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Chevy 400sb.... opinions?

4K views 15 replies 11 participants last post by  TEX 
#1 ·
I just bought a '1976 GMC that has a 400 small block in it. I've never come across this before, i've seen 305, 350, 454's and the occasional 383 stroker comes up for sale..... but i've never even heard of a 400 before and now I own one. It has unknown mileage and burns oil, but the power.... a lot more power than your typical oil burning 350. The thing will drive straight up a tree, starting off in 3rd gear, it's crazy.

So what kind of a reputation does the 400 have? Why are there so few around? All engines have their weak spot, what is the weak spot in this one? If it dies (i'm hoping it won't anytime soon but you never know) is it worth rebuilding or should I be happy to be rid of it?

I have heard from a couple of people that they have a tendency to overheat. If this is true are there any changes I can make or modifications that can help prevent this?

Thanks in advance,
Newbie collecter of big rusty oil - burning things,
250girl
 
#2 ·
The weak points in my opinion are over heating and the rods are really short in them which causes allot of side load on the pistons . My girlfriend's dad has had a few in his nova... he revs them too about 7k and has problems with the block flexing. His engine guy said a 383 is a better choice

400ci is just too many cubes in the Chevy small block sized block IMO without extensive modification.
 
#4 ·
Like mentioned above they have siamese cylinder walls which means there is no coolant running between the cylinders whicn can lead to them siezing up from the cylinder walls getting too hot.

Honestly I had one in a 77 chevy k10 with a decent sized cam, full roller rockers, headers, edelbrock intake, and a edelbrock carb and it ran awesome. Definetely had tons more tq down low than a 350 but it didn't like to spin up much more than like 5k, you'd hit like 4800rpms then it would slow down how fast the rpms went up drastically. I had 37" tsl's with a welded up rearend with 3.73's and it would break them loose from a dead stop with a factory stall converter.
 
#5 · (Edited)
Most REAL high performance smallblocks are based on the 400 rather than the 350. Any issues with it are more fear than fact & any competant engine builder can make a 400 last just as long as any other small block.

383 FWIW, is probably the worst bang for the buck. You arrive at that displacement by overboring a 350 and installing a 400 crank and rods. Studies have shown that if you were to take the $ used to convert a 350 to a 383, and use it on better parts for your 350 instead, you'll make more power.

edit: I have a .40 over 400 (408") with stock crank & rods that I've chipped at 6,400 to keep the revs down (hit 7 grand once when my wife missed a shift).
 
#6 ·
Thanks for all the advice so far,

I figured, being a bigger motor that revving the crap out if it was a bad idea. Plus, I have no reason to rev it up that high with the amount of power I have down low.... oh... and i'm cheap... so watching the gas gauge drop that fast causes pain. Serious pain.

Right now it's actually giving me some trouble. When I bought it I test drove it a bit and the thing started and ran GREAT. Drove it up a mountain, it didn't miss a beat. Now that I have it home it'll start fine, run fine for about 2 -3 minutes and then start running real rough, idle right down, and then stall. Doesn't wanna run much after that. I think it's flooding... which would be a carb issue. Carb is a Holley 650.
 
#8 · (Edited)
Right now it's actually giving me some trouble. When I bought it I test drove it a bit and the thing started and ran GREAT. Drove it up a mountain, it didn't miss a beat. Now that I have it home it'll start fine, run fine for about 2 -3 minutes and then start running real rough, idle right down, and then stall. Doesn't wanna run much after that. I think it's flooding... which would be a carb issue. Carb is a Holley 650.
Yep, sounds like carb trouble. Is that Holley 650 a Vacuum Secondary or a double pumper? Did it pop back through the carb before it started running rough? It may just be time for a rebuild. Just in case you didn't know, there should be a filter where the fuel line goes in to the carb. The fuel line should go into a larger brass fitting, that fitting should have a washable brass screen filter in it.
 
#10 ·
If you were to build it at some point, buy either 5.7" rods or 6.0" rods, you will have to clearance the block and buy the corosponding pistons, and id recomend getting the bottom end balanced,(not buying a kit that says its balanced they say they are balanced but the few we have checked have always been off about 3 grams) if your worried about the cylinders getting hot drill and tap the back of the intake or buy one that already is to get the coolant from the back of the heads through the intake to the front ports ive seen it lower temps on 400sb 10-15 degrees. also here is some more info if you havnt already found it. here
 
#15 ·
Yes the 4 core is on my "to do list". It's only got a 2 core in it. And, oh joy of joy, the truck's temp gauge doesn't work. It's stuck at a pretty high temp and doesn't move. Although I feel i'm pretty adept at knowing just how hot my truck is by how it's running and how it smells. When my 305 (also a fried temp gauge) starts smelling bad and billowing smoke up from under the hood.... I stop and let her cool down for a while. Haha, fingers crossed for no smoking from under the hood on my 400!

Previous owner said the truck has never overheated for him.... but previous owner also had the front brake calipers on the wrong sides, and drove it with a shot universal clattering away until it took out the CV joint too. Sigh.....
 
#12 ·
ive been running a 76 400 for about 4-5 years so far. great engine, coming from mostly running 305s and 350s very torquey engine. i had that engine in a suburban and when i pulled a car you barely knew it was there. ive been told the whole overheating thing too and have never overheated this engine, and its been in nothing but undergeared heavy offroad trucks
 
#13 ·
I have run 350s, 383s and 400s. The latest flavor is a 406 in my wife's S10. It went 10.51 @127 first time out. We have some tuning to do and it has lower 10s in it. I have not had any overheating problems with them. We did blow a head gasket in one before but that was it. Have a good cooling system and don't rev the shit out of one with stock rods and you should be happy with it.

Here are some shake down vids after we got it running.
Launching my wife's S-10 - YouTube
Mele truck first t brake - YouTube
 
#14 ·
a friend of mine had one in his YJ and he hated it. it ran great plenty of power but couldn't keep it cool. he ended up have to buy a 4 core aluminum radiator, a 1300 CFM fan, and put loovers in the hood to cool it down. and he didn't have inner fenders either. not to mention the in cab heat from all the under the hood heat.
 
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