What does a peaky engine mean? What makes an engine more drivable?? What does
mid range power mean?? Isn't power a function of how hard the throttle is pressed?
More throttle = more power?? What does it mean when you compromise high end power
to get more mid range power in terms of acceleration and performance? What kind of modifications to engine are made to get low end or high power (e.g. cam profile, timing,
etc.)??

jackbbot51 - 07 July 2008 11:53 AM

What does a peaky engine mean? What makes an engine more drivable?? What does
mid range power mean?? Isn't power a function of how hard the throttle is pressed?
More throttle = more power?? What does it mean when you compromise high end power
to get more mid range power in terms of acceleration and performance? What kind of modifications to engine are made to get low end or high power (e.g. cam profile, timing,
etc.)??

There are two basic components to engine power. One is torque -- the amount of twisting force the pistons and connecting rods exert on the crank shaft. This is the force that gets the car moving.

The other is horsepower -- torque applied over time.

The equation for calculating horsepower from torque and rpm is:

HP = Torque x rpm
___________5252

where HP is horsepower, RPM is the specific RPM you're looking at, and 5252 is a constant that comes from the derivation of the equation.

Ideally, an engine's torque curve would be flat -- You get the same amount of torque whether the engine is idling or running at full throttle. In fact, this doesn't happen. There are many factors, from the engine's bore and stroke, breathing characteristics and cam timing to intake and exhaust tuning and possibly phases of the moon all affect torque at various rpm.

The result is a torque curve (torque plotted at several rpm levels) that looks a bit more like an inverted parabola. Because horsepower is directly related to engine rpm and torque, the horsepower curve reflects the torque.

A "broad power band" would come from a flat torque curve. You get useful torque and a steady increase of power over a wide rpm range. A car with such an engine feels like you can put your foot down at any rpm and get power. It would be easy to drive at all speeds.

A "peaky" engine" is the opposite -- an engine's torque curve is much better at one narrow rpm range than at others. More to the point, this happens at the high end of the rpm range. This kind of car would feel "gutless" in the low to mid rpm range. Then it comes on with a bang when you get into the fat part of the torque curve. You would be shifting gears frequently to stay in the power band and have a tendency to bog down or stall the engine if you're not careful with the clutch.

Next question: What makes an engine "torquey" or "peaky?"

Torque comes from a long stroke and relatively small cylinder bores; this is called an "under-square" engine. As you know, the longer a lever arm you have, the more torque you can generate. The drawback of a long stroke/small bore engine is that it can't rev very fast. Imagine trying to turn a three foot long crank, for example. The valve timing tends to be short, with little overlap between intake and exhaust, and there isn't all that much attention paid to intake and exhaust breathing.

Peak horsepower comes from the opposite characteristics: The cylinder bore diameters are larger than the stroke is long -- a condition called "over-square." In Formula One engines, they are very oversquare, with the bore 40% or more larger than stroke length. The cam shafts have high lift to allow a lot of air to flow into the engine. They also have considerable overlap, when both intake and exhaust valves are open, to allow cross-flow -- the exhaust gas leaving the cylinder creates a vacuume that draws the intake into the cylinder.

Further, the intake and exhaust systems are carefully tuned to make the best use of the intake and exhaust pulses. This link http://www.speedtv.com/forums/viewthread/272122/ discusses the effect of sonics on engine breathing.

The over-all effect is an engine that produces prodigious power, thanks to its ability to turn at high rpm, but does not have all that much torque. That's good for running at high speed, but not so hot for accelleration.

Compare the Formula One 2.4 litre V-8 with a six litre NASCAR engine. Both produce about the same amount of power. However, the F1 engine does it by turning 19,000 rpm and can't get out of its own way below about 15,000 or so. The driver has to shift frequently to stay in that narrow power/torque range. This is what you'd call a "peaky" or "high end" engine because it needs the high rpm.

The NASCAR engine is more than twice the displacement and turns at half the maximum rpm. It also has enough torque to turn the world backwards. It's more of a "mid-range" engine because the torque curve if flatter and broader. You can put your foot down at any place in the rpm range and get a solid jolt of torque. Because the engine has only two valves per cylinder (albeit, very large ones) its torqe curve falls off at high rpm. You can run them over 10,000 rpm, but you won't get all that much power. But between, say, 5,000 and 9,000, it's an absolute bear.

A diesel engine is a "low-end" "high torque" powerplant. A truck engine may run to 2500 rpm and even the Audi R-10s peak at 5,500 to 6,000 rpm and about 600 bhp. But they also generate an immense 800 lb-fte of torque. The trick here is not keeping the rpm up, but not lighting the tyres every time you push the accellerator!

-how come the driver must change gears often to stay in power range of a peaky engine??
- what happens to the engine rpm in relation to the transmission rpm on upshifts and downshifts??
- so does the driver shift up to the next gear to reduce the engine rpm back under the upper limit??

jackbbot51 - 12 July 2008 08:51 PM

-how come the driver must change gears often to stay in power range of a peaky engine??
- what happens to the engine rpm in relation to the transmission rpm on upshifts and downshifts??
- so does the driver shift up to the next gear to reduce the engine rpm back under the upper limit??

1) Because a 'peaky' engine has a very narrow rpm range in which it makes near peak power, and a much more broad range of speed throughout a lap.

2) The transmission has two simultaneous rpm ranges: input and output. Input rpm is always the same as the engine rpm; output rpm is determined by the gear ratio selected.

3) Yes, while accelerating; likewise, he shifts to the next lower gear on deceleration to keep the engine rpm above a lower rpm/power limit.