The GMPP HT383, similar to the traditional 350/400 production-based strokers, is externally balanced, but the balance factor is different from either a 350 or a 400. GM lists a specific damper, PN 12498008, for the new HT383 application, while at the output end a PN 14088765 flexplate, common to '86-and-later one-piece rear-main-seal small-blocks, is specified. Follow the directions from GM and proper balance will be taken care of.
Production-based custom strokers typically use eccentrically weighted production 400 dampers and flexplates to achieve balance, often employing way too much eccentric weight. Better too much than not enough, though, since the assembly can be brought into balance more easily by removing weight than by adding it. Expensive custom balancing can be one of the hidden costs of a pieced-together stroker. Some aftermarket stroker cranks have a "split balance," combining an externally balanced 400-style flywheel with a neutrally balanced harmonic damper to achieve overall balance.
While at least partial external balance is the norm for 383s, internal balance may be achieved by the use of dense Mallory metal inserted into the crank's counterweights, rather than using the eccentrically weighted, external-balance dampers, flywheels, or flexplates. What's the difference? Externally balanced engines make up the deficiency in counterweight at each end of the crank, which serves the purpose of bringing the total rotating assembly in balance. However, this arrangement is not without compromise. With the eccentric weight hung at the extremities of the crank rather than at the counterweight directly in line with the piston and rod weight they are charged with offsetting, a bending movement is imparted to the crank. With a low-rpm slug like a production 400 small-block, the magnitude of this force is negligible, making an externally balanced assembly an effective approach to bringing the engine into balance. In very-high-rpm applications, however, an internally balanced engine will hold an advantage in minimizing these bending motions, which become more significant as rpm escalates. As a consolation, high-rpm race engines are more likely to hang much lighter components at the crank journal, making internal balance much more viable. On the other hand, in a typical street/strip rpm range, we'd hang our hat with the Detroit engineers and simply externally balance it and not get too hung up about it.
Pudding SkinnerI own a '95 Chevy 4x4 fullsize pickup with a 5.0L V-8, four-speed overdrive automatic trans, and TBI induction system. This combo hasn't got the power to pull the skin off rice pudding! I intend to swap out the engine with a late-model 350ci Vortec-head engine or put in my 400ci early model engine. I would like to keep the EFI, so as not to blow the budget. What problems lie ahead?Bob RedmondMarshfield, MO
You could use the 400, but that would require a new flywheel, as early engines with two-piece rear main seals have a different crankshaft mounting flange. Also, the 400 is externally balanced, requiring a specifically weighted flywheel. For these reasons, let's look at using a late-model 350. This swap would be relatively straightforward, and the power gains should be significant, if spec'd right.
Using Vortec heads is an excellent idea, as they're cheap, efficient, and offer substantial performance gains over conventional factory heads. The only obstacle here is that your existing intake manifold will not bolt up to the Vortec heads. Vortecs use a unique intake bolt pattern not found on other Chevy cylinder heads. The intake bolts go straight down through the manifold, meeting the cylinder-head mounting surface at an angle, somewhat like the arrangement used on Ford small-block V-8s. Some machinists have devised ways of re-drilling Vortec heads to accept conventional Chevy intakes, but this will result in a port mismatch that will diminish the power potential of the head swap.