9 replies to this topic

[ray b]

as flow in and out of a cylinder improves and HP goes up torque goes DOWN!!!
WHY?????????
example my ride has a GM v-6 2.8 and if exaust manafolds are opened up by removing
block to flow [ lips sticking in to edges] HP goes up by 4-5 BUT torque drops 2-3ft-lbs
so how does eazyer exit of exaust gas HURT torque?????
same effect if a bigger intake is used MORE HP BUT LESS TORQUE so better flow seems to hurt torque!!!

[Top Fuel F1]

To be totally accurate I need to look at my reference material (which I don't have with me); but basically there is an ideal cross section area for both the intake and the exhaust paths for any given engine design at any given RPM. Just making these paths larger (less restrictive) is not the answer. The right air/exhaust gas velocities are important in making the engine reach it's greatest performance potential.

[testarosa]

the increased flow increased torque higher in the rpm range, giving the increase in power, even if there is too much flow in the midrange. Remember that the area under the power graph and the torque at the wheels is what's important, not the peak torque at the flywheel.

[ray b]

actualy peak torque droped in exaust mod on the 2.8
hi-er RPM torque did not drop off as much and
torque peak moved up range a few 100 RPM
but hyd-lifters limit RPM anyway to about 6000.
full tube headers are worse a little more HP up hi
and more loss of peak torque with a move up in RPM


what torque rates are claimed in F-1 now or in resent past?

[palmas]

Originally posted by ray b
as flow in and out of a cylinder improves and HP goes up torque goes DOWN!!!
WHY?????????
example my ride has a GM v-6 2.8 and if exaust manafolds are opened up by removing
block to flow [ lips sticking in to edges] HP goes up by 4-5 BUT torque drops 2-3ft-lbs
so how does eazyer exit of exaust gas HURT torque?????
same effect if a bigger intake is used MORE HP BUT LESS TORQUE so better flow seems to hurt torque!!!

It is all about filling the cillinders. Top power is achieved with "all you can get" inside the cilliders, the better the flow, the more you can get in, specialy in top rpm, when you haven't got much time.

When you have time to fill the cilinder (low rpm) you need some turbulence to improve fuel pulverization and mixture homogeneity. And turbulence means worst flow.

Many engines have air manifold (?) lenght variation to get the best of both worlds.

[Tec Freak]

Originally posted by ray b
as flow in and out of a cylinder improves and HP goes up torque goes DOWN!!!
WHY?????????
example my ride has a GM v-6 2.8 and if exaust manafolds are opened up by removing
block to flow [ lips sticking in to edges] HP goes up by 4-5 BUT torque drops 2-3ft-lbs
so how does eazyer exit of exaust gas HURT torque?????
same effect if a bigger intake is used MORE HP BUT LESS TORQUE so better flow seems to hurt torque!!!

I believe the problem your discussing is a variation on the venturi effect. Air, fuel, exhaust or whatever accelarates as it is forced or drawn through a narrow apperature. Thus, at low rpm you actually put more mixture into your cylinders with a small throttle body and manifold. This trick only works up to a point, because eventually the capacity of the small body and manifold is reached and the engine is starved for more fuel (this will happen at higher rpm, right?). Of course, since hp is directly related to rpm (and torque is not) this practice restricts rpm and thus hp.

The same principle works in reverse on the exhaust end. If the exhaust manifold is narrow the venturi effect essentially "sucks" exhaust gas from the cylinder. Thus, a smaller manifold usually results in better low-end power. As rpm and total exhaust volume increases, however, the smaller manifold eventually reaches capacity, back pressure builds, and power decreases.

As for why hp goes up and torque goes down, remember that hp = (rpm x torque)/c. If your engine gains intake volume or looses back pressure it should gain some rpm, which will offset the decrease in torque. HP can increase even if torque is decreasing, so long as the rate in change of rpm is greater than the rate of change(downward) for torque.

As for "what torque rates are claimed for F1," nobody publishes torque figures. I calculate that at 850hp and 18,000 rpm an F1 motor only produces 119 ft/lbs of torque. Not that surprising really, if you consider the stroke is only about 41cm.

[Top Fuel F1]

Basically one would design the size of the intake and exhaust cross sectional areas to maintain the highest volumetric efficiency in the areas of RPM of most interest. Bigger and bigger openings are not better. The correct gaseous velocities must be maintained.

[MRC]

The original question/assumption is that torque goes down, but horsepower goes up, with improved flow in and out of the cylinder. I find this to be an extremely broad statement.

Improving mass flow through the cylinder, improves torque throughout the rev range. Intake valve closure timing is very important to volumetric efficiency. The pressure and velocity slightly before valve closure is important, thus the timing of the valve closure is important. As TFF1 and others have said, the velocity through the valve will have a large effect on the VE (vol eff). So will the pressure.

TFF1 makes the important point that the port (and everything else doing any breathing on the engine) must be designed for the RPM range of interest. Port size (diameter and volume) will effect velocity, but will also effect frictional effects. These two effects must be weighed in together. Let me also say that changing any pipe or port diameter will have an effect on the tuning resonant frequency. So, the exhaust ports or pipes have been opened up. What gave the change in torque? If flow was improved, was the AFR brought back into spec?

A venturi in the flow does not help to pull gases from the cylinder. This is a out of date and incorrect theory that has been shown to be inaccurate. Look at a simple steady flow case in a venturi tube to convince yourself of this. There are some venturi effects in collectors. But take a single cylinder isolated exhaust pipe, put a venturi in it, and the venturi is not sucking any gases from anywhere.

Turbulence in the manifold does not always mean "worst flow". Generally not a good thing on a large scale, but turbulence in proper areas can be employed to your benefit in some cases. In-cylinder turbulence is generally a good thing. Something to keep in mind is that for more swirl (and tumble to a much lesser degree) you will pay for it in lower flow into the cylinder. Everything is a trade off and nothing is free.

Tec Freak, you may want to check your calculations on the F1 torque. My guess is that your off by, close to a factor of 2, from the bhp and rpm numbers that you gave. I think you meant 4.1cm or 41mm, not 41cm??

[palmas]

Originally posted by MRC


Tec Freak, you may want to check your calculations on the F1 torque. My guess is that your off by, close to a factor of 2, from the bhp and rpm numbers that you gave. I think you meant 4.1cm or 41mm, not 41cm??

torque should be over 350Nm and stroke should be between 4 and 5 cm!

[lateralforce]

Originally posted by Tec Freak


the stroke is only about 41cm.

I think u meant 4.1 cm.