Bore, Stroke, and block strength

[peejay]

Forced induction application.

I had a rambling line of thought that small bore/long stroke may be more beneficial for block strength and main bearing life. To make the same amount of torque, you can make that with a longer lever arm (more stroke) or more area for cylinder area to push on (more bore) or more boost (more pressure). However it occurs to me that the main webs are going to be affected mainly by the latter two, if you look at bore area and boost as pushing harder on the wristpin (which is ultimately resisted by the main webs) versus stroke, which just gives you more leverage against the flywheel.

So I am thinking, for engines that are run in normal RPM ranges and have marginal block strength, it makes sense to go with a longer stroke to increase torque, rather than upping the boost.

I was thinking about this a bit because Volvo went to an extra long stroke on their high performance 5 cylinder even though it also had a higher RPM range, and I was considering what engineering rationale there was for that decision. But then it got me to thinking about the Ford V8s with tiny 3" strokes that blow their bottom ends apart, and wondered if a 347 with less boost to make the same power would ultimately be friendlier to the block.

[PackardV8]

Some research will disabuse you of the notion of long stroke/torque relationship.

My limited experience is the main webs don't know how the torque/time loading is being developed.

[strokersix]

Take it to extreme, say a 1 inch stroke and corresponding huge bore for illustration purpose. Conrod pushing on crankpin must be resisted my main pushing back. I think there may be some truth in the assertion.

[peejay]

Take it to extreme, say a 1 inch stroke and corresponding huge bore for illustration purpose. Conrod pushing on crankpin must be resisted my main pushing back. I think there may be some truth in the assertion.

Yes, that is kind of what I am getting at... a longer stroke crank allows for less "wristpin pressure" for the same amount of torque at the flywheel, which should be easier on the block and the bearings.

Kind of like the difference between pushing really hard on a stubby ratchet, or more gently on a breaker bar. Same amount of torque will be produced but one will be a lot easier on your joints!

Is it enough of a difference to be worthwhile to pursue? Not sure.

[tt 383]

Aren't the Volvo mains basically like a girdle? Also having more mains per cyl and no opposing or intersecting crank forces seems like an imherent strength advantage.

[Juho_]

Yes it is.
Also crank has quite large journals.

[PackardV8]

[quote=peejay post_id=831985 time=1571507968 user_id=11953]
[quote=strokersix post_id=831977 time=1571504333 user_id=7639]
Take it to extreme, say a 1 inch stroke and corresponding huge bore for illustration purpose. Conrod pushing on crankpin must be resisted my main pushing back. I think there may be some truth in the assertion.
[/quote]

Yes, that is kind of what I am getting at... a longer stroke crank allows for less "wristpin pressure" for the same amount of torque at the flywheel, which should be easier on the block and the bearings.

Kind of like the difference between pushing really hard on a stubby ratchet, or more gently on a breaker bar. Same amount of torque will be produced but one will be a lot easier on your joints!

Is it enough of a difference to be worthwhile to pursue? Not sure.
[/quote]
Do you have a refererence to substantiate this assertion? That 'logic' is backward in that the fastener is the main bearing and sees the same torque and makes no difference how it was applied.

[MadBill]

2X.
If generating the same crankshaft torque, a big bore short stroke engine will apply exactly the same vertical reaction force to the mains as a small bore long stroke one.

[econo racer]

I still think that the bigger bore with a blower makes much more HP. Plus you can get bigger valves in. Using the longer rod might be better but be careful where your top ring land is.

[Geoff2]

The longer stroke will load the bore wall than a short stroke; so, no thin flexible bores.

[strokersix]

I think a free body diagram would be helpful here...

[strokersix]

Sure, torque output is proportional to displacement, independent of stroke and bore relationship.

However, forces involved are directly related to stroke and bore relationship. This was the OP question.

[cardo0]

Forced induction application.

I had a rambling line of thought that small bore/long stroke may be more beneficial for block strength and main bearing life. To make the same amount of torque, you can make that with a longer lever arm (more stroke) or more area for cylinder area to push on (more bore) or more boost (more pressure). However it occurs to me that the main webs are going to be affected mainly by the latter two, if you look at bore area and boost as pushing harder on the wristpin (which is ultimately resisted by the main webs) versus stroke, which just gives you more leverage against the flywheel.

So I am thinking, for engines that are run in normal RPM ranges and have marginal block strength, it makes sense to go with a longer stroke to increase torque, rather than upping the boost.

I was thinking about this a bit because Volvo went to an extra long stroke on their high performance 5 cylinder even though it also had a higher RPM range, and I was considering what engineering rationale there was for that decision. But then it got me to thinking about the Ford V8s with tiny 3" strokes that blow their bottom ends apart, and wondered if a 347 with less boost to make the same power would ultimately be friendlier to the block.

This reads like you are trying to solve a lack of strength in areas of a Volvo block by choosing a different stroke/bore combination. So I thought I should mention many of the overseas racing classes where there are no stronger aftermarket blocks the engine builders will add metal to weak areas of the block to increase block strength. Plates and gussets welded into to main webs and lifter valley in strategic locations. Often thick deck plates are glued and screwed onto the block deck for not only strength but added stroke.

Sometimes you have to work with what you got. Block modifications for strength are nothing new and actually popular in some racing classes.

[hoffman900]

A longer stroke would add more bending / twisting movement into the crank, which would have its own set of problems as it relates to the block.

To me. The tune, and detonation control seems to be a bigger variable here thank stroke vs. bore.

[MadBill]

A longer stroke would add more bending / twisting movement into the crank..

Except that the force acting through the rod is lower for the long stroke due to the smaller piston area, exactly cancelling the greater moment arm of the longer stroke and so creating the same crank torque. It's like applying 50 lbs. force on a 12" long wrench vs. 25 on a 24" one; the result is a torque of 50 lb-ft. in either case.