jimlab

Untrue, unless they have different cams and breathing capacity. However, this is a common misconception, so don't feel bad.

The only difference between two 5.0L engines of the same type, one achieved with stroke, the other with bore, is piston speed, all other factors being equal. Unless they have different cylinder heads or cams, there is no difference in their power potential or in the shape of their power curves, or in their rpm potential, assuming quality components are used. Both could easily turn 7,000+ rpm, if need be.

Torque does not come from stroke. Torque comes from displacement, and the potential to burn more fuel and air in a given period of time. The reason the big blocks and stroker engines make so much low end torque is not because they have huge strokes. It's because they can consume a hell of a lot more air at low rpm than a smaller displacement engine can.

The real advantage of an "all-bore" engine over a stroked version is that the increase in bore size unshrouds the valves further and potentially allows the use of larger valves, increasing high rpm breathing capacity. In addition, reducing piston speed results in less stress on parts and therefore potentially greater durability and longevity.

teamstealth

Then whats with all the rave about piston speed? I remember Car and Driver were RAVING about the piston speed of the E46 M3's engine when the car debuted, and the # they produced was only 7 ft per second slower than the F1's engine....what gives?

DamonB

An engine that can survive with very high piston speeds implies it uses advanced materials and design.

Piston speed is literally how fast the piston is traveling up and down inside the bore and it is NOT solely dependent on rpm. Take two engines with equal displacement; one with a big bore and shorter stroke and one with a smaller bore and longer stroke. For each crank revolution the piston will travel from the bottom to the top of the bore and back. The motor with the longer stroke has further for the piston to travel and since the piston completes one round trip per revolution the piston with the longer stroke must travel farther in the same amount of time. That means this piston must travel at a higher speed even though the engine RPM and displacement of the two engines is identical.

For a given RPM the shorter you make the stroke the slower the piston speed is. This is why F1 engines have such short strokes; they have to keep piston speeds in check.

jimlab

It gives them something that sounds impressive to brag about, although if they did say that, I think they may want to recheck their numbers.

piston speed (fpm) = stroke (inches) x rpm / 6

Maximum piston speed for an E46 M3 would be:
3.43" x 8,000 rpm / 6 = 4,573 fpm

Maximum piston speed in my stroked 396 LT1 would be:
3.875" x 7,500 rpm / 6 = 4,844 fpm

Maximum piston speed in the 2003 BMW P83 F1 engine is quoted as 40 meters/second. 40 meters is approximately 131.234 feet. Multiply by 60 and you get about 7,874 fpm.

Excellent explanation.

The distance the piston travels from top dead center (TDC) to bottom dead center (BDC) is equal to the stroke of the crankshaft, FWIW.

clayne

Thought the Honda B18C5 (Type R Integra) was still king of piston speeds in production car engines?

jimlab

That's simple to figure out. The stroke was 87.2mm, or ~3.43", and they redlined at 8,400 rpm.

3.43" x 8,400 rpm / 6 = 4,802 fpm

However, the pre-2004 Honda S2000s had an 84mm (~3.31") stroke, but redlined at 9,000 rpm.

3.31" x 9,000 rpm / 6 = 4,961 fpm

In 2004, though, they increased the stroke by 6.7mm to 90.7mm (3.57") and lowered the redline to 8,000 rpm, resulting in a piston speed of "only" 4,761 fpm... making the same 240 peak horsepower 500 rpm earlier.

Piston speed is about as meaningful as horsepower per liter as a bragging point. Neither one means anything by itself, and neither one necessarily wins races.

Ohnigiri

You can have all of the power you want under the hood, but if you can't get it to the pavement in an effective means it is useless. More horsepower WILL generate more torque. Most drivetrains were not designed to handle the amount of torque applied after a ton of mods. Any one ever blow up a diff? Balance is key. It really depends on the driving you will be doing doesn't it. I run narrow mountain roads. Don't need much torque. If I were only going 1/4 mile at a time I would probably beef up my car to handle more torque.
(What was the subject anyway...... )

Ayreel

Very good stuff, but I think this is not the end of it. Its not just the HP you need to care about. And its not just the torque you need to care about. It is the combination of both, and to what degree depends on what RPM and speed you are running or what kind of driving you do.

Another excellent technical discussion that I found to be more correctly well rounded and complete can be found here:

http://member.rivernet.com.au/btaylo...ginePower.html

What I take from all of this, and some personal experience is that for racing conditions, HP is more useful. For typical daily driving/street conditions, torque is more useful.

I drive a 2WD Toyota pickup with a V-6 that is rated at 150+ HP, but not particularly great torque. (my rx-7 is in the driveway awaiting an engine.) My wife drives a VW Jetta with the TDI (Turbo Diesel). with a rated horsepower of 90 HP, but an excellent relative torque figure (its a diesel!). Which is more fun to drive? Hands down the diesel Jetta. Sure, the weights are a little different, but not enough to account for the difference in HP.

My point being that for typical driving on the street, not on a race course with high revs, but with stop signs, stop lights, traffic, etc...torque is more 'fun.' Getting on the highway is a different story, and becomes more like race conditions where the revs can come up consistently. A friend of mine drives a Subaru WRX. Starting from a stoplight, getting onto an open highway, I surge ahead and drop him for the first hundred yards, until he finally winds up and blazes by.

It all depends on the use and your desired results, hard speed numbers or seat of the pants fun.

My plans for my RX-7 are to be both a daily driver and a weekend canyon chucker. I don't believe you can say that torque or HP is the only factor I need to look at when making performance decisions. I wish it were that simple, but few things in life are.

Again, read the link I mentioned earlier. It fills in some of the gaps I felt existed after reading some of the tech stuff here.

Just drive!

Therotaryrocket85

Ive had a corvette with close to 350 horsepower and a gslse rx7 with about 200....... I dont care what the nonsense arguments are about but the point of the matter is (since im on both sides of the spectrum) the more reliable motor is the rx7 motor because of lack of moving parts. Honda has built a reputation for reliable hi reving motors that rival american v8s. And someone is saying v8s are better? I love corvettes as i love rx7s but when i compare the cars the motor in an rx7 is far superior to a corvette just because its more precise on driving ability and you can put it through alot more than a torque monster. Both drag racing and autocrossing which ive done with both prooved that the rx7 is a better car for the job. Drag racing is more fun with torque though( its an american thing In the end both rotaries and f1 motors are fast, as are smallblock chevys and nascar motors. I love it all and its all great, but in the end we are just comparing apples and oranges. This is why we have to just understand the variables, like suspension, gearing, weight, & atmosphere. Theres a whole spectrum to create different applications of all this. Why are we argueing, at least were not all driving around volkswagon busses

jimlab

Gee, thanks for clearing that up.

Mazinger

Thanks a lot rxrotary2_7, for the valuable information, its not that you pass on this stuff everyday on the net.

DamonB

Finally found a great article.

Torque and Power

In a nutshell: Power is what moves the car; not torque.

Kento

Phew...take about your long-winded exercises in vocabulary...

While I don't disagree with any of his bullet points, I'd have to disagree with his assertion that an inertial dyno is a "more realistic test of an engine's ability to accelerate a vehicle..." Modern eddy current dynos are able to run through their load programs at lightning speed; his description of all brake dynos gives the impression that they force the engine to sit at steady state rpm/loads for noticeable periods of time-- they don't. The only calibration necessary is the initial loading to begin the run that last for several seconds; after that, the run progresses just like any inertial dyno. You can hold the engine under load at a specific rpm for any amount of time if you so desire, which allows you to make fueling adjustments impossible to perform on an inertial dyno. You can also program the actual run and final readings to replicate an inertial dyno if you so desire.