Something that has been bugging me and none of my classmates have been able to answer: why do people prefer higher RPMS when racing or building a track car? If you have a 383 that has a powerband at 4k and a little turbo 4 banger with a power band at 6.5k, and both make the same HP, why do people always want the one with more RPMS? I would think the 383 would be preferable because if you can get to your powerband quicker, you can get your full power sooner.

Another question that's related: with manual transmissions, people always mention that they have the advantage if they have more RPMs before shifting. I don't drive a manual so I don't know anything about it when it comes to racing. I would just like to clear up some of this stuff, I always thought lower power band = good, and RPMs are irrelevant as long as you are in your powerband.

 

Take two engines that make the same power at different speeds. If you put them in two identical cars and gear them both down so that they're doing the same road speed at their preferred engine speeds, the engine turning more RPMs will need more gearing, which means more torque multiplication, which means more torque at the rear wheels. Applying this torque to the ground is what accelerates the car.

 

How does the higher revving car have more torque multiplication? Wouldn't that depend on what gearing the car has? If Car A only needs 4 gears to stay in it's power band and Car B needs 6, car B just has to shift more. I'm not following how that means more mechanical advantage with gears.

 

Yes, because to do the same road speed with different engine speeds means they'll have to have different gearing.

 

But you lose torque multiplication as you climb gears and then get smaller until you hit 1:1 and then overdriven gears...If 1 car can stay in his 3.73 1st gear to 60 but a second needs to shift to a smaller gear to hit 60 I don't see the advantage there other than lost time shifting...

 

Which car do you think will have to shift first? The one with its peak at low speed, or the one with its peak at high speed?

 

Well technically that depends on which one can spin up to it's powerband first....my 305 is a bog after 1st gear for reaching 4.5k but I know not every 4 banger spins right up to 6k either.

 

Think in terms of RPM, not time.

 

Well then the larger V8 would. Which is a good thing, right? Reaching your powerband and having more usable power quicker...

 

Lets say they're both V8s of the same displacement... one reaches peak HP at low RPM, one reaches it at high RPM.

 

I'd still go with the lower RPM one...less wear on parts and quicker usable power. Sorry, I'm not getting where you are going with this...maybe I need to drive a manual to understand.

 

Think of it in simple terms. The higher it spins the more fuel/mixture is being displaced over time which makes more HP. Every cycle of the crank cylinders fire which makes the power and spins the crank. If they fire harder (more fuel/air per bang) which makes the crank spin harder and it makes more power.

A cylinder can only produce so much power per RPM. So to meet goals you will have to spin it to whatever that is. If you want a 500hp 350 it can't do it NA @ 5000RPM, it's not doing enough work.

It's the same reason you have to spin a 4cyl to the moon to make any decent power NA. It's doing as much work as a V8 305@4500 when it is going 9000 (all things equal of course).

 

That explains why a small displacement engine has to spin higher to make more HP, but he wants to understand why it's more desirable in a racing application to make HP at higher RPMs rather than lower.

 

If we assume the rear-end gearing is the same, then the engine with the lower power band upshifts at a lower road speed. That takes it to a gear with less torque multiplication and lower acceleration. The engine with the higher power band stays in a lower gear with better acceleration for longer.

We can also say we'll change the rear end gears so that both cars will shift at the same road speed. The car with the higher power band will need steeper gears to do this, so it will have higher torque multiplication and better acceleration at any given speed

 

That makes sense. But even though one motor may be in a gear longer, ins't it still not putting out peak HP while the other one is? Wouldn't that counter-act the benefit of longer torque multiplication?

 

What do you mean when you say it isn't putting out peak HP while the other one is?

 

If I hit peak HP at 4k as I get near 4k and hit it I'm putting out my Peak HP. Even if you have a larger gear you can stay in longer your powerband is 6k and you are still at 4k you are not putting out peak HP yet while I am. In that time I will pull ahead until you hit 6k.

 

If I have better rear gearing, then it's possible that thanks to the multiplication I'll have more rear-wheel torque than you, even while I'm off my peak.

If we have the same rear gearing (and same transmissions, of course), then you might be accelerating faster than I am through your first gear, but I'll be accelerating faster than you after you shift, and on every shift after that.

 

These numbers are exaggerated, but look at it this way.

You have one 350 making 400 lb-ft of torque at 4000 RPM

You have another 350 making 400 lb-ft of torque at 8000 RPM


Lets assume they have the same gear ratios in the transmission:

-2.94:1 First gear
-1.94:1 Second gear
-1.34:1 Third gear
-1:1 Fourth gear

If they both have the same gear ratios in the rear end (We'll say 3.23), the higher revving motor can accelerate to twice the speed in first gear as the lower revving motor. The higher revving motor stays in 1st gear, for a combined torque multiplication of (2.94 x 3.23 = 9.50) 3,800 lb-ft of torque)

The second motor must shift into second gear much sooner, and only has a torque multiplication of (1.94 x 3.23 = 6.27) 2,500 lb-ft of torque, while the other motor is still putting out 3,800


Or the more practical reason higher revving motors are faster:


Because the second motor can rev to 8000 RPM, you can also use a higher numerical gear ratio in the rear end. In this example (Again, numbers are for illustration only), the 8k motor could run a rear end ratio that is double the 4k motor, and they would reach the same speed at the end of each gear.

In second gear, the 8000 RPM motor is making a torque multiplication of (1.94 x 6.46 = 12.53) 5000 lb-ft of torque

The 4000 RPM motor in the same gear, is making a torque multiplication of (1.94 x 3.23 = 6.26) 2500 lb-ft of torque

 

Consider also that the engine peaking at 8000 RPM will probably at no point be running as low as 4000 RPM.

 

That is what torque converters are for. You build a motor that peaks higher then stock then you need a converter that stalls higher then stock to get the most out of it. Higher stall and gear will put you into your power band instantly.

A manual trans gives you full control of what RPM you engage the trans. A 5sp or 6sp manual also leaves you higher in RPM after a shift because of the tighter ratios and being more gears. The closer to peak torque after a shift the faster you accelerate, ideally you want to begin the gear 500rpm before peak.

The higher you spin a motor the higher the potential for HP gets if it's designed to support it.

 

Read this
http://www.hardtail.com/techtips/hpandtorque.html

 

If you build a 383 and build it intentionally so it runs the best at 2000 to 4000 RPMs it will never be as fast as one that has a power band from 4000 7000 RPMs.

Displacement (or volumetric efficiency depending on how you look at it, I will say displacement for the sake of simplicity) creates torque... F1 cars with the 900 hp 2.4 liter V8's make less peak torque than an L98 because even with the astronomical amounts of air those engines can flow, they cant make any horsepower at low RPMs... because Torque x RPMs = horsepower.

Diesels make a ton of torque, if you could make that diesel spin at 10k RPMs it'd make ridiculous amounts of horsepower. But they cant - they're tractors. They can pull stumps up all day long, but they cant do it quickly.

If you have a 383 and you limit the powerband to 2000-4000 and use the same or at least equivalent top end parts as one that you tuned to work from 4000-7000, the second engine will ALWAYS make more horsepower. Displacement is static, volumetric efficiency is comparable since we will assume the heads are the same, the only way to significantly increase the amount of power made is to alter the valve timing and shift the powerband up and make all that torque higher in the RPM range. Because torque x RPM makes horsepower, and horsepower is what wins races. Torque doesnt win races, spinning torque as quickly as possible does. Whens the last time you saw a tractor at a drag strip racing?

So in order to make an engine that at 4000 RPM can hang with a well-built 383 that redlines at 7000 RPM, you have to increase the displacement or significantly increase the volumetric efficiency. Shove more air into the engine at low RPMs - think supercharger or turbo. Horsepower is torque multiplied by RPM (and a constant), and for a given displacement and airflow capability, the engine that's tuned to spin faster will ALWAYS be faster.

So this is the dilemma. If you build an engine... a 383. If the only major difference is the camshaft, carb size, and maybe the intake.... not a huge money difference... are you gonna want to spin it fast and make 500-600hp, or do you want to give it a 2000-4000 RPM powerband and make 275hp?

You build it to make torque at high rpms and you make power. That's juts how it works, because torque multiplied by RPMs (and a constant) is what horsepower is.

 

But torque decreases as HP increases, so how can you make good torque at higher RPMs?

 

No it doesn't. Torque decreases as volumetric efficiency decreases. Build an engine that fills the cylinders efficiently at high speed, and you'll have good torque at high RPM.

 

If you're talking about looking at a chart of horsepower and torque over the RPM range, then yeah, peak torque is going to decrease fairly early, but the torque is decreasing more slowly than RPMs increase, therefore the horsepower is still higher. Proof is that peak horsepower is gonna happen far past the peak torque in most (I hesitate to say all) power curves.

If you're referring to building a high horsepower engine and losing low end streetability torque, then the previous post is what you need to look at. You tune the engine to operate better at higher RPMs (more aggressive valvetiming essentially, to make sure the cylinders fill up with air even though the engine is operating at high RPMs) and all you're doing is tuning the engine to make more torque at higher RPMs instead of lower ones. See the expression "low-end torque", low-end being the key point. Torque isnt necessarily low RPM power, just low-end torque is low RPM power.

So to kind of get back to the point with your initial point... The more displacement you have, the slower you have to spin them to make good power, but the rotating assemblies also get heavier. What invariably happens is most people find the inherent power/reliability/cost equilibrium point on whatever platform their working with and generally build it to the limit because... we're all adrenaline hot rod junkies. No sense in building a 383 that peaks at 3000 RPM, you're leaving tons of power on the table by doing that, you ideally take your parts and estimate what RPMs they can safely handle, and then you put the power band as far up as you can with your parts to make the most power. Also remember most racing classes have displacement limits. Formula 1, old Trans Am (hence the 302 sbc), and NASCAR, etc. And when you're displacement limited all you can do is spin it faster. And lets face it, most of us here dont feel like going through the trouble or money to build a big block, or do an engine swap to a different type of engine. many of us are, for practical reasons, displacement limited. We dont have the money or patience to find a bbc or an LSX to swap into our cars. So we are stuck with 350s and 383's... and the only way to make more power out of those than the next guy is to either spin them faster or make them flow better. And since we're all hot rodding adrenaline junkies - we do both.

 

Don't forget, torque is work and HP is how fast the work is done. Even as the torque drops off, the increasing HP as the rpms increase means the work is being done quicker even if there's less work being done.

Torque gets you moving, HP gets you down the track.
Torque is what you feel when driving stoplight to stoplight. HP is what you feel when you pass a car on the highway.

These high reving 4 cylinder cars can make a lot of HP but don't make as much torque as a V8 at the lower rpms where torque is needed. I laugh when I see car commercials bragging about the amount of HP these small cars have. They never show you the torque figures. Give them 5 or 6 speed transmissions so they can multiply the small amount of torque they produce.

 

So it's better to move the peak torque and HP up higher in the rpm range so HP will have a higher peak as well because of torque X RPM = HP. This is getting more complicated than I thought..I figured I'd just rebuild a 350 for a lot of low end torque and it would be fine all around...

 

You're not the only one, I couldn't care less about the horsepower a 4-banger puts out. If I was comparing models, torque, gearing & weight are what I would be concerned with. But horsepower is what sells cars, and the companies know this.


As for building a motor... There is nothing wrong with building a motor for low end torque. It'll generally run & idle better, be easier to drive, put out better emissions numbers to keep the tree huggers happy Not to mention cost less to build

For a street car, you'll probably be happiest with a motor that puts the powerband in the low to mid range. It just won't be as fast as a motor build for higher RPM's at the dragstrip. It'll still feel fast if it's built right, according to your butt-o-meter

 

You asked about building racing engines... For the street, low to midrange torque is more desirable, as long as you don't make it too low and end up having to drive it like a truck.

 

Oh sorry. yea I was asking about racing engines. But not like Formula 1 stuff, just track cars. I wanted to know why they do that and what aspects of it to bring to my streetable motor so it's still a good performer.

 

The main reason in a race car is that the highest HP is at high rpms. Since HP is how fast the work is done, you want the engine to stay in that high HP powerband.

Using my engine as example. It makes peak torque around 4500-5000 rpm but peak HP is somewhere just over 7000 rpm. I've launched at 4500 rpm but the car isn't as quick as when I launch at higher rpms. My converter stalls at 6200. When I shift at 7400 rpm, the rpms drop back to 6400 then climb to 7400 where I cross the finish line. Keeping the engine operating in the peak HP range produces the fastest times going down the track because that's where the work is being done the fastest. The goal is to get the engine into it's highest powerband as quick as possible and keep it there with gearing.

As for your original question of why we want the one with more rpms, higher rpm means more power strokes per second which means the work is being done faster. If both cars have the same HP, the one with the higher rpm engine will be faster.

Now in a big truck, it's all about torque. A big diesel usually only makes around 500 HP but also makes 2000+ pounds of torque at 1000 rpm. Even with a multiple gear transmission, it takes a long time to get up to speed but can move huge amounts of weight with little effort.

As I mentioned above, for stoplight to stoplight driving, you want torque. That's why RV cams were so popular. They're designed to make a lot of torque below 2000 rpm where a street car does most of it's driving. There's always going to be a trade off. Low rpm torque or high rpm HP. It's hard to get both. With gearing through the transmission, you can always multiply torque.

You can go to an extreme by looking at a Comp Eliminator engine. They scream down the 1/4 mile at 10,000 rpm. An Indy engine screams down a straight at closer to 20,000 rpm. These cars are very quick but don't need to produce a huge amount of torque down low.

Now there's also a drawback to high rpm engines. They wear out a lot faster. A Jap motorcycle which can easily reach 10,000 rpm doesn't have a long life expectancy while a Harley cruising around below 2000 rpm will last for years.

A highway truck with a big diesel normally operates below 1800 rpm. It's not uncommon for these engines to go well over 1 million miles before needing a rebuild.

 

It's always a compromise between making high RPM horsepower, and low RPM torque.

If you want both in a street engine (Low end torque doesn't matter in a race engine), the easiest "no compromise" way will always be forced induction, or greater displacement. Same effect, just different ways of getting there.

 

Ok this clears it up a lot for me. I didn't stop to think that a car designed for a track will want it's peak torque moved up since it will always be revving high. I was more or less thinking "How can I take a 350 and make it comparable to my friend's Audis (A4 and A6) without it being just a stop light to stoplight car." They all haul *** when they get above 80, I'm surprised how quick those things can accelerate at higher speeds.

I like big V8s and gobs of torque cuz it's fun to drive and has good streetablity but I don't want to completely suffer at high speeds. I wasn't sure where the compromise line is or if you can move the torque curve up and not really feel it much on the low end. I just don't want to feel like I'm driving a ferrari motor that revs to 8k but has shitty torque at low end because unlike those...I don't have a fast spinning rotating assembly.

 

I've read all the posts and what comes to mind is the races ive seen with a v8 305 against a lambo on a 1/4 mile run. The 305 beats the lambo pushing out 500 hp but like Aperion was suggesting once those shifts take place the one with the higher rpms walk the 305. So far i've beaten a few few bmw m3s and some bmw with an i(All imports look the same to me) in street races. But like you i suffer with higher rpm loss of torque. I'm pretty sure there's a way to increase the torque and red line limits. I'm also interesting in eating some audi's