Clever Gearing

[Knighton]

I just read an article on hp vs. tq posted by Steve Toner. http://www.g-speed.com/pbh/torque-and-hp.html

"It is better to make torque at high rpm than at low rpm, because you can take advantage of *gearing*."


I am having difficulty wrapping my head around this statement. Doesn't high torque at low rpm determine that feeling of acceleration (flat torque curve) until redline? HP helps top speed but torque is what accelerates the vehicle, correct?

So, if the above statement is correct, what is an example of proper (clever) gearing to take advantage of 7,500 rpm 331 or 306 producing 500 hp and 350 lbs torque, for example, or any high rpm motor?

Use shorter gearing? 35 mph first gear or use a Getrag or similar ratios? Yuck /ubbthreads/images/graemlins/tongue.gif

Would this be similar to using a stroked 408 with 500hp/500tq that uses taller gearing?

Then again, I have heard that high tq motors like a stroker make the car somewhat undriveable because of the rear end stepping out under acceleration in the corners.

It is all a bit confusing /ubbthreads/images/graemlins/confused.gif.

 

[Ian Clark]

Knighton,

I'd have to say after reading the article that it's strange but true. However, the concept is based on theoretical applications and not the real world.

Most of us drive our cars on the street. The power band of typical V8's is about 3000prm. You should keep the power band in the rpm range you intend to use.

For street driving an engine that pulls from 4,000 to 7,000 rpm won't be any fun from 1500 to 4500 which is where you drive.

So there's my 2cents worth. Build a engine with a smooth broad power band and get the power in before 6,000rpm. That's where most good crate motors are anyways

 

[ChrisL]

[ QUOTE ]
I'd have to say after reading the article that it's strange but true. However, the concept is based on theoretical applications and not the real world.

[/ QUOTE ]

Its true even in the real world. Over time engine design and material design have allowed engines to rev higher and more smoothly. Fifty years ago, the only engines that would cruise at 6000RPM were race engines, now its common place. Most modern motorcycle engines, because they are small and have a small rotational mass, can be made to reliably rev even harder and cruise happily at 9000 RPM. Subsequent designs of production engines have seen engine revs go up (never down) and will continue to do so as long as people keep inventing technologies that to allow them to be built economically.

 

[Ian Clark]

Hi Chris, Love the colours on your car by the way. I'm with you on design and materials allowing higher reving smoother engines. Variable valve timing, variable veturi's, variable intake runners all contribute to modern engines ability to turn faster. I really like the advantages of variable valve timing to broaden the power band, however in the case of a small block Ford pushrod engine in a GT40 application, we have none of the above to play with. Hence I maintain the original point being to keep the power band in the rpm range you intend to use the most. This will give you the most driving pleasure.

 

[ChrisL]

Canuk40,
Thank you for your kind remarks about the colour of the car. I kinda like it too, but then again I'm biased /ubbthreads/images/graemlins/laugh.gif

Incidentally, I fully agree with your remarks about placing the peak torque at the rpm point that you intend to use most. My comments though were not about individual engines so much as a general observation that even in the real world an engine builder would look to place the peak torque as high as possible after taking into account all factors such as cost and sweet points etc. The final location of the torque peak would then define the preferred revs rather than the other way around.

To take this point to its extreme, three 302s could probably be built so that each had the same peak torque at 2000RPM or at 4000RMP or at 6000RPM. However, if you were to build the 2000 version, I think that we would all agree that you would be cutting yourself short. If you were to build the 6000 version, you would either need to spend large amounts of money or live with an engine that has moved well beyond its sweet point. Therefore, the rpm point that you might choose is 4000 because it is the highest livable rpm range for a reasonable amount of money.

 

[Kevin M]

After looking at the article you listed, I would have to say that it is right in theory, but falls into the same trap a lot of magazine articles fall into of treating Torque and Horsepower as if they are different things entirely.

Horsepower and torque are directly related and are different ways of looking at the same thing. In truth, when one takes gearing into account much of this cancels out. You can have an engine making 250 ft. lbs. of torque at 7,000 rpm or a motor making 500 ft. lbs. At 3,500 rpm and you are actually getting exactly the same amount of work (horsepower) out of them. Since the car needs to go a fixed speed, all that happens is that you use gears for the 3,500 rpm engine that are twice as high as for the 7,000 rpm engine, and since gearing multiplies torque, you end up with identical torque figures at the rear tires for both engines.

The main advantage of a high torque, low RPM motor is that it is generally much more reliable and easier to live with in a street car.

Kevin

 

[Gary Gibbs]

My feelings exactly. I can make a high HP motor with a hyd cam and great reliability with large displacement and good heads. Then you use taller gearing and a lower redline and make the same torque at the wheels.

 

[ChrisL]

The question I would then ask is, if reliability is the only consideration then why aren't we still building engines that redline at 1000rpm as we did at the start of the 20th century? The article actually makes an extended effort to show that it is not the absolute theoretical numbers that count, but the practical application made possible by the improved characteristics of a high RPM engine.


With regards to reliability at RPM... that is a function of the engineering and design techniques available at the time rather than the absolute RPM value. One thing that the article didn't mention though is that higher RPM levels give you diminishing advantages, whereas unreliablity levels increase exponentially.

 

[Kevin M]

Like the cars we love (GT40s) I’m afraid the motors that go with them are also relics.

The smalblock Ford is now over 40 years old, and while we have better ports and cams to make more power. The basic design and its inherent limits hasn’t changed.

In the longer stroke motors we are still limited by the loads on the bottom end to about 4,000 to at most 4,500 FPM average piston speeds. (Unless you are going to use Titanium rods, etc. which would be a little silly on a 2 valve pushrod engine).

In the shorter stroke motors, we are RPM limited by the rather ancient pushrod valve train technology and our ability to control valve motion and get the motor to breath at very high RPMs. (Even if the bottom end would hold up.)

Since we are not racing in a class that limits displacement, it is much easier to build a larger, lower revving motor as opposed to a small high RPM one. They will both make the same power, but the larger one will be much more reliable and far better for street driving.

This is even in the spirit of the original GT40s. When they were racing they had very inefficient engines compared to the cars they raced against. The rules didn’t limit displacement so we used 7-liter engines against 3 and 4-liter cars. Because it was endurance racing that helped give the GT40s better reliability (For the engines, the weight caused all sorts of problems with the brakes.)

It is certainly possible to design much better, higher revving engines today. Unfortunately, Ferrari V12s are hard to come by used and would cost more than the rest of the car put together. Plus if one is that serious about the performance then the GT40s outdated aerodynamics and suspension would not be acceptable anyway.

In the final analysis, we are all having great fun reliving the past with these cars, but they are by their nature limited to the technologies of that past as well.

 

[Gary Gibbs]

That is my philosophy. Pick the power level you are interested and then try to accommodate with the use of reasonable component selection. I also agree that for the given power level a relatively larger displacement will let you build that power more reliably. You can get away with hyd. roller cams and decent idle quality for example.

 

[Ron Earp]

I don't know if I agree that we're really limited by the SB. The SB with modern heads and components can easily make 7000RPM and be reliable. They can also make in excess of 450 hp if needed, and if stroked and poked 600+. How much more do we need? There are Dart SB blocks coupled with proper rods and cranks that can take whatever RPM you would like to make. There are heads that can flow whatever you need. top end valve trains can be very reliable with rail mounts, good springs, lifters, etc. Coupled with any number of modern EFI systems these motors are very tractable and very powerful. Heck, GM still uses the basic motor and is now in excess of 500 smogged hp for the new generation Vette - same old pushrod technology etc. and I bet the car will compete with Europe's finest (as much as I hate to say it since I'm not a big Vette/GM fan).

Now, if you're talking about using a 289 straight from a standard Mustang of that era and hopping it up, then you are limited. But you definitely aren't if you're building the motor with what is available to you on the market.

R

 

[Gary Gibbs]

I also agree Ron (I am beginning to sound like a brown noser). We have made huge strides given the basic engine design. It is incredible what you can do with these engines. I we haven't really talked much about power adders (NOS, Blower, Turbos).

 

[Kevin M]

I would agree completely that we can make as much power with the smallblock Ford as the car could possibly use.

My thoughts were relative to Chris L’s comments about the article in question.

Compared to a modern dual overhead cam 4-valve engine, the smallblock ford is ancient technology. We have done a lot with it, but it becomes somewhat analogous to making a stone axe with a carbon fiber handle. It is better, but it will never match a chainsaw.

There is an interesting article about the BMW formula 1 engine in the current issue of Racecar Engineering magazine. The BMW turns to 19,200 rpm, makes over 900 hp out of 3 liters (180 cubic inches) and has BMPs as good as engines that turn one third that high.

That is a good example of a modern, state of the art engine.

The point of my answer was that the smallblock Ford is not a high RPM engine in the modern sense of the term, and that the best way to make power with one is to build it larger and go for torque rather than trying to go small with very high RPMs. That is what the engines were originally designed to do. You can go for very high RPMs, but you are working against the basic limitations of the engine design when you try.

 

[Mark Worthington]

The Boss 302 was a high-rev version of the small block, and a successful one at that. Mix a Boss 302 block and crank with good heads and valvetrain components and it can reliably make 400+ hp at 7,000 rpm. On the other hand, if you take the 4.6 DOHC and tweak it to get the same level of horsepower and reliability you will be out a lot more money and you will have a much larger and heavier engine. You will also have a much less reliable engine - I personally know of 14 local DOHC engines that have blown up, including mine. So while Ford took one step forward with an aluminum-block 32 valve DOHC engine, they took two steps back with a small bore spacing, weak-not oiling and windage systems, and crappy valvetrain components, like stamped cam gears.

A small black Ford engine is correct in a GT40, and it can make as much power as you want, either as a low-end grunt machine or as a high-rev screamer. The low-rev, high-torque variant will give you more reliability at a lower cost. On the other hand, the sound of a GT40 at full song through a crossover exhaust only gets better with rpm.

How fa$t do you want to go?

 

[Kevin M]

The Ford 4.6 and 5.4 are something of a special case.

That engine was designed primarily as a truck engine. It is sold in the V8 Explorer and all of Fords pickup trucks. They sell about 20 of those for every Mustang, so you can see where the priorities were.

As you mentioned it is a small bore, long stroke engine. Which is perfect for making torque in a truck, but the opposite of what you want in a high performance motor. Building a set of 4 valve heads for it was a nice gesture, but it didn’t change the basic layout of the engine.

Ford has had a lot of problems trying to make a performance engine out of it. Notice that for the Cobra they had to go back to an iron block because the aluminum one wasn’t strong enough. It was never intended to be used that way.

The Boss 302 was a great engine. My first really good car was a 1970 Mustang that I bought used in 1972, so I have very fond memories of those cars. Still, as good as it was, new high performance engines like the Ferrari 4 liter V8 have much better power output for their size.

Here we come back to the same thing again, you can get a lot of power out of a smallblock Ford, but the best way to do it is by going larger and taking advantage of the superb torque production.

The Boss 302 was a poor street motor. It had little low-end torque and the ports and valves were way too large for a street engine. When they stroked it in 71 and made the Boss 351 using the Cleveland block, it was a much better engine. Drivability was very good and it would accelerate the larger and 400 lb heavier 71-73 body Mustang considerably faster than a 70 – 302 combination.

Again, I am not saying that there is anything wrong with the smallblock Ford. It is a good engine. It’s just that like any engine, it was designed to work best in a certain section of the powerband. And it is at it’s best when you don’t try to push it to work outside of the basic range it was designed for.

You can build an 8,500 rpm 302, but it will be a very unpleasant street engine and will need constant maintenance to keep it healthy. Much easier to build a 6,500 rpm 347 or a 6,000 rpm 393 and use the motor in the way it was designed to be most efficient.

That said, these cars aren’t about practicality (If you want practical, buy a Toyota Camry or a Honda Accord). If you love the sound of a 302 at 8,000 rpm, and it makes the hair stand up on the back of your neck when you hear one, then by all means, that is the motor you should have in your car.