nsu

I'm reading this book and there's a chart that says V8 is slightly smoother than a rotary. Any thoughts?

8 Maniac

iTrader: (1)

Depending on each engine, that could be true.

edit: what's the definition of smooth they're talking about?

shizzlesled

prob. engine vibration to rpm ratio

nsu

They just said "smoother". I assume they mean free from vibrations, friction and coarseness.

dos

Is apple pie better than cherry pie? I say apple pie is, my girl says cherry.

8 Maniac

iTrader: (1)

that post = fail. Whether they mean a smoother power deliver or overall feel of the engine vibrations in the car, it's something that can basically be measured or rated, regardless of opinion. If it was really close, you could sum it up to inaccuracies, but it's not really an opinion thing.

b'Eight'

It depends upon the crank and firing order. Ferraris are not balanced the same way as an American V-8.

m477

Rev that V-8 up to 9500rpm and then ask...

I guess it depends how they measure it, but I doubt in real life driving any V-8 would "feel" as smooth -- the only piston motor that is inherently balanced is the I-6 (and V-12). The V-8 configuration is not inherently balanced.

RyansRx8

I have that book too i recommend it to everybody it tells you everything you need to know...I got very familiar with maintaining and increasing hp safely from that book...the knowledge anyway i still need the funds to actually do it

dillsrotary

Drive the new m3. Redline is pretty high.

Huey52

iTrader: (2)

In theory an opposed action cannot be smoother than a rotary action.

Vlaze

iTrader: (14)

Not sure why one would care if a V8 was smooth or not. With a V8 you want the roar of the motor and sheer power of the beast unlocked. That includes the trembling and vibrations from the excess power. For a rotary smooth is nice, but for a V8 no danke.

8 Maniac

iTrader: (1)

I'd wager that there are V8's that are smoother than currently available rotary engines. In theory, I would think that a rotary has more potential smoothness.

rodjonathan

iTrader: (6)

i dont think v8s were meant to be "smooth" so to speak but thtas may just be me

dynamho

A well-balanced V8 is slightly smoother than a 2 rotor based on torque fluctuation.

This graph is from a Mazda technical document.

bse50

V8s are meant to have a smoother power delivery than the other engines, same applies for the v12. Then you have firing orders, cranks and whatever to throw in that could mix the cards. I think that they're talking about stock or race tuned engines, not the usual dyno queen\drag strip 200000hp rustang.
American engines in the v8 family were not "well designed" from a sports car point of view but if you take into consideration many of the european engines (smaller displacements, higher power outputs NA vs NA) then you can see that even with a high redline they have a nice smooth power band and torque curve. You could make a comparison out of a Ferrari F430 and a Rx8 to see what the torque delivery is like. The v8 in the new m3 is another good example.
We're talking in percentages though, those engines are way too different!

8 Maniac

iTrader: (1)

See that's the issue I'm having... what do they mean by smooth? I'm guessing the V8's in F1 cars could be considered smooth in a sense

b'Eight'

V-8 Ferrari's are poorly balanced due to firing order and crank shaft. American V-8's are better balanced because of the reverse BUT don't rev near as high as a result.

I-6's (slant 6 included) are a very well balanced engine but they often pose packaging concerns for front wheel drive cars. Plus its hard to have a common plenum with equal length runners.

Four cylinders sucks with balancing and that is why the bigger a four cylinder is, the more vibration is inherent in the motor. Some four cylinders have counter rotating internal balance shafts to counteract vibration.

V6's are ok balanced in the 60 degree V configuration vs the 90 degree. Volkswagen has a 15 degree compromise V angle. It functions much like a straight 6 but with better under hood packaging with the slight V.

12 cylinders is the way to go with smoothness but then you have greater internal friction to overcome, particularly when cranking the motor.

V10's lots of power---Poorly balanced.

Everything is a trade off. Rotaries have smooth power delivery because of the law of physics: You don't have pistons that constantly accelerates and then deaccelerate. But, rotaries have a poor elongated shaped combustion chamber.

The advantage to the rotary really is its HP power to weight ratio. Smoothness may be a plus but a light engine allows for a light car that corners on rails. It's definitely not fuel efficient though because of the no torque because there are no con rods acting on the crank to multiply torque. And you still have the poor combustion chamber.

I'm sure somebody will come by and say I'm an idiot spouting half truths but okay, whatever, just add to the dialogue and save the insults.

bse50

Yep, but if you take the average torque into consideration from idle to redline then you have a smooth power delivery
The firing order and crank shaft designs are a pretty needed trade-off when you consider that those cars should be driven in their appropriate rpm range, when you start driving them then they're smooth... another thing is if you buy one to attend your cool happy hours and dinners

It would be cool to make the same comparison between a 20b and a v8 or a v12 to see how the added rotor behaves.

b'Eight'

No disagreements. Everything is a trade off. I wish the rotary had more torque but it doesn't and never will.

bse50

After the edit your comment is much more complete and i like it even if it is a bit generic. The thread is generic too so that's fine.
Rotaries with higher torque numbers are possible but changing the e-shaft's eccentricity and housing depths means huge investiments. There's another problem that's displacement related too.
Will a new generation rotary be as smooth as a v8? we shall wait and see.

Anyway i think that part of the problem is also created by the weight of the rotors. Spinning them around at low speed requires a bit of effort while a piston has got what you call "torque multipliers", the rods. Dunno if you understand what i mean though.

At least that paper didn't take boxer engines into consideration!

m477

I know.... it's not 9500rpm.

8 Maniac

iTrader: (1)

Neither is the RX-8. The redline on the 8 is 9k. The rev limiter on the other hand, is slightly higher. I'm not sure if it quite goes to 9500 stock though. The tach lags a bit so it's typically a couple hundred rpms lower than what it looks like. Either way, it doesn't matter since you have less power in the 9k range. You can rev to 9k+ (and it's still fun), but not very useful as far as power goes. The 8 is best shifted at 8700 rpm. The m3 has peak power at its redline (8300 rpm) and a rather flat torque curve up until that point as well. My point is, the redline means nothing. I realize the point of this discussion is smoothness, but the thing about the V8 redline is that it's most likely making significantly higher power at that redline. How many renesis engines are putting down more power than the M3? What rpm do they stop making power at?

Razz1

Well... if you drive luxuray cars all the time....

I would say the rotary is smoother in normal operting driving conditions.

reddozen

iTrader: (1)

http://www.mazda.com/mazdaspirit/rotary/story/p6.html