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Just want to double check that I understand TDC correctly.

I have the number 1 piston all the way up and at zero on the tab. First revolution my intake valve for piston 1 opens and closes. Second revolution my exhaust vavle opens and closes. The when my piston #1 is all the way up again. Thats when #1 is firing? Just want to double check and not assume anything
 

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Just want to double check that I understand TDC correctly.

I have the number 1 piston all the way up and at zero on the tab. First revolution my intake valve for piston 1 opens and closes. Second revolution my exhaust vavle opens and closes. The when my piston #1 is all the way up again. Thats when #1 is firing? Just want to double check and not assume anything
#1 fires near TDC after the intake valve closes.
 

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your engine fires 8-36 degrees before the piston reaches the top of the compression stroke, depending on rpm and advance settings.. Both valves will be closed at that point..if both valves are closed and the mark shows 0 * on the timing tab, thats Top Dead Center for #1 cylinder
 

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#1 fires near TDC after the intake valve closes.
Powerlabs has got you covered.

As you rotate the engine through it's strokes, starting a TDC power, the exhaust should open first, followed immediately (and even slightly simultaneously) by the intake valve. The TDC after the intake closes, is firing position.
 

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Stroke #1 (TDC)
Piston going down Intake valve is open (Exhaust Valve closed).

Stroke #2
Piston going up Compression stroke (Both Valves are closed).

Stroke #3
Power stroke Piston going down (Both Valves are closed).

Stroke #4
Exhaust stroke Piston gong up. Exhaust valve is open (Intake valve closed).
 

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stroke #1 (tdc)
piston going down intake valve is open (exhaust valve closed).the exhaust valve closes just as the piston starts to descend.

stroke #2
piston going up compression stroke (both valves are closed). the intake valve closes shortly after the piston starts to ascend.

stroke #3
power stroke piston going down (both valves are closed).the exhaust valve starts to open as the piston approaches bdc.

stroke #4
exhaust stroke piston gong up. Exhaust valve is open (intake valve closed).the intake valve starts to open before the piston reaches tdc and while the exhaust valve is still open ( the overlap period).
race on!!!
Jim
 

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Just want to double check that I understand TDC correctly.

I have the number 1 piston all the way up and at zero on the tab. First revolution my intake valve for piston 1 opens and closes. Second revolution my exhaust vavle opens and closes. The when my piston #1 is all the way up again. Thats when #1 is firing? Just want to double check and not assume anything

If you don't understand where TDC is how did you go about setting your valves? TDC is when your timing marks line up right after your intake valve closes.
 

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Not too long ago I had to verify what stroke my 454" was on. As I already had it's valve covers bolted on I screwed my compression tester into it's #1 spark plug hole then bumped the engine over until I saw pressure building. Then I barred the engine over a bit more until the timing marks on my damper and timing tab lined up.
 

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Discussion Starter #11
Thanks guys
I have found TDC many times but it was always because I had removed the distributor noting where the rotor was and corresponded that with the timing mark.
Never really fully understood the relationship between intake valves opening/closing vs the spark. But know I do understand completely.
 

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Now you under stand 4 stroke.
2 Stroke will screw you up how they work what valves.
 

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Thanks guys
I have found TDC many times but it was always because I had removed the distributor noting where the rotor was and corresponded that with the timing mark.
Never really fully understood the relationship between intake valves opening/closing vs the spark. But know I do understand completely.

Ed, if you'll look at a cam timing chart you'll see the intake valve closes just past bottom dead center. Then when the piston rises the air/fuel mixture gets compressed and the spark plug ignites it just before the piston gets all the way to the top. When the piston reaches the top after the next revolution the exhaust valve is still partially open at the same time the intake valve is beginning to open and that's known as the "overlap" period. There are two kinds of duration. One is the "actual" duration (that includes the opening and closing ramps) and the other is the "effective" duration. The effective duration is the best way to judge a cam's performance as it's measured at .050" lift and airflow has measurably began. Cams that use a large valve lash (like .030") have to have much longer opening and closing ramps and therefore have a very long actual duration.
 

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ed, if you'll look at a cam timing chart you'll see the intake valve closes just past bottom dead center. Then when the piston rises the air/fuel mixture gets compressed and the spark plug ignites it just before the piston gets all the way to the top. When the piston reaches the top after the next revolution the exhaust valve is still partially open at the same time the intake valve is beginning to open and that's known as the "overlap" period. There are two kinds of duration. One is the "actual" duration (that includes the opening and closing ramps) and the other is the "effective" duration. The effective duration is the best way to judge a cam's performance as it's measured at .050" lift and airflow has measurably began. Cams that use a large valve lash (like .030") have to have much longer opening and closing ramps and therefore have a very long actual duration.
it is measured at .050" tappet rise, not valve lift. Which may or may not be enough valve lift for airflow to start.

Race on!!!
Jim
'
 

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it is measured at .050" tappet rise, not valve lift. Which may or may not be enough valve lift for airflow to start.

Race on!!!
Jim
'

Back in the late '50's and early '60's the published camshaft durations were all stated in actual degrees of duration. But it was soon realized the actual duration wasn't a true indication of the camshaft's performance because of the very long opening and closing ramps used on some solid lifter camshafts. So the camshaft grinders got together and decided to use the duration starting (and ending) at .050" tappet lift to give the buyer a better picture. Today almost all camshaft grinders give us both of the figures so we can make a more informed decision when we select a camshaft.
 

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Reed Valves ;)



And direct injection too... Even the 2-Strokes have moved on with times ;)
Wel if you want to go that way you have.

1.Reed Valves

2. Power Valve Exhaust valve.

3. Rotax Valve

4. 2 stroke Detroit diesels If I remember they were developed for the US Military so they can just about run any thing though them so they will run, I think they had intake valves only they were a wild set up it the one I seen had a turbo charger on top a super charger.
 

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The crazy thing about 2 strokes they breath and blow out of the Exhaust and Intake at the same time when they run.

Remember there no valves in them to stop the reverse flow of intake or exhaust flow that why it can breath thought the exhaust pipe when its running and blow exhaust out the intake also when running.

Back in the day when Dirt bikes ran mostly down pipes I have killed many bikes getting to deep of water or mud sucking back up though the exhaust system.
 

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. 2 stroke Detroit diesels If I remember they were developed for the US Military so they can just about run any thing though them so they will run, I think they had intake valves only they were a wild set up it the one I seen had a turbo charger on top a super charger.
I worked at a Nuclear Power Plant and we had two HUGE ones of those as backup emergency power. They used two pistons per cylinder, eight cylinders, each one big enough to fit a man. They had a roots blower that was the size of a big block V8, and a turbo the size of a beachball. Only ran 3 or 5PSI; the boost wasn't to make power as much as to keep the engine savenging linearly. CRAZY engines, and a real sight to behold when they fired up; the startup requirement was from "cold start" (they were kept warm at all times with electric heaters) to 100% rated power in 30 seconds or less!
 
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