Go Back   The Ford Barn > General Discussion > Model A (1928-31)

Sponsored Links (Register now to hide all advertisements)

Reply
 
Thread Tools Display Modes
Old 01-13-2017, 11:04 PM   #1
ericr
Senior Member
 
ericr's Avatar
 
Join Date: Apr 2012
Posts: 1,542
Default Understanding Timing Principles

I did a search for this but could not find an answer.

if the combustion occurs at top dead center with the spark fully retarded, which is the position that gives the most power, why would there ever be a spark adjustment that causes the explosion to occur before the piston hits the top of its travel? doesn't that reduce the amount of power generated?

I suspect someone will comment that this is some relationship between electrical sparking, which is instantaneous, versus mechanical travel, which is slower; but that still doesn't explain it well to me.
ericr is offline   Reply With Quote
Old 01-13-2017, 11:22 PM   #2
redmodelt
Senior Member
 
redmodelt's Avatar
 
Join Date: May 2010
Location: Portland OR
Posts: 6,339
Default Re: Understanding Timing Principles

That's how all most all engines run. The gas exploding/power is not as instantaneous as the spark. As the RPMs increase, you need to have the spark and the start of the combustion of fuel before the piston travels over top dead center so the full expansion/power of the exploding fuel is on the down stroke.
__________________
I know the voices aren't real but damn they have some good ideas!
redmodelt is offline   Reply With Quote
Sponsored Links (Register now to hide all advertisements)
Old 01-13-2017, 11:27 PM   #3
Art Newland
Senior Member
 
Art Newland's Avatar
 
Join Date: Mar 2016
Location: Lynden, WA
Posts: 1,564
Default Re: Understanding Timing Principles

It takes time for the fuel to completely burn.
Art Newland is offline   Reply With Quote
Old 01-13-2017, 11:59 PM   #4
DesmoDog
Junior Member
 
Join Date: Aug 2016
Location: SE Michigan, USA
Posts: 20
Default Re: Understanding Timing Principles

Quote:
Originally Posted by ericr View Post
if the combustion occurs at top dead center with the spark fully retarded, which is the position that gives the most power,
Where did you get the info that max power occurs with timing at zero degrees advance? That's where it's easier to start the engine (especially if starting with a hand crank or kick start) but certainly not where max power is developed at higher RPM.

As already mentioned, if you fire the plug at TDC, the piston is already on it's way down the bore when the pressure from combustion is building. Normal combustion is not an explosion, it's not instantaneous, there is a flame front. Instant would be detonation, which is bad.

Normal combustion takes time. Start it at TDC and you miss the party. Also keep in mind that as the piston nears TDC it is slowing down, so it isn't moving as much per degree of crank rotation as it does mid stroke so pressure from combustion isn't fighting mechanical compression as much as you might imagine.
__________________
-Craig
DesmoDog is offline   Reply With Quote
Old 01-14-2017, 12:28 AM   #5
tbirdtbird
BANNED
 
Join Date: May 2012
Location: inside your RAM
Posts: 3,134
Default Re: Understanding Timing Principles

let us know how you make out
__________________
'31 180A

Last edited by tbirdtbird; 02-03-2017 at 12:28 AM.
tbirdtbird is offline   Reply With Quote
Old 01-14-2017, 01:08 AM   #6
40 Deluxe
Senior Member
 
Join Date: May 2010
Location: now Kuna, Idaho
Posts: 3,774
Default Re: Understanding Timing Principles

Sponsored Links (Register now to hide all advertisements)
Quote:
Originally Posted by tbirdtbird View Post
Wow! Thanks, tbirdtbird! That is some excellent information! Should answer ericr's question beautifully.
40 Deluxe is offline   Reply With Quote
Old 01-14-2017, 11:20 AM   #7
Bob Bidonde
Senior Member
 
Bob Bidonde's Avatar
 
Join Date: May 2010
Location: Long Island, NY
Posts: 3,432
Default Re: Understanding Timing Principles

Embellishing on the answers above, it takes time for ignition to occur, travel of the flame front across the combustion chamber, combustion and a rise in pressure into the thousands of PSI.

Thus as the engine's crankshaft speed increases from idle, the ignition spark must advance before any piston reaches TDC so the maximum pressure occurs at or just past TDC, and on the downward stroke of the piston.

At idle, there is sufficient time for combustion pressure to reach its maximum a few degrees after TDC because the is piston is relatively slow moving.

By the way, a piston instantaneously stops at TDC, accelerates to a maximum speed at 1/2 of its downward stroke, then decelerates to a stop again at BDC. This is due to the natural reciprocating motion of a crank.
__________________
Bob Bidonde
Bob Bidonde is offline   Reply With Quote
Old 01-14-2017, 11:28 AM   #8
tbirdtbird
BANNED
 
Join Date: May 2012
Location: inside your RAM
Posts: 3,134
Default Re: Understanding Timing Principles

see article from #5
Max cylinder pressure has been determined to be needed at 14° ATDC for any motor
the crank pin has to be over center
__________________
'31 180A
tbirdtbird is offline   Reply With Quote
Old 01-14-2017, 12:45 PM   #9
jm29henry
Senior Member
 
jm29henry's Avatar
 
Join Date: Sep 2010
Location: michigan
Posts: 1,126
Default Re: Understanding Timing Principles

Watch utube how to time your model a by Jack Bahm great video easy the do and get perfect timing
jm29henry is offline   Reply With Quote
Old 01-14-2017, 12:48 PM   #10
jm29henry
Senior Member
 
jm29henry's Avatar
 
Join Date: Sep 2010
Location: michigan
Posts: 1,126
Default Re: Understanding Timing Principles

https://www.youtube.com/watch?v=mwirH7f0a9o Try this it should be the link
jm29henry is offline   Reply With Quote
Old 01-14-2017, 12:58 PM   #11
jm29henry
Senior Member
 
jm29henry's Avatar
 
Join Date: Sep 2010
Location: michigan
Posts: 1,126
Default Re: Understanding Timing Principles

I could be wrong but I think the video is made by a ford Barner Jackson!!!,he is a great guy and a really smart model a mechanic
jm29henry is offline   Reply With Quote
Old 01-14-2017, 08:53 PM   #12
Mitch//pa
BANNED
 
Join Date: May 2010
Location: Bucks County, PA
Posts: 11,454
Default Re: Understanding Timing Principles

Quote:
Originally Posted by tbirdtbird View Post
That should help set the record straight....

Google or an internet search is a valuable tool which can answer many questions
Mitch//pa is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Sponsored Links (Register now to hide all advertisements)


All times are GMT -5. The time now is 09:31 PM.