Author: Robert Hyatt
Date: 13:22:29 08/28/03
Go up one level in this thread
On August 28, 2003 at 14:52:20, Tom Kerrigan wrote: >On August 28, 2003 at 00:31:23, Robert Hyatt wrote: > >>>Well, as the saying goes, you can either have high displacement or high RPMs but >>>not both... unless you're Ferrari. How many I4s redline at 8000-9000 these days, >>>and how many V8s under $100,000 redline in that range? >> >>Not many stock motors that appear in _any_ vehicle redline up there. IE, >>for comparison, my son's SHO taurus redlines at about 7K. The current 4.6 >>ford V8 redlines in the same area. As did the SVO I4 with turbo. >> >>A bit more exotic is the Honda S2000 at near 9K, but it has no torque, >>after driving one. > >Honda and Toyota both make engines like that. S2000, RSX, Celica, probably some >others. If the S2000 puts down as much torque at 9000 as some other car at 7000, >does it matter if it lugs at 1000? Yep. When starting from 0.0 mph, one has to go through 1000 RPM to get to 7000. :) That's why they put big V8's in trucks that need to pull at low RPM. Or big diesels. Torque gets you going. > >>>>We are talking pure motor issues here. Horsepower is important. But so is >>>>torque, which is a different way of measuring horsepower. 8 cylinders will >>>>produce more torque, even at the same displacement. A V10 is better still. >>> >>>That may generally be the case, but torque technically depends on the shape of >>>the cylinders. I'm sure it's easy to make a V6 that has more torque than a V8 >>>with the same displacement. >> >>No idea what you are talking about when you say "shape of the cylinders." The >>_only_ shape being used is a perfect circle of varying radius, with a taller >>or shorter swept area controlled by the stroke of the crank. > >Isn't it obvious that I was talking about bore/stroke? Knowing about rotary engines, _nothing_ is obvious to me. :) > >>However, it is definitely impossible to make more torque with fewer cylinders, >>if all else is the same. Just compare the difference between a power pulse >>every 1/4 revolution vs every 1/3 revolution. That's why v12's exist. More >>torque. Smoother since power pulses are closer together in terms of 1/6 of >>a revolution between power pulses vs 1/4 or 1/3... > >I don't have the time to go looking for any specific datapoints, but if your >stroke is much longer you're going to get more torque per cylinder, right? So >why do you think it's impossible for a V6 with a really long stroke to make more >torque than a V8 with a really short stroke? Surely there's some crossover >point. Simple. We were talking about "equivalent". IE yes, a 6 liter v6 should make more torque than a 3.4 liter V8. But nobody does that. IE if you use a v6 bore/stroke of 4.0/3.0, I'm going to use the same in my V8, and have 33% more displacement. And 33% more power strokes. And more torque. Or I'll buy a 500 CI V-10 from Chrysler and _really_ make a ton of torque. And horsepower too of course. > >>>>The problem with the turbo is lag. The V8 supplies torque _right now_. The >>>>I4 is going to die at low RPM, and waits until the turbo spools up to get >>>>moving. A good example is the old SVO mustang with an I4. Yes, it runs like >>> >>>Which is fine, because if you're drag racing you can spool up the turbo before >>>launching, so turbo lag is not an issue unless your car is so poorly tuned that >>>you experience lag after you upshift at redline. Turbo lag is only significant >>>for rolling starts and actual race courses. >> >>The turbo won't spool up. It depends on exhaust volume. A free-wheeling >>engine won't produce the exhaust gas volume that an engine under load >>produces, you simply don't pump as much air through the engine, nor as much >>gas, which translates into less exhaust volume. > >No exhaust (not "less exhaust") means the turbo doesn't spool. I know from >experience that you get at least some boost from an Eclipse when you launch at >5000 RPM. And plus, what car takes any significant amount of time to get to ~10 >MPH, where a typical turbo will be hitting max boost in 1st gear? (Which is the >only time you should even possibly be seeing turbo lag in a drag race.) Yes, but do the math. If at the end of 1 second, I am doing 25 mph, and you are doing 15, you are _never_ going to catch me. I will be pulling away for the rest of the race, even if our acceleration is identical after 1 second passes. That's why drag racers worry so much about their "60-foot times". The first 60 feet takes about 2.1 seconds in a factory car. Slicks and good driving can take that to 1.5. During that 1.5 seconds most races are won or lost. > >>>That's great, but the gearing could have been different, the turbo could have >>>been tuned poorly for the 1/4, and the friend might not have been launching >>>right. Actually, unless he had a race clutch, I doubt he was launching right. >> >>This was a regular drag racer, using a comp clutch and pressure plate, using >>drag radial tires. He knew what he was doing. I drove the car myself and we >>were within .1 of each other... > >Fine, there are still other variables. It's easy to find circumstantial evidence >to prove my point: check out the Evo 8. 2L turbo I4 making 271 HP and it's >pretty much exactly as fast as a Corvette with a 5.7L V8 making more (345?) HP. > >-Tom I don't believe it for a minute. IE the Ford cobra (2003) makes almost 400 horsepower blown. It can't take a C5 corvet (non-Z06). See this month's Motor Trend magazine. 271 horses won't touch a corvette. The Cobra is .1 second slower in the 1/4 mile.
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