I've been interested in cars for a while, mainly in modifying my Corvette. Sooner or later if you modify enough you'll end up taking your car out to the dyno and playing around. Long story short I've run my car on the dyno a lot so I understand what the graph is telling me.

For example, look at the graph below:
*note, actual image tag removed to observe forum limit of 4 images per post

http://www.wku.edu/~nathan.plemons/g...no_rockers.jpg

This is a dyno graph for my car. Ignore the A/F for this part, it's not important. A dynojet works by measuring horsepower, then taking the RPM input and back calculating torque. It can do this because horsepower and torque are mathematically related via the following formula HP=(Torque*RPM)/5250. As such you'll notice then that no matter what engine you're dynoing, provided that HP and torque are plotted on the same scale, HP=TORQUE @ 5250 RPM's.

Anyway, the graph above is what I'm used to seeing for a relatively free revving V-8. Torque is relatively flat, or builds, and thus HP builds the faster you spin the engine.

That's all well and good, no big news to any gearhead. Well one day we were playing around on the dyno and my friend showed up with his F-150. Just for fun we decided to dyno it and see what it did. We strapped it down and made a few passes and I saw something very interesting. The car made ~110 HP, clear across the board, it didn't build, it didn't gain, it just stayed flat at virtually the same level. Unfortunately the dyno wasn't able to pick up on the RPM signal. As such the graph was plotted against MPH instead of RPM. I didn't really think about it until I got home. Since I knew MPH and I knew the tire size, final drive ratio, and gear ratio's in the trans, I could actually back calculate torque based off of the HP information I was given. When I put it into Excel I was a little amazed at what I saw. Plug it all in and this is what you get:

*note, I don't actually have the dyno graph anymore, so this simulated graph isn't perfect, IE the engine probably didn't rev to 6000, but it illustrates the point.



Notice how the torque looks like it follows some kind of decay function. The people who say that above 4400 RPM's you're just making noise, are absolutely right.

Anyway, that got me to thinking about some other stuff. My friend had told me "You can't seem to put this truck in the wrong gear." Now that I own one of these engines, I thought I would play with some more numbers. I used my spreadsheet to calculate the torque applied to the wheels for any given MPH, for any given gear in the transmission.

Now if you take a car with a "normal" power curve, you end up with a graph that looks something like this:



Instead though if you take the I6 with it's very flat power curve, you end up with this:



Now THAT is what I call very interesting indeed. Look at what this graph is telling us. Since every gear overlaps the previous one perfectly, notice something. Take say 30 MPH. It doesn't really matter if you're in first gear or if you're in second, you're putting the same amount of torque to the ground. The same goes for every possible overlap area. My friend was right, you really can't put this truck in the wrong gear!

*Note. The perfectly flat 115 HP that I put on that simulated dyno graph is idealized. There was naturally some fluctuation in the numbers with RPM, but it was VERY minimal. Unfortunately I no longer have the dyno printout, I was just going off of memory. In the real world the numbers don't work out quite this perfectly, but they're pretty dang close.

Anybody who has ever driven the I6 can verify that the above is the nature of the truck, now you know why!

 

Except that's not what the torque curve for a 300 looks like.

It begins to rise virtually off-idle and is up to 225-230 ftlbs by 1500 RPM
It peaks at 260 ft lbs @ 2000 RPM and holds near that value, dropping back down to just over 230 @ 3500. Above that it turns downwards quickly and just sort of falls off the edge of the earth somewhere around 4500.

 

Not to bag on you, but how come the graph shows no data at all below 2000 RPM? I've heard the i6 goes kinda like a plateau shape. Rises quickly from idle to around 2000, hovers there until about 3500, then starts to fall off past that.

But I do agree with the "overlapping" gears bit. Going around town, I can pretty much put the truck in 3rd and forget it. I can start off in 3rd(not saying I do this all the time, I know it's bad for the clutch), cruise around, get up to about 40 or so if need be. I really like the Mazda trans, I don't feel the need to be constantly shifting like in some other manual cars I've driven.

 

I disagree. What it does below 2000 RPM's is irrelevant for the purposes of my illustration because it only starts at 2000. Again, the horsepower number isn't completely constant, but it is relatively flat, flat enough to illustrate the point. I am also well aware of the fact that it doesn't rev to 6000 and falls on it's face, it is just on the graph to emphasize the effect.

When you dyno a car you have to be on the rollers and moving at a certain speed, usually we start dyno runs at 2000 RPM's for sports cars. As such we dyno'd this truck in the same fashion, and the above is a representation of what we got.

If the endpoints of the graph trouble you guys, think about it this way. Consider it a snapshot of the middle, throw away the scale on the bottom altogether. The point I'm was trying to make is that most cars don't have a flat horsepower curve at all, it's either rising or falling, very rarely is there a steady plateau. In order to create the steady HP line torque has to fall very strangely with RPM which creates the very interesting driving characteristics of the engine.

I didn't intend this to be a definitive debate on the shape of the power curve, but rather just some interesting observations that prove mathematically what you can feel when you drive the truck. If you've ever driven the I6 and felt like it just didn't feel like anything else you've ever driven before, you're probably right.

 

I know the hp and torque meet at 5250 rpm, but my 4.9L will never see rpms that high. lol.

The thing that is so killer about the 4.9L is the fact that it reaches it's maximum torque at 2000 rpm. That's why, from a dead stop, a 4.9L F150 will always beat any other V8 F150 (with the rest of the driveline equally equipped) at least across the intersection, often farther. You reach your maximum power sooner. This is also why you can drive them "in any gear". In a conventional engine, you reach your peak torque at a higher rpm, which restricts the choice of gears more closely with vehicle speed. But with a peak torque at 2000 rpm, you will be very close to your maximum power in any gear, at any speed, just about.

 

But the torque doesn't "fall very strangely with RPM." The torque builds well below the 2000 RPM where your dyno begins to measure, so you've already missed a substantial (and important) portion of the torque curve. You need a better dyno. The 300 actually uses the RPM range from 1000 to 2000. The torque holds fairly steady until the 34-3500 HP peak and starts to trend downwards fairly rapidly above that point. The 300 was never intended to run in the RPM range in which you're measuring and/or calculating.

 

Again, I disagree. The dyno is acurate and my numbers are as well. There are no numbers below 2000 because WE didn't start measuring below that point because we didn't know the nature of the engine. On most cars there is absolutely no point, this one happens to be the exception. We could have started the run off idle if we wanted to. The reason we didn't make a run any lower was because we did not see the torque curve at first because the dyno didn't get an RPM reading. This is NOT a problem with the dyno itself, but rather with trying to get a good RPM reading from the vehicle, which is a very common problem.

As for the engine was never meant to run in the range I'm measuring, that's incorrect. The I6 has a rock solid bottom end and will spin to the moon with no problem. It might not make the best power, but it is certainly capable of operating in that range with no harm at all.

The reason the engine doesn't make any power in the upper RPM's is because of the stock intake and exhaust system as well as camshaft selection. A slightly more aggressive cam, a better intake, and a free flowing exhaust would have that engine making power up to 6000 RPM's and you'd still have plenty of low end because of the long stroke.

 

It goes WAY deeper than intake, exhaust and camshaft.
Every feature of the 300 was designed to maximize volumetric efficiency at low RPM. The long stroke, the port sizes, valve and bowl diameters, the very long intake runners of the EFI version, etc.

6000 RPM? Better take a look at the piston speeds. At 6000, you're running 3980 FPM. That's getting well into the grey area for a long and happy life. That's the same piston speed that a 302 would see at 7960 RPM or a 351 at 6823 RPM.

 

I agree that the 4.9L was designed and intended to be a low rpm torque engine, HOWEVER....





I saw a modified truck chassis, with a carbureted, N/A 4.9L in it, run an 8 second 1/4 mile at the NHRA Finals in Pomona in 2000 or 2001.

I swear to God.

 

Steve Ambrose runs one in a Comp Eliminator car that is easily that quick as are Mike Nahan, Ralph Hope and Glen Treadwell (before he switched to C***y). Race engines that get rebuilt after every 6-8 hours are hardly a model for either practicality or longevity.

 

Which is well within the realm of a properly built 351.

I never said that the engine would run at 6000 RPM's for hours at a time, but brief bursts up to that speed are perfectly feasible. Anyway, that's getting off topic of the original point.

My numbers that I used are from memory from several years ago, and I admit that they are not exact, rather an idealized representation. If that troubles you I'm sorry. If you want to argue the point then please by all means post a dyno graph of a fuel injected I6 with a 5-spd that is contrary to what I've posted here and I'll be happy to discuss it with you.

 

You're preaching to the choir.

It still blew my mind when I saw it happen.

It was followed by a fullsize dragster that was powered by nothing but 12 volt car batteries, which also ran an 8 second 1/4 mile. It was so quiet, if you weren't watching, you never would have known it ran.

 

Soooo, how about them Mets!

 

Two known points courtesy of Ford:
260 ft-lbs @ 2000 RPM = 99 hp
150 HP @ 3400 RPM = 231 ft-lbs.

It's the torque curve that approximates a flat line in the 1500-3000 range. The HP increases 50% between the torque peak and the HP peak, almost like any allegedly normal engine.

 

Courtesy of Ford, meaning their published values that were done on an engine dyno that was run under idealized conditions. Those numbers never line up with what you get on a chasis dyno.

GM rated the following as a maximum horsepower of 300 @ 5000 RPM's and 330 lb-ft of torque @ 4000 RPM's.


The chasis dyno shows that although they got the HP more or less correct, account for driveline loss, there is no reason to say the torque peak is at 4000 RPM's. In fact 4000 RPM's is right where the torque actually begins to fall, it makes the same torque from 2200 RPM's all the way up to 4000. This is not "just my car" either, you can dyno a thousand of these cars and you'll get the exact same graph. The point is that engine dyno's and published numbers may have very little meaning in the real world.