Increased RPM's ??

[TheSaint71]

Hey Guys, I need your input. What is the idea of increased RPM's , what real purpose does this serve? I have heard alot about it on another site and am quite intrigued..

Jamin

[Excaliber]

Pretty simple. The higher the rpm the more horse power your engine makes and arguably the longer between shifting points.

Ernie

[SCOBRAC]

Period cars generally limited to four forward gears, most always ending with a final ratio of 1:1 rely heavily on the rotation that assembly for top speed.. Gear ratio is obviously a big factor, that and a specified tire diameter, in most of our cases of 25.5-26.5" limit top speed to the top speed of all the parts rotational capacity, of which engine rpm is the biggest factor..

If an engine can turn 5400 rpm you divide that by the final drive ratio, or in the case of a toploader typically 1:1, and again by the ratio distributing that rpm and then by the size of the tire.. Remember pi.. 3.14159+/- which gives you a finite speed..

A car that can only turn 5400 rpm with a final drive ratio of 1:1 and a gear ratio of 3.54:1 with a tire size of 26.5" (295/50 15) has a maximum speed of 128 mph.. Not accounting for aerodynamic influences.

The same car with a maximum rpm of 7200 will give you a top speed of 165 mph.

Modern sports cars with 5 and 6 speed gear boxes are less hindered by rpm.. A modern vehicle turning 4800 rpm with a six speed final ratio of .72:1, a rear end ratio of 3.45:1 with a 375/45 tire has a top speed of 172 mph..

Horsepower is a big factor but only to the point at which aerodynamically it becomes physically impossible or impractical to push an object through the air.. Conversely a 900 hp engine that will turn 7200 rpm will still only push a car as fast as a 450 hp car will given the same rpm.. It will however get it to that speed more rapidly.. Not twice as fast but nearly so.



Hope this helps..

[Ron61]

Jamin,

Call Isky in Los Angles and get one of their small Ratio Computers. I think they cost a dollar. I have four that I keep in the cars and they are very accurate and quick for everything to finding out tire sizes, gear ratios, RPMs. speed, and displacement. I think other companies have them also. My first one came from Ford with the 1969 Cobra.

Ron

[427 S/O]

Jamin, every conceivable calculator is on your puter', just do a simple search.

[PatBuckley]

Jamin -

Are you asking what purpose increased rpm's mean as far as engine power output or increased vehicle speed?

PatBuckley

[Excaliber]

I've often noticed in cars with an overdrive that top speed is still "limited" to the max engine rpm in the 1:1 gear. 5400 rpm, top loader 1:1 4th gear being 128 mph.

OK, so lets say you have a 5 speed. At 128 you shift because engine rpm is at it highest. RPM falls to say 4800 rpm (estimated), but top speed REMAINS about 128 or even FALLS to 126.

Like SCOBRAC pointed out: At some point "aero drag", tire friction, rotational forces in the drivetain, these all "eat up the horse power" and you can't "get there from here".

I recall the LeMans Porsche 908 series cars (I think it was 908) had about 350 hp (not "that much" for a race car) but it could do 200 mph! The 917 had about 1000 hp? Their top speed was very close to the same. So there is a lot more going on here than just rpm! No doubt the 917 had a lot more "down force" at that speed and was more stable. I bet the 908 was "on the edge" and cleared for takeoff at that speed.

I guarentee you will NOT do anywhere near 200 in a Cobra with only 350 hp. But you might "approach" that number with, say, 600-700 hp? 600 hp being enough to let your engine turn up to max rpm. Or does it make 600 hp BECAUSE it turns high rpm? Both!

But 350 hp aint enough with a Cobra "brick" body to do 200! I don't care WHAT gears your running or how much RPM your engine is "capable" of! 350 hp at 5000 rpm or 8000 rpm is STILL 350 hp. But I bet the 8000 rpm motor is way smaller than the 5000 rpm motor and there both making the same hp!

Now Racer "X" would prefer the smaller, lighter, higher reving motor for his race car! Others may prefer the "torque" (pulling power like a "truck") at lower rpm the big motor would have.

If your "pulling stumps", go for torque! If you want to get there "in a hurry" go for high rpm (horse power).

Ernie

[Ron61]

Ernie,

You are very correct. My horsepower is at 5800-6200 RPM and in 5th gear I can't get above 3300 RPM which is 115 MPH because of my gearing and that wall of air. I have 3:27 gears and I chose the wrong split for 5th which kicks the gears up to 2:10 and with this brick I can't even begin to get into my power range. In 4th I can run around 145-150 if I push it. So the rpm, tire size, gears, and about everything including aerdynamics play into what speed you can do at what RPM.

427/SO

You are correct about every conceivable calculator being on a computer. However I like the small ones in the car because we do a lot of comparisons of gears, tire sizes & etc. when we are far away from a computer. My old junker just isn't equiped for the modern options.

Ron

[427 S/O]

Considering the fundamentals, I believe the FE427 is the perfect combination of torque and HP.

This is Interesting reading, forgot where I found it.

HARD AS IT MAY BE for those of us brought up to think of horsepower as the ultimate measure of an engine's worth, we need to rethink our emphasis. Maybe we should add the word "power" to torque because that's really what we're after. It's the torque that gives us the "feels good" sensation in seat-of-the-pants evaluation of acceleration. Going back to the most basic understanding of an internal-combustion engine, we all know that it is the reaction of an ignited air/fuel mixture that drives the pistons down the bore, causing the crankshaft to turn. Obviously, the more energy released in that reaction, and the more energy used (rather than lost), the greater the force on the piston and the more twist there is on the crank. Torque is a measure of the "twisting force" of the engine. One of the ways engineers evaluate the power of an engine is by looking at the Brake Mean Effective Pressure (BMEP). The higher the BMEP, the higher the torque. Torque is a function of cylinder pressure and displacement, which is the scientific origin of the old speed merchant's axiom, "You can't beat cubic inches." The reason engineers look at the BMEP is that, unfortunately, with current rod design the peak pressure is reached when the piston is just past TDC (10 to 15 degrees) and decreases as the piston goes down. That peak pressure near TDC arrives at a time when the pistons/rods have the least amount of mechanical advantage for twisting the crank. As the piston moves downward, the volume increases, creating a cooling/refrigeration effect when we really want heat! In terms of engines in street and racing trim, where an engine gets power is all important. An engine with a low BMEP has to run higher rpm to make the same power as an identical displacement engine that features higher BMEP. Shifting the maximum torque to a lower rpm range really makes the engine feel bigger. A good low-end torque motor always has the advantage from stoplight to stoplight. Within the usable rpm range of most street engines, good torque is much more valuable than peak horsepower. Horsepower is rpm-related. In a racing situation, where the desired speed and rpm are known, high horsepower is good, so racing engines are tuned to make their power at high rpm. Outside of Bonneville, 95% of the driving public may only see peak power in their engines momentarily. Within each gear we get up to maximum horsepower briefly and then have to shift before power drops. During acceleration through the gears, we're only getting perhaps 60% to 70% of the potential power to the ground because the engine must go through a range in each gear, and maximum power is available only for an instant. The rest is a compromise. The "perfect engine" would run at a single, specific rpm. Then everything, from cam timing to carburetion, exhaust design, intake manifolds, compression ratio, and combustion chamber shape could be designed to make optimum power at that speed. Engines that must operate within an rpm range are designed with compromises. What works best at 5000 rpm is not going to work as well at 4000 or 6000 rpm. Perfect engines can be easily built today. They are already in place as stationary engines all over the world, providing pumping power and running electrical generators.

[speed220mph]

Boiling it down into simple terms, there's no point in turning an engine past its appetite for air. More power will not result because the engine is on the downhill side of the power curve. However, if you install a head that flows more air, then you have to turn the engine at higher rpm. However, if the valve train and bottom end can't handle the higher rpm . . . broken parts in the oil pan . . . then there's no gain. Better yet, keep the revs at a safe level, but have a valve/port that flows better at lower lifts, then you'll have improved torque and horesepower at a tolerable rpm.

About increasing all out vehicle speed, be aware that aerodynamic drag increases as the square of speed. Regardless of power, a car such as the Cobra that has the aerodynamics of a brick, is not a high-speed car. That was the reason Shelby developed the Daytona Coupe. Make that sucker slick and it will achieve a much higher speed with the same power. The GT40 is another excellent example of this. One thing we know from running at Bonneville is it's better to work on aero along with building more power.

On the other hand, power and the light weight of the Cobra is what kicks you in the rear at lower speeds, or less that 100 mph. That's the delight in driving a Cobra. So go for power, but at a safe rpm.

[Excaliber]

I think there are a couple of cars on the market right now with "perfect" motors, and incredibly boring performance.

Makes me think perfection, per se, is not my goal, LOL!

What cars are they? They have that "snow mobile" style transmisson (shiftless) with a steel belt and variable pulley type arrangement. Another one might be the new electric\gas engine hybrids?

Ernie

[TheSaint71]

Whoaaa.... thanks guys, you all just taght me the world history of this aspect of car engines, thanks...You guys really know alot about this stuff... I was wondering about that because I've read articles on a cobra that weighed 2600lbs and had a 351 in it, but the thing could do 200 miles per hr, and the horsepower was around 545... I was intriged because ive seen 660hp SB engines in cobras that weight alot less than 2600lbs and couldnt do 200.. man thats fast.



Jamin... thanks.. now I get to ask more questions hehe

facinating

[Sizzler]

Where exactly did you read this?

I'd question anything else read in that same publication if that's what they're printing. Bloody unlikely is putting it mildly.

[Excaliber]

I'd be very interested in seeing some documentation on a Cobra doing 200 mph.

Sizzler,,,,,I almost hate to ask. The last time this came up a major fight broke out, lol.

Ernie

[mr0077]

Saint71, weight and horsepower relate to acceleration, but drag and horsepower relate to top speed (unless your power/weight ratio is so low you don't have room to accelerate to top end). John Cobb's Railton Mobile Special weighed over 4000 lbs when it set the world land speed record (in the 1950's?) at over 400 mph.
RPM and horsepower are related to torque by the formula, horsepower = torque x RPM / 5252. So, if you have 400 lb/ft of torque at 5000 RPM, you're making 381 horsepower at that RPM...but if you can move the same 400 lb/ft of torque up the RPM band to 8000 RPM, you're now making 609 horses (at 8000 RPM), with the same size motor. The naturally aspirated Formula 1 engines are a good example of this...not much torque, but running at maybe 11,000 RPM(?), they make big horsepower numbers with small cubic inch displacements. Problem is, it gets VERY expensive to increase RPM, not nearly so expensive to increase cubic inches...or to increase effective size of the motor with forced induction.

[SCOBRAC]

Saint..

Do us a favor.. Go to the "other site" and tell them we are smarter than they are and we have cooler cars..

Regards,

[SCOBRAC]

For fun do the math on a 2003 Viper.. on paper the top speed is like 312 mph.. kinda fun to imagine.. Damn air.. Maybe the liberals will get around to banning it some day..

[Harris]

Saint,
I'm not trying to sound like the village idiot but RPM is how many revolutions per minute your engine is turning. At 6000 rpm your crankshaft is turning over 100 times per second. At that incredible speed you can see how critical engine balance becomes. Your valves and rocker arm are a blur.

Fellas, am I right? Seeing those figures on paper floors me!

Harris

[Excaliber]

A Blur? Man there in a different time zone, the twilight zone! It is hard to grasp the concept of all that metal "doing the dance" at even moderate rpm!

SCOBRAC,,,,,your right you know,,,,,you are. Now go tell 'em Saint,

[Excaliber]

double threader.....and were good looking too!