSS]

Im running a 406 small block with a 700R in my truck. I also have the TBI still fueling it, RV cam, factory heads, etc. and it gets on down the road and is smooth in overdrive. Dont know what gears I have in the rear though. I say go for it.

[chevymotocross]

I have an 87 with the 305 fuel injected engine, 700r4, and 3.08 gears. I absolutely hate the 3.08 gears. I find myself not even using OD since it constantly shifts in and out.

Unless my truck is not loaded with anything in the bed, on highway with no inclines, doing atleast 65, and with no one to pass, I can cruise along in OD.

First gear is awesome with the 700r4. As soon as it shifts into 2nd, it falls flat on its face due to the 3.08 gears and probably my weak 305.

[jchav62]

I agree with the others. With an overdrive trans, go with a 3.42 or a 3.73 gear. Even with a 400, in OD with a 3.08, seems that the RPM's would be way too low, and it'll constantly be shifting. In my '62 with a mild 327, I used to have 3.08's and a th350..it did fine on the highway. Now I have 3.73's with a GearVendor overdrive. Off the line it's a lot quicker now compared to the 3.08's, and on the highway with the OD on it does about the same as it did with 3.08's.

My Final answer:

th350 with 3.08's
700r4 with 3.42's or lower.

[Hazieview]

Well I'm waking up this thread again. I've decided to pep up my sb400. I'm going to put an air gap on it, new heads and turn it into a full roller. (I was going to spend the money on forged rods and pistons but decided to roll it instead)

So full roller sb400 350th trani and .308 rearend.

The 400 should be able to handle a little higher rpms so we'll see what happends.

I would love to have a sb400 bullet proof 700r4 and .373 posi. However that would cost thousands for the trani and rearend so we'll see what happens!!!

[slugish]

Originally Posted by Hazieview

Well I'm waking up this thread again. I've decided to pep up my sb400. I'm going to put an air gap on it, new heads and turn it into a full roller. (I was going to spend the money on forged rods and pistons but decided to roll it instead)

So full roller sb400 350th trani and .308 rearend.

The 400 should be able to handle a little higher rpms so we'll see what happends.

I would love to have a sb400 bullet proof 700r4 and .373 posi. However that would cost thousands for the trani and rearend so we'll see what happens!!!

Iagree with you 100 % .Ive had a 3.73 -a 3.08 & a 3.42 in my 408 / 350 .

currently a 3.42 ,but on the highway cruisin still too many rpms > usually Ijust slow down & run only 60-65 mph at approx. 2500 rpms .

yea, just beef up that ole 400 & stick with your 3.08 > best of both worlds . If you do this let me know what you think by a pm . Good Luck

[joe231]

The rear gearing is totally dependant on tire size. What tire size are you running?
gchemist is running 255/60/15's with a 355-700r4 combo and 3.08 gears. he is happy with it.