Re: low/high end torque

["MO ( Steve Mick)" <micher@xxxxxxxxxxxxxxx>]

Thanks Dooner, I did find that interesting--I will add that to my list of 
"keepers".................................MO
----- Original Message ----- 
From: "Dooner" <doonerfunderburk@xxxxxxxxx>
To: <1962to1965mopars@xxxxxxxxxx>
Sent: Tuesday, December 29, 2009 7:48 PM
Subject: low/high end torque



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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Please address private mail -- mail of interest to only one person -- directly to that person.  I.e., send parts/car transactions and negotiations as well as other personal messages only to the intended recipient, not to the Clubhouse public address. This practice will protect your privacy, reduce the total volume of mail and fine tune the content signal to Mopar topic.  Thanks!

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