Building a 400 for 400 Horses



What do I need to do with my 400ci small-block Chevy to get an honest 400 horses out of it?

Garry Smith
Kemp, TX


Luckily, this isn’t as hard as it used to be. The improvements gained by extensive research into cylinder heads and camshafts have brought greater power to the streets, especially where the small-block Chevy is concerned.

Start with a good set of cylinder heads (around 195–200cc intake ports would be good for you), add a matching camshaft (like the proven Comp Cams’ Xtreme Energy 262 or 268), team it with a good intake manifold (like Edelbrock’s Performer RPM Air Gap), and top it with a 750 cfm carburetor. Make sure your compression ratio is in the 9.4:1–9.6:1 range, you’ll be really close to making 400 horses! Naturally, a good set of exhaust headers is essential (we recommend either 1.5-inch or 1.75-inch diameter primary tubes) and you’ll want a good electronic ignition. A fresh GM HEI will get the job done, but if firewall clearance is an issue, small-cap options exist from MSD and others.

The key is matching the overall package to work from 2,500–6,000 rpm, and a good set of aftermarket heads (or even ported GM Vortec factory heads) will get you there. The budget-friendly Vortec heads will require a Vortec-specific intake manifold, so know this before buying parts that won’t bolt up together.

Also, because it’s a 400, don’t forget to drill the steam holes (photo) into the cylinder heads. These steam holes are located between the bores in the block and are unique to the 400. You can use the new head gasket as a template to mark them, and drill through the deck of the cylinder head into the water jacket. The huge bores on the 400 made it necessary for these holes to be added, and if you don’t drill them into the heads, you may experience cooling issues. Once you’ve drilled them, you shouldn’t have any problems.

Mark Simpson
Managing Editor

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13 Responses to “Building a 400 for 400 Horses”

  1. Steve Pleasant

    Have a 1976 Eldog and a 1941 Plymouth coupe good condition needs excessive Restoration Please help

    • Customer Service

      Hello Steve,

      Great question! The ‘Ask an Expert’ section is currently for members of our online community. By becoming a member, you will have access to our expert’s knowledge. With your membership you will also receive discounts on products and hours of Premium video content.

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  2. Eddie taylor

    I did almost this,,195s 262 comp,,I used summit blow through,,edelbrock carb in my 67 firebird,74,sbc 400,,don’t know exact numbers but it’s good,,let’s me think I might have known a little bit went I put together,,jmo

  3. Jay Wilson

    A previous owner installed a 383 in my 1984 c 10 pick up. They removed the stock 305 and left the stock automatic transmission and I assume the stock flywheel. Will this cause problems? Thanks…

    • Customer Service

      Hi Dave. The steam holes are drilled in the 400ci block and the original heads that were bolted on 400ci blocks, but if upgrading the heads to larger ports or aftermarket heads the steam holes are not there as they are unique to 400ci heads, but by drilling the steam holes any set of Small Block Chevy heads can be fit to the motor.
      Mark-Classic Car Restoration Club

  4. Jay Kimmal

    ‬What options do I have for a 1963 283ci SBC? is it worth spending the dime and time to put new heads, intake and carb, or will it still not perform? will vortec heads work on this bore size and bolt pattern?

    • Customer Service

      Hi Jay,

      Here is the answer from the expert:

      Clearly you’re not afraid of taking your car away from stock… A better solution may be to pull the stock motor and leave it stock.
      Then pickup a nice 350 crate motor, throw on some power adders and go have fun…. then you can always put it back to stock if your needs change.

      Wrench Safe,

      Classic Car Restoration Club

  5. Manny

    I have a 350 v8 from mid 70s. Sbc. Is this about the same process for my engine. I have HEI MSD already, 750 edelbrock but need headers? And better intake?

    • Customer Service

      Hello Manny,

      The Chevrolet 400 cubic inch small block is unique among Chevy’s Small block engines, in that the bore is so large that the cylinders are siamesed together and the coolant can not travel between them.
      That is why the 400’s require steam holes drilled into the heads for cooling. You’re 350 block does not have these issues so there is no need for steam holes.
      Having had both in my hobby cars I believe the 350 is a better choice as they run cooler than the 400s do, or you could compromise and put a 400 crankshaft in your 350 and build a 383 cubic inch stroker motor.
      Just be aware that the stock Chevy 400 crankshaft is externally balanced and you’ll need a 400ci. Chevy flywheel/flexplate and harmonic balancer.
      Your 350 is a great motor, have fun with the build!

      Wrench Safe,

      Classic Car Restoration Club Video Membership

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    • Customer Service

      Hi, Richard. Measuring the size of an intake port on a head is done the same way as measuring the size of a combustion chamber. It’s a process that involves using a glass plate (or plexiglass) to seal off the intake port then filling the void with a measured amount of fluid. Here is a link to a kit that can help get the job done: Note: Most Intake runners are larger than the 100cc Buret offered in this kit, however the Buret could be filled more than once or source a larger one to fit your needs.