FamilyGuy

I have a few questions about how engines 'rev'.

First, does horsepower or torque rating have anything to do with how fast the needle moves up the tachometer? If not, is there any other measure that tracks the rate of increase of RPMs? I think it's a really important aspect of a car's acceleration, and I'm not aware of any way to judge it on paper.

Let me give an example: I have a ~3200 pound Impala with a 3.4 liter pushrod V-6 engine, FWD, rated at 180 hp, 205 torque. My wife has a ~3150 pound Honda CRV with a 2.4 liter DOHC 4 cylinder engine, AWD, rated at 160 hp and torque. You'd think my car would be faster, but there's basically no difference in acceleration between them. If you look at edmunds.com, for instance, they're both rated for 8.7 seconds 0-60 mph.

As far as I can tell, this is why they're about the same speed despite the apparent power gap: when you slam on the gas in my car, it sounds like you woke up a bear. The engine starts grumbling, the car downshifts, and you can hear it slowly climb the revs. (Unfortunately, I'm only guessing since I don't have a tach on the dash.) With the CRV, putting the pedal down almost immediately puts you at 4000 RPM, and if you hold it down the needle moves towards the redline pretty quickly.

I'm guessing that this is one of the things for rotary engines that make the horsepower and torque ratings deceptive. The Renesis may only be rated for 162 torque, but I'll bet it goes from 1000-8500 RPMs a hell of a lot faster than my 205 torque 3400 series V-6 goes from 1000-5500 RPMs. Am I correct?

First question, part 2. Is this governed by the type of camshaft you have, or are they unrelated? I mean, the most obvious difference between the engines of the two vehicles in question is pushrod (Over head valvle) vs. Dual Over Head Cams.

First question, part 3. Does a rotary engine have a camshaft, per se? I mean, obviously it has intakes, exhaust, and valves. But does camshaft apply to the rotary layout?

Second question. This one is more vague than my first question. Does revving a car within, say, 1000 RPMs of the redline frequently have any detrimental affect on engine life? I like gunning the engine in my bucket of bolts but I sure as hell can't afford to have it fall apart before it's paid off (in 4 years). Anyone care to hazard a guess?

Thanks in advance.

Hercules

Buger?

ZoomZoomH

iTrader: (1)

I suspect the difference in GEARING of the transmissions have more to do with how 'fast' your rpm rises than actual power. torquey engines that don't rev very high, such as GM's pushrod V6 will usually have very tall gearing ratio to reduce gas consumption, at the cost of better off-line acceleration.

so, for your first question, yes, hp and torque rating are deceptive because it does not tell HOW THAT POWER IS SENT TO THE WHEELS. Proper gearing of the transmission to the engine it's mated to is very important in determining how well the power of the engine is utilized.

2nd question, no, 'cams' have no direct corrollation to how fast an engine revs, though one can argue that the OHC design is inherently superiour than OHV design for high rpm operation. (though both still trails the rotary engine in that category :D )

3rd question, NO, rotary engines do not have cams or valves as seen on a piston engine. The only moving parts of a rotary engine are the rotors and the eccentric shaft, and that's the beauty of it, how simple the concept of operation a rotary engine truly is.

Salo

I concur with ZoomZoomH. The ability of an engine to increase rpms depends on its ability to accelerate the load (car and drivetrain weight). This inturn depends on the output of the engine and gearing.

Buger

Hi Family guy,

Below are a couple of factors that make a big difference:

1. HP/weight ratio
2. AWD vs FWD
3. Gearing
4. Auto vs Manual?

First of all... the HP/weight ratio looks to be a little better for the Impala based on your numbers. If your CRV and Impala were both geared to shift at the same speeds, the Impala would have a definite advantage.

But... your CRV has AWD which may help traction off the line if it is used. This changes things quite a bit.

And... I suspect that the CRV is geared quite a bit shorter. Would I be correct in assuming that your auto Impala shifts from 1st to 2nd gear at a higher speed than your CRV does? Perhaps the Impala shifts to 2nd at approx 60 mph while the CRV shifts to 2nd at approx 42 mph? Shorter gearing will increase wheel torque.

You mentioned that "the car downshifts" so I assumed your Impala had an auto transmission. I'm not sure if your CRV is an auto or manual. Manuals usually have an extra gear that allows for shorter gearing for 1st gear.

The relatively low torque of the rotary is a little deceptive. HP is a more reliable indicator of acceleration performance than engine torque because a higher HP car can be geared to have more wheel torque. The rpms on an RX-8 will increase very quickly because it is geared to have a good amount of wheel torque for it's weight. If you were towing a 2000 lb boat with your RX-8 however, the revs will not increase very quickly because you will not have a good amount of wheel torque for the weight. The mistake that many people make is that they mainly look at hp and torque numbers without thinking about their relation to weight.

Different sources may also test acceleration differently. One source might just stomp on the gas of an auto car while another might use brake torque to get faster times. Try doing some informal tests yourself with your 2 vehicles and see what you come up with.

Check out rotaryengineillustrated.com for more specifics on rotaries. No cams, camshafts, valves, etc and revving a renesis to it's 9000 redline means that the rotors are only spinning at 3000 rpms. I would hope that you wouldn't have to worry about your Impala v6 falling apart if you take to redline a lot. I would trust revving my RX-8 beyond redline much more than I would trust revving a piston engine to redline though.

Brian

[Edited correct car name]

lefuton

So I sit here thinking about it, and in a way, some rotaries do have a sort of cam-like effect... take honda's vtec or toyota's vvt system which adjusts valve timing, isn't that sort of like a 6 port rotary that has different ports open/close at different rpms?

-nate

ZoomZoomH

iTrader: (1)

very nice breakdown, but he drives an IMPALA with the old school GM 3800 PUSHROD V6!!! :D :D :D

not that an Intrepid would perform drastically different from an Impala...

ZoomZoomH

iTrader: (1)

similar idea, yet completely different implementation.

both are designed to improve engine breathing at high rpm, one adjusts the valve opening/closing timing via a cam profile change, while the other just opens up additional intake ports to allow extra air to enter.

heh, even just typing that up makes it sound like the rotary engine is a much simpler design to achieve the same effect

Buger

But that's what I said! That's what I meant at least. :D I edited my post to say Impala. :D

Buger

Btw, the 159 ft-lb RX-8 is a relative torque monster compared to the 205 ft-lb Impala. :D Sure the Impala has a higher peak engine torque but looking at the wheel torque in first gear (or any gear really) tells a different story.

RX-8
engine torque ..... : 159 ft-lbs
1st gear .......... : 3.76:1
final drive ....... : 4.444:1
total gearing ..... : 3.76 * 4.444 = 16.71 (in first gear)
approx wheel torque : 2106 ft-lbs (in first gear)
approx weight ..... : 3000 lbs ?

2003 auto Impala
engine torque ..... : 205 ft-lbs
1st gear .......... : 2.92:1
final drive ....... : 2.86:1
total gearing ..... : 2.92 * 2.86 = 8.35 (in first gear)
approx wheel torque : 1182 ft-lbs (in first gear)
approx weight ..... : 3300 lbs ?

You can see that the RX-8 is geared about twice as much as the Impala in first gear (16.71 vs 8.35). The end result is that the RX-8 has almost 1000 ft-lbs more wheel torque than the Impala. If that wasn't enough, it is also quite a bit lighter. :D

Brian

zoom44

i love it when buger talks like that:D :D mmmmm rotary + gearing goodness.....

FamilyGuy

:D
Thanks for the explanations, ZoomZoomH and Buger. Helps explain a lot. I would have responded sooner, but I accidentally turned off email notification.

Oh, and I've got the 3400 series V6. My grandfather used to work for GM, so I got the car at dealer invoice price. The catch was, I had to buy something the dealer had in stock. He didn't have any Impala LS's in stock, so I was out of luck. Oh well, I've been consoling myself with cheaper insurance and better gas mileage.

Oh, and - big surprise - Buger, you called it almost exactly. My Impala tops out around 60 in 2nd gear and the CRV usually shifts to 3rd before hitting 50. The mileage for the CRV is a pretty consistent 22/25, while the Impala is more like 24/30... so I would say the gearing is taller in the Impala.

Those torque to the wheels comparisons between the Impala and RX-8 have renewed my enthusiasm for rotaries. I didn't realize how huge the gap was.

I would hope that redlining my engine on occasion wouldn't be a problem, but none of the Big Three automakers have a stellar reputation for reliability. I wouldn't care if I was driving a Japanese car, but I'm not.

CraziFuzzy

I believe the actual Radius of the Drive Wheels have something to do with how that Torque rating applies force to the road (therefore, moving the car)... Wheel Torque is a rotational measurement, so therefore, leverage play in when considering the actual linear force applied ot the street.... of course, there is only a 0.6" difference in the diameter of the RX-8's 225/40R18's and the Impala's 225/60R16's... but hey, I'm an engineer... I'm **** about such things.. :-)

Buger

A lucky guess. Many people make uninformed decisions based on engine torque figures. Hopefully, this discussion board can educate many of us.

You're absolutely correct Crazifuzzy. :D I was pretty careful to mention wheel size as a factor in many of my comparisons in previous posts but I forgot to mention it here.

Brian