Ya get some, ya lose some? You gotta be kiddin me?!?

[HavockWK]

Okey dokey! I am new to the performance market. Before I got my Titan, I never had more than a 4 banger and performance was a fantasy, not a possibility. (I don't subscribe to pocket rocket digest) Now, I keep hearing all this talk of performance mods that increase horsepower and/or torque at the cost of low end torque. My uneducated brain is not comprehending how this actually works out. I have done a little reading up on HP v torque and keep finding the same equation on how to convert between the two:

(Torque x RPM) / 5,252 = Horsepower

I flipped the equation around to:

Torque = (Horsepower * 5,252) / RPM

And:

RPM = (Horsepower * 5,252) / Torque

(I like to check out different view points)

Now I know that when you get up around the high end of the RPM scale, the resulting HP & torque start to drop as I have seen on dyno charts and can be seen in the equations. But down at the low end of the RPM range, HP & torque both vary in direct proportion to the RPM. Therefore if you increase HP at low RPM, torque goes up too and vice versa. This shows in the equations, but I don’t recall seeing any dyno charts that go into the lower RPM range (say 1000-2500 RPM). Any body have a complete dyno workup of the Titan’s complete RPM range from idle to redline?

Now are you trying to tell me that if I put a mod on my truck that increases HP, but costs me low end torque that is somehow shifts the entire power band of the Endurance engine up and to the right? So I can go faster, but it will take me longer to get up and go? Please explain….

Oh yeah, I have heard some people say that some back pressure is needed. I have found no evidence of this outside of the fact that you do not want raw air sucked back into a cylinder of the muffler. Beyond this I have not seen anybody post anything explaining why back pressure that has a negative performance impact, is required to maintain performance and engine “harmony”. Can anybody explain to me how constricting the ability for the engine to exhale benefits the engine? After all the principal behind exhaust scavenging is based on negative back pressure to suck more exhaust from the cylinder.

[juma]

This is from a self taught layman of the first degree...

First of all, its all about what rpm your engine has airspeed and pumping efficiency. With a mod as you specify to move power to the high rpm, you move pumping efficiency to a higher rpm and take it away from lower end. and thus the inertia of the spinning motor is less at low end than at high end and the motor is harder (slower) to accelerate and requires more power to maintain a low rpm due to defective airspeed/pumping efficiency that you changed from stock. whereas, if you spin the mother up to high rpm with your mod, it has a hell of lot more inertia equalling pulling power from its ability move more air at that speed and that is felt as hosspower. got it? they don't say an engine is just an air pump for nothing.

The reason a motor tuned for low rpm usually has exhaust restriction and needs it to be able to generate torque is because the exhaust restriction, within a certain rpm range, generates scavenging and thus airspeed and pumping efficiency (volumetric efficiency). Once you get beyond that rpm range the exhaust restriction really becomes an air moving restriction.

A motor with our present on hand technology can only be tuned to run well in a segment of the whole rpm band. You add an exhaust to get more power and you lose low rpm airspeed built into the design but you gain more airspeed at the top end and get more power there.

somehow getting variable valve timing, valve lift, intake, and exhaust capability would greatly enhance horsepower/torque. A supercharger overcomes all the restrictions with just plain old air pressure.

juma

[PHOEBISIS]

Havockwk,as you probably already noticed HP always equals torque at 5252 RPMS-on all engines,always.I looked at that equation 1 million times and never noticed that-someone told me.Torque is always more than HP up to 5252 RPMS,and HP is always more than torque over 5252-they always cross right at 5252.Of course,some motors don't rev that high,so they never cross.
The RPM where you get peak torque is usually considered the point where the cyl filling is the best- where the vol efficiency is the highest-The Head mentioned this.Really good motors can get more than 100% vol efficiency at their torque peak.This is because of the speed-inertia of the incoming air-fuel mixture will keep "ramming" it in even after the piston has reversed itself,and is coming back up.The column of air is moving,so it keeps moving.This is why there is so much emphasis on velocity of the air-fuel.
Finally,it is HP(and gearing,of course)-not torque-that makes you accelerate fast,and reach high speeds.If you want to know which vehicle accelerates fastest look at the HP and the weight(assuming the gearing is correct).With my legs,and a 4 ft lever,I can produce 1000 lb ft,but I sure as heck can't do a 7 sec 0-60-it is always the HP.
On your question-if I lose low end,but gain high end-won't it take me longer to get up and go?You might(but you won't if you are a racer) lose a tiny bit of acceleration while you work 1st gear thru the the whimpy RPM range.But you won't because a "racer" won't"start" at idle(800 rpms).With a manual transmission he will start at say 3000 RPMS-partially drop the clutch feeding in clutch till the wheels just barely start to spin.The whole time he will be keeping the RPMS up high-above the whimpy part of the power curve(the part that was "weakened" when he went for more high RPM power).He will use the clutch to feed in as much power as traction will allow,till he fully drops the clutch.Now an automatic will sorta do the same thing,but not so well in stock form-it won't initially lock up solid.In real life,with motors that don't have enough power to break the tires lose-you will lose lower speed-lower RPM acceleration when you go for peak power at the expense of real world power.This "slipping"is hard on the clutch or auto transmission.Now,after 1st gear,gets you to 40 mph or so,the drop in RPMS to second gear shouldn't drop you into the whimp RPM power range.
The reason you never see Dyno charts from idle on up,is that the motors just can't smoothly drive the dyno at low RPMS and full throttle.When they would be adding the load to the rear tires,the motor would start jerking-it just isn't designed to work at full load -full throttle-1000 rpms.They usually won't drive the dyno smoothly till about 3000 rpms.It would also "lug" the motor and maybe damage the bearings.Lugging is that jerky feeling you get in a manual transmission car when you have it at low RPM in too tall a gear-you sorta feel the motor slowing down between piston pulses.Thanks,Charlie

[rholland]

"HP is always more than torque over 5252-they always cross right at 5252.Of course,some motors don't rev that high,so they never cross."
B--- S---

[CoachBoone]

Now I remember why I keep coming back here......thanks for the class guys, really!!

[JAOHN]

Originally Posted by rholland

B--- S---

HUH?? how could it be BS when if you look at the dyno you posted, at 5250 RPM the HP is about 77/78 by the HP scale on the left. Amazingly, at the same rpm, the torque scale on the right says that the torque line is also at about 77/78.

[PHOEBISIS]

Look at the equation,they have to cross at 5252.Now they don't measure at exactly 5252 RPMS,so sometimes it will appear the graphs don't cross,but HP=TQ at 5252 -ALWAYS-.Ask Coach Boone-I have seen him give some math corrections here before.Math teacher??Thanks,Charlie PS I didn't tumble to this,someone told me,and them it seemed obvious.
TQx5252/5252=HP cancel out the 5252 and TQ=HP at 5252RPMs always.
RHOLLAND-the right hand and left hand vertical scales are different,so that is why the curves don't cross.If they were the usual-both TQ and HP on the Y axis(left hand vertical),they would cross at 5252.

[rholland]

Now I see, you're right.

[Bob]

Originally Posted by PHOEBISIS

The reason you never see Dyno charts from idle on up,is that the motors just can't smoothly drive the dyno at low RPMS and full throttle.When they would be adding the load to the rear tires,the motor would start jerking-it just isn't designed to work at full load -full throttle-1000 rpms.They usually won't drive the dyno smoothly till about 3000 rpms.It would also "lug" the motor and maybe damage the bearings.Lugging is that jerky feeling you get in a manual transmission car when you have it at low RPM in too tall a gear-you sorta feel the motor slowing down between piston pulses.Thanks,Charlie

Another reason you don't see Titan chassis dyno numbers from 1000 to 2500 rpm is, unless you're dynoing in first gear, the transmission will downshift at that low of an rpm unless there's a "hold" button that I'm not aware of

[loufish]

It's important to remember that we measure torque, then calculate HP using the RPM where the torque occurs.
When you dyno an engine, you plot torque on the graph, then come back and calculate the HP for every rpm point on the graph. You multiply the torque by the RPM, and then divide that by 5252, now you have HP.
No matter how much Torque you make, once you multiply that by any RPM below 5252, the answer (which will be HP) has to be less then the torque value for that same RPM.

We have a saying in racing, Torque moves metal...The heavier the veh, the more important the torque numbers

[rholland]

I must say that you guys have given me some of the most usefull info, to help me tune my bike for next season, that I have ever seen on any forum. I will reset my cams to move the torque curve higher into the rpm range. In the 1/4 mile the engine never drops below 8000 rpms. I shift at 10,500.
Thanks a bunch.

[HavockWK]

Juma and Phoebisis, thank you for your replies.

Phoebisis:
I understand that in drag racing, you rev up the engine before take off to make sure you start the race in the higher end of the power band. But what about other kinds of racing like Cart, Formula 1, etc where there is a lot of racing done throughout most the RPM range due to things like hair pin turns where they gotta downshift and down the RPM’s to get low end torque to blast ‘em back into the straight away to the next turn without breakin’ the back-end loose like you can get away with in a drag race? And for those of us with a 5 speed auto tranny, big tires and no clutch?
As for the dyno thing, I can understand that there are flaws in the ability for a dyno to test the full range. This just means we need a better dyno! But, what about the engine dynos that motor manufacturers use? For example, Nissan says “Fully 90% of its torque kicks in below 2,500 rpm for power you'll have at your beck and call. Even more so, 80% comes in at 1,000 rpm, putting it among the top of its class.” There’s gotta be a way to measure and track this kind of information to verify claims such as a mod increases the ability for the engine to make horsepower, but reduces its ability to generate toqure. No, I don’t want to go pull my engine every time I make a mod change, but I would like to see numbers/charts from the big mod manufacturers (or anybody) to verify the claims. (seeing is believing whether proving or disproving) If all we ever get is a HP/torque chart above 3000 RPM, then how can we be sure of how the low end is being affected?

Juma:
From what you are telling me that because my engine doesn’t have to work as hard, it can’t work as well? I’m not really sure where you are trying to go with the inertia thing. There are too many real life factors that play with inertia inside the engine, especially since part of Newton’s laws specify that an object in motion tends to stay in motion, in the same direction it is traveling. This would fit it we look at the entire mass of the truck being accelerated down a straight dragstirp, but the physics gets weird when you start talking about circles. Also when you think about the pistons, they have to come to an absolute stop at both the top and bottom of their swing. Also, the faster you have to accelerate the piston, the more power it takes and consequently the more force it takes. If we look at these equations:

Work= Force * Distance
Power = Work / Time
Then combine to get: Power = (Force * Distance) / Time

Distance is the linear measure of how far the piston travels from the bottom to the top (or vice versa) of its swing. Time is how long it takes to move the piston from the bottom to the top (or vice versa) of its swing. So basically it takes more power to keep the engine at a higher speed that a slower speed. However, what I am looking for is the low end info. Same equations come into play at the low end, it is just that we are focused more on decreasing the Time variable and consequently Power is increased if the Work value stays constant. Of course if the time stays constant, but the Power is increased, then the Force component of Work will increase because in our case we know that the Distance is constant. Crap, I’ve gotten off track!

Let me get back to the basis of my question… If I use a mod, say big pipes on my exhaust, some people claim I will lose low-end torque, but gain horsepower. Let’s also mention things like scavenging and backpressure, free-breathing and strangulation. If we look at the engine as an air pump as you said, then let’s consider the intake to be capable of a constant max volume ( standard cfm so we can ignore the temp factor) for the entire discussion and only the exhaust volume (cfm) will change through pipe restrictions or lack there of. OK, let’s say that the natural flow balance for the pump is 1:1 (1cfm in, and 1cfm out) for every 1 unit of work. Now if we put an exhaust pipe on the pump that is capable of 2cfm, the pump will still output 1cfm, with 1 unit of work because the exhaust pipe allows the pump to breathe freely. If we put an exhaust pipe on the pump that will only handle .5cfm, then the pump will be required to increase the amount of work to be able to continue to output 1cfm because of backpressure. If it is not able to continue to output 1cfm, then the pump will be slowed and perform less work because the exhaust air passage is constricted causing the pump to be strangled. And if we remember that work is based on force, which in our case is torque, and the constant distance, then you cannot say that back pressure increases low end torque because of backpressure. You can only say that the back pressure will either reduce the output of the engine, or that it requires more torque to do the same work as the free-flowing exhaust. Don’t confuse potential for work with actual work. Just because an engine doesn’t require as much torque to generate power, that doesn’t meant that the engine cannot generate that torque, it just doesn’t have to.

As for scavenging versus backpressure, what I have read is that scavenging is good because it helps draw exhaust out of the cylinders. How does it do this? By the same method as you clean your carpet, be creating negative pressure or a vacuum inside the exhaust system between the exhaust valve and the X-pipe, H-pipe or whatever is your source of scavenging. Backpressure is positive pressure is this same space of the exhaust system. Because the backpressure does not allow the piston to easily push all of the exhaust gases out of the cylinder, and consequently not have as clean of a burn on the next fire, the engine loses its potential energy because it can’t get as much fuel/air mixture in the cylinder. Then when the spark ignites the fuel/air mixture, the bad gases from the remaining exhaust will not allow the burn to be as efficient and therefore you lose the ability to produce kinetic energy that actually generates the force to make torque in the engine.

Basically, if we put on mods that allow our engine to produce more power without requiring as much force, it doesn’t mean the engine is not capable of producing that force. Just think of the difference between driving the truck on the road vs jacking it up and letting the tires be accelerated without having to push the truck. When the truck is on the road, it takes more force to make the tires rotate, but with it jacked up, it takes very little force from the engine to make the tires rotate. I could come up with several scenarios where the amount of force required to do work changes based on alterations of the efficiency of the machine and/or load conditions with a constant maximum force capability.

[Tim Sloper]

A reduction of backpressure will not by itself create a loss of torque in the lower RPM band. First, most people only think they have lost torque and never actually do an apples to apples before/after dyno comparison (along with A/F ratio readings). If there is a loss of torque it will directly be attributed to a significant change to the A/F ratio. If you add headers the fuel trims will begin to go positive. This means that the factory calibration is not correct for the new configuration and the computer is having to add a compensating factor to the injector pulse widths. This is a good thing but it also can have a negative effect at wide open throttle where the factory already tends to delivery a richer A/F ratio than is ideal from a performance point of view. Being too rich will cause a loss of power.

I've been tuning (PCM calibrations) and modifying GM LS1 based vehicles for 6 years and have never seen a loss of torque result from a reduction in backpressure. Once the A/F ratio is brought back inline you will always make more power if there was a restriction in the first place. Another common thing is that because the exhaust note has become greater in amplitude the feel doesn't necessarily equal the new sound. Have you ever seen the guy at the track that sounds like top fuel but only runs 13s?

Tim

[juma]

Originally Posted by Tim Sloper

A reduction of backpressure will not by itself create a loss of torque in the lower RPM band. First, most people only think they have lost torque and never actually do an apples to apples before/after dyno comparison (along with A/F ratio readings). If there is a loss of torque it will directly be attributed to a significant change to the A/F ratio. If you add headers the fuel trims will begin to go positive. This means that the factory calibration is not correct for the new configuration and the computer is having to add a compensating factor to the injector pulse widths. This is a good thing but it also can have a negative effect at wide open throttle where the factory already tends to delivery a richer A/F ratio than is ideal from a performance point of view. Being too rich will cause a loss of power.

I've been tuning (PCM calibrations) and modifying GM LS1 based vehicles for 6 years and have never seen a loss of torque result from a reduction in backpressure. Once the A/F ratio is brought back inline you will always make more power if there was a restriction in the first place. Another common thing is that because the exhaust note has become greater in amplitude the feel doesn't necessarily equal the new sound. Have you ever seen the guy at the track that sounds like top fuel but only runs 13s?

Tim

but, you did see the torque peak shift up the rpm curve, right. the loss of low end torque is essentially from moving it up the rpm curve, not out the window and never to be seen again. juma

[PHOEBISIS]

I've never been clear on why a loss of "backpressure' would cause a loss of low RPM power or torque.
Tell me why this is wrong.
Very low restriction exhaust-low RPMS-Open Exhaust valve at 120 degrees ATDC on the power stroke-all the gases stream out immediately,and I am left with no pressure to push the piston for the last 60 degrees of the power stroke
High restriction exhaust-low RPMS-Open exhaust valve at 120 degrees-gases flow out more slowly,leaving more high pressure gas to push the piston the last 60 degrees.
Shouldn't this scenario mean that you do lose low end torque with a very low restriction exhaust?
Don't the Motorcycle Manu build adj exhaust restrictors into their pipes for just this reason??
Correct me on this-I'm just not clear on it,Charlie PS I am clear that as the time to empty decreases,and the volume emptied per unit of time increases-then the low restriction exhaust will "pay for itself".