Twin Plugging a Stock Air Cooled 911
07-20-2012, 01:12 PM
#1
Race Car
Thread Starter
Twin Plugging a Stock Air Cooled 911
To achieve maximum torque and thus maximum horsepower, both the
ignition timing advance and the AFRs must be set close to their ideals for
a given octane level. Because of the offset location of the spark plug in an
air cooled 911 engine, the ideal ignition advance cannot be attained because
of the increased likelihood of detonation versus a center positioned spark
plug for a given octane. Thus the maximum torque is partially compromised
for the stock air cooled 911 engine as a result of the less than ideal ignition
timing advance setup to provide a margin of safety for engine reliability under
all driving conditions.
The amount by which the maximum torque is compromised can be reduced
by using higher octane fuels to decrease the likelihood of detonation, and
thereby allowing the ignition timing advance to approach the ideal value for
maximum torque. This approach, though, becomes impractical with the
limited availability of higher octane fuels over time and with its marginal
effect because of the inherent issue of the offset spark plug.
An alternate approach to the problem is to twin plug the stock 911 air cooled
engine. The allows the fuel charge in the combustion chamber to be ignited
from both sides of cylinder reducing the likelihood of detonation for a given
ignition timing advance and octane level. This approach was adopted by
Porsche on the 964/993 engines when they increased the CRs to 10.3/11.3,
respectively, to reduce the likelihood of detonation and to not additionally
compromise torque, i.e. because of the higher CRs requiring even less timing
advance than the lower CR engines.
Since with twin plugs the burn time of the fuel charge will be somewhat
reduced, given the simultaneous ignition, the ignition timing advance can
be reduced by a small amount, i.e. versus the single plug setting, while
achieving the original level of torque using the same octane level. The key
point here being that the simultaneous ignition of the fuel charge has
reduced the likelihood of detonation, i.e. a volatile secondary pressure
ignition, and not the small reduction in ignition timing advance because
of the second spark plug being used.
Because the likelihood of detonation has been reduced with twin plugs while
achieving the original level of torque, i.e. using the same octane level, the
ignition timing advance can now be increased approaching the ideal value.
This results in a less compromised ignition timing advance to achieve a
maximum torque which was not previously attainable with the single plug
setup. The overall effect of the twin plug setup for an air cooled 911 engine
is a more efficient engine similar to the center positioned spark plug of the
996 engine.
Therefore thru the use of twin plugs on a stock air cooled 911 engine,
an increased timing advance, i.e. nearer the ideal, can be used resulting in
an improved torque level, and thereby more horsepower. And this without
necessitating a corresponding octane increase to prevent detonation and to
maintain the engine's margin of safety as would be required for a single plug
air cooled 911.
References:
911 Carrera 4 Service Information, Porsche, WKD 495 121, pg 2-13
"911 Carrera 4 has two spark plugs for each cylinder to ignite the air/fuel
mixture. In this manner the burning through time of the air/fuel mixture
is shorter because of shorter spark travel and there is a less tendency of
knocking, through which it is possible to increase the compression ratio
and therefore improve the thermodynamic efficiency of the engine."
Porsche 911 Performance Handbook, Bruce Anderson, pg 151
"The improved combustion of the twin-ignition system reduces the engine's
sensitivity to octane, and you can usually get away with a compression ratio
about one point higher."
ignition timing advance and the AFRs must be set close to their ideals for
a given octane level. Because of the offset location of the spark plug in an
air cooled 911 engine, the ideal ignition advance cannot be attained because
of the increased likelihood of detonation versus a center positioned spark
plug for a given octane. Thus the maximum torque is partially compromised
for the stock air cooled 911 engine as a result of the less than ideal ignition
timing advance setup to provide a margin of safety for engine reliability under
all driving conditions.
The amount by which the maximum torque is compromised can be reduced
by using higher octane fuels to decrease the likelihood of detonation, and
thereby allowing the ignition timing advance to approach the ideal value for
maximum torque. This approach, though, becomes impractical with the
limited availability of higher octane fuels over time and with its marginal
effect because of the inherent issue of the offset spark plug.
An alternate approach to the problem is to twin plug the stock 911 air cooled
engine. The allows the fuel charge in the combustion chamber to be ignited
from both sides of cylinder reducing the likelihood of detonation for a given
ignition timing advance and octane level. This approach was adopted by
Porsche on the 964/993 engines when they increased the CRs to 10.3/11.3,
respectively, to reduce the likelihood of detonation and to not additionally
compromise torque, i.e. because of the higher CRs requiring even less timing
advance than the lower CR engines.
Since with twin plugs the burn time of the fuel charge will be somewhat
reduced, given the simultaneous ignition, the ignition timing advance can
be reduced by a small amount, i.e. versus the single plug setting, while
achieving the original level of torque using the same octane level. The key
point here being that the simultaneous ignition of the fuel charge has
reduced the likelihood of detonation, i.e. a volatile secondary pressure
ignition, and not the small reduction in ignition timing advance because
of the second spark plug being used.
Because the likelihood of detonation has been reduced with twin plugs while
achieving the original level of torque, i.e. using the same octane level, the
ignition timing advance can now be increased approaching the ideal value.
This results in a less compromised ignition timing advance to achieve a
maximum torque which was not previously attainable with the single plug
setup. The overall effect of the twin plug setup for an air cooled 911 engine
is a more efficient engine similar to the center positioned spark plug of the
996 engine.
Therefore thru the use of twin plugs on a stock air cooled 911 engine,
an increased timing advance, i.e. nearer the ideal, can be used resulting in
an improved torque level, and thereby more horsepower. And this without
necessitating a corresponding octane increase to prevent detonation and to
maintain the engine's margin of safety as would be required for a single plug
air cooled 911.
References:
911 Carrera 4 Service Information, Porsche, WKD 495 121, pg 2-13
"911 Carrera 4 has two spark plugs for each cylinder to ignite the air/fuel
mixture. In this manner the burning through time of the air/fuel mixture
is shorter because of shorter spark travel and there is a less tendency of
knocking, through which it is possible to increase the compression ratio
and therefore improve the thermodynamic efficiency of the engine."
Porsche 911 Performance Handbook, Bruce Anderson, pg 151
"The improved combustion of the twin-ignition system reduces the engine's
sensitivity to octane, and you can usually get away with a compression ratio
about one point higher."
07-21-2012, 11:35 AM
#3
Race Car
Thread Starter
As you mentioned, the cost is the key drawback.
If one is going to do a top end rebuild, the major cost
is for the distributor. Other than that, there's no real drawback.
If one is going to do a top end rebuild, the major cost
is for the distributor. Other than that, there's no real drawback.
07-22-2012, 12:01 AM
#4
Rennlist Member
It really is a simple conversion on a Motronic engine. I went with the Andial splitter, and welded up a coil mount. It isn't cheap, but it is well done, although I'm sure someone could build their own for a lot less.
07-22-2012, 12:58 AM
#5
Race Car
Thread Starter
"I went with the Andial splitter"
Or use the OEM 993 dual coil driver, i.e. about $150.
Any questions about actually doing a twin plug on a 911 3.2, ask Ed.
He knows all the nut-and-bolts of the process and the final results.
Ed basically did what Bruce Anderson talks about in his book,
i.e. a 911 3.2 to 3.4 with twin plugs without any CR change.
Ed will have to speak about what his final CR was. But in any case,
Bruce Anderson indicates for his mod the results were impressive.
Or use the OEM 993 dual coil driver, i.e. about $150.
Any questions about actually doing a twin plug on a 911 3.2, ask Ed.
He knows all the nut-and-bolts of the process and the final results.
Ed basically did what Bruce Anderson talks about in his book,
i.e. a 911 3.2 to 3.4 with twin plugs without any CR change.
Ed will have to speak about what his final CR was. But in any case,
Bruce Anderson indicates for his mod the results were impressive.
Last edited by Lorenfb; 07-22-2012 at 01:50 AM.
07-22-2012, 10:15 AM
#7
Rennlist Member
Well, I did mine with a bump to 98mm bore, 3.4L and 10.3:1 compression, which were the reasons for my switch. It ran really nice on 91 octane and made pretty decent power.
Trending Topics
07-26-2012, 06:28 AM
#8
Since with twin plugs the burn time of the fuel charge will be somewhat reduced, given the simultaneous ignition, the ignition timing advance can be reduced by a small amount, i.e. versus the single plug setting, while achieving the original level of torque using the same octane level.
http://forums.pelicanparts.com/porsc...93-octane.html
There he repeatedly claimed (incorrectly) that twin plugs require more the timing advance.
My advice is to get tuning advice from someone who actually tunes motors. There is a good explanation of twin plugging here:
http://rennsportsystems.com/letstalk...ignition/#Twin
