In the ‘80s, GM used a 4.3L 90-degree V6 in many different cars. I know that this engine is basically a small-block Chevy with two cylinders removed. This got me to wondering why they haven’t built a 90-degree V6 using the 5.3L or 6.0L LS engines?
Jeff Smith: This is a great “what if” question. I remember a quote from Competition Eliminator racer Bill Maropulous who made a name for himself building 90-degree V6 Chevy engines in the ‘80s. When asked “What’s it take to build a Chevy V6 engine?”
His response was “A band saw and a heliarc.”
He was cutting up small-block Chevy cylinder heads to adapt them to the 90 degree Chevy V6–an enormous and complex task. The answer to your question is more about engineering. The first thing we have to talk about is 90-degree banked engines. This is the angle between the two banks of cylinders. With a four-stroke cycle of 720 degrees of rotation, a V8 will fire every 90 degrees. With a 90-degree bank angle, engine designers can create a common crankshaft pin sharing two rods per crank pin. When we reduce the number of cylinders to six, we now have 120 degrees of time between firing pulses. The smoothest running engine is a 60-degree bank angle engine, which allows a common crank pin. A smooth firing order of 120 degrees per cylinder won’t work with a 90-degree banked angle, necessitating a split pin crankshaft where the crank has individual journals for each rod that are paired, yet offset by 15 degrees in opposite directions to create the proper firing differential, creating firing impulses of 105 and 135. This unfortunately creates a vibration front to rear. Passenger car engines spend much of their time idling, which is when the amount of time between cylinder firings is the greatest, which makes this difference the most pronounced. Chevy engineers tweaked this by building their original 90-degree V6 engine with pin angles of 108 and 132. As you can imagine, the engine still vibrated. Since production engines spend a majority of their time at low engine speeds, it was my impression that these 90-degree band angle V6 designs had become outmoded unless the OE’s were willing to spend money on balance shafts that are both costly and don’t contribute to making the engine any more efficient.
Current engine technology demands that the engine idle so smooth that it is difficult to tell if the engine is running at all. This is the main reason why current LS engines use such wide lobe separation angles of 116 to 120 degrees for the camshafts. My guess would have been that GM would rather go with a smoother engine design, but when I did my research just to make sure, I made an interesting discovery:
Beginning in 2014, GM debuted a 4.3L 90-degree V6 for the Silverado trucks based on the LS engine architecture!
The engine is a direct injection engine running 11:1 compression and rated at 285 horsepower and 305 ft.-lbs. of torque. Good low-speed torque is probably the biggest reason they have retained the pushrod design. That and they can build these engines on similar production lines as the V8 engines. Note that the new GM V6 is much larger at 4.3L (262ci) than the 2014 Ford 3.7L (230ci) EcoTec normally aspirated 4-valve V6. The GM 4.3L uses the LS’s small-block Chevy-based 4.40-inch bore spacing. This allows them to build an engine with a big 3.92-inch bore. This leaves room for relatively big valves and, combined with the greater displacement, means it can make more towing torque at a lower engine speed. The GM 4.3L is rated at 305 ft.-lbs. of torque at 3,900 while the smaller Ford engine makes 278 ft.-lbs. at 4,000 rpm. While 27 ft.-lbs. might not sound like much, that’s 10 percent more torque. I then divided torque by cubic inches to compare the engines and the Ford is actually more efficient at 1.21 ft.-lbs./ci compared to the GM’s 4.3L effort of 1.16 ft.-lbs./ci. Because the GM engine is direct injected, its claimed fuel mileage is 1 mpg better than the Ford. This is probably due to the EcoTec3’s half-point higher compression ratio and improved direct injection combustion efficiency. In the real world, the larger GM engine will probably not beat the Ford in mileage only because the larger engine will probably still use more fuel in the daily grind.
So there are some technical tidbits inside the new GM 90-degree V6. This will probably only appear in trucks and SUV’s.