Hi everyone
I've been watching this thread with some interest- Ricks latest post is
especially good
My experiences with very long stroke passenger car engines has been mixed to
say the least- Seventies engines in particular seem to have some odd
responses to ignition timing and advance. In short stroke very large bore
engines (say Chevy 164, 283, 396) generally the idea of advancing the timing
as far as possible without ping under pretty much all circumstances seems to
work well, but the big undersquare engines (Pontiac & Olds 455, Chevy 400,
Buick 430/455) they seem to have a 'sweet spot' beyond which extra advance
doesnt improve performance on any level. In short, they dont respond
positively to maximum advance, which is characteristic of many oversquare
engines.
I'm not really sure why this is. The whole thing is further muddied by
primitive emissions control measures employed on a lot of seventies
powertrains, and even more primitive measures employed to correct
drivability problems resulting from the original tuning compromises. Engines
using ported vacuum are quite prone to dieseling due to the minimal advance
at idle and wide throttle opening neccessary to maintain idle speed, as one
example. Its also often very hard to determine where the timing is at some
point on the rpm/vacuum scale.
Here's what I have more or less come up with, by no means is it the actual
explanaition, but just my theories; They are in no special order.
Apparently big inch, low compression engines can tolerate terrific initial
lead on the static timing *IF* the distributor centrifugal advance isnt
coming in early, and when the vacuum advance is modest and slow acting.
16-18deg BTDC can work on regular gas in many GM big inch engines at sea
level. Some combinations (catalyst 455's as example) originally spec'd
timing in this vicinity. The good part is with lead this high initially,
theres good throttle response from a stop, and on a cold engine, and they
aren't prone to dieseling much, but the bad part is they cant be started if
the battery's weak, as they'll often kick back against the starter.
Somewhere around 12-14 BTDC seems to be the real point of diminishing
returns in this last respect.
The ported advance seems to be largely a way to curb emissions, by reducing
advance on high speed closed throttle coasting situations. The downside of
it is it may be in tandem with a bit of an 'overacheiver' vacuum diaphagm,
eager to make up lost time you might say, which advances too agressively
when it does activate.
In engines using the GM cam, the centrifugal advance curves are a pretty
good match for Wide Open Throttle. Very little improvement seems possible in
this respect. If vacuum advance is disconnected and the initial timing is
advanced to the point of *almost* knocking it will stay right on the hairy
edge of knock all the way past 4500 rpm. One thing I would emphasise is the
cam timing and distributor centrifugal advance are a combined system, which
the cam companies really gloss over in their literature.
Its the cam more than anything else that determines the advance an engine
can tolerate, as cam profiles determine to a large extent what the cylinder
pressures are in the combustion chambers at different speeds at WOT. This is
why most distributor centrifugal advance curves either advance early and
completely by 2600-2800 rpm, or a long slow advance beginning ~1500 rpm,
terminating around 4200 rpm. Its not usually practical to advance fast or
far in the area of the torque peak, which is in practical terms, the point
of greatest average cylinder pressures, and the point the engine will be at
its most senstive to knocking. Changing a cam will move this point of high
pressures up or down the rpm scale, and usually require recalibration of the
centrifgal advance.
GM in particular seems to favour the slow advance to ~4200 concept with
modest initial advance, and a quite agressive vacuum advance device.
I've also noticed vacuum advance units with identical part numbers can vary
extremely widely between individual parts in thier reponsiveness. This is
further complicated by the normal wear and wobbliness of the
mechanism/connections in the distributors. Often its worth trying several
different VA units out one after the other to find one that 'feels good'
once you've got the rest of the tuning more or less figured out.
For a dead stock configuration engine, I confine modifications to
redirecting the VA hose to manifold vacuum, generally with a different VA
unit that flattered the engine better (namely, not too much total advance
possible, but responsive at part throttle) and attempting to discover a
point of initial lead that the engine seemed 'powerful' at in the 2000 rpm
region. My experience suggests this point is a few degrees before theres any
knocking issues on regular 87 octane fuel, usually 8-12 degrees. The
original ported vacuum installations are very sensitive to light
acceleration pinging, and ping in hot weather at highway speeds and medium
vacuum, say 14-18"Hg, when theres actually no ping at WOT.
It seems the advance units are just too much, too early. Unfortunately,
when they're on ported vacuum, they also arent too responsive, either. That
hiccup as you start from a standing stop in vehicles using it is probably
Ported VA. The saggy feeling under hard acceleration at highway speeds is
late timing also, which is another byproduct of a too-little, too-soon
mechanical and initial advance. Having pretty consistently advanced timing
and a very responsive VA unit gives a progressive response.
The large number of GMC owners getting 7 mpg and feeling the need for
premium fuels suggests more than a few coaches must have some really awkward
ignition timing issues. Theres certainly no mechanical reason those 455's
cant run perfectly on 87 gas and deliver good economy and power, aside from
specific troubles on a particular engine, like sharp edges hanging in the
combustion chambers etc. Some combinations of aftermarket parts or fuel
systems may also effect the situation negatively.
One last thing I'd be curious about- has anyone here experimented with a
fixed metering system on a Q jet? Seems a ~44 jet on the primary side and no
needles might work, Cadillac did something like that on some 429's, rather
than having power enrichment, it just relied on the secondaries apparently.
Hope this is food for thought
Brent Covey
Vancouver
I've been watching this thread with some interest- Ricks latest post is
especially good
My experiences with very long stroke passenger car engines has been mixed to
say the least- Seventies engines in particular seem to have some odd
responses to ignition timing and advance. In short stroke very large bore
engines (say Chevy 164, 283, 396) generally the idea of advancing the timing
as far as possible without ping under pretty much all circumstances seems to
work well, but the big undersquare engines (Pontiac & Olds 455, Chevy 400,
Buick 430/455) they seem to have a 'sweet spot' beyond which extra advance
doesnt improve performance on any level. In short, they dont respond
positively to maximum advance, which is characteristic of many oversquare
engines.
I'm not really sure why this is. The whole thing is further muddied by
primitive emissions control measures employed on a lot of seventies
powertrains, and even more primitive measures employed to correct
drivability problems resulting from the original tuning compromises. Engines
using ported vacuum are quite prone to dieseling due to the minimal advance
at idle and wide throttle opening neccessary to maintain idle speed, as one
example. Its also often very hard to determine where the timing is at some
point on the rpm/vacuum scale.
Here's what I have more or less come up with, by no means is it the actual
explanaition, but just my theories; They are in no special order.
Apparently big inch, low compression engines can tolerate terrific initial
lead on the static timing *IF* the distributor centrifugal advance isnt
coming in early, and when the vacuum advance is modest and slow acting.
16-18deg BTDC can work on regular gas in many GM big inch engines at sea
level. Some combinations (catalyst 455's as example) originally spec'd
timing in this vicinity. The good part is with lead this high initially,
theres good throttle response from a stop, and on a cold engine, and they
aren't prone to dieseling much, but the bad part is they cant be started if
the battery's weak, as they'll often kick back against the starter.
Somewhere around 12-14 BTDC seems to be the real point of diminishing
returns in this last respect.
The ported advance seems to be largely a way to curb emissions, by reducing
advance on high speed closed throttle coasting situations. The downside of
it is it may be in tandem with a bit of an 'overacheiver' vacuum diaphagm,
eager to make up lost time you might say, which advances too agressively
when it does activate.
In engines using the GM cam, the centrifugal advance curves are a pretty
good match for Wide Open Throttle. Very little improvement seems possible in
this respect. If vacuum advance is disconnected and the initial timing is
advanced to the point of *almost* knocking it will stay right on the hairy
edge of knock all the way past 4500 rpm. One thing I would emphasise is the
cam timing and distributor centrifugal advance are a combined system, which
the cam companies really gloss over in their literature.
Its the cam more than anything else that determines the advance an engine
can tolerate, as cam profiles determine to a large extent what the cylinder
pressures are in the combustion chambers at different speeds at WOT. This is
why most distributor centrifugal advance curves either advance early and
completely by 2600-2800 rpm, or a long slow advance beginning ~1500 rpm,
terminating around 4200 rpm. Its not usually practical to advance fast or
far in the area of the torque peak, which is in practical terms, the point
of greatest average cylinder pressures, and the point the engine will be at
its most senstive to knocking. Changing a cam will move this point of high
pressures up or down the rpm scale, and usually require recalibration of the
centrifgal advance.
GM in particular seems to favour the slow advance to ~4200 concept with
modest initial advance, and a quite agressive vacuum advance device.
I've also noticed vacuum advance units with identical part numbers can vary
extremely widely between individual parts in thier reponsiveness. This is
further complicated by the normal wear and wobbliness of the
mechanism/connections in the distributors. Often its worth trying several
different VA units out one after the other to find one that 'feels good'
once you've got the rest of the tuning more or less figured out.
For a dead stock configuration engine, I confine modifications to
redirecting the VA hose to manifold vacuum, generally with a different VA
unit that flattered the engine better (namely, not too much total advance
possible, but responsive at part throttle) and attempting to discover a
point of initial lead that the engine seemed 'powerful' at in the 2000 rpm
region. My experience suggests this point is a few degrees before theres any
knocking issues on regular 87 octane fuel, usually 8-12 degrees. The
original ported vacuum installations are very sensitive to light
acceleration pinging, and ping in hot weather at highway speeds and medium
vacuum, say 14-18"Hg, when theres actually no ping at WOT.
It seems the advance units are just too much, too early. Unfortunately,
when they're on ported vacuum, they also arent too responsive, either. That
hiccup as you start from a standing stop in vehicles using it is probably
Ported VA. The saggy feeling under hard acceleration at highway speeds is
late timing also, which is another byproduct of a too-little, too-soon
mechanical and initial advance. Having pretty consistently advanced timing
and a very responsive VA unit gives a progressive response.
The large number of GMC owners getting 7 mpg and feeling the need for
premium fuels suggests more than a few coaches must have some really awkward
ignition timing issues. Theres certainly no mechanical reason those 455's
cant run perfectly on 87 gas and deliver good economy and power, aside from
specific troubles on a particular engine, like sharp edges hanging in the
combustion chambers etc. Some combinations of aftermarket parts or fuel
systems may also effect the situation negatively.
One last thing I'd be curious about- has anyone here experimented with a
fixed metering system on a Q jet? Seems a ~44 jet on the primary side and no
needles might work, Cadillac did something like that on some 429's, rather
than having power enrichment, it just relied on the secondaries apparently.
Hope this is food for thought
Brent Covey
Vancouver
