low/high end torque

[Dooner <doonerfunderburk@xxxxxxxxx>]

I was looking around to find out more of what different performance 
parts do to improve an engine. I found this and thought it was 
interesting. Maybe you will think so too.

To increase torque is simply to increase the amount of air and fuel 
burnt with each stroke, this is in the realms of regualr engine tuning, 
i.e. getting as much air in the cylinder as possible.

What I think you possibly meant was how do you shift the torque curve up 
and down the rev range?

Lets take a simple 2 litre four pot engine. This engine has a rough sine 
wave torque curve, i.e. the peak is around the middle of the rev range, 
say 4.5k out of an 8k max. One thing to remember here is that while the 
engine speed varies greatly, the speed at which air can accelerate and 
change direction doesn't. This is largely about optimising the engine to 
work with the natural resonances within the air.

First, we want to shift the peak torque down the rev range, to about 2k. 
There are a number of ways to do this. The inlet manifold is a good 
starting point. Ideally, we will want to increase the manifold runner 
length. What this will serve to do is increase the time it takes for one 
pressure pulse to bounce off the closing inlet valve, back up the 
runner, bounce off the back of the plenum and return down the runner to 
the valve opening. With the stock runner length, the amount of time this 
movement took was synchronised perfectly with the valve opening at 4.5k. 
While this effect is in sync, the pressure pulse will raise air pressure 
as the valve opens, creating a small boost in volumetric efficiency. 

By increasing the distance and thus time it takes to travel this 
distance within the runner, the pulses will become synchronised with the 
valve openings at lower rpms.

The next thing we can do with the manifold is decrease the throat width 
of the runners. This serves to speed up the velocity of the air mass 
within the runner. This air mass in turn generates more kinetic energy 
as it travels down the runner, and creates a ram effect as it is forced 
into the cylinder at a point where it would otherwise be full at 
atmospheric conditions.

The exhaust manifold runners can also be tuned in length for a similar 
effect. Longer runners will increase low range torque, which is why a 
4-2-1 style header will yeild better low speed gains than a 4-1 type. 
What the ?2? in the 4-2-1 header is doing is effectively increasing the 
length of the primaries and thus increasing the pulse length.

We can also alter the valve timing to suit a lower torque peak. In the 
standard engine, the inlet valve will open a few degrees before TDC, and 
close a few degrees after BDC, and the exhaust valve will open a few 
degrees before BDC and close a few after TDC. To shift the peak torque 
lower in the revs, we will want to decrease the amount of time the 
valves are open, especially the period when both valves are open 
together, otherwise known as the duration of the camshafts. By opening 
the inlet valve later, i.e. closer to TDC than standard, we are reducing 
the possibility of the upwards stroke of the piston forcing exhaust gas 
into the inlet port, as at low speeds, this will easily overcome the 
pressure and velocity of the intake charge. By closing it earlier we are 
avoiding a similar scenario, where the upwards motion of the piston on 
compression forces the fresh intake charge back out of the cylinder and 
into the inlet port, albeit with the addition of considerable heat soak.

By opening the exhaust valve later, we can maximise the amount of time 
the high cylinder pressure is forcing the piston down before releasing 
the burnt gases out of the exhaust port. (It may or may not be obvious, 
but the exhaust gases largely exit the cylinder under their own 
pressure, not by the upwards motion of the piston.)

By closing it earlier, we are reducing the oppourtunity for inlet charge 
entering the cylinder during the valve overlap period to exit straight 
out the exhaust port without being burnt.

These are the simplest ways to shift torque down the rev range. 




To shift the peak torque up the rev range say to 6.5k, we want to use 
the same principals, but in reverse.

The manifold runners want to be short in length, this will decrease the 
time it takes one pressure pulse to reverberate back down to the valve, 
and so will be in sync with the valve opening at high rpms. Likewise, 
the throat widths of the runners will need to be wider. Whilst at low 
rpm we needed to boost the velocity of the inlet charge by restricting 
the throat width, at higher rpm, the inlet charge already holds 
significant veolcity, and so such a restriction would only suffocate the 
engine at high speeds. Thus a wider bore in the runners will allow the 
engine to breathe easier at high speeds, boost the upper range torque.

Valve timing will need to be changed also. In the low speed engine, we 
opened the inlet valve late, now we want to open it early. By opening it 
earlier, the air mass in the port is given more time to begin moving 
forward into the cylinder, and the pressure and velocity of the inlet 
charge is enough to overcome the residual exhaust gas pressure in the 
cylinder. If we opened the valve to late at this speed, the inlet charge 
would not have sufficient time to move into the cylinder, and we would 
lose VE.
By closing the valve later, we are utilising the high speed ram effect 
of the inlet charge to cram in more air particles than is theorically 
possible at atmospheric pressure, even as the piston begins to rise in 
the cylinder.

We will also shift exhaust valve opening forwards, this gives the 
cylinder time to ?blow down?, whereby the the exhaust gases exit the 
cylinder under their own pressure. Most of the useful work is done on 
the power stroke by about 60°, and so we are sacrificing little by 
allowing the spent gas out early, however what we gain, is a reduction 
in pumping losses, generated when the piston has to physically force the 
remaining gases out of the cylinder. This is an increasing possibility 
as the engine speed increases, so by opening the valve early, we are 
ridding the cylinder of as much pressure as possible.

By closing the valve late, and leaving the inlet and exhaust valves with 
a long overlap in which they are both open, we are allowing the vacuum 
effect of the exhaust gas pulse travelling down the exhaust runner to 
?suck? fresh inlet charge into the cylinder.

These modifications will serve to improve high end torque, at the 
sacrifice of low end.

Some systems, like variable inlet manifolds such as Toyota?s TVIS system 
allow two different optimisations, the same with variable valve timing 
systems such as Honda?s VTEC. By ultimately, even with these, the toque 
can only be optimised in two very narrow parts of the rev range, the 
rest remains a comprimise, albeit a slightly broadened one.

There are of course plenty of other ways to alter the engine 
characterstics in design, but these are the simplest modifications to an 
existing design.

Hope someone thought this was interestin too.


Dooner
'64 Plymouth Belvedere 4dr Sedan
Poly 318 + Push Button Auto Trans

Contact me to redneck your radio.


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