Double check dot to dot..
03-31-2018, 07:33 PM
#21
Hi 351, I will state another VERY COMMON asked question here at LS-1 Tech as the example.
The Piston/Valve distance question is common : "is it safe" to run this camshaft ?
The cylinders are numbered with the piston position of #1 AND #6 being at TDC BOTH at the SAME TIME.
THUS one cylinder will be FIRING (compression) AND the other cylinder will have the inlet/exhaust valves "off the seat" OVERLAP.
This IS were the P/V distance is of CONCERN !
The Valve Lift is caused by the Camshaft Lobe Lift, an item that can be measured AT THE LIFTER.
The "proud" statement is ABOUT that distance "out" of the lifter bore with EASY MEASUREMENT from the crankcase deck/lifter plunger.
I measure with a Dial Indicator Gauge standing on the block deck with the shaft of the indicator placed at the lifter push rod register.
I believe you question is VERY VALID, thanks.
SO now, how did I do ?
Lance
The Piston/Valve distance question is common : "is it safe" to run this camshaft ?
The cylinders are numbered with the piston position of #1 AND #6 being at TDC BOTH at the SAME TIME.
THUS one cylinder will be FIRING (compression) AND the other cylinder will have the inlet/exhaust valves "off the seat" OVERLAP.
This IS were the P/V distance is of CONCERN !
The Valve Lift is caused by the Camshaft Lobe Lift, an item that can be measured AT THE LIFTER.
The "proud" statement is ABOUT that distance "out" of the lifter bore with EASY MEASUREMENT from the crankcase deck/lifter plunger.
I measure with a Dial Indicator Gauge standing on the block deck with the shaft of the indicator placed at the lifter push rod register.
I believe you question is VERY VALID, thanks.
SO now, how did I do ?
Lance
I'm confused at how either the #1 or #6 could have valves "off the seat" when the other is firing. With both being at tdc simultaneously, wouldn't their valve events also take place simultaneously?
03-31-2018, 10:00 PM
#22
wouldn't their valve events also take place simultaneously?
Consider the 4-stroke engine cycle.
On any given cyl, at firing, the piston is more or less @ TDC. During the power stroke the piston then descends to the bottom of the bore, during 180° of crank rotation. At the bottom the exh valve opens. The piston returns to the top (another 180°, for a total so far of 360°, aka 1 full rev), pushing out the spent gases; the exh valve continues opening during this stroke, reaching its max lift somewhere near halfway up, then begins closing again, closing as the piston nears the top. Once the piston gets there and is once again @ TDC, the exh valve nears full closed and the int begins to open. The piston descends again (180° more, 1½ rotations), with the int valve opening as it goes and sort of "following" it, reaching full open around halfway down. As the piston reaches the bottom the int closes and the piston once again ascends, reaching firing at the top. When it gets back to the top and fires again the crank has rotated 2 full turns since the last firing.
4 strokes: power (down), exh (up), int (down), compression (up).
The 2 cyls that are 4 apart in the firing order (in a V8... other cyl count are different of course) experience their valve events 360° (one full crank revolution) apart from each other. 1 & 6, 8 & 5, 7 & 4, 3 & 2. If it wasn't that way, that is if the valve events on #1 & #6 occurred at the same time, then #1 & #6 would fire at the same time. Which of course doesn't happen. Thus when #1 is firing, #6 is in "overlap" and 1 full crank rev away from its firing, and vice-versa.
The "overlap" period is when PTV clearance is the least, and should be checked. Max lift is totally irrelevant to this. What matters is the distance the valves are open as the piston passes through TDC as the exh is just about to close and the int has just started to open. Therefore duration and cam advance/retard are what matter to PTV interference, not max lift. Retarding the cam brings the exh closer to the piston; advancing it brings the int closer.
Last edited by RB04Av; 03-31-2018 at 10:16 PM.
04-01-2018, 10:59 AM
#23
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Cylinder Firing for Off Road Racing Hi RBO4, thanks for more input, GOOD for the P/V questions NEVER understood by many.
NOW, I have changed YOUR answer for better "pulling power" up a Sand Wash.
I HAVE timed the cams in 3.8 Porsche AND Ford 4.0 V-6 Ranger (Both OHC) engines so that the complement cylinders FIRE at the same time !
The Rolling Road test brought ALL of the people in the dyno/engine shop for car observation caused by the VERY LOUD noise of a THREE cylinder firing song. (hurt their ears)
Lance
NOW, I have changed YOUR answer for better "pulling power" up a Sand Wash.
I HAVE timed the cams in 3.8 Porsche AND Ford 4.0 V-6 Ranger (Both OHC) engines so that the complement cylinders FIRE at the same time !
The Rolling Road test brought ALL of the people in the dyno/engine shop for car observation caused by the VERY LOUD noise of a THREE cylinder firing song. (hurt their ears)
Lance