Stage 3-4
STAGE 3 to 4
NOTE: Installation of 428 heads on 390 blocks requires either tube-type headers or 428 cast iron manifolds.
Tube Type Headers Add 15 Horsepower
Bigger exit tubes are a must if you want to push a lot of mixture in the top of your 390. Headers will add about 15 bhp @ 5000 rpm. Tube type headers come in a variety of shapes and sizes. But the criteria for good ones is very simple; separate tubes for each exhaust valve, equal length from valve to collector, and maximum efficient diameter for good exhaust flow. The headers that will best "tune" your 390 will observe these basic dimensions:
STEEL TUBE HEADER DIMENSIONS| PRIMARY PIPES | COLLECTORS | ||
| Length | 36" | Length | 12" Manual 16" Automatic |
| Inside Diameter 2" | Inside Diameter 31/2" |
OPTIONAL CAST IRON MANIFOLDS
If you don't have the coin to go the steel tube headers route, you can get just about as much improvement (5 bhp increase over stock 390 manifold t in scavenging exhaust gases with a set of cast iron EXHAUST MANIFOLDS C8OZ-9430-C (R.H.) and C8OZ-9431-B (L.H.). They are specially designed for the 428 Cobra Jet heads of Stage 4. When installed with the 428 heads they can be used with any big block engine.
STAGE 4
21 More Horses; It's All In Your Heads
428 COBRA JET HEADS
To complete the hnpressor Kit and really let the other guy know "big breather is watching you" install 428 CJ CYLINDER HEADS C8OZ-6049-K 2 req'd) on your 390. These bigger, better breathing heads will let you install 428 CJ valves that are larger than the stock 390's. 428 CJ INTAKE VALVES C9OZ-6507-U (8 req'd) have a 2.097" head diameter and 428 CJ EXHAUST VALVES C9OZ-6505-N (8 req'd) have a 1.660" head diameter compared to the stock 390's valve diameters of 2.037" and 1.566" respectively.
UPPING THE COMPRESSION RATIO
If you install 428 CJ heads on your 390 without any changes, you'll find that because of their larger chamber size the compression ratio has fallen to about 9.6:1. To raise the compression ratio to the 428's 10.7:1, mill 0.030" off the heads and install STEEL HEAD GASKETS C3AZ-6051-B (2 req'd). These gaskets are 0.020" thinner than the gaskets that come with the heads (which should be discarded}. This modification has the effect of reducing head thickness a total of 0.050", thus boosting compression ratio. The end result is a 21 bhp gain @ 5200 rpm.
Milling the cylinder heads means you must also mill the intake manifold, to keep all ports in proper alignment. Take 0.030" off each mounting surface and 0.042" from the bottom of the manifold.