[Diy_efi] Timing

Sun Oct 9 23:33:00 UTC 2005

Hugh,
  The 5-10 deg value is actually point of 50% mass fraction burn.  I think I
may have accidentally stated that it was peak pressure.  Peak pressure can
be anywhere from 5-15 degrees, based on time on the dyno, working with very
high end racing engines (i.e. engines from $60k on up), and watching what
happens to the power output as that location of peak pressure moves around.
In most cases, however, it will be no later than about 12 degrees.
Basically, the faster the overall cylinder burn happens, the earlier you can
make the peak pressure and still get the power output.  If you have a
relatively slow burn (i.e. stock engine or mildly modified type
applications), you have to delay the occurrence of peak pressure.

  The reason for this is fairly simple.  As you all know, you start the burn
before TDC, so as the cylinder charge is burning and combustion pressure is
building, the piston is still moving up.  Your growing combustion pressure
is fighting piston movement, costing you horsepower.  This is the main
reason multiplug heads, heads with a centrally located plug, etc are able to
make more power on an otherwise equivalent motor.  Your ignition timing can
be retarded, providing less combustion pressure before TDC, so there is less
pressure to overcome, while still providing that peak pressure soon after
TDC for maximum effective torque.  I think from this, you can see that the
optimum location of peak pressure DOES vary, depending on the fuel, whether
the engine is boosted, and if so how much, cylinder head type, etc, etc.
That's why I mentioned the range.  However, that range is typically more for
racing engines where a fast charge burn occurs.

-----Original Message-----
From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org] On
Behalf Of hugh at sol.co.uk
Sent: Sunday, October 09, 2005 4:27 PM
To: diy_efi at diy-efi.org
Subject: Fw: [Diy_efi] Timing

John,

Many thanks for your recent input to the list, I and many others appreciate
the time you have taken to pass on some of your knowledge.

I would like to ask about the most effective timing for peak cylinder
pressure. The number I have seen most quoted is 14? after TDC, do you have
any point of reference to back up the 5? to 10? ATDC to make maximum power.

Also is the most effective peak cylinder pressure timing the same for all
fuels and high boost turbo applications?

Many Thanks

Hugh 
----- Original Message ----- 
From: John Gross 
To: dh at busb.com ; diy_efi at diy-efi.org 
Sent: Thursday, October 06, 2005 12:25 AM
Subject: RE: [Diy_efi] Timing

Under normal combustion conditions, the energy to ignite a given volume of
the charge comes predominantly from the heat and pressure generated by the
burning of the air-fuel mixture around said volume.  The temperature of the
flame front in a gasoline fueled engine is roughly 4000 deg F, and peak
cylinder pressures in a good naturally aspirated engine can top 1200 psi. 
This is significantly higher than the heat and pressure added from
compressing the air-fuel charge by piston motion and latent heat
absorption.  

Also, I assume that you meant that the flow into the cylinder is directly
related to the throttle position, not inversely related, as such a
condition would mean that a closed throttle resulted in maximum airflow
into the motor.  And yes, manifold vacuum decreases with greater throttle
opening, resulting in higher absolute pressure values and greater air flow.

The timing advance at WOT vs part throttle is due to the relative dynamic
compression.  The more dynamic compression you have, the greater the
turbulence and the faster the overall charge will burn, therefore the less
timing advance that is needed.  Part throttle operation results in lower
dynamic compression because you are blocking air mass from entering the
engine with the throttle.  Lower dynamic compression means more timing
advance.  The engine will run fine at part throttle with less advance, but
will see more efficient operation with more advance.

The overall goal with timing advance is to achieve peak cylinder pressure
and ~50% of your air-fuel charge mass burn at roughly 5-10 deg ATDC. 
Dynamic compression (determined by such things as static CR, forced
induction boost pressure, throttle position, etc, etc), type of fuel used,
cylinder head material, spark plug location, spark plug number, cylinder
head cooling efficiency and uniformity, combustion chamber shape, and many
other factors all influence how fast the net cylinder charge will burn once
ignited.  You adjust the timing advance on the dyno until you find maximum
power (assuming your each maximum power before the onset of detonation). 
If you had the data acquisition system required to measure in-cylinder
pressures on the running engine, you would find your 50% mass fraction burn
and peak cylinder pressure would fall at about 5-10 deg ATDC when you make
maximum power.  This is your MBT timing (maximum brake torque timing).  At
part throttle, you can increase the ignition advance to help keep your peak
pressure and 50% burn at the same general crank angle, and find an
improvement in efficiency, using less fuel, etc, etc.  That is why auto
makers spend so much time on the dyno at part throttle.

Your comment on the failed vacuum advance DH is an interesting one, and
quite correct.  The reason behind this is that the trailing end of the
combustion process is still occurring at the exhaust valve opening point,
because the burn was started too late.  There are times, however, when such
a situation, for a brief time, is advantageous.  Turbocharged engines will
benefit from drastically retarded ignition timing *during gear changes and
slight throttle lifts*, as the super late combustion greatly increases the
velocity of the exhaust gases, helping to keep the turbo spooled.  ;-)

-----Original Message-----
From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org] On
Behalf Of dh at busb.com
Sent: Wednesday, October 05, 2005 6:39 PM
To: diy_efi at diy-efi.org
Subject: Re: [Diy_efi] Timing

I'm assuming that by more pedal you mean to press on the gas pedal and
depress it toward the floor.  The energy to ignite the charge can come from
many sources but a significant source is compression heat.  As the
compression heat is increased the charge burns more quickly, therefore less
advance is required.  But, compression heat is a function of the charge
initial volume and that is proportional to the flow into the chamber which
is inversely proportional to throttle position.  Vacuum is increased as the
throttle is closed for a given engine speed.

I'm guessing you meant that Manifold vacuum decreases and carb flow
increases.  Remember that as the manifold pressure gets lower (absolute)
the vacuum increases.

To simplify, WOT requires less timing than part throttle operation.

If the vacuum advance on a Chevy is defective the headers will glow from
the heat generated by inadequate advance.  Also note that there is no
vacuum advance at start when mimimal advance is required.

dh

----- Original Message ----- 

From: Bret Levandowski 

To: diy_efi at diy-efi.org 

Sent: Wednesday, September 28, 2005 6:33 PM

Subject: Re: [Diy_efi] Timing

>From what I've seen on the vacuum advance on Chevy's, more pedal is more
vacuum to the advance pod. Manifold vacuum decreases but carb vacuum
increases.  Ski

Rick McLeod <dunvegan at sbcglobal.net> wrote: 

> If I make a quick stab at the
> pedal, the only real change before downshift is
> vacuum increase causing the vacuum component to
> increase but I'm still reducing just the base? 

shouldn't this cause a reduction in vacuum? Therefore
causing a reduction in advance toward base?
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