Check my math please

[flyinlow]

Say I am out cruising around in the Charger and some guy in a Rustang Girls Type does not show the proper respect, and I am forced to defend the honor of Mopar. The petals on the floor and at the first shift the tack hits 6000 RPM.... It should not work.
100 revolutions per second . .005 second for the power stroke. Combustion speed range in a gasoline engine 6-16 meters per second. I picked a speed in the middle of 400 inches per second. From the sparkplug to the end of the combustion chamber is about 2 inches.  2/400= .005 second to burn across the chamber. I give the flame a 36* BTDC head start above 2500 rpm.  (.0001 sec. at 6000 rpm) With the piston moving away faster then the flame front at mid stroke and the exhaust valve opening before the piston reaches BDC. How can this engine make the power it does? Is my math wrong? Or have I missed something? Why don"t you keep increasing the ignition advance all the way to redline?

It was a cold rainy day and I was garage racing with some buddys.

[Domino]

You're missing Wiebe.  Pressure acts on the piston area, not the flame. 

[flyinlow]

OK, gas pressure on the piston crown is what makes the car go. However it appears the combustion process does not have time to finish. My engines exhaust valves start to open 51* BBDC . I should be blowing flames out the exhaust ports.

[Cooter]

This kinda falls in the same category as the "How can a high compression engine live with a HUGE overlap cam at idle, yet still make power with the same camshaft at 6500 RPM, if it was used to bleed off compression?"
Things that are hapening at low RPMS are not nessesarily the same things that are happening at high RPMS.

If you had the headers/manifolds off the engine, you WOULD see flames coming out the ports. Bout the only way you would see a flame all the way back to the tailpipe is if the jets just fell completely out the carb...

[frederick]

You're assuming the air in the cylinder is stationary.

[flyinlow]

You're assuming the air in the cylinder is stationary.

So the air/fuel mixture swirling around speeds the combustion process enough to make it work?

The stock centrifugal advance curve slowly advanced until almost 4800 rpm. I recurved it to have full advance by 2500 rpm but nothing more after that as the combustion process has less and less time to finish. If 36*BTDC works well at  2500rpm , why not more at 5000 rpm?

[firefighter3931]

If 36*BTDC works well at  2500rpm , why not more at 5000 rpm?

Increased timing at lower speeds helps with power but there is an optimal timing setting for each motor which depends on several factors. As piston speed increases the engine requires less lead time and catches up to the spark advance.


Ron

[flyinlow]

[quote . As piston speed increases the engine requires less lead time and catches up to the spark advance.
Ron
[/quote]


Trying to get a better understanding of that statement. Most of the wedge headed V8s I have played with run total mechanical timing some where in the 30*-40* range. I understand that 60*-70* advance would increase pumping losses, but at high rpms, you would think that the extra lead time would make getting peak combustion pressures to occur in the 15* ATDC for max torque easier.   

Craig

[frederick]

Around 3000rpm the need to increase the timing is almost canceled out by the increase in turbulence.
Read this page and the next for some good info.
http://books.google.com/books?id=mX1-OJBQ6ngC&pg=PA19&lpg=PA19&dq=Effect+of+Engine+Speed+on+Flame+Speed.+The+most+important+effect+shown+is+the+fact+that+the+variation+in+crank+angle+occupied+by+flame+travel&source=bl&ots=ilinXC4tvQ&sig=JU01FKAYb6FtkQ_CPRcYajmsEhQ&hl=nl&ei=JrqmTcz1BcSZOuiT3NEJ&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBUQ6AEwAA#v=onepage&q=Effect%20of%20Engine%20Speed%20on%20Flame%20Speed.%20The%20most%20important%20effect%20shown%20is%20the%20fact%20that%20the%20variation%20in%20crank%20angle%20occupied%20by%20flame%20travel&f=false

More turbulence means you don't need as much advance.
Factors that increase turbulence are:
-Higher rpm
-Smaller squish gap
-More swirl
-More tumble
Though swirl and tumble are not something you'd want on a high performance high rpm engine, cause they decrease the volumetric efficiency at higher rpm.


Frederick

[flyinlow]

Thanks for the replys and the link 

[Challenger340]

It has to do with the better Volumetric Efficiency, and higher initial pressures to incipient burn, that the camshaft produces higher in it's sweetspot.