In cylinder turbulence/ burn rate

[rookie]

Then you throw in Darts new wet flow Platinum series
here is an article David did on them
Cylinder Head Wet Flow Testing - Dart Cylinder Heads - Super Chevy Magazine

[automotivebreath]

I find it interesting that the new Dart chamber resembles the RHS or LT1
design. I have always believed that the compact chamber has benefits
of increased squish to bore ratio with improved combustion potential.

[rookie]

Here is a unported RHS chamber.

I think your right on the small chamber thought but with the extra power seen with every little change I wonder how far it will go.

[automotivebreath]

It tempting to look a power alone, but there's more. On another forum the pro's
talk about the need to run two sets of plugs, one set for warm up and one set
to run, what a pain. During warm up the engine is cold and is running outside of
the intended RPM range, the relates to poor fuel vaporization and incomplete combustion
requiring a hotter "warm up plug" . What can we do to improve the combustion
characteristics at low RPM to eliminate the need for two sets of plugs?

As for WOT power I believe the benefits come when the engine requires reduced
ignition advance, generating reduced cylinder pressure before TDC and yet a
higher percentage of the charge burned early in the power stroke.

[rookie]

I bought a scangauge for my 2001 Chevy Truck and it will display what your timing doing while you are driving and interestingly at 60 mph and about 2000 rpm my timing is at 47 to 48 degrees and when I stab the throttle it drops to low 30's and then climbs back up to high 30's as rpm increases.

[automotivebreath]

Originally Posted by Pinhead

Wow, there's a LOT of info in this thread already. Can't wait to see the article!!

Hi Pinhead, welcome to the forum. Feel free to jump right in.

[automotivebreath]

Originally Posted by rookie

I bought a scangauge for my 2001 Chevy Truck and it will display what your
timing doing while you are driving and interestingly at 60 mph and about 2000
rpm my timing is at 47 to 48 degrees and when I stab the throttle it drops to
low 30's and then climbs back up to high 30's as rpm increases.

Rookie, there’s two keys in you post that changes all of the rules in regards
to ignition advance, those are throttling and load.

Lets look at throttling first, basically what we have is a method of controlling
volumetric efficiency of the engine, open the throttle and the VE goes up,
close it and VE goes down. What does this have to do with ignition advance?
The more air/fuel we let into the cylinder the higher the cylinder pressure
gets, the higher the pressure gets the faster the burn requiring less ignition
advance.

Secondly we must factor in engine load, we know that load increases can
bring on detonation. Years ago this was evident when we would climb a hill
and the engine would rattle like crazy. I have given this a lot of thought and
what I come up with is as engine load increases, the resistance to push the
piston down the bore at the beginning of the power stroke increases. This
resistance causes a rise in cylinder pressure by demanding more force from
the expansion of the combustion gasses to push the piston down.

So the engineers at GM programmed the engine management system to
sense the increase in VE and engine load and respond accordingly to reduce
ignition advance. How does turbulence and burn rate factor into all of this?
Your turn.

**********************************

There's a third factor I totally forgot, that’s RPM. As RPM goes up , the time
for flame travel goes down, twice the RPM means 1/2 the amount of time for
combustion to complete.

[rookie]

Originally Posted by automotivebreath

So the engineers at GM programmed the engine management system to
sense the increase in VE and engine load and respond accordingly to reduce
ignition advance. How does turbulence and burn rate factor into all of this?
Your turn.

**********************************

There's a third factor I totally forgot, that’s RPM. As RPM goes up , the time
for flame travel goes down, twice the RPM means 1/2 the amount of time for
combustion to complete.

With increased throttle opening and increased rpm you now have increased airflow and turbulence this creates better air fuel mixture for better atomization so your burn rate speeds up, so now you don’t have to ignite the mixture as soon.

[tommurphy73]

Added to that with increased throttle opening you get an increase in the amount if mixture in the cylinder leading to an increase in dynamic compression ratio which also increases the burn rate.

I am still not 100% on why exactly an increase in turbulence prevents detonation. If an increase in swirl and turbulence increases the burn rate will not that in turn speed at which in cylinder pressure increases leading to a greater chance of detonation.

I have read many explanations of detonation but am still not convinced

Regards
Tom

[rookie]

With faster air speed you have less localized heat, this is why a tighter quench area will allow more compression before you encounter detonation.

As the mixture improves and burn speeds up then you ignite it later, so now your burn is concentrated on pushing the piston down the bore instead of being compressed.

What I wonder is at a higher rpm you now don't have time to burn the whole mixture unless you progressively lean it down some or add some timing back in.