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Ask Away! with Jeff Smith: Getting 400+ HP From an Old Small Block Chevy

This is a set of Vortec heads we’ve modified by machining the guides, and adding bigger springs, a set of Summit stainless steel valves, and ARP screw-in studs. Our total investment was a little more than half the cost of a set of aluminum aftermarket heads. (Image/Jeff Smith)

I want to build the cheapest, least expensive small-block 350 Chevy that can make around 400 to 450 horsepower. How would I do that? I have a 350 Chevy truck engine short block with a one-piece rear main seal. I really don’t know what it was originally in but I’m pretty sure it was a truck. I’d like to put this engine in a ’69 C-10 pickup to terrorize the streets. How much power could I make? — B.C.

Jeff Smith: Before we get started, let’s just say that the terms “lots of horsepower” and “cheapest” are at opposing ends of the same discussion.

It’s not easy. Frankly, the easiest way to make 400 to 425 horsepower would be to buy a 5.3L truck engine and put on a carbureted intake manifold, a 750 cfm Holley carb, headers, and stick in a mild hydraulic roller like Summit’s Stage 2 truck cam with 218 degrees of duration.

That’s not what you asked, but that would be the easiest way to achieve 400 hp. Of course, adapting an LS to your truck would be expensive and not part of your plan. So now let’s address your question.

It’s not easy to make 400 hp with a basic 350 Chevy using stock heads.

The advent of modern engines that make 400 hp or more has colored the reality of the past. In our experience, you must have a decent set of cylinder heads in order to make 400 hp on a basic, low-compression 350 c.i.d. small-block Chevy.

Let’s start by assuming (here we go) that your short block is in decent shape. That means you can’t feel an edge at the top of the cylinder wall from bore wear. If you can feel a ridge, then you are wasting your time to attempt to make power. A ridge of any thickness means the cylinder bore is worn and the rings won’t seal and all your money spent on good parts will only pressurize the oil pan when that cylinder pressure leaks past the rings. But we’ll play along and assume the short block is sound.

The biggest item that will improve power is a good set of heads. The more air the heads can move through the combustion space, the more power the engine will make. But aftermarket heads cost money that you don’t want to spend. The best iron production cylinder head that Chevrolet ever produced was the Vortec that came on truck engines from 1996 up through when the LS engines first appeared in 1998.

The Vortec head is a bit of an offshoot from the traditional small-block Chevy head. It demands its own, specific intake manifold and features smaller 1.94/1.50-inch valve sizes and a compact 64cc chamber.

This smaller chamber helps add power by adding compression. You can still find these heads in the junkyard but be careful as engines with a long service life can suffer cracks that make them unserviceable.

Let’s start with the first benefit to using these heads — the smaller chamber. Nearly all traditional heads for a mid-70’s and later 350 c.i.d. small-block came with 76cc chambers. This was done to reduce compression so that Chevrolet could achieve the emissions requirements.

With the smaller 64cc chamber Vortec heads, this means more power. Let’s assume that your 350 comes with typical four-eyebrow pistons that measure about 0.020-inch down from the deck.

If we use a stock composition gasket at 0.041-inch thick with a 76cc chamber, we end up with a compression ratio about 8.5:1. In most production 350 Chevys, this is actually high. Loose production tolerances often limit these engines to barely 8.0:1. By simply adding a 64cc chamber head, the compression jumps to 9.6:1. That’s worth 10-12 hp with no other changes.

But the real beauty of the Vortec is its additional airflow even using 1.94/1.50-inch valves. The best thing to do here is make sure the valve seats are in decent shape and seal properly and leave them alone. Our flow bench tests from back in the day reveal these heads to flow around 230 cfm at 0.500-inch lift at a time when a production iron small-block head generally couldn’t flow 200 cfm without porting. Plus, the Vortec will add 20 to almost 30 cfm in the mid-lift valve positions like 0.300-inch and this is where the additional cylinder filling really crams the air in for a mild street engine. Peak flow numbers are only achieved at one point, but mid-lift valve lift occurs twice — once on the way up and again as the valve is closing.

By adding the Vortec head, we have the makings of a more powerful engine with more compression and airflow.

Next, we add a camshaft. Your engine may be equipped with the fixtures in the block to allow a hydraulic roller cam, but since we don’t know if this is the case, we’ll assume that it is a flat tappet engine. Since your budget is limited, that points us to a flat tappet hydraulic cam package.

Back when we were testing these combinations, our favorite was a conservative cam from COMP called the Xtreme Energy 268H. With advertised specs of 268/280 it offered 224/230 degrees at 0.050-inch tappet lift with valve lift numbers of 0.477/0.480. This added valve lift creates a problem for those Vortec heads unless changes are made to either the valve springs or machining the heads to improve retainer-to-seal clearance. So let’s touch on that.

The Vortec heads use a very tall valve guide boss. With stock springs and retainers, there is only about 0.475-inch of clearance between the bottom of the retainer and the top of the valve guide seal. This limits valve lift to an absolute maximum of 0.450-inch lift. There are two possible remedies to allow additional valve lift.

The first involves machining  the valve guide height by using a COMP cutter (PN 4726) and arbor (PN 4732) and a ½-inch drill motor or drill press. Cutting the height down by roughly 0.050-inch or more will be sufficient. This also cuts the guide diameter down to use a 0.530-inch diameter seal, which means new seals are required. If you are considering this, practice using this tool on a junk head first to make sure you get all the dimensions correctly cut. There have been reports that some of the cutters may make the guide O.D. too small for the 0.530-inch seal.

Of course you could pay a machine shop to perform this work, which isn’t that much more than doing it yourself and may be the best solution if you don’t have experience with machine tools.

When we performed this little machining exercise, the cutter worked perfectly and produce sufficient clearance for a 0.500-inch lift cam.

If you decide to change springs, COMP offers a 26915 beehive spring and matching retainer that will produce a little more clearance to allow up to roughly 0.500-inch of valve lift. We haven’t checked that lately so if you go this route, it would be worthwhile to verify those numbers.

The Vortec head also requires several components that are different from the traditional small-block Chevy parts list. Let’s start with these heads require guided rockers that have two small ridges that straddle the valve stem tip. This was done to eliminate the need for pushrod guide plates. The Vortec head also requires its own unique intake manifold, centerbolt-style valve covers, and spark plugs.

For your application, we’d suggest an Edelbrock Performer RPM intake. Remember that this will also require a Vortec style intake gasket.

The combination of the heads, Xtreme Energy cam, Performer RPM manifold, 750 cfm carburetor, and headers will push your 350 into the 410 to 420 hp range — again assuming the rings are in decent shape.

Of course, if all this chasing around looking for a set of used heads and then spending time and money modifying them seems like a lot of work, Summit has a solution as a brand new pair of iron Vortec castings that are fitted with all new valves and springs.

Summit’s version of the Chevy Vortec head sports 1.94 / 1.50-inch valves, a 67cc chamber, and a 175cc intake port. A pair of these heads are perhaps among the most affordable heads that can push a small-block well into the 400 hp zone. (Image/Summit Racing)

The Summit Vortec sports a 175cc intake port, screw-in rocker studs, the same 1.94 / 1.50-inch valve sizes and a 67cc combustion chamber. Flow numbers are better than the production heads and the heads are capable of accommodating a 0.500-inch-plus lift camshaft. For the money, this might be the simplest and easiest way to get to your 400 to 450 hp goal.

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19 Comments

  1. Pingback: Ask Away! with Jeff Smith: Getting 400+ HP From an Old Small Block Chevy

  2. Mike Brown says:

    I’m surprised that one of the many top end kits wasn’t recommended. Yes, they’re pricey as an initial investment. On the other hand, any Vortec head that I’ve seen come from a typical (there because it’s used up, not a low mile wreck) junkyard vehicle is pretty much guaranteed to be cracked. Time spent chasing good junkyard parts is also money spent. The top end kits,as I’m sure you know, have parts matched to work together, no guessing it this will work with that.

    • Drew Hunt says:

      What’s a good top end kit for a sbc 406 with a 555 lift cam?

    • Why does nobody ever give any attention to rods in Chevy engines? I could build a 400hp 327 Chevy using stock heads and cam with efi from a vortec truck 350. The key is in the road length. You need a rod length of about 1.9 to get easy top end on a has engine.

  3. Dean Steward says:

    I can vouch for your recommended combination. The sbc’s for street that I have built differ only in that the cam is the next more duration comp cam. They were used in Camero’s and performed excellent.

  4. A 350 ABC motor is a 5.7l not a 5.3l that’s a 305 or 307.

      • I believe the intent of Jeff’s comment was how to make 400hp, not that a 5.3L has a 350CI displacement. A junkyard 5.3L (323CI) if found in decent shape is easier to make horsepower due to the higher flowing head, but the expense of the other needed swap parts can make the overall costs go way up.

        • Jeff Smith says:

          Thaks Carl for making that more clear. Yes, that was the intent of the comment – I should have been more clear that a 5.3L engine is not the same displacement as a 5.7L 350 Chevy. Y intention was to point out, as you mentioned Carl, that a smaller displacement engine can make more power using the highly efficient LS version heads.

  5. James DeWilde says:

    I have pretty close to the same setup, except I have a Victor Jr. with a Holley 750 vacuum secondaries and Dart (world products) SR torquer heads with 2.02/1.6 and 76CC chambers. like the article, with the head gasket I figured I lost 1.5 compression points so I swapped out gaskets for steel shim .015 thick. the cam is a lunati bracket II. This is going into a Datsun 620 truck about 2350 lbs. I’m being on the safe side and thinking somewhere in the 12s but my friends say 11s easy.

  6. Neil Millen says:

    How big a thumpr cam can i put in a 91 l-98? Vacuum not an issue going in a ratrod

  7. Shawn Haley says:

    What is the part# for the Summit vortex heads featured in this article?

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  11. Why does nobody ever give any attention to rods in Chevy engines? I could build a 400hp 327 Chevy using stock heads and cam with efi from a vortec truck 350. The key is in the rod length. You need a rod length of about 1.9 to get easy top end on a gas engine.

    • Jeff Smith says:

      Actually Josh, this is a common misperception. David Reher of Reher-Morrison racing engines and an NHRA Pro Stock champion has done rod length dyno tests and came to the conclusion that rod length has a near-zero affect on power. It will move the power curve around slightly but very little else. Rod length was a big controversy about 30 years ago in the magazines and urban legend has evolved from all that nonsense. Bottom line is connect the crank to the piston with whatever rod length fits and is strong enough to survive. There is no power to be had from the rod length to stroke ratio. It’s just a number.

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