piston friction, valve springs

[dan miller]

With a given displacement, just for conversation let’s assume 4" X 4" @ 402 cid, I wonder how much moving the bore and stroke around changes the piston assembly friction.

If the stroke is shortened, the bore has to become larger, so there would be more ring surface and skirt surface rubbing, countered by less distance traveled per engine revolution. And, of course, vice versa.

Whatcha think?

Here’s another question, for extra credit: How much extra power is required for valve springs that are too strong (roller lifters and rockers)? We preferred to err on the “way strong” side, as that would protect against overrevving, and we didn’t have a really good feel for how much was enough. I feel that it’s important to have as little as possible, as the strong springs almost certainly have a negative impact with regard to harmonics (cam wind up). Thinking about installing a harmonic damper on the rear of the cam. Too much coffee does strange things to the mind.

Thanks, Danny

[Stan Weiss]

With a given displacement, just for conversation let’s assume 4" X 4" @ 402 cid, I wonder how much moving the bore and stroke around changes the piston assembly friction.

If the stroke is shortened, the bore has to become larger, so there would be more ring surface and skirt surface rubbing, countered by less distance traveled per engine revolution. And, of course, vice versa.

Whatcha think?

Here’s another question, for extra credit: How much extra power is required for valve springs that are too strong (roller lifters and rockers)? We preferred to err on the “way strong” side, as that would protect against overrevving, and we didn’t have a really good feel for how much was enough. I feel that it’s important to have as little as possible, as the strong springs almost certainly have a negative impact with regard to harmonics (cam wind up). Thinking about installing a harmonic damper on the rear of the cam. Too much coffee does strange things to the mind.

Thanks, Danny

When you play with stroke what happens to rod length?

When you increase the bore what happens to piston weight?

Note these are all with GAS PRESSURE = 0

Green Line
Bore = 4.0 Stroke = 4.0 Rod Length = 6.8 RPM = 7000
Wrist Pin Offset = 0.0
Piston Weight = 500.25 Rod Weight = 500.5
Small End Rod Weight = 166.8333 Big End Rod Weight = 333.6666

Red Line
Bore = 4.2 Stroke = 3.628117 Rod Length = 6.8 RPM = 7000
Wrist Pin Offset = 0.0
Piston Weight = 500.25 Rod Weight = 500.5
Small End Rod Weight = 166.8333 Big End Rod Weight = 333.6666

Blue Line
Bore = 4.2 Stroke = 3.628117 Rod Length = 6.8 RPM = 7000
Wrist Pin Offset = 0.0
Piston Weight = 550.25 Rod Weight = 500.5
Small End Rod Weight = 166.8333 Big End Rod Weight = 333.6666

[dan miller]

Hello Stan

Thanks for the input. But, I have to admit, I don't get it. Could you possibly 'splain it to me? lol

I don't believe that rod length necessarily has to change, as the deck could be changed.

I would be interested in a comprehensive answer, but my question was seeking a simple and general answer.

Thank you, Danny

[Old School]

Pipemax has a feature that calculates friction loss.

One engine 4.00 x4.00 friction hp is 208.6 hp at 7000.

Engine two at 4.28 bore x 3.5 stroke friction hp is 170.5 at 7000. At about 7500 did this engine experience 208 friction loss.

Both had 6 inch rod inputs. Pipemax defines friction hp as the amount of mechanical and fluid friction losses, along with pumping losses ....based upon research by GM, Honda, Superflow equations, and others.

Maybe Larry M. will comment. Hope this helps.

Would this get more replies if it was in the engine or advanced engine section?

[David Redszus]

Friction loss is not a function of area but rather of load. For a given load, a large surface and a smaller surface will have the same friction loss.

Lateral loading of a piston can be easily calculated knowing the engine geometry, piston speed and combustion pressure.

[Stan Weiss]

Hello Stan

Thanks for the input. But, I have to admit, I don't get it. Could you possibly 'splain it to me? lol

I don't believe that rod length necessarily has to change, as the deck could be changed.

I would be interested in a comprehensive answer, but my question was seeking a simple and general answer.

Thank you, Danny

Hi Dan,
Let me start by saying what is show is piston side force. Based on RPM (speed), stroke, rod length, rod weight, and piston weight. Note these are all with GAS PRESSURE = 0, think of this as if there is no head on the short block. In all 3 examples the RPM and the rod length, rod weight (both small and big end), and engine size 402.12 ci are the same.

Green Line
Bore = 4.0 Stroke = 4.0 Rod Length = 6.8 RPM = 7000
Piston Weight = 500.25 Rod Weight = 500.5

Red Line - This is the same as the Green Line accept that the bore has been increased and the stroke decreased
Bore = 4.2 Stroke = 3.628117 Rod Length = 6.8 RPM = 7000
Piston Weight = 500.25 Rod Weight = 500.5

Blue Line - This is the same as the Red Line accept that the Piston Weight was increased 50 grams
Bore = 4.2 Stroke = 3.628117 Rod Length = 6.8 RPM = 7000
Piston Weight = 550.25 Rod Weight = 500.5

I hope this makes some sense.
Stan