Rod ratio and dwell time... doesn't make sense

[Belgian1979]

Depending what you use in terms of stroke and rod length, you always reduce the demand for airflow through the intake tract when you reduce the piston speed. In an undervalved engine like most us type V8's this seems important to me.

[ptuomov]

Depending what you use in terms of stroke and rod length, you always reduce the demand for airflow through the intake tract when you reduce the piston speed. In an undervalved engine like most us type V8's this seems important to me.

I like this thread because it's been trying to move towards estimated magnitudes of various effects.

[Belgian1979]

Depending what you use in terms of stroke and rod length, you always reduce the demand for airflow through the intake tract when you reduce the piston speed. In an undervalved engine like most us type V8's this seems important to me.

I like this thread because it's been trying to move towards estimated of magnitudes of various effects.

Yes, but it seems that a lot of people have opinions but no clear facts to compare things. If such a thing is possible at all. Let's put 2 examples on the table in terms of a SBC with a similar head and similar cam : 4.000X3.480X5.700 and 4.125X3.250X6.250 which are almost identical in displacement. The latter will have reduced substantially it's piston speed and demand on airflow through the head. So you're left with a choice :
- reducing camshaft to improve low rpm operation.
- keeping the cam and gearing accordingly.

But like I said everyone has an opinion on the subject, me included.

[Stan Weiss]

Depending what you use in terms of stroke and rod length, you always reduce the demand for airflow through the intake tract when you reduce the piston speed. In an undervalved engine like most us type V8's this seems important to me.

I like this thread because it's been trying to move towards estimated of magnitudes of various effects.

Yes, but it seems that a lot of people have opinions but no clear facts to compare things. If such a thing is possible at all. Let's put 2 examples on the table in terms of a SBC with a similar head and similar cam : 4.000X3.480X5.700 and 4.125X3.250X6.250 which are almost identical in displacement. The latter will have reduced substantially it's piston speed and demand on airflow through the head. So you're left with a choice :
- reducing camshaft to improve low rpm operation.
- keeping the cam and gearing accordingly.

But like I said everyone has an opinion on the subject, me included.

Not really. In this case piston speed has nothing to do with it. Give the 4.125 x 3.250 one the same rod stroke ratio and they will have the very close to the same piston air flow demand curve.

Stan

[Stan Weiss]

I like this thread because it's been trying to move towards estimated of magnitudes of various effects.

Yes, but it seems that a lot of people have opinions but no clear facts to compare things. If such a thing is possible at all. Let's put 2 examples on the table in terms of a SBC with a similar head and similar cam : 4.000X3.480X5.700 and 4.125X3.250X6.250 which are almost identical in displacement. The latter will have reduced substantially it's piston speed and demand on airflow through the head. So you're left with a choice :
- reducing camshaft to improve low rpm operation.
- keeping the cam and gearing accordingly.

But like I said everyone has an opinion on the subject, me included.

Not really. In this case piston speed has nothing to do with it. Give the 4.125 x 3.250 one the same rod stroke ratio and they will have very close to the same piston air flow demand curve.

Stan

[Belgian1979]

I like this thread because it's been trying to move towards estimated of magnitudes of various effects.

Yes, but it seems that a lot of people have opinions but no clear facts to compare things. If such a thing is possible at all. Let's put 2 examples on the table in terms of a SBC with a similar head and similar cam : 4.000X3.480X5.700 and 4.125X3.250X6.250 which are almost identical in displacement. The latter will have reduced substantially it's piston speed and demand on airflow through the head. So you're left with a choice :
- reducing camshaft to improve low rpm operation.
- keeping the cam and gearing accordingly.

But like I said everyone has an opinion on the subject, me included.

Not really. In this case piston speed has nothing to do with it. Give the 4.125 x 3.250 one the same rod stroke ratio and they will have the very close to the same piston air flow demand curve.

Stan

Stan,

Air has mass and will have to follow your piston when it moves and displaces volume. So although the volume displaced between both is similar, the speed at which the air has to flow is different. If it would follow perfectly the displacement of the piston, no vacuum/pressure drop would be created.
I'm note sure how to explain this other than if you have a piston moving in a cylinder without a cylinderhead the air mass to has to replace the volume displace by the piston and would have no problem filling the void. If you have to move that same mass through an orifice it makes a lot of difference whether that same piston tries to accelerate the same mass of air faster or slower. In this case the force necessary to accelerate the air increases. F=m*a where the acceleration here is created by the piston. Since the piston only creates so much force to draw in the air the only factor that can change in my opinion is the amount of mass that can move in the same time or the speed at which the air moves. In a cylinderhead you would therefor reach your choke point earlier.

[Stan Weiss]

We total disagree on how this works. Weather it is a graph of piston flow demand or cylinder volume change per degree of crank rotation those 2 engine with the same rod stroke ratio will be almost identical (difference is the displacement is not identical). The cylinder volume change per degree of crank rotation is what causes the pressure differential and this pressure differential is what pushes the air into the cylinder. Based on this I say the force is the same.

Stan

[swampbuggy]

I'm believing it a motor that stays wound up and operates at high RPM range would like the Long Rod motor more so than one that operates over a wider RPM range anybody agree

[groberts101]

I'm believing it a motor that stays wound up and operates at high RPM range would like the Long Rod motor more so than one that operates over a wider RPM range anybody agree

Unless something's changed more recently?.. that seems to be the trend used by nascar on various tracks. Superspeedways with big sweepers get shorter stroke/longer rod type deals compared to shorter tracks sharper turns getting longer strokes/shorter rods.

It's also been said that the cylinder head and induction requirements can change slightly due to the way the piston is drawing on it. Makes me wonder how the cylinder heads might change between the short and long tracks too.

[Belgian1979]

We total disagree on how this works. Weather it is a graph of piston flow demand or cylinder volume change per degree of crank rotation those 2 engine with the same rod stroke ratio will be almost identical (difference is the displacement is not identical). The cylinder volume change per degree of crank rotation is what causes the pressure differential and this pressure differential is what pushes the air into the cylinder. Based on this I say the force is the same.

Stan

That's correct, but I stand by what I said. BTW : if you look upon fueling considerations as used in FI, it's clear that everything is based on mass of air vs mass of fuel. If the change of volume would be the basis, we would be working on completly different calculations.

The force that you talk about is what pulls the air molecules apart and is what reduces it's density. That in itself says enough.
In the equation F=m*a, when the force stays the same an the acceleration becomes higher, you have to reduce the mass in order for this to remain true.

[Belgian1979]

I'm believing it a motor that stays wound up and operates at high RPM range would like the Long Rod motor more so than one that operates over a wider RPM range anybody agree

Unless something's changed more recently?.. that seems to be the trend used by nascar on various tracks. Superspeedways with big sweepers get shorter stroke/longer rod type deals compared to shorter tracks sharper turns getting longer strokes/shorter rods.

It's also been said that the cylinder head and induction requirements can change slightly due to the way the piston is drawing on it. Makes me wonder how the cylinder heads might change between the short and long tracks too.

correct and the reasons are imo that you have better breathing capabilities of that same engine in terms of how much mass of air it can cram into the cylinder.. Which in its basic form boils down to how many air molecules you end up with in your cilinder during the intake stroke.

[ptuomov]

The most interesting question that has been answered in this thread to some extent is whether any of these directionally correct statements are in any sense quantitatively meaningful in terms of magnitudes. It is my reading of the thread that, within reason, rod-to-stroke ratio has only a very small impact on the knock resistance of the engine, breathing of the engine, and stresses on the connecting rod. It's also the case that completing a larger fraction of heat release during the burn before the combustion chamber expands more is not going to be a large effect. Not zero, but "if I've got 99 problems R/S isn't one."

Now, based on the numbers seen in this thread, the piston side loading might actually be a quantitatively significant effect in terms of friction and wear, but I am not sure. Would be interested in understanding more, but that would require friction coefficient estimates etc. I am definitely interested in that topic.

[David Redszus]

Yes, but it seems that a lot of people have opinions but no clear facts to compare things. If such a thing is possible at all. Let's put 2 examples on the table in terms of a SBC with a similar head and similar cam : 4.000X3.480X5.700 and 4.125X3.250X6.250 which are almost identical in displacement. The latter will have reduced substantially it's piston speed and demand on airflow through the head

.

Let's put some numbers in place. Two engines with the same camshaft, valves, port area and rpm:

Engine 1
B = 4.0
S = 3.48
R = 5.7
cv = 717 cc
Mean piston speed = 23.6 m/s
Max piston speed = 38.6 m/s
Piston air demand =663 CFM
Max air flow = 643 CFM
Reversion flow = -80 CFM
Time * volume = 31486 CFM

Engine 2
B = 4.125
S = 3.25
R = 6.25
cv = 711.7cc
Mean piston speed = 22.0 m/s
Max piston speed = 35.7 m/s
Piston air demand = 651 CFM
Max air flow = 632 CFM
Reversion flow = -84 CFM
Time * volume = 31238 CFM

The above numbers should be self-explanatory. The last number (T * V) represents the aggregated flow through
the valve for the time the valve is open. It is the area under the flow curve measured at the valve/port.

Given that the difference between these two engines in real flow is less than 1%, perhaps we should look elsewhere
to find more performance.

[Warp Speed]

I'm believing it a motor that stays wound up and operates at high RPM range would like the Long Rod motor more so than one that operates over a wider RPM range anybody agree

Unless something's changed more recently?.. that seems to be the trend used by nascar on various tracks. Superspeedways with big sweepers get shorter stroke/longer rod type deals compared to shorter tracks sharper turns getting longer strokes/shorter rods.

It's also been said that the cylinder head and induction requirements can change slightly due to the way the piston is drawing on it. Makes me wonder how the cylinder heads might change between the short and long tracks too.

correct and the reasons are imo that you have better breathing capabilities of that same engine in terms of how much mass of air it can cram into the cylinder.. Which in its basic form boils down to how many air molecules you end up with in your cilinder during the intake stroke.

No difference in bore or stroke between tracks.

[RDY4WAR]

Yes, but it seems that a lot of people have opinions but no clear facts to compare things. If such a thing is possible at all. Let's put 2 examples on the table in terms of a SBC with a similar head and similar cam : 4.000X3.480X5.700 and 4.125X3.250X6.250 which are almost identical in displacement. The latter will have reduced substantially it's piston speed and demand on airflow through the head

.

Let's put some numbers in place. Two engines with the same camshaft, valves, port area and rpm:

Engine 1
B = 4.0
S = 3.48
R = 5.7
cv = 717 cc
Mean piston speed = 23.6 m/s
Max piston speed = 38.6 m/s
Piston air demand =663 CFM
Max air flow = 643 CFM
Reversion flow = -80 CFM
Time * volume = 31486 CFM

Engine 2
B = 4.125
S = 3.25
R = 6.25
cv = 711.7cc
Mean piston speed = 22.0 m/s
Max piston speed = 35.7 m/s
Piston air demand = 651 CFM
Max air flow = 632 CFM
Reversion flow = -84 CFM
Time * volume = 31238 CFM

The above numbers should be self-explanatory. The last number (T * V) represents the aggregated flow through
the valve for the time the valve is open. It is the area under the flow curve measured at the valve/port.

Given that the difference between these two engines in real flow is less than 1%, perhaps we should look elsewhere
to find more performance.

The advantages of Engine #2 would be in reduced friction and lower inertia from the reduced piston speed, reduced shrouding from the bigger bore, and slightly larger quench area.