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.
We brought our stock 350 to Westech Performance Group's dyno facility in its baseline stock configuration, complete with an iron intake manifold and Q-Jet carb. To facilitate dyno hook-up, the engine was configured with a set of Westech 1 3/4-In. sprint-car style dyno headers, which are equipped with the required fittings for EGT connections and a wide band Lambda sensor. Unfolding our plans began with running the engine for a baseline, which would give a chance to validate the stock power output. Before long, the engine was running on the SuperFlow dyno, and up to operational temperature. Our initial pulls showed a disappointing 214 hp and 307 lb-ft. This was a good 76 hp short of the 290 hp the crate engine is rated for, so what happened? It didn't take long to spot the anomaly on the dyno sheet-- it had recorded the intake manifold vacuum reaching a peak of 8.8 in-hg at 5,300 rpm!
There is only one explanation for that, the secondaries in the Quadrajet were not opening. We re-adjusted the tension at the secondary air valve, setting it loose, and made certain that the choke lock-out linkage was clear, and finally got our valid baseline, 299 hp at 5,000 rpm, and 350 lb-ft at 3,400 rpm. Considering the difference between the SAE correction factor used by OEM manufactures (from which this engine's ratings were derived), and the STP correction factor used by the performance aftermarket, the numbers were now right in the expected zone.
Our first mod was to unbolt the heavy iron lung from between the heads, and bolt in the sweet Edelbrock AirGap intake. To top the manifold, we pulled out a Mighty Demon 750 cfm carb. The Mighty Demon line is aimed at serious street performance, but is packed with most of the features of Demons Race Series carbs--mechanical secondaries, replaceable air bleeds, a contoured air horn, and no choke. Being from mild Southern California, we never miss the choke, but we certainly appreciate the power delivered by these carbs. We were nothing short of impressed with the results of the induction change, with the dyno now registering peaks of 348 hp and 370 lb-ft of torque, at 5,300 and 4,200 rpm, respectively. Frankly, the increase was about the biggest gain we've seen in a carb and intake swap, proving once again the effectiveness of this outstanding combination. Torque was up by 20 lb-ft, while power was boosted an incredible 49 hp. This is definitely hardware worthy of consideration for any high performance effort.
With the induction system tested, we were left with the RHS cylinder heads and COMP hydraulic roller cam. Our inclination was to move to the cam swap first, and test the effect of a cam change alone, however this proved to be problematic. We knew that the valvesprings would need to be upgraded in order to run the hotter cam. We found that the exhaust retainers would also need to be changed, since the engine was equipped with exhaust valve rotator style retainers, which reduced the installed height to the point of making coil bind an issue. The clincher was finding insufficient retainer-to-guide clearance to accommodate the cam's lift. It would take removing the heads, and machining the guides to get the job done. All in all, it would take a significant amount of reworking to get the stock iron heads set up to handle our cam choice, including upgrading the studs, and machining the spring seats.
While these hurdles can certainly be crossed, we decided the best course of action was to consider the cam and head swap as a single package. After all, the full benefit of the cam would not be realized without cylinder heads that flow well at higher lifts. On the flip side, it seemed like an unlikely combination to bolt on aftermarket heads with the small factory cam. The cam just wouldn't make use of the heads, and negate much of the available power potential. Either the cam or the heads alone would be a questionable combination, and unlikely in the real world. What works is a matched cam and cylinder head combination that complement each other, and these two parts were originally selected to work well together.
We opened the 350 up for the cam and head swap, completing the installation with the COMP Pro Magnum 1.6:1 rockers. We found the performance parts were easy to install, replacing the original pieces perfectly. The retrofit hydraulic roller arrangement replaces the stock flat tappets with paired lifters joined with link bars to establish the roller alignment, unlike the OEM hydraulic roller arrangement that employs alignment yokes and a spider assembly. With the RHS heads, the 350 took on a much more purposeful performance look, but the bottom line would be the power output. We were pleased to find the idle quality was more than acceptable for a street engine, with a mild lope at an 850 rpm idle, and about 13-14 in-hg of manifold vacuum. The smaller combustion chambers of the 64cc RHS heads added a solid point of compression ratio, which is needed compensation for the longer valve events of the COMP cam. We found 165 psi of cranking compression with the new combination, low enough to be happy with even lower grade pump gas, but high enough for decent low-end torque and power.We were shooting for a solid 400 hp from this basic and street-friendly 350. The dyno would soon tell us if the combination would be enough to reach our goal.
The willing small-block easily covered that mark, posting a peak of 412 hp at 5,600 rpm, with torque now reaching 421 lb-ft at 4,100 rpm. These are good numbers for a real street 350 based on a typical production-style low-compression bottom end. The cam and head swap was worth a solid 64 hp gain in horsepower, an increase of over 18 percent. Torque was up a substantial 42 lb-ft, and the torque was up everywhere in our test rpm range by about that amount. It is unusual to find big torque gains across the board, through a broad rpm range. The engine was clearly much happier with the new combination.
We felt that the engine's output struck a very good balance of useable power for a street driver that operates in a moderate rpm range and would run happily on low-grade fuel. For some added sizzle, we figured the engine could reliably handle a 100 hp hit of nitrous. We like the idea--412 trouble-free and reliable horsepower, packing an extra "C-note" of power when the occasion calls. We tapped ZEX for one of their jettable perimeter plate nitrous units, wired up the solenoids, and routed the trigger switch to the dyno control center. Just for additional safety, we pulled 6 degrees of timing, dropping the total timing from 36 degrees to 30 degrees, which reduced the normally aspirated power by about 10 hp. Ingesting nitrous, the results were as predictable as they were satisfying. We hit the squeeze at 4,200 rpm, and power spiked with a huge torque surge to peak at 584 lb-ft at 4,500 rpm. Horsepower cranked up to nail the 500hp mark, and just hovered at that power level all the way to 5,700 rpm, where we let go of the nitrous trigger. This engine seemed to just love the modest shot of nitrous.
With a few well-chosen bolt-ons, our "Plain-Jane" 350 gained 112 hp in normally aspirated form, and over 200 hp with a livable little shot of juice. Even better, it gained that power without killing the drivability and usefulness of the engine in a real street application. On the motor, it made enough power not to have to make excuses. With a modest shot of nitrous, we could shove the power upward to where it would match power levels normally seen with very radical and raucous small-blocks. It's a nice combination to have, without the drawback of living with and feeding a temperamental rascal on a daily basis.
The retrofit hydraulic roller...
The retrofit hydraulic roller lifters are linked in tandem, keeping the roller wheels in constant alignment with the cam's lobes. The conversion components just dropped right into our stock block.
Some checks such as coil bind...
Some checks such as coil bind clearance, and retainer to guide clearance, told us that the high-lift cam was not going to play nice with the production iron heads. Always make these checks when making changes, or be prepared to suffer the expensive carnage.
Rather than fight the clearance...
Rather than fight the clearance problems, we opted to go right for the RHS heads at the same time as the cam change. Really, these two items will work best together, each complimenting the other. Abbruzzese hoisted the stockers and tossed them on the pig-iron pile next to the manifold.
With the heads off, we had...
With the heads off, we had a chance to take a look inside the stock short-block. These low-compression dished cast pistons are not normally what performance legends are made of, but we did make some power.
Our 180cc RHS heads are assembled...
Our 180cc RHS heads are assembled with top-quality COMP Cams components, including the valves, dual springs, and 10-degree locks and retainers.
Our heads came with COMP rocker...
Our heads came with COMP rocker studs and guideplates. It's important to use thread sealant on the intake studs, since the bolt hole opens into the intake port. Put them in dry, and high port vacuum will suck in oil like you wouldn't believe.
Instead of the standard 1.5:1...
Instead of the standard 1.5:1 ratio rockers, we went with COMP's 1.6:1 Pro Magnum set. With the leverage of a higher ratio, we gain about 8 percent in lift, without having to increase duration. It's a nice way to tap more high lift-flow than you'd normally get within a given duration range. The heads and cam made a stout little small-block, easily topping the 400 hp bragging barrier.
With 412 hp and 421 lb-ft...
With 412 hp and 421 lb-ft of torque, we had plenty of power in a streetable package. Is that enough? We know this engine can swallow 100 hp worth of nitrous without spitting parts, so why not bolt on a unit for some standby reserve? A ZEX plate kit was an easy addition.
Snorting just a tickle of...
Snorting just a tickle of the go-fast gas, our 350 made some monster torque, belting out over 580 lb-ft of torque. Horsepower spiked to the 500 mark, and just held there until the button was released.
The 180cc RHS heads feature...
The 180cc RHS heads feature a compact combustion chamber packed with 2.02/1.60-inch valves, with ample quench areas at either side of the chamber. These heads can be had with 64 or 76cc chambers, with either straight or angled plugs. We went with the straight plugs and the small chambers to add compression.
Beneath the valves you'll...
Beneath the valves you'll find a nice open bowl form, with minimal valve guide intrusion. All in all, a nicely cast head.
RHS 180cc Head Flow
RHS 180cc Straight Plug Aluminum
Tested On SuperFlow 600 Flow Bench
Tested By Steve Dulcich
CFM @ 28 inches Water Depression
|Lift||Intake ||Exhaust* |
|*Exhaust tested with flow pipe |
Dyno TestingDyno Results
Small-block 290 HP GM Crate Engine
Tested On SuperFlow 902 Engine Dyno
Tested At Westech Performance Group
STP Correction Factor
|Torque lb-ft |
|Horsepower (STP) |
|RPM||Test 1||Test 2||Test 3||Test 4 |