An incredible deal, GM's Vortec L31 heads are available fully assembled for about $200 eac
With just the cam swap, would you believe we saw a 41.5hp gain with virtually no loss of peak torque? That's with 16 degrees of initial timing. The Xtreme Energy cam made more power above 3,300 rpm, with its advantage increasing as it moved up the rpm scale. Its 269.8hp peak occurred at 5,100 rpm, 400 rpm higher than with the stock cam. Above-3,500-rpm torque was way up too. All this and the engine still made 14.5 inches of vacuum idling at 950 rpm. We tried a 1-inch spacer plate, but it made no significant difference.
The Class Of The Heads
Chevy 305s came with only 1.84-inch intake and 1.50-inch exhaust valves. Fortunately, there is an effective upgrade available in the form of '96-'97 Chevy truck L31 Vortec heads with 1.94-/1.50-inch valves. The cast-iron heads' intake and exhaust ports are similar to the late-model Corvette aluminum LT1 heads, but unlike the LT1 heads, they bolt onto conventional small-blocks. The Vortec's greatest improvement is on the intake side, which is why we chose the dual-pattern cam with its slightly larger exhaust lobes.
There are several minor hassles when using Vortec heads. First, they have a unique intake manifold bolt-pattern requiring a dedicated (and more costly) GM or aftermarket intake. Also, early small-blocks must switch to '87-and-later center-bolt valve covers and late-model guided rocker arms. The heads' 64cc chambers must be milled to avoid compression loss on a 305, and the larger-diameter chamber outline requires 350 head gaskets, which could also cause reduced compression. To solve this, we milled the heads 0.045-inch and used thin 0.015-inch head gaskets and got just over 10.0:1 compression. We had no detonation problems on the dyno even with 89-octane gas.
With the same Holley 750 double-pumper on Edelbrock's new Super Victor single-plane intake, the Vortec heads blew away the stockers at power points above 3,400 rpm. They churned out 316.5 lb-ft at 4,800, raising the torque peak by 1,100 rpm in the process. Torque production was over 300 lb-ft between 3,600 and 5,600 rpm, nearly as flat as a billiard table. At peak, the Vortec combo made 324.7 hp at 5,800 rpm, a gain of 54.9. And like the torque curve, it maintained the power output, producing over 300 hp from 5,100-6,100 rpm. Under-3,000-rpm numbers were down slightly, probably because the Super Victor is optimized for the midrange and upper rpm band.
With loose-tolerance pushrod clearance holes and no provisions for guideplates (the studs
Finally, we tried 1.6:1 rockers in place of the 1.5:1 stockers. They showed no significant gains.
Gain Without Pain
At this point, our 305 was up 127 hp and 55 lb-ft from the original dyno baseline, and 180 hp and 77 lb-ft over the stock net ratings. Backed by low gears and a loose converter, this engine ought to surprise plenty of 5.0L Mustangs. At least it'll get you by while you save up to build a 350!
Another 305 Option
Once we put together the 305 combo outlined in this story, we realized that many of you already have a 305 with typical bolt-ons in a GM A-, G-, or F-body. For this type of application, World Products offers 305 S/R Torquer cylinder heads that may save you some bucks. They run about $599 complete from Aerohead-that's more than the GM Vortec heads, but the price may only be higher if you're trying to duplicate our 305 engine combo from scratch. The bolt-on World Products will save money if you already have an aftermarket intake and rocker arms and don't want to pay extra for the dedicated intake, rockers, and late-model valve covers required by the Vortec heads. The World heads are available in either the early or late intake bolt-pattern and come with 58cc chambers so they don't need to be milled to maintain a good compression ratio on a 305. Vortec heads definitely need to be milled.
After pulling the cover to swap cams, we noticed this stock 80,000-mile timing chain was a
With the Xtreme Energy cam, Vortec heads, Super Victor intake, and 750 Holley, our tired 3
For a Vortec-compatible intake manifold, we chose Edelbrock's new Super Victor for Vortec