Timing and tuning questions
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Teching In
Joined: Apr 2002
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Timing and tuning questions I keep reading where people need more tuning or gain more from tuning. My question is this, once the AF is straightened out, where else is there a gain? Timing? How much will timing make a difference? Say you advance the timing from 24-26? Then from 26-28? 28-30? Assuming you have no detonaion or KR, won't you consistently see gains? Use one of the 380-400 rwhp head and cam cars as a model. If one produces 390, and the AF ratio is steady at ~13:1, woulkdn't there always be a gain with more timing (again assuming noKR or detonation is present)? I've been reading here for a while,but am used to SBCs (Old skool), do the same basics apply to LS1s?
TECH Senior Member
Joined: Nov 2001
Posts: 9,246
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From: Troy, AL
Re: Timing and tuning questions The lower the timing, the more you gain from a degree of timing. For example, you will see good gains in going from 22-26 degrees. But, go from 26-30 and you will see only small gains. You can go from 30-32 and see no gains at all and above that you will lose power. This is all assuming no detonation.
Just shoot to have 28-30 degrees of timing with NO detonation. On pump gas, you will likely find your limit at 28-29 degrees.
Just shoot to have 28-30 degrees of timing with NO detonation. On pump gas, you will likely find your limit at 28-29 degrees.
Re: Timing and tuning questions What the colonel is saying is that there is a diminishing return on additional timing. This is true. TQmngmt is a culprit as well, have this defeated. The amount of timing is specific to each vehicle and the year of that vehicle for example a 98 can take 31 deg @ wot @ pk HP and a 02 26 @ pk HP the more cyl head psi the less timing it needs, ALL Z06's can take is 24 deg @pk HP
Re: Timing and tuning questions Timing positions peak cylinder pressure relative
to crank (piston) position. There is a sweet spot
where total impulse energy to the flywheel is
maximized. This is "maximum (spark for) best
torque", MBT. It is generally accepted that this
point lies just before onset of spark knock.
This is why cars that are "tuned to the bone"
need knock sensors. Yer buddy the EPA basically
requires that all new cars be tuned right up
against it (best torque is not far from best
fuel efficiency, given proper mixture, and you
have constraints on both - CAFE and emissions).
Advancing from the low side, you are creeping up
on the MBT point. Coming down from the high side,
you are reducing knock events / KR.
Mods that increase cylinder scavenging / filling
are going to increase pre-spark cylinder pressure
and decrease burn time. Decreased burn time needs
(and tolerates) less advance. A fast burn will let
you run higher RPMs more efficiently (deliver full
impulse before BDC / exhaust open). A faster burn
delivers higher peak pressure (harder on hardware)
than a slow burn. Not sure about power-added
efficiency if both are completed before BDC.
It's an area-under-the-curve thing with bleeders
for thermal relaxation etc.
You can cram more fast burns into a second than
slow ones, and that's RPM & power if the rest of
the chain can keep up.
to crank (piston) position. There is a sweet spot
where total impulse energy to the flywheel is
maximized. This is "maximum (spark for) best
torque", MBT. It is generally accepted that this
point lies just before onset of spark knock.
This is why cars that are "tuned to the bone"
need knock sensors. Yer buddy the EPA basically
requires that all new cars be tuned right up
against it (best torque is not far from best
fuel efficiency, given proper mixture, and you
have constraints on both - CAFE and emissions).
Advancing from the low side, you are creeping up
on the MBT point. Coming down from the high side,
you are reducing knock events / KR.
Mods that increase cylinder scavenging / filling
are going to increase pre-spark cylinder pressure
and decrease burn time. Decreased burn time needs
(and tolerates) less advance. A fast burn will let
you run higher RPMs more efficiently (deliver full
impulse before BDC / exhaust open). A faster burn
delivers higher peak pressure (harder on hardware)
than a slow burn. Not sure about power-added
efficiency if both are completed before BDC.
It's an area-under-the-curve thing with bleeders
for thermal relaxation etc.
You can cram more fast burns into a second than
slow ones, and that's RPM & power if the rest of
the chain can keep up.
