RPM vs # of Cylinders

[kc5f]

I've been pleased that my Versa w/CVT runs at 60mph at 2000rpm and 75mph at 2500rpm. Seemed reasonable compared to what I remembered from my 5-speed Probe in 5th gear, even if the mileage isn't as good.

Then yesterday I drove my wife's Odyssey minivan to work while she took the Versa in for its 7500 mile service. The Oddyssey is an automatic, but at 2000rpm it's going 70mph, and 2300rpm will get you 80mph. (I didn't try 2500rpm on a 70mph speed limit highway!)

I know the 3.5L 6-cylinder Odyssey engine has to be using more gas at 2000rpm than the 1.8L 4-cylinder Versa at 2000rpm, but is this the sole reason it can have a gear ratio that allows it to run slower but still have plenty of headroom for acceleration?

- Steve in WNC

[Ever Victorious]

It takes so much power and torque to move a vehicle from A to B mph. On a lighter, lower powered car, it's pretty easy to take off at low speeds but high speeds it just doesn't have the grunt to get it done. V6's and V8's have an easier time, because comparatively, they almost always have a better power-weight ratio than a 4-cylinder. Not comparing a WRX to a minivan, of course.

[motoguy128]

I wish they published the torque curves for engines on the window sticker. The torque curve is much more important that peak horsepower.

Simply put, a larger displacement engine, no matter how many cylinders will generally produce more torque at lower RPM's.

Here;s a few things ot consider in engine design and performance:

The dimension of the cylinders is important... almost as much as the total displacement. A longer stroke will typically give the engine better performance and lower RPM's because it has more leverage on the crankshaft and provides higher air velocities at lower RPM's because of smaller valves. However at higher RPM's, the piston must move faster compared to a shorter storke, and the smaller valves become a liability and limit high RPM performance. Motorcycle engines for example are designed for maximum power form a small displacement, but lower RPM performance isn;t improtant because the vehicle is so light. They can have up to 50% larger piston diamater, compared to the stroke. This is known as being "over square". In automobiles, you typically see a larger stroke than piston diameter, or on smaller engines up to maybe 10% over square. (the Versa is slightly over square).

To complicate this, other factors effect engine performance. These include compression ratio (a 10:1 compression engine, if the engine has a 100mm stroke, when fully compressed at top dead center, there will be only 10mm of clearance to the valves), the length of the intake runners (longer = better low end performance), the size of the valves (smaller gives better low end performance), and the valve timing. Remember air is a fluid and has inertia, at higher speeds, the air has momentum, so you need to leave the valves open longer to allow air to keep filling the cylinder even after the pistion begins to move upward. Finally, the shape of the intake and exhaust as well as the amount of resistance in the intake and exhaust will affect performance. Further, the angle of the cylinders in a V configuration motor also affect how the engine performs. More often a V6 is used because its more compact and gives the best overall performance characteristics for engines over about 2.7 liters. A Inline 6 has is better in may ways. A V8 generally gives the best perfomance over 4.0 liters. A V10 or V12 are used over 6.0 Liters. Exceptions are when special performance characteristics are needed. Diesels are optimized for low end performance and use inline 6's for displacements exceeding 8 or 12 liters. IN FOrmula 1 racing, V10's or V8's are used under 3.0 liters.

I know this is a lot of information, but the bottom line is that no one factor makes an engine better, or explains how it performs. Gear ratios are optimized by the engineers ot give the best overall compromise in economy and performance for the vehicles intended purpose.

[kc5f]

Simply put, a larger displacement engine, no matter how many cylinders will generally produce more torque at lower RPM's.

Thanks, both of you! The above explains a lot, and the rest helps, too. I knew folks here would be able to make sense of it for me!