Crankshaft offset

[itslow]

Originally Posted by Bored&Stroked

Hey Gang - for those of you who are looking for dimensional details . . .

I've just been going through the original Ford drawings for the 59AB and the 8BA engines (from the Ford archives)....

This is the key. Use the ENGINEERING drawings from the archives. The two drawings posted on the HAMB thread are simply illustrations for other purposes. Folks are mistaking them as the former instead of the latter.

[JSeery]

Originally Posted by thirt4

Ok... I am so cornfuzzed I just gotta ask...

IF this is all true, and it works on Henry's flatheads, would it work on an OHV engine?

This is accomplished on modern engines with other techniques such as offset piston pins.

[ford38v8]

More good info on this thread than you can shake a stick at! I have another related question now~ There has been discussion about timing being affected by the offsets. I'm thinking that no matter where the bores are in relation to the crank, it remains that TDC is still TDC, which principle would also apply to offset wrist pins. This much is universally agreed going in, so how, pray tell, could there be a timing difference? The crank navigates the full 360º without skipping a beat, so I can't see a timing issue for the life of me. Is there anyone convinced of the opposite that can explain their thinking on this? Don't misunderstand me, I've learned a hell of a lot on this thread myself, and I just want to understand the other point of view, please?

[cmbrucew]

Too much information, System overload.
Much more than I need to know.
Good night
Bruce

[Mike B]

Originally Posted by ford38v8

More good info on this thread than you can shake a stick at! I have another related question now~ There has been discussion about timing being affected by the offsets. I'm thinking that no matter where the bores are in relation to the crank, it remains that TDC is still TDC, which principle would also apply to offset wrist pins. This much is universally agreed going in, so how, pray tell, could there be a timing difference? The crank navigates the full 360º without skipping a beat, so I can't see a timing issue for the life of me. Is there anyone convinced of the opposite that can explain their thinking on this? Don't misunderstand me, I've learned a hell of a lot on this thread myself, and I just want to understand the other point of view, please?

TDC would always be TDC, I'd think?...take it to an extreme, move the crank over an inch,,,yes, would assume one bank of pistons would not get as high in the bore as the other bank, but still would reach a TDC.

I would think valve timing would be the same, albeit lower lift on one side vs the other if cam was moved the same...less angle to promote lift.

My ramblings

Edit:

Wait a sec on my valve observation, as no push rods are used in the FH...must think harder here, but I'm like Bruce, late fore me, as I start the day early

[BillM]

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Timing: Timing is referenced to TDC, but with the offset of the crank or wrist pins the crankshaft passes through its TDC point slightly before the piston reaches its TDC. So the timing mark on the crank pulley is slightly advanced. Which is so slight that it makes no practical difference.

[ford38v8]

Bill, I'm not seeing it yet. The piston being related to the crank by the rod, both TDCs have to be simultaneous no matter where the crank is centered. Two aspirins and a good night's sleep!

[Flat32]

Given a 4" stroke, 7" rod length and a .265" crank offset what happens is BDC becomes 183.63 degrees after TDC.

Has me wondering about cam cards referencing BDC. Seems like they take it as 180 degrees away from TDC, which is fine, but technically not correct for our Flatheads. I don't know why they even reference BDC when everything can be referenced to TDC and be very clear as well as correct.

Flat32

[Mart]

My brain hurts.

[JSeery]

TDC is not determined by the position of the crank throws, it is when the piston is at Top Dead Center. Crank offset, piston pin center, etc are not factors, it is the position of the piston when it reaches the top of the cylinder and reverses direction. The offset has to do with the angle of the rod and nothing to do with cam or ignition timing. As far as the ignition point of the fuel mixture, this does not normally occur at TDC, it occurs prior to TDC, that is what ignition advance is. The fuel mixture is ignited while the piston is still on the compression stroke and moving up in the cylinder. The fuel/air mixture in the cylinder does not explode, it burns. It has a burn rate and a flame front. Ignition advance allows time for the flame front to move across the combustion chamber to develop maximum pressure for the power stroke (the flame front can continue past TDC). There is also a dwell time for the piston. From BDC the piston starts to accelerate and reaches maximum speed when the crank throw is around 90 degrees to the cylinder centerline. It then starts to slow down. It stops its upward movement and reverses direction at TDC. Because of this stopping then reversing and accelerating in the opposite direction there is what is referred to a piston dwell around TDC. This dwell and rod angles etc are all involved in the differences between long and short stroke engine designs. The amount of advance an engine can use is determined by many factors, big ones being combustion chamber shape, fuel type and CR. Crankshaft offset is a mechanical issue involving the physical angles of the crank and the rods in relationship to the cylinder centerline and does not change TDC, cam events or ignition timing.

[Ol' Ron]

I think JSeery just accomplished what many have tried before, and failed. And he didn't even need a stick. Well done.

[ford38v8]

Ol' Ron is demonstrably the sharpest tool in the box. He declines to comment until the definitive answer is posted so he can then agree with it!

[Jim in Wisconsin]

I think I'll never get this - if putting the crankshaft off to one side has some effect on one back of cylinders, why wouldn't it have the opposite effect on the other bank?

[ford38v8]

Jim, I think that's the whole point in a nutshell as explained by JSeery: The offset has no effect on the timing.

[Jim in Wisconsin]

What would be the reason for the offset then? Or are we back to where we started?

[Mart]

Think of it this way. The side loading on the piston is much higher on the power stroke than the compression stroke. By offsetting the cylinder slightly to the US drivers side the rod angularity is reduced on the power stroke and hence the side loading on the thrust side reduced during the power stroke. The increased rod angularity during the compression stroke is tolerable because the load during compression is less than when firing.

People prefer the 7" rod length over the 6-7/8" French rods for the same reason. Less angularity equals less load on the thrust side.

Mart.

[cmbrucew]

Yeah
What Mart said.
Bruce

[Jim in Wisconsin]

Does this apply to both banks of cylinders though?

[ford38v8]

Originally Posted by Jim in Wisconsin

Does this apply to both banks of cylinders though?

yes

[Fordors]

Originally Posted by Jim in Wisconsin

Does this apply to both banks of cylinders though?

Yes, both banks see the same result.
I would like to clarify one thing, the crank is not shifted, it is the center lines of the cylinders that was shifted. Shifting the crank would necessitate the deck heights be different. The center lines of the cylinders were shifted toward the driver's side .265 with a datum point on the horizontal plane. That c/l extended on the 45° cylinder angle moved the cylinders to the driver's side .187.
To accommodate the cylinder c/l shift the lifter angles were made different so that the valve pocket to cylinder bore dimension remain equal and also the dr. side cylinder head was located lower on the deck.