Ask Away with Jeff Smith: Building Blocks for a Big Block Chevy

I am tackling a 1969 Corvette. The 390 HP L36 427 was rebuilt but now needs to go to a 0.060-inch overbore. My buddy and I have been considering building a 454 stroker to give it a little more power. We used to go to open chamber square port heads in the early days, partly due to the better shape of the chambers and reduced detonation. Is this new oval port stuff really that good, and not as susceptible to detonation like the old iron heads?

I would like to make around 500-plus horsepower. I know Edelbrock advertises their oval port head can produce around 540 HP on a 454. I may just get pistons and rods to do the long block, but I haven’t found a good OE 0.060-over forged piston.

 J.J.

This sounds like an adventure and you have friends (psychologists call them enablers!) who will help. Your 500-plus horsepower estimate is close with an overbored 427, but you’ll have to spin it a little higher to achieve that power. Since this is not a daily driven vehicle and the Vette already has to lug around all that Rat mass, a stroker will easily make the power.

Here’s my recommendation based on a recent engine I built. We built a 540 cubic inch Rat with Edelbrock oval port heads, a mild hydraulic roller cam, and a single four-barrel that made 660 lbs.-ft. of torque and 630 HP. It was down about 25-30 HP at the peak because the cam was a short duration version we chose for this mild street engine. That engine idled at 900 RPM at around 10.5 inches of idle vacuum–and that was after spending less than a minute adjusting the idle mixture.

Based on the success of that engine package, I’d say you could come close to this with a 0.030-over 454 block by using a SCAT 4.250-inch stroker crank and SCAT 6.385-inch I-beam connecting rods. You’d end up at 489 cubic inches at an extremely attractive price.

Attach the rods to a set of Mahle Motorsports 4032 alloy pistons with 18cc domes. When combined with a set of 118cc chamber heads, the dome will create a 10.2:1 compression ratio, which is near ideal for a street engine on 91 octane pump gas.

The Mahle pistons use a 1.5mm/1.5mm/3mm ring package that is slightly thinner than the typical 1/16/1/16/ 3/16-inch ring package on most off-the-shelf performance pistons. The big point is the difference in oil ring thickness. The oil ring package generally represents about half the total ring drag produced by a three-ring piston. A 3/16-inch standard oil ring measures 0.1875-inch. By reducing that to 2mm (0.078-inch), thickness is cut by more than half, which guarantees good oil control while reducing friction. That equals to adding perhaps 5-10 free horsepower, especially with the longer travel of a 4.250-inch stroke.

If you really want to improve combustion efficiency, consider adding a set of Total Seal rings with a lateral gas port top ring. These rings are quickly becoming a hot deal since they add cylinder pressure behind the ring to load it more tightly against the cylinder wall. This improves ring seal and also reduces blow-by, which is crankcase pressure. Plus, there appears to be some small horsepower advantages as well. These are not inexpensive rings, but we’ll leave it up to you to evaluate the return on the investment potential.

Let’s move to the camshaft. The right way to go would be with a hydraulic roller. On the 540 I built, we used a Comp Xtreme Energy hydraulic roller (see spec box) with 236 degrees of duration at 0.050-inch tappet lift. This cam was a bit on the small side for peak horsepower, but made up for it by making 1.22 lbs.-ft. of torque per cubic inch, which equates to 660 lbs.-ft. of total torque–and that was with a single plane intake manifold, which is notorious for hurting low-speed torque.

This cam is quickly becoming my favorite cam for street big blocks because of its great idle and off-idle characteristics. Your engine will be slightly smaller in displacement and this might push the peak horsepower level up slightly. Our simple horsepower calculator formula indicates your engine with a decent set of heads (we’ll get to that next) should make around 600 lbs.-ft. of torque and 575 to maybe 590 HP somewhere near 6,000 RPM.

Now we can address your cylinder head question. We built our 540 Rat with Edelbrock E-CNC 325cc factory-ported cylinder heads. These heads come with 2.25/1.88-inch valves and 118cc combustion chambers. These are essentially “roval” or rectangular oval port heads that fill in the port entry corners that do not contribute to airflow anyway. On our 540 package they helped made gobs of torque.

Top this off with a Victor Jr. single plane intake, an 850 cfm Holley HP Ultra carburetor, and a set of 1 7/8 inch primary tube headers and your biggest problem will be keeping rear tires on the car. With a 5-speed manual transmission, this would make a killer street car that would have all the power you need for track days at any road course in America.