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New Dog, Old Trick

Introducing GM Performance Parts' new carbureted intake for LS1s

Jul 22, 2004

Carburetors are a thing of the past for many performance enthusiasts. This is not striking news to anyone who's been to a show or race in the last couple years to witness the amount of late-model swaps or aftermarket EFI systems being used. Of course this makes perfect sense since carbs haven't been used to mix the air and fuel from an OEM car in nearly two decades. It's all part of the performance evolution with the availability of aftermarket EFI systems, user-friendly components, and late-model junkyard scores.

Don't get us wrong, we're well aware that EFI is not for everyone, nor will it ever be on every street machine. EFI is to a carburetor as the Internet is to a magazine in that there's a lot more information available but sometimes it's just too much technology and you just don't need it all.

By disregarding the thought of putting EFI in your next project, however, you're missing out on some pretty cool late-model engine swaps such as the Chevy LS1, Modular Ford, and now the new Hemi. These engines feature cutting-edge engineering and if you want to reap the rewards of this modern engine technology, you have to accept EFI and all of its sensors, wiring, and high-pressure fuel line glory--until now.

The folks at General Motors Performance Parts realized that EFI isn't for everyone so they went about seeing what it would take to fuel their Gen III small-block with a carburetor. The result is a cast-aluminum open-plenum intake manifold that drops right in place of their nylon plastic EFI set up and accepts a square-bore carb. Now how cool is that? We found it to be very cool indeed and made tracks to the dyno to see how the high-tech Gen III engine would fair with this old-school approach.

Third Time's The Charm

The Gen III small-block is generally referred to as the LS1, primarily because that was its code when it was first introduced in the '97 Corvette. By '99 however, there were a number of variants based on the Gen III block such as the 4.8L LR4 and 5.3L LM7 cast-iron truck blocks, the iron 6.0L LQ4 (320 hp) and LQ9 (345hp) used in Escalades, and of course the aluminum LS6 that powers the zoomy Z06 Vettes. With the extra cubic inches that the 6.0L offers, you know that racers and engine builders will be taking advantage of the bigger-is-better motto, especially since there's said to be an aluminum version available in the upcoming LS2.

Rather than attempt to find a 6.0L or LS6 version, we stuck with the popular LS1. Again, these engines have been available since '97 and are becoming easy to find at swap meets and bone yards. Our test engine came in crate form from GMPP (PN 25534322) equipped with EFI, exhaust manifolds, flexplate, waterpump, Camaro oil pan and the ECU complemented by the wiring harness. If you're easily intimidated by wiring, this harness would be like Indiana Jones falling into that pit of snakes. It's huge and there are wires shooting off in all directions and sizes just waiting to tangle around you. Actually, if you were to spread it all out and take your time, you would find that everything is labeled to make the installation as easy as possible.

This engine specs out at 346 cubic inches and is conservatively rated at 320 horsepower at 5,800 rpm and 330 lb.-ft. at 4,400 rpm in full accessory and restrictive exhaust dress. The compression is right at 10.25:1 and is equipped with a hydraulic roller bumpstick with 0.500/0.500-inch lift with 119.5-degrees separation. It's a great combination that most anyone would be happy with tucked between the fenders of their favorite street car.

On the Dyno

We ran the engine at Westech Performance on their Superflow engine dyno. The crew there has already been running the Gen III engines and were ready for us. A set of fuel rails from Wilson Manifolds was installed as they were already plumbed with a return line connected to their fuel pump (which we ran at 52-psi). We also used a set of long-tube 1 3/4-inch headers from Hooker with 3-inch collectors. The long-tube design has been found to really wake up the LS1's performance. Up front, an electric water pump was installed to ease the hassle of running a belt, especially since our engine didn't come with an alternator or other belt-driven accessories.

The engine fired right up and we let it run for a while at a variety of different rpm to let things get seated and broken in. Also, this gave us a chance to check things over before we started dropping a load on the motor. When Westech's chief dyno jockey Steve Brule gave his official thumbs up, we did a number of pulls to establish a base. The average was much higher than the SAE spec that GM lists in their catalog. Our peak corrected STP numbers turned out to be 429hp at 5,600 rpm and 425 lb.-ft. at 4,800 rpm which we were extremely pleased to see.

To confirm consistent numbers we flogged the engine numerous times and each run was right on top of the other. The headers were a huge help in achieving these numbers plus the lack of a belt drive along with the 92-octane fuel surely had to help. Also, the ECU we used was a model that Westech had removed the factory rev limit through the LS1-Edit program. It still retained the factory set curves, but the fuel cut-off rev limiter was erased from the program. This allowed us to run the engine up until the valve springs couldn't stand it any more, which occurred at around 5,850 rpm. Brule even pulled up another chart from a stock LS1 to find that the numbers compared similarly.

Old Schoolin'

With our numbers checked, tested, and rechecked it was time to peel off all that modern age high-tech EFI stuff and give the age old carburetor a try. We disconnected connectors, pulled wires, and unbolted the intake from the aluminum block. We used a set of intake manifold gaskets from Weiand and bolted the single-plane intake in place. Since the intake has no coolant pumping through it and there's no distributor to deal with, the swap only took a short time.

The fuel mixing duties were given to a Barry Grant 750 Mighty Demon. These carbs work very well and are priced right as you basically get the center section of a Race Demon plus have changeable air bleeds. Without the ECU on board, the next hurdle in converting to a carburetor is how to fire the individual coil packs. This is where MSD and Edelbrock come into play.

Edelbrock worked with MSD Ignition to develop an ignition control that would drive the eight coils. The ignition is an inductive spark design, unlike the common capacitive discharge ignitions that power most race cars. This features factory connectors that simply plug into the main coil connector for each bank of cylinders, the crank sensor, the cam sensor, and into a Map sensor. There is also a power and ground wire to connect. One other feature is that you can choose between six pre-programmed timing curves by simply plugging in a different module (just like MSD's rpm modules). MSD is said to be working on their own system that will incorporate their Pro-Data+ software for easy programming through a PC.

With everything connected, we gave the throttle a couple pumps and hit the key. The LS1--an engine that was never intended to be crowned with a carburetor--fired right up. The throttle response seemed crisp and we were on our way. After a few light pulls, the engine was shut off so we could take a look at the plugs. It seemed to be a little lean so we set about swapping the jets. The Demon was supplied with 75 primary jets and 83 in the secondary. After a couple different changes we stuck with 79s up front and a pair of 90s in the back. With everything set, we revved it up and performed three more pulls to gain an average to compare to the EFI.

The results were surprising. We kind of expected to see a drop in power and torque at lower rpm due to the single-plane design, but the carb stayed right in there with the EFI system across the entire rpm range. The EFI peaked torque at 4,900 rpm with 425 lb.-ft., while the carb tipped 420 lb.-ft. at 5,000 rpm. Power wise, the two induction systems were equal at about 430hp at 5,600 rpm.

Trying to compare the two systems is apples-to-oranges as the runners of the EFI system are longer, yet they are only carrying air and the squirt of fuel has been optimally positioned. The open plenum of the GMPP carb intake seemed to be able to easily fill each of the shorter runners with plenty of air/fuel mix. We felt that the intake would really begin to shine at higher rpm, but we were limited due to the valve springs.

More Upgrades

With that, off came the coil packs and valve covers so we could install a fresh set of LS6 springs from GMPP. (These are supplied with the Hot Cam that you'll read about shortly.) Like the rest of the engine, GM did their homework on creating a lightweight, compact, single-coil valve spring. Unique materials combined with a honeycomb design are the reason the new springs work as well as a multi-coil spring. We pulled the original equipment off in favor of a set of 'blue' springs that were used on the '01 LS6 engine platform.

The new springs did the trick! No longer did we have a built-in 5,800 rev control. The LS6 springs allowed the engine to wind up easily to about 6,600 rpm before the power numbers began to dip again. We were now reaching upwards to 454hp at 6,400 rpm while torque dipped but remained in the upper 370 lb.-ft. range.

We later found out that if you change the springs, it is recommended to step up to a stronger push rod as well. Remember, any OEM engine is built to meet certain performance goals but more importantly, it first must meet strict emission and economy standards. Parts are designed to work within these specific parameters plus they must be cost effective for mass production, not to mention deliver incredible endurance and performance. When you take any engine and begin over stepping the specific boundaries that it was built to perform within, you're going to need to upgrade to parts made to step over that line. This is why we have a thriving aftermarket and is the reason you're reading PHR in the first place.

Our final step over the factory's performance limit came in the tune of a fresh GMPP Hot Cam. The cam is even a little step over and above the LS6 cam specs. The Hot cam offers a lift of 0.525-inch on both intake and exhaust with a duration of 219-degrees and 228-degrees with a lobe separation of 112-degrees.

Cam Swap is a Cake Walk

Once again, our hats off to the engineers behind the Gen III small-block. It took us longer to swap the valve springs than it did to change the camshaft. You still have to remove the rocker arm and the push rods, but the intake stays right in place. Before the cam is pulled out of the engine, you simply need to spin it around a couple revolutions. This pushes each lifter up into a tray that holds it up out of the way so the cam can be removed. How cool is that? It was also recommended to swap to a heavy-duty timing chain with thicker alloy side plates, which GMPP offers in their catalog.

After pre-lubing the new cam and getting it installed in the engine, be sure each push rod gets centered on the lifter as it slides back down into position on the cam. You can confirm that everything is assembled correctly by putting the lobe at about 50 percent lift and inspecting the position of the rocker tip. It should be in the middle of the valve stem and at max lift the tip should not be off the top of the valve stem. This is more important when you get into bigger cams but it never hurts to check things twice.

The engine fired right up again and we noticed a slight difference in its tone. It had a slight rumpety-rumpety chop in its idle that signaled that this was no longer a stock LS1. Again, we ran the engine to let the new parts get some heat and break in properly before we started rolling the dyno again.

The Hot Cam certainly lit a fire in our LS1. Across the board we saw substantial power and torque number increases. Before the cam swap, the engine was done at 6,400 but now the engine pulled up through 6,600, registering a stout 503 horsepower. The torque peaked at 5,000 rpm at 449 lb. ft. as compared to the 420 lb. ft. with the stock cam. The increases were everywhere through the rpm range and definitely would be felt behind the wheel of any car.

It was great day of dyno testing. Gen III is a fun engine to work with, and the new GMPP carburetor intake worked better than we anticipated. Being able to install a carb on these engines is really going to open a lot of doors, or shall we say hoods, for the most advanced small-block GM has ever built. There are a number of performance parts coming for these engines and that number is going to continue to grow. In fact, GMPP is even going to be offering a front assembly that will allow you to run a distributor and a mechanical fuel pump on the Gen III engine. You can bet that when this block was being designed and tested, carbs and distributors were the furthest thing from any engineer's mind. With the results we achieved with a few bolt-on parts, we can't wait to see more old school parts for these modern day powerplants.


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For anyone who is intimidated by wiring and excessive sensors and controls, the GMPP carbureted intake is a godsend. You will still incorporate the individual coils, crank sensor, cam sensor, and a few other inputs to a new MSD controller (available separately) but going old school on a new Gen III small-block is easy.

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The new GMPP intake is a single-plane design that accepts a square flange carb. GMPP felt that a dual-plane intake would end up being too restrictive when considering the lightweight internal components, the unique and great flowing port design, compression, and the efficient quench design of the combustion chamber.

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Due to their tall and skinny design, the Gen III intake ports are sometimes referred to as 'cathedral' ports. There is no performance secret to their unique design, it was simply the only way GM engineers could achieve the proper port volume while contending with the in-line valvetrain design and the four head bolts around each cylinder. The nylon EFI intake is only 3mm thick so you can't do much massaging to it, but the new cast aluminum intake will allow for some polishing.

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We first ran our crate LS1 in its out-of-the-box configuration with the factory electronic fuel injection. Westech had a set of Wilson Manifold fuel rails set up for the Gen III engines as well as an electric water pump so we wouldn't have to contend with the belt system. Also, a set of long-tube Hooker Headers were used throughout the testing.

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Once we broke the engine in and ran a series of tests, the plastic intake was peeled off the LS1. An interesting note is that since 2002, all of the LS-rated engines share the LS6 style intake manifold. What's the difference you ask? Read on...

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How do you know if you have an LS6 or LS1 intake manifold? The lower housing of the LS6 version is flat and comes closer to the valley cover on the block (shown on the left). The early LS1 version, shown on the right, also has a little swoop in its base that is easy to spot.

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The single-plane GMPP intake manifold is supplied with new metric hardware (all the bolt holes on this block are metric). We used a set of gaskets from Weiand, though GM will be providing gaskets with their intake manifold. What a simple swap! No distributor, no gasket sealer or coolant to mess with.

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To mix up the air and fuel we chose a 750 Mighty Demon from Barry Grant. The Mighty Demon line is recommended for high-output crate engines with higher compression and good flowing intakes. Our LS1 meets all of those criteria (and if it doesn't yet, it will soon).

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The new controller from Edelbrock and MSD provide the ignition and timing chores. This control allows you to choose from six different timing curves, much like setting the centrifugal advance in a distributor. The curves are changed with plug-in modules and we selected a mild curve that was recommended for stock configurations.

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The Edelbrock/MSD ignition controller uses factory-style connectors and plugs into the factory coil pack harnesses, crank sensor, cam sensor, and a MAP sensor if you would like to take advantage of a vacuum advance.

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The crank sensor is nearly hidden behind the starter. Note that if you use a GMPP crate engine, you'll need to buy a starter motor separately.

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Once we fired up the engine with the carb and intake in place, we did a couple short pulls and checked our tune up by reading the spark plugs. The mix appeared to be a little lean so we did a little tuning with jets and ended up with a set of 79s in the primary and 90s in the secondary.

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After dialing in the jets and timing, we made another three pulls to come up with an average to compare against the EFI. As you can see, the carburetor and intake are nearly even with the EFI across the entire rpm range. The valves began to float around 5,700-5,800 as you can tell in the graph.

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To swap the valve springs the coil packs need to be removed. Each coil is mounted to a common bracket that is bolted to the valve cover. Currently there are no higher output coil upgrades, largely due to the fact that the drivers that control the firing of each coil are built into each coil. This makes it a challenge to aftermarket engineers in cost and design. It is said that the coils used on trucks have slightly larger heat sinks making them more desired for higher horsepower applications. Truck coils are more triangle-shaped than the rectangular LS coils and the plug wire tower is in the middle of the coil compared to the bottom.

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The next upgrade to our LS1 was to swap out the rev-limiting valve springs. GMPP recommended a set of blue LS6 springs. The slick spring removal tool is from Moore Motorsports and made the replacement job a breeze. The new springs helped the engine rev right up to about 6,700 rpm without experiencing valve float.

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See anything missing from the snout of the crankshaft? There's no keyway for the balancer! Thanks to precise internal balancing and no need for timing marks, the balancer does not require being keyed to the crank. The one-piece front cover easily unbolts from the engine and seals to the pan with three lower bolts. Not having to worry about the oil pan seal was a great comfort and made changing the cam a much more pleasant experience.

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Just like any other small-block Chevy, you do need to align the dot on the cam gear with the one on the crankshaft gear before removing the cam. Note the gerotor oil pump that is driven off the front of the crank. This design reduces friction and mechanical inaccuracies compared to the standard camshaft-driven pump drive.

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Like any engine that uses a hydraulic roller type camshaft, a thrust plate must be incorporated. The cam we installed is a GMPP Hot Cam, with a 0.525/0.525 lift and 112-degree lobe separation. The factory cam was 0.500/0.500-inch with 119.5-degree separation.

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Before you remove the cam, spin it around a few times by hand. This pushes all of the lifters up where they are locked in place in a lifter tray. This means you do not have to pull the intake and valley cover off to remove each lifter! Removing and installing the cam is straightforward. Be sure to use the supplied lube on the cam bearings and each lobe. Note that the Gen III cams are hollow for rotating weight savings and feature larger diameter bearing journals.

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In the fall, Cartech Books will release a new book by Will Handzel all about the Gen III engine. We were able to get a sneak peek at "How to Build High-Performance Chevy LS1/LS6 V8s" and its pages are chock full of example build ups, the history of the engine, and is a wealth of knowledge for part numbers and specifications. If you're going to get into these engines, you'll need this book.

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For our final series of dyno pulls, we swapped the timing curve chip for a slightly more aggressive curve. Thanks to the heavy-duty valve springs and Hot Cam we were able to wind the LS1 up to 6,600 rpm where the peak power crested at 503 horsepower. At just over 6,700 we experienced severe valve float so 6,600 rpm was now our limit. Peak torque crested just short of 450 lb.-ft. at 4,900 rpm. GMPP noted that the push rods would also extend the rpm range of the engine, which we'll be sure to install before we make it back to the dyno.


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