Camshaft changes or not with stroke increased.

[Fatman]

You would need to make a huge change in duration, and lobe centerlines

what changes would you make?

[PRH]

It seems my thinking is the opposite of Walters.

The smaller intake valves & smaller bores(possibly causing some flow deficits), along with the increased demand due to the quicker increasing swept area of the longer stroke and shorter rod ratio......... I feel like you’d want to get a head start on the flow, and maybe get a little assistance from the exhaust port too.
So....... slightly quicker intake lobe on a tighter lsa.

[Stan Weiss]

Is there an RPM limit for this class?

Stan

ab-pfd-361.gif

[CamKing]

You would need to make a huge change in duration, and lobe centerlines

what changes would you make?

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.

[blackflag]

The 2.055 intake valve was recommended by my head guru. 2.100” intake valve was too large for the 3.915” bore. I’m thinking more swept volume and more piston speed of the longer stroke will probably want a bigger camshaft. We were running a lot bigger cams in our short stroke engines when we first started building them. Made big power for a 360 turned them 8800 rpm. They were like an on off switch! We are about ten to twelve degrees smaller duration and about 2 degrees smaller on LSA than we were in 2010. They are really a lot better with the smaller cams.

[Geoff2]

I don't see where 'more swept volume' comes from. Both combos are 361 cu in!

[houser45]

What cylinder head are you using?

[Walter R. Malik]

I don't see where 'more swept volume' comes from. Both combos are 361 cu in!

Mean piston speed away from TDC and BDC at whatever RPM, not swept volume, will be the determining factors here.

[Stan Weiss]

Yes Volume above the piston ATDC and ABDC are the same but the volume is different for different degrees of crank rotation between those points.

Stan

ab-cv-361.gif

[blackflag]

Yes it’s definitely the same total swept volume. But the demands to fill the cylinder as the pistons are in motion are totally different. Stan’s data shows a good difference at 90 degrees crank angle. Awesome information by the way. Big changes to bore and stroke change hence a big change in camshaft duration and LSA as camking stated. I love speed talk thanks camking for saving it.

[RevTheory]

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.

[CamKing]

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.

[David Redszus]

If the RR stayed the same, and the displaced area per degree of crank rotation was the same........ why would the longer stroke need a different cam?

The OP hasn’t filled in the details of the rods used for both combos, but I suspect the RR between the two combos is different.

Maybe when that gets revealed, we can see how the two combos compare in displaced volume in that first 90* of rotation.

Edit: I see where Stan used 6” rods for both combos in his sim.

I’ll have to look that over.

Rod ratio has no meaningful effect on piston position, piston velocity, or piston acceleration near TDC, and can be ignored.
Stroke, however has a very large effect on the above, regardless of rod length (within reason).

[PRH]

If the RR stayed the same, and the displaced area per degree of crank rotation was the same........ why would the longer stroke need a different cam?

The OP hasn’t filled in the details of the rods used for both combos, but I suspect the RR between the two combos is different.

Maybe when that gets revealed, we can see how the two combos compare in displaced volume in that first 90* of rotation.

Edit: I see where Stan used 6” rods for both combos in his sim.

I’ll have to look that over.

Rod ratio has no meaningful effect on piston position, piston velocity, or piston acceleration near TDC, and can be ignored.
Stroke, however has a very large effect on the above, regardless of rod length (within reason).

Unless?

If the rod stroke ratio is the same for both engines then the displacement for each degree of crankshaft rotation is the same.

I guess what I’m getting at is, fundamentally....... does it matter that the piston velocity moving away from TDC is quicker with a longer stroke if that added movement doesn’t displace any additional area compared to the big bore/short stroke combo?
Is the piston velocity what creates the pressure differential, or the volume displaced?

If the RR is the same for both long and short stroke combos, and the area displaced per degree of crank rotation is equal...... how much does the velocity of the piston matter?

The original combo is 3.28” stroke with a 6” rod.....RR of 1.829.
To retain the RR with a 3.75 stroke the rod needs to be 6.858”.

So, the long stroke version will still have the piston accelerating away from TDC with more velocity per degree of crank rotation, even with both having the same RR.......but it doesn’t create area in the cylinder any quicker.

Does the incoming column of air care how that area was created?

Bigger piston moving slower vs smaller piston moving faster?

[PRH]

Real world example:

BB Mopar 528.0 vs 526.7 ci

528 - 4.500 bore x 4.15 stroke, 6.76 rod, 1.628 RR
526 - 4.380 bore x 4.375 stroke, 7.100 rod, 1.622 RR