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LT1 vs. LT4 Corvette
By Robert Gross
Photography by Richard A. Lentinello and the author
(these guys were involved with getting GM Tech Tech Performance mag started)
Track testing the performance of the two late-model small-block powerplants.
These stock Vettes look almost identical on the outside, but the real difference is under the hood. The red LT1 has a solid 300 hp, while the Black LT4 boasts 330 hp. Let's see how they do.
To discover the differences between the LT1 and LT4 powerplants, we decided to take a stock 1994 LT1 Coupe and put it up against a stock 1996 LT4 Coupe, and see just how much of a variance a few minor internal engine modifications could make. This track test will also put to rest that endless rumor: "... It's just a set of heads and an intake manifold bolted onto an LT1 bottom end." In a sense, that's true. However, there are many more subtle differences in the bottom-end assembly that people seem to overlook. On the surface, it doesn't really make sense to manufacture an engine, such as the LT4, for only one year of production. But with the four-valve LT5 already killed, and the new LS1 reserved for the C5, the engineers were told to redesign the existing small-block to produce at least 10 percent more power than the LT1 to satisfy the immediate need for extra power.
First, let's start with the obvious-the cylinder heads. As you may already know, the LT4 heads aren't reworked LT1 heads. They are a completely different casting.
GM Powertrain engineers decided to take the easiest and most effective route toward more power by increasing the volume of the air/fuel movement through it, so they built an all-new cylinder head.
In comparison to the LT1 head, the new LT4 head has had its intake roof raised by 0.100-inch and the radius, from the port to the valve bowl, has been smoothed out for better flow. To improve the flow of hot exhaust gases, the exhaust ports have been made wider, while the rear walls have been reshaped to allow quicker travel of the gases from the seats to exhaust flange. In addition to the modifications of the ports, the combustion chamber has been redesigned to improve flow to and from the ports, and larger 2.00/1.55 intake/exhaust valves have been fitted.
The LT4 intake valves are not only larger, but due to their hollow stem design, they're also lighter. The intake valves weigh 85 grams each (25 fewer than the LT1 valves), while the exhaust valves, which are partially filled with sodium and potassium to promote better heat dissipation, weigh 75 grams apiece (20 fewer than the LT1's).
One of the most obvious features of the LT4 over the LT1 is the higher 6300 rpm redline. Since the redline was increased 500 rpm, the engineers had to improve the valve springs significantly. They did this by using an egg-shaped, single-coil spring with 100 lbs. of seat pressure, as opposed to the LT1's 85-lb. pressure. The closed seat pressure is 260 lbs.
The 1.6:1 roller fulcrum, roller-tipped rocker arms are a modified version of Crane's design. By using these extruded aluminum rocker arms, the engine effectively increases the lift with the added advantage of less engine friction and a smoother idle than the LT1. The rocker arms sit on special shouldered 10mm studs, and are tightened down until they bottom out. This design eliminates the need for any kind of adjustment.
To further maximize the performance potential of the lightweight valves and roller rocker arms, a larger camshaft was selected. The LT4 cam lift profile is .476 intake/.480 exhaust, while the duration is 203 degrees intake/210 degrees exhaust at 0.050-inch lift. It also has 115-degree lobe centers and is installed one degree retarded, which raises the engine's redline by 150 rpm.
Since the LT4 heads have been reshaped, a new intake manifold had to be designed; the LT1 intake will not fit. The new intake's runners are matched to the bigger ports in the heads and large 3.5-gram injectors replace the LT1's 3.0-gram injectors. In addition, the external EGR hardware has been eliminated.
There are also several lower end modifications for increased strength. The notches in the LT4's piston to clear the valves are 3cc smaller than the LT1, resulting in a compression ratio increase to 10.8:1, from the LT1's 10.5. The top compression rings in the LT4 are slightly chamfered, causing them to dish when compressed in the cylinders. This new design produces a better seal under positive compression loads to handle the engine's higher cylinder pressure. Other bottom-end modifications include undercut and rolled journals in the crankshaft, steel crankshaft drive with an 8mm chain and a dual mass torsional damper.
With the subtle differences between the LT4 and LT1 in mind, we selected a bone-stock model of each to see just how wide the gap in ET slips would be. Gerard DeSantis, who owns the black '96 LT4, met up with David Tortolani's red '94 LT1 at Raceway Park in Englishtown, N.J., so we could determine if those engine modifications really make a difference in the quarter mile.
The LT1 puts out 300 hp at 5000 rpm and the same 340 lbs.-ft. of torque as the LT4, but at 4000 rpm.
1994 LT1
Owner: David Tortolani
Engine: 5.7-liter SFI V8 LT1
Horsepower: 300@5000 rpm
Torque: 340 lbs.-ft.@4000 rpm
Block: Cast iron, 4-bolt mains
Crankshaft: Cast iron
Pistons: Cast aluminum
Compression: 10.5:1
Cylinder heads: Aluminum with 170cc intake port volume
Combustion chambers: 54.4cc
Valves: 1.94 intake/1.50 exhaust
Valve spring pressure: 85 lbs.
Rocker arms: 1.5:1 adjustable self-aligning, stamped steel
Camshaft: Roller
Lift: .447 intake/.459 exhaust
Duration: 203° intake/210° exhaust@.50-inch
Intake manifold: Aluminum
Torsional Damper: Single mass
On David's first pass, the engine temperature in his gleaming red Coupe was at 160 degrees. After a short burnout to clean off the stock tires, David launched his Vette at 2500 rpm and powershifted the T56 at 5700 rpm. He ran a 14.02 ET at 102.24 mph. His 60-foot time was 2.48, due to excessive tire spin. Since the tires broke loose, David decided to drop the tire pressure in the rear to 26 psi. It seemed to help, because his second run was the best one of the day. He let the engine cool to 150 degrees before making his next run. When the green light flicked on, David brought the revs up to 2500 rpm again, but bogged out of the hole. His 60-foot time was 2.10, and he shifted the same as he did during the first pass; the result was a 13.68 at 101.80. After the second pass, David dropped the rear tire pressure even lower to 20 psi and let the engine cool to 140 degrees. This time, he launched at 3000 rpm, but that was way too high for the stock tires. With a 2.18 60-foot time, the result was a 13.87 at 102.70.
1996 LT4
Owner: Gerard DeSantis
Engine : 5.7-liter MFI V8 LT4
Horsepower: 330@5800 rpm
Torque: 340 lbs.-ft.@4500 rpm
Block: Cast iron, 4-bolt mains
Crankshaft: Cast iron, undercut and rolled journal fillets
Pistons: Cast aluminum
Compression: 10.8:1
Cylinder heads: Aluminum, with 185cc intake port volume
Combustion chambers: 54.4cc
Valves: 1.94 intake/1.50 exhaust
Valve spring pressure: 100 lbs.
Rocker arms:1.5:1 adjustable, self-aligning, stamped steel
Camshaft: Roller
Lift: .447 intake/.459 exhaust
Duration: 202° intake/208° exhaust@.050-inch
Intake manifold: Aluminum, matched runners
Torsional Damper: Dual mass
The LT4 puts out 330 hp at 5800 rpm and 340 lbs.-ft. of torque at 4500 rpm.
All Gerard DeSantis wanted to do was get his stock black Coupe into the 12s. That was his goal of the day. With the LT4's water temperature registering 185 degrees and with 24 psi in the rear tires, Gerard left the line at 4200 rpm and powershifted at 6500 rpm. Going through the traps in third gear, he ran a 13.00 ET at 107.13 mph, with a respectable 60-foot time of 1.96 seconds. Gerard didn't do any tuning to his Coupe between passes. He liked the way it hooked up and decided to leave everything status quo. On his second run, and with the engine temperature reading a higher 195 degrees, Gerard launched at 4500 rpm. The tires grabbed well, yielding a 1.93 60-foot time, resulting in a 12.95 at 104.89. Having achieved his 12-second goal, Gerard tried to lower his ET still further. After letting the engine cool down to 185, Gerard went out and backed up the run with a 12.94 at 104.89-his best effort of the day. The Vette hooked with a solid bite and he snagged a 60-foot time of 1.92.
After Gerard returned from the 12.94 pass, he was very excited and wanted to make a back-to-back run. The temperature gauge was showing 200 degrees, but he didn't want to wait for it to drop. He launched at 4500 rpm and experienced clutch slippage, which yielded a 1.99 60-foot time with a 13.10 at 105.90.
Conclusion
No doubt a set of drag radials on each car would have been a great help. As a result, the LT1 would probably see an ET reduction down into the low 13s, while the LT4 would hit mid-12s. But even in their stock form, the changes provided by the LT4 have shown positive results.
Being .7 seconds quicker down the quarter-mile proves that extra horsepower can always be found in the cylinder heads. The GM Powertain engineers knew this, which is why the majority of the modifications that were designed into the LT4 centered on the heads themselves. These were design changes that worked, and worked well.
Nevertheless, a bone-stock LT1-powered Corvette that can run mid-13s, all the while providing excellent handling and cornering performance, is an achievement that most other cars will never lay claim to.
Special thanks to Vette Magazine for the article!
MFG. Mike
`02 Trans Am WS6 Collectors Edition 6-Speed
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