Camshaft changes or not with stroke increased.

[David Redszus]

A novice question: Isn't the change in the valve size by far the most important change here in terms of the camshaft? I'm just thinking out loud that the the rod ratio change would have a smaller effect and the change in the bore vs. stroke (keeping the displacement the same) an even smaller effect. Curious about the magnitudes and what issue to focus on.

528 - 4.500 bore x 4.15 stroke, 6.76 rod, 1.628 RR
At 14 deg ATC:
piston position.....2.04mm
piston vel...........10.43m/s
piston accel........2633G
Maximum piston velocity...34.66 m/s

5

26 - 4.380 bore x 4.375 stroke, 7.100 rod, 1.622 RR
At 14 deg ATC:
piston position.....2.15mm
piston vel...........11.0m/s
piston accel........2778G
Maximum piston velocity...36.55 m/s

For the two engine examples shown above, the areas under the flow curves are:
32,031 CFM and 32,061 CFM respectively. Given the same valves and cam, they flow virtually the same CFM.

The small changes in stroke are offset by the small change in rod length. There would be no need to change valve curtain area, valves, lift, or cam lobe shape.

[Stan Weiss]

David,
We are all responding to a few different combinations posted in this thread. How do your calculations work out on the original OP's combinations?

bore = 4.185"
stroke = 3.28"
rod = 6"

bore = 3.915"
stroke = 3.75"
rod = 6"

Stan

[FuelieNova]

The cam's duration is related to the FPM of the piston, not the RPM of the crank. The lift rate of the profile is related to the rod to stroke ratio.

Hey Rick
I also am glad to see you back on here. You spent a lot of time with me and made me a cam.
It was probably the best running cam I have had in that small block.
Tom Griffin

[David Redszus]

David,
We are all responding to a few different combinations posted in this thread. How do your calculations work out on the original OP's combinations?

4.185 bore x 3.28 stroke, 6.0 rod, 1.83 RR
At 14 deg ATC:
piston position.....1.57 mm
piston vel.............8.08 m/s
piston accel........2031G
Maximum piston velocity...27.14 m/s
Area under flow curve...24,238 cfm

3.915 bore x 3.75 stroke, 6.0 rod, 1.60 RR
At 14 deg ATC:
piston position.....1.85 mm
piston vel...........9.46 m/s
piston accel........2388 G
Maximum piston velocity...31.2 m/s
Area under flow curve...24,275 cfm

The maximum flows are almost identical at 495 CFM vs 500 CFM, and max reversionary at -55 CFM vs -57 CFM

[digger]

I the two following images, I have change only the bore and stroke values, keeping the same peak HP RPM. I've turned off the "AutoAdjust" on the port velocity, curtain velocity and VE% values so they can remain the same for cam comparison. This will give you a good indication of how the stroke affects the "Seat Duration". Since the rod length did not change the rod to stroke ratio did, which will affect the rate of lift / profile shape.

SpeedTalk Fun 1.png

SpeedTalk Fun 2.png

can you post the case where the 3.75 stroke has the same RR as the 3.25 keep everything else the same?

[Walter R. Malik]

Seems to me that all this fictitious, theoretical stuff about rod length is kind of immaterial here ... you can only use a rod which fits into the space allowed.

[GARY C]

David,
We are all responding to a few different combinations posted in this thread. How do your calculations work out on the original OP's combinations?

bore = 4.185"
stroke = 3.28"
rod = 6"

bore = 3.915"
stroke = 3.75"
rod = 6"

Stan

I was kind of wondering how the OP's question got side stepped for theoretical RR discussion?

[GARY C]

Less flow of the smaller valve affected the lift. It doesn't affect the duration. But the curtain velocity is higher than the mean port velocity because I picked a bad peak HP RPM for this example. So because the valve is smaller than the minimum required we need to adjust the curtain velocity to or below the mean port velocity.
SpeedTalk Fun 3.png
In order to do that we raised the valve lift some more.

If I read you screen shots correctly it does effect overlap?

[blackflag]

Seems to me that all this fictitious, theoretical stuff about rod length is kind of immaterial here ... you can only use a rod which fits into the space allowed.

Exactly what I was thinking. Same with the valve size on the 3.915” bore. Every combination is different. Thanks

[PRH]

Seems to me that all this fictitious, theoretical stuff about rod length is kind of immaterial here ... you can only use a rod which fits into the space allowed.

As I’m sure you’re aware, the two BB Mopar and BB Chevy examples I put forth aren’t fictitious at at.
All 4 are fairly common builds.

The main reason the RR was brought into the conversation was to illustrate that even though the piston speed is higher on the long stroke/small bore combo, that doesn’t necessarily translate into a higher demand on the intake port at any given point, if the RR remains the same and the engine displacement doesn’t change with the added stroke.

[GARY C]

If I read you screen shots correctly it does effect overlap?

Correct, without changing the seat duration or total net valve lift, it increases the area under the curve and obviously the over lap area.

I was just wondering if I was reading that correct, playing around with DV's old version of CM it used a different LSA change along with a seat duration change for overlap but stayed with a lift that would utilize (for lack of a better term) all of the head flow.

Just pondering the different approaches.

[digger]

can you post the case where the 3.75 stroke has the same RR as the 3.25 keep everything else the same?

SpeedTalk Fun 2b.png

SpeedTalk Fun 2b Graph.png

There you are.

thanks

so the RR had a degree or two difference?
which would seem to be relatively negligible IMO. So the difference is because of the aspect ratio (bore/stroke)?

this seems to suggest that it assumes pistons speed is more important than rate of change of displacement?

[digger]

thanks

so the RR had a degree or two difference?
which would seem to be relatively negligible IMO. So the difference is because of the aspect ratio (bore/stroke)?

this seems to suggest that it assumes pistons speed is more important than rate of change of displacement?

I think the difference in profile area was 1%. AT 800 HP, that could be 8 HP.
That is what it is all about, getting 1% or .5% from each part in the engine.

The difference is strictly caused by the rod to stroke ratio / piston motion.

As far as piston speed being more important than rate of change of displacement. The math does not favor one over the other, in fact they are related to different parameters. Mean Piston Velocity is a determining factor for the total length of the valve opening, while rate of change in cylinder volume is a dictating factor for the lift rate of the valve opening curve.

unless i'm reading wrong or you are talking something else both are 361CI, both are 1.83RR the 3.75 stroke wants ~ 20 degrees more duration than the 3.28" stroke at low lift points. So RR has zero to do with it. the piston displacement from TDC per crank angle is different but the volume change per degree is identical.

why is MPS dictating the total length of valve open (presumably this is seat duration) and not the rate of change of volume? i dont understand the logic behind that? i see the signal as being created by the volume change

[digger]

Let me backup to your post where you asked me to post the software screen shot of the 3.75" stroke but with a 1.829:1 r2s ratio.
So, I posted the "2b" image which had the 3.75" stroke with the longer rod.
And I also posted the Graph screen comparing the valve motion graphs of the 3.75" stroke with the 1.6:1 r2s and the 1.829:1 r2s.

SpeedTalk Fun 2b.png

SpeedTalk Fun 2b Graph.png

There you are.

thanks

so the RR had a degree or two difference?
which would seem to be relatively negligible IMO. So the difference is because of the aspect ratio (bore/stroke)?

this seems to suggest that it assumes pistons speed is more important than rate of change of displacement?

Reading "so the RR had a degree or two difference?", I assumed you were talking about the 3.75 stroke cam durations between long rod and short rod. Which was 1 degree different at .050" and 2 degrees different at .200".

If you were talking about two different engine models. I'm sorry for the mistake
Please, start over from "thanks". I usually get it on my second try.

comparing the 3.75" stroke the long and short rod were different by a couple of degrees.

comparing 3.75 and 3.28" stroke with the same RR and CI the requirements for duration are around 18 degrees different at lower lift areas give or take. the difference has nothing to do with RR as its the same value. i was curious why the difference is so large when the rate of change of volume per degree is identical. mean piston speed is a key driver when i would have thought the signal is caused by the rate of change in volume.

my recollection was that Vanniks 1D program predicted the VE curve to be the same when you traded bore and stroke to maintain the same CI provided you also adjust the rod length to also maintain the RR. I may need to revisit. your program is saying that the engine should peak earlier unless you increase duration with the longer stroke option

[John Wallace]

your program is saying that the engine should peak earlier unless you increase duration with the longer stroke option

With more stroke (of same engine), the VE of engine is at a lower RPM for max HP.