Machine Design, Editorial Comment
May 4, 2000
When I first learned to drive, our family had a variety of cars and pickup trucks, all with in-line six-cylinder engines. My ear became accustomed to their exhaust sound as being normal for anything automotive.
Then one day a friend gave me a lift in his Ford V8, and I suddenly realized that V8s were a lot different from sixes. Where a six labored its way through the gears and settled into an annoying drone above 50 mph, a V8 eagerly climbed through high revs in each gear and settled into a pleasant purr on the highway. Drivers who put Hollywood mufflers on their V8s got a tone similar to the deep burbling noise made by an inboard marine engine exhausting through water.
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I've always wondered why the tone of a V8 is so distinctive. I thought it had something to do with exhaust pulses alternating in either tone or phase, fooling our auditory sense into thinking it is hearing only half the exhaust pulses an engine is actually producing.
But what causes this effect? Is it connecting rods attached to common crankshaft journals sweeping around cylinder banks that are 90° apart, leaving 270° before the pair of pistons again sets up a firing sequence? Or is it an acoustical phenomenon from exhaust being fed from two cylinder banks?
When I did my own tune-ups with breaker-point ignitions, I noticed that spark towers on distributor caps of V8s are evenly spaced, just like they are with sixes, so firing interval can't be responsible. Moreover, V8s, especially those with Hollywood mufflers, sound much the same whether they feed crossover pipes or separate dual exhausts. When V6s became popular, I noticed they sounded like straight sixes, not V8s. So, overall, exhaust coming from two cylinder banks apparently didn't have much to do with engine tone. All of this really puzzled me, and I never was able to figure out what causes the distinctive acoustical signature of V8s.
Now fast forward the tape to late March of this year, and associate editor Amy Higgins has just handed Lee Teschler her article on the Indy 500. Because of new 180° crankshafts being used at Indy this year, the article has a segment on the acoustical signature resulting from the cranks. Lee thought Amy's explanation of this might not be technically accurate, and he referred the matter to me. I agreed that this part of her article might have some technical flaws.
Poor Amy. Lee and I asked her to delve further into the matter and verify what she had written. That took a lot of time and caused her a lot of additional work, but guess what? She had been correct from the start. The problem was that Lee and I didn't know as much about engines as we thought we did.
So now you're thinking Amy's article finally answered my questions about the acoustics of V8s. Well, you're wrong. The article answers some of them -- up to a point. But additional technical explanations from Oldsmobile, which you won't see in the article because it isn't germane, left me as confused as ever.
Here's the problem. Conventional automotive V8s with so-called 90° crankshafts have irregular intervals between firing, which explains the split tone. But engineers at Oldsmobile go on to say that the cylinders also fire at 90° intervals regardless of whether you have a 90 or 180° crankshaft. So you get irregular exhaust intervals from a regular firing order. If you can figure that out, you're smarter than I am. What makes me feel better, however, is that an Oldsmobile spokesman says all of this is almost impossible to visualize. I believe it, and still wonder exactly what it is about automotive V8s that make them sound so good.
--Ronald Khol, Editor