Hi,

I have access to a 400 small block, complete out of a running car. The motor will probably need a rebuild though. Anyway, I don't know much about the 400... I was thinking of a 383 build, but would a 400 be worth considering? What kind of combos (rod length, etc) do people do with the 400? Right now my car has an almost dead 350 in it and a new 700-r4, 2.77:1 posi. What kind of power compared to a similar 350 does a 400 produce? Is the gas milage similar? Sorry for all the questions! I'm just trying to get all the ducks in their rows. Any info is appreciated, thanks.

 

PLEASE don't use it's guts for a 383
Definately about the best smallblock to build without stroker kits or other high$$$ alternatives.
Most 400 blocks are 2 bolt,but thats enough to support a lot of power,studding the mains is a good alternative if you have any doubts about a 2 bolt bottem end.
Most 400 blocks get pretty thin after any more than an .040 overbore,and if aftermarket heads are used,or any non 400 head for that matter,they should get steam holes drilled in the decks.
The most common and affordable setup is a 5.7 rod with corresponding pistons.These pistons are available in cast,hypers,or forged.There are also lots of pistons available for 5.85 and 6 inch rods,but IMHO,5.7" is about ideal for the avg. street performance engine.
Bottom line,these are getting super hard to come by,and are very much worth building,as there is a lot to be gained over the avg. 350.(over a 1/4" of stroke and a good deal larger bore)
As far as fuel economy,that's debatable.About 12 years ago I had a very mild 400 in my car(stock heads,about 9.3:1 compression,very mild dual pattern hydraulic cam,performer intake,600cfm Edelbrock carb)It ran tons better than the stock TPI 350,and still got around 22MPG on long highway trips at 65-75mph.Would an identical 350 do better?I can't honestly answer that.

I'm not against 383s by any means,but if the 400 is in usable condition,definately a better option.

 

What about using 083 heads on it till I can afford some better flowing ones? I know 400s like to breathe, but would it still be decent up to around 5000 rpm?

 

your 083 heads will work fine.
They will work much better if you fully home port them and install new 2.02x 1.60 valves.
If you do a thourough job you likely won't need aftermarket heads to get the performance you want.
400 blocks and heads have 4 extra steam holes in the deck. you can drill your 083 heads for steam holes using a 400 head gasket.

If you use stock GM 350 5.7" rods in a 400 the rod bolt head and edge of the rod needs to be clearanced ground to clear the camshaft lobes on a few cylinders.
Stock 400's us a shorter 5.565" rod. The benefit of going to a 5.7" rod is well woth the time required to grind the rod bolts for clearance. this is much, much less work than grinding a 350 block for rod clearance on a 383 stroker.
A 383 would be a step back. the 400 is the way to go.

To get a pump gas friendly compression ratio on a 400 using 64cc heads ( your 083's) calls for a dished piston.
Keith Black makes 2 nice sets of 5.7" rod hypereutectic D dish pistons that fit the bill one has 18cc dish and the other has 21cc dish volume. The 400 will require crank balancing. the stock 400 cast crank is plenty strong.
federal mogul also makes dished pistons for this motor and 5.7" rods. Again look for a 18-21cc dish volume.
A 400 will only have slightly less gas mileage than a simular built 350. Will make 20+ hp more and a ton more torque. The big gain in torque is well worth any real or imagined fuel mileage penalty.
My 400 is very reasonable on fuel consumption. Especially when you consider it makes +/-450hp and 500ft/lbs torque.

if you build the 400 with a "0 deck height" and KB168+.030 21cc dished pistons. (deck height machining) using a .039" 400 gasket and 64cc heads you'll get 9.8:1 compression and optimal .040" quench clearance.
Keith Black piston require wider than stock (typical) top ring gaps. Follow their recomendations. Other than that these pistons work just fine.