350 Chevy Bolt-On Power Mods
Big power from a truly streetable combo
From the February, 2009 issue of Popular Hot Rodding
By Steve Dulcich
Sure, we've been around the game long enough to know that bolting parts on a 350 Chevy small-block has been done before--but the difference is that we want to do it better. With any project like this, there is a wide range of possible starting points, so the first question to answer is where will we begin? Maybe it should be a stock 350, like the one in your buddy's '70s Corvette, or the reman-ed stocker in your dependable old pick-up. The common theme here is average, everyday, and totally accessible-- call it every-man's small-block Chevy. What potential might be waiting in an average 350? We all know the sky's the limit if the plan includes a mega shot of dough, and basically starting over, filling the block with a new crank, rods, pistons, and a big, hard-hitting cam. That's not what we were after.
We wanted to see what a regular 350 could do with a well-thought out selection of aftermarket parts. That's easy, but we also wanted to keep the combination street-friendly enough to drive everyday without having it drive us crazy. Gloriously high compression ratios and belligerent cam specs make for great bragging on power numbers, and we're all for that, just not in a vehicle that we need to run day in and day out. So let's get down to the basic premise: what kind of practical power levels can be achieved with a stock low-compression 350, without going overboard and making it so temperamental that it's harder to live with than the ex-wife?
First, we needed a suitable test mule. Here we specifically avoided the glamorous and fancy stuff, and took a hard look at GM Performance Parts PN 12499529 290hp crate engine. GMPP certainly sells higher end versions of the 350 crate engine. In fact, this is one of the cheapest they offer. Basically, the engine is a fresh replacement engine for the early style small-block, with conventional Gen 1 architecture. Having dished cast pistons, it replicates generations of low-compression factory offerings, and swallows 87-octane rotgut without feeling pain. The heads are 76cc iron castings, with 1.94-inch intake valves and 1.5-inch exhaust valves, just like any run of the mill 350 of an earlier vintage. About the only edge it might have over the masses of stock 350s on the road is a somewhat healthy hydraulic flat-tappet camshaft, featuring 0.450/0.460-inch lift. Gm rates the engine package at 290 hp at 5,100 rpm, and 326 lb-ft of torque at 3,750 RPM, using a gross SAE rating regime.
We considered this engine to be representative of a basic 350. If you've got a good-running stock 350 under the hood, you can substitute your engine for ours and expect similar results.
Before any wrenching begins, it is important to take a mental inventory of what you have, and where you want to go. Simple as it seems, this type of analysis is what gives a project direction, discipline, and the best chance of success. First we had to weigh the capabilities of the basic package, and the shortcomings. From a performance standpoint, the low-compression dished pistons are the biggest handicap. With the factory 76cc heads, the ratio works out to around 8:1, which is low in anyone's book, even for a low octane performer. Low-compression makes it difficult to get the full effect from most high performance tricks. When short on squeeze, adding too much cam, or too big of a cylinder head will ruin the bottom end, and compound the compromises in idle quality due to the copious clearance volume. Sure, new flat-top pistons would fix that, but re-pistoning the engine changes the direction of the effort from bolt-on improvements, into the realm of a full rebuild.
The obvious answer here is to build-up the ratio with a smaller chamber volume. True the factory heads can be milled, but the potential there is limited. Aftermarket heads are the better answer, since they can be commonly ordered with 64cc chambers, for a net reduction in volume of 12cc, and a bump in compression ratio to approximately 9:1. That's still not heroic territory, but it is certainly workable. Replacing the heads can offer much more than just a gain in ratio; there are serious improvements in airflow to be had. The head swap would be a pivotal element in our build-up plan. Since we were after good street performance with a relatively mild combination, a moderately sized cylinder head with the factory 23-degree architecture would be the best way to go. No need to break the bank on a wild set of race heads for a 9:1 cast piston 350.
We settled upon the new RHS aluminum heads, PN 12022. These heads feature 64cc chambers to provide the compression ratio bump we needed, and 180cc intake runners, with high performance ports that promise the flow to really add some power. A set of aluminum heads may seem extravagant to some, but considering the costs of milling, porting, machining, changing valves, springs, retainers, and possibly guides, it really makes much more sense to swap heads than to rework the stockers. We flowed a set of the RHS 180s and found nearly 250 cfm on the intake side, and 205 on the exhaust (see Sidebar: RHS 180 Port Flow).
Next up for consideration was the camshaft. The 290hp crate package already comes with a mild performance hydraulic camshaft--a lot hotter than your typical stocker. With the added flow of the new cylinder heads, especially at higher lifts, it seems foolish not to step-up the cam specs. Going overboard with the camshaft can net some impressive numbers at the high rpm end of the scale, but here we were after a much more balanced approach. We decided that in keeping with the street theme, we'd select a camshaft that's short enough in duration to maintain a livable idle quality and drivability, and yet add significantly to power within a moderate upper rpm range. One way of achieving this is to use a camshaft profile with very fast ramps, giving a high level of lift for a given duration level. Though there are fast rate flat-tappet cams available, a hydraulic roller got the nod here.
By design, a hydraulic roller provides more duration at the upper lift levels than a comparable flat tappet, meaning that the valves spend more open time at the higher lift ranges where the heads offer the most flow. Hydraulic rollers have an enviable record of longevity and reliability with aggressive profiles. Finally, a hydraulic roller can tolerate the higher spring loads demanded by higher rpms and aggressive lobes with ease, whereas a flat-tappet always represents a balancing act between spring loads and longevity. To gain advantage of these benefits, we decided that an update to a hydraulic roller set up would be worthwhile, however, we would opt for a relatively short duration 'shaft to keep rpms under control, while maximizing torque and drivability.
We selected COMP's XR276HR grind, a retrofit hydraulic roller with 276/282 degrees gross duration, 224/230 duration at 0.050-inch tappet rise, and a rated 0.502/0.513-inch lift. To provide even deeper access to the high lift flow available from the heads, our set-up would include 1.6:1 ratio COMP Pro Magnum rocker arms instead of the factory 1.5:1, upping the lift to 0.536/0.547-inch. A set of COMP pushrods finished the cam and valvetrain combination. That may seem like a lot of lift, and compared to traditional street flat tappet it certainly is. All that lift is nothing to fear, though, since the overall duration has a far greater effect on idle quality and drivability than lift does. The high valve lift won't make for an evil and harsh combo; actually with the moderate duration, this cam would be very roadworthy.
Finally we had the induction to consider. Our initial configuration consisted of a factory iron two-plane intake manifold, topped with a Quadrajet carb. This arrangement is pretty much the universal setup used by Chevrolet from the late '60s into the '80s. Here, again, we were not looking for an all-out drag race induction, but rather a system to yield the fattest torque curve our 350 could deliver. In past testing, we have found the combination of an Edelbrock Performer RPM AirGap in conjunction with a four-barrel Demon carburetor virtually unassailable for street performance. It seemed like just the complimentary package to get the most from our street-bound 350 package.With that we had our basic battle plan laid out--a simple bolt-on upgrade consisting of heads, a cam and valvetrain combo, all topped with a free-breathing induction. It's about as traditional as performance package can get. The only unanswered question remaining was what kind of dividends these components would add to bottom-line horsepower output. A dyno test would tell the story.
What kind of power can be...
What kind of power can be tapped from a regular low-compression 350? Our GMPP PN 12499529 Crate engine comes speced out with 8:1, dished pistons, a mild flat tappet hydraulic cam, and plain old iron heads. Kind of a familiar set-up to anyone who has turned a wrench. The engine comes from GM without an intake or carb.
For a baseline configuration,...
For a baseline configuration, we bolted on this factory iron Quadrajet intake manifold, which is pretty typical of the units installed by the General for decades.
Keeping with the OEM theme...
Keeping with the OEM theme for our baseline testing, we topped the intake manifold with a nice Quadrajet carb, which was standard GM issue before the advent of fuel injection.
Although our 350 was outfitted...
Although our 350 was outfitted in pretty modest wear for the initial testing, we made a few compromises compared to a bone-stock layout. For access to EGT information on the dyno, we opted for these swoopy dyno headers, with 1 3/4-inch primary tubes, and the required bungs for the EGT and Lambda sensors.
The engine also had some help...
The engine also had some help in the form of a CSR electric water pump, again to facilitate the dyno installation. These crate engines come sans damper, so we bolted a SFI rated Rattler damper from TCI to the nose of the crank.
Handling the ignition requirements,...
Handling the ignition requirements, the 350 was equipped with a complete MSD ignition system, including their billet distributor, 6AL control box, and wires feeding current to Denso Plugs.
In baseline form, our 8:1...
In baseline form, our 8:1 350 Chevrolet made about the power we'd expected from this package, twirling the dyno's impeller to read 299 hp at 5,000 rpm, and reaching premature torquegasm at only 3,400 rpm, with 350 lb-ft reading on the dial.
The first item to get the...
The first item to get the heave-ho was - you guessed it, that iron intake manifold. What went in its place is the proven Edelbrock Performer RPM AirGap, a consistent power maker, a change that clearly made Westech's Steve Abbruzzese really happy.
To top the intake, we selected...
To top the intake, we selected a capable Mighty Demon 750 carb. It might seem like kind of an aggressive choice at the current mild power level, but these carbs work well in a variety of applications, and will provide plenty of capacity to keep up as the power is increased.
Back in the control room,...
Back in the control room, Abbruzzese again put the 350 against the dyno, and we were all impressed with how the 350 responded. The iron intake must have been suffocating, since we found a good 20 lb-ft, and a shocking 49 hp gain from the change.
We tore down the engine on...
We tore down the engine on the dyno for the next round of mods. The flat tappet hydraulic camshaft was removed in favor of COMP's XR276HR hydraulic roller cam. We did not go overboard with cam duration, since we want to keep the engine's nice street characteristics and partially as a concession to the relatively low compression ratio.