Camshaft changes or not with stroke increased.

[David Redszus]

The following data is intended to clarify the effect of rod ratio on flow dynamics.
The bore, stroke, valve, port, cam lift, duration, rpm are exactly the same except for a change
in rod length. The data for piston position, velocity and acceleration are at 14 deg ATC.

4.0 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..........2388G
Maximum piston velocity...31.37 m/s
Area under flow curve...24,680 cfm

4.0 bore x 3.75 stroke, 7.1 rod, 1.89 RR
At 14 deg ATC:
piston position.....1.78 mm
piston vel...........9.11 m/s
piston accel........2306 G
Maximum piston velocity...30.96 m/s
Area under flow curve...24,646 cfm

Note that the areas under the flow curves are virtually the same.

When we overlay the piston velocity curves, we find that the short rod (1.6rr) has greater velocity up to
90 CS degs, after which the longer rod gains a velocity advantage till BDC. The net flow result is virtually a wash.

The critical factors to consider do not involve rod length. For a given bore, stroke and valve curtain area are the parameters upon which to focus.

Flow is determined by piston area and piston velocity, not acceleration.
Area (M^2) * Velocity (M/S) = Volume flow (M^3/S).

[travis]

Since both engines are the same CID, the long stroke engine will have to turn the same RPM. With the longer stroke, and shorter rod/stroke ratio, you need to increase duration to make peak hp at the same RPM as the short stroke engine.
With the smaller valve diameter, you need to increase lift, to fill the cylinder. Because of the higher velocity, with the smaller valve, you need widen the LSA, and install the cam on a later ICL.

My brain tends to think you'd want a head start on presenting flow into the cylinder with an earlier ICL along with all the exhaust pull you could get. I'll ponder your response today to see if something clicks.

*No, with the smaller valve, the velocity is higher, so you don't need to open it as early*. The smaller valve won't fill the cylinder as fast, so pressure in the cylinder will stay below the pressure above the valve, for a longer amount of time, so to take advantage of that, you leave the valve open longer.

Maybe I’m taking this out of context, but I can’t seem to wrap my brain around what I underlined there. Why would you not want to start the intake open event earlier to get flow moving through the more restrictive port earlier? The rest of it makes sense to me, just not that part

This is one area that I’ve noticed that you (Camking) and DV don’t always agree on, especially when you stroke an engine for more displacement but keep the heads the same. I’m just trying to follow the thought process is all.

[CamKing]

Why would you not want to start the intake open event earlier to get flow moving through the more restrictive port earlier?

Opening the valve earlier, before TDC will only "get the flow moving" if the pressure below the valve is less then the pressure above the valve.
Since the piston isn't helping, and actually hurting at this point, you have to rely on the exhaust to have scavenged enough to drop the pressure below the intake valve enough to be below the pressure above the intake valve. The earlier you open the intake valve, the less time the exhaust has to drop the pressure below the intake valve.

[travis]

Why would you not want to start the intake open event earlier to get flow moving through the more restrictive port earlier?

Opening the valve earlier, before TDC will only "get the flow moving" if the pressure below the valve is less then the pressure above the valve.
Since the piston isn't helping, and actually hurting at this point, you have to rely on the exhaust to have scavenged enough to drop the pressure below the intake valve enough to be below the pressure above the intake valve. The earlier you open the intake valve, the less time the exhaust has to drop the pressure below the intake valve.

So...since you needed to add duration, the same or tighter lsa is going to have the intake opening a lot earlier, which is going to considerably increase reversion up the intake port, which means that flow in the right direction won’t start until the piston is farther away from TDC...so you end up with a rougher idle, worse fuel economy, more of a “light switch” type power curve (harder to control if traction is ever an issue), reduced low end off idle torque, etc. Am I on the right track?

Would adding some exhaust duration help with the earlier opening intake? That’s going to start blowing down the cylinder earlier, but it also adds overlap which is maybe getting excessive at this point?

[GARY C]

Rick, what head are you using to get your numbers? Based on the numbers I am guessing it's not a traditional 23°

[digger]

i dont understand the difference between these. looks like the headflow input is different?

download/file.php?id=22022&mode=view
download/file.php?id=22043&mode=view

[GARY C]

Using intake flow numbers from Chads heads and fabricating exhaust numbers to match Ricks this is what I get from DV's 90's vintage Cam Master. This is the one I was looking at. download/file.php?id=22015&mode=view

[Stan Weiss]

Both Gary C and Rick. Shouldn't you be using a 2.055" for the 3.915" engine?

Stan

[GARY C]

Both Gary C and Rick. Shouldn't you be using a 2.055" for the 3.915" engine?

Stan

Yes I was just trying to use a known 23° head with numbers close to Rick's to see what DV's program spit out. Just changing the valve size but leaving everything else the same it goes to a 313/323 on a 109.6 and increase overlap to 100. I am not sure how well a head sized for a 2.055 valve would support 8000 rpm at least on a 23° head but thats above my pay grade.

[Walter R. Malik]

Both Gary C and Rick. Shouldn't you be using a 2.055" for the 3.915" engine?

Stan

Yes I was just trying to use a known 23° head with numbers close to Rick's to see what DV's program spit out. Just changing the valve size but leaving everything else the same it goes to a 313/323 on a 109.6 and increase overlap to 100. I am not sure how well a head sized for a 2.055 valve would support 8000 rpm at least on a 23° head but thats above my pay grade.

Way back in the 80's I ran a Modified Production small block Chevy with Iron bow-tie 23 degree heads having 2.02" diameter intake valves on a 292 cubic inch engine, (.060" overbore 283 having a 3.935" bore), shifted at 9,200 RPM and went through the traps at 9,800 RPM.

Apparently you don't think much of the 23 degree architecture cylinder head capabilities. I am sure that the longer stroke would not tax the cylinder heads as much as the reciprocating assembly because a whole lot better 23 degree heads are available these days...

[GARY C]

Both Gary C and Rick. Shouldn't you be using a 2.055" for the 3.915" engine?

Stan

Yes I was just trying to use a known 23° head with numbers close to Rick's to see what DV's program spit out. Just changing the valve size but leaving everything else the same it goes to a 313/323 on a 109.6 and increase overlap to 100. I am not sure how well a head sized for a 2.055 valve would support 8000 rpm at least on a 23° head but thats above my pay grade.

Way back in the 80's I ran a Modified Production small block Chevy with Iron bow-tie 23 degree heads having 2.02" diameter intake valves on a 292 cubic inch engine, (.060" overbore 283 having a 3.935" bore), shifted at 9,200 RPM and went through the traps at 9,800 RPM.

Apparently you don't think much of the 23 degree architecture cylinder head capabilities. I am sure that the longer stroke would not tax the cylinder heads as much as the reciprocating assembly because a whole lot better 23 degree heads are available these days...

Yes I know guys were doing that back in the day, Grumpy for example, It's just not something I have ever done so I would not want to assume what ingredients it would take to do it. I know in some cases there was a lot of welding and angle milling.

[Stan Weiss]

Both Gary C and Rick. Shouldn't you be using a 2.055" for the 3.915" engine?

Stan

Stan,
Everything we have been doing in this post is theoretical, theoretical engines and theoretical math.
If you want real...we will need a whole bunch of REAL engine values which we can then use with the real math.

Rick,
While a number of different combinations have been posted in this thread and I have posted answers to them. I still try to go back to what was posted in post #1

Stan

[PRH]

It looks as though a fair amount of head data would be required for Ricks program.

I got the impression(which could be totally wrong) that the heads for the new small bore combo weren’t done yet.

[blackflag]

These are engines that get run quite often. I love trying different combinations. We have a 4” stroke 3.625” bore 360 that we run on pavement. It’s very capable of winning but has a rookie driver. The big bore short stroke engines are proven winners and have been our main 360s on pavement and some dirt since 2008. They will go when you can keep the pipes lit. They don’t peddle good in the dry slick. They are like a high strung two stroke. The 3.915” engine is complete and ready to dyno. All engines have 23 degree heads. I’m going to dyno 5 engines when time permits.

[PRH]

It looks as though a fair amount of head data would be required for Ricks program.

I got the impression(which could be totally wrong) that the heads for the new small bore combo weren’t done yet.

I guess I totally read that situation wrong!!