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.
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.
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.