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Old 08-05-2010, 10:33 AM   #1
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Default Glow plug rating vs engine power vs runtime ?

Ok couple questions:

Now I understand as I move from say a #7 to a #8 plug I am increasing run time as I am able to run engine on leaner setting

However am I at same time decreasing peak engine output by running the colder #8 plug?

Seem some mixed info on this looking for some feedback.

Where is Roelof! Ha!
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Old 08-05-2010, 10:37 AM   #2
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Originally Posted by senna555 View Post
However am I at same time decreasing peak engine output by running the colder #8 plug?
Not necessarily. Too hot a plug can cause detonation. Very bad for power and engine life so a colder plug will make more power in this situation.
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Old 08-05-2010, 11:04 AM   #3
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A colder plug will make better peak horsepower because the detonation of the fuel happens much later in the stroke of the piston.

The fresh fuel mixture is going to detonate at some point in time when the piston reaches TDC. A hotter plug will make this happen much sooner, this will cause a loss of HP when it happens too advanced in the stroke.

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Old 08-05-2010, 11:15 AM   #4
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Thats why most drivers change the compression and correct the changed ignition timing back with the plugnumber. Changing the compression is giving the change in milage.
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Old 08-05-2010, 02:13 PM   #5
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Quote:
Originally Posted by senna555 View Post
Ok couple questions:

Now I understand as I move from say a #7 to a #8 plug I am increasing run time as I am able to run engine on leaner setting

However am I at same time decreasing peak engine output by running the colder #8 plug?

Seem some mixed info on this looking for some feedback.

Where is Roelof! Ha!
there is a danger with using too cold plugs.
if you running a very cold plug you have to lean the engine so it makes power at low-mid range
and you might end up with a too lean mixture which may damage the engine.

It needs some experience to mess with shims and compression but Roelof is right.
I would start with a mid plug and make a hard run. Then, inspect the
plug wire. If it is still shine and motor works well i would stay with it. If plug
wire is not so good and engine seems to maintain rpms at the end of the
straight when throttle is off or maintain high rpms at idle while your needles

are not set too lean, it is good to try moving up a #plug.

When weather is hot we may end up with a colder plug or if the track has a very long straight.
It is always recommended to use a colder plug when moving to higher %nitro.
Also, a hot plug will give you definetely more bottom-mid power while a cold plug will give you more top end.

i am not sure that a colder plug will help you with runtime because if the engine
doesnt produce enough power exiting corners you may end up using more throttle
and this definetily affects runtime.

I think that you have to run the plug that runs better with the engine and search
other factors affecting runtime such as gear ratio or pipe model.

just my experience.
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Last edited by gfilos; 08-05-2010 at 03:14 PM.
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Old 08-05-2010, 04:24 PM   #6
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Quote:
Originally Posted by Roelof View Post
Thats why most drivers change the compression and correct the changed ignition timing back with the plugnumber. Changing the compression is giving the change in milage.
Can you go into more detail with examples..please.

Thanks
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Old 08-05-2010, 04:49 PM   #7
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If I am right then lowering the compression will give a better mileage, it does give a later ignition so changing the plug from 7 to 6 could be needed to bring back the ignition on time.

Lowering the compression will loose bottom power and does give a slight more RPM
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Old 08-06-2010, 07:09 AM   #8
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sorry to sidetrack a little: example

a .12 engine usually comes stock shimmed for 16%nitro using a #6 plug.
what's the environment that the engine is stock for? i assume it's for sea level usage but what about temperature and humidity?
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Old 08-06-2010, 09:18 AM   #9
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Quote:
Originally Posted by Roelof View Post
If I am right then lowering the compression will give a better mileage, it does give a later ignition so changing the plug from 7 to 6 could be needed to bring back the ignition on time.

Lowering the compression will loose bottom power and does give a slight more RPM
Experience dictates that just by lowering the head (more compression) you gain more power especially in lower rpm range, idle quality can suffer, but the engine also runs cooler . Also, that a higher head (less compression) will increase top rpm speed on bigger tracks.
A decrease in head shims (an increase in compression ratio) will increase torque because as the compression ratio goes higher, the actual ignition timing occurs sooner. However there is a point of diminishing returns where detonation occurs or engine temps can soar, and if this happens a colder plug can help.
A colder plug will also increase torque, except in the instance of a colder plug the ignition is slowed until a greater point of compression build occurs.
When you increase head shims (a decrease in compression), top end is enhanced as the ignition timing is retarded and occurs later. Generally a hotter plug is needed to advance the ignition cycle so that timing does not occur to late in the cycle, but at this point you end up over leaning the engine to get it to rev properly and the engine life will suffer dramatically.

On a .12 engine we would only advise going 0.10mm over or under 0.46mm, so that is 0.36mm or 0.56mm of total head clearance at sea level.
Generally on a .12 engine 0.10mm will change the compression ratio about 3/4 to 1 point..

We should also state that the comments with regards to plugs and head shimming are when both are used together. Used alone a hotter plug will rev harder and a colder plug will make more torque.

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