Exhaust / intake flow ratios on LS heads

[MadBill]

The consensus seems to be that engines can briefly experience sonic flow early in the blow down cycle. However I believe the correct response to this knowledge is: "so what?"

[GARY C]

The consensus seems to be that engines can briefly experience sonic flow early in the blow down cycle. However I believe the correct response to this knowledge is: "so what?"

Have you been watching old Andrew Brieghtbart videos?

[MadBill]

I've heard the name, but don't get the implication..

[RW TECH]

The consensus seems to be that engines can briefly experience sonic flow early in the blow down cycle. However I believe the correct response to this knowledge is: "so what?"

So if it stays that way for too long the piston may wind up with a bunch of pressure above it near TDC overlap at low exhaust valve lift. Maybe that's why some of the "weird" LS cams exist.

[MadBill]

From everything I've read, it could only happen at EVO for perhaps less than 20° after effective flow begins and so would have ended long before BDC.

[groberts101]

Not sure I fully subscribe to sonic flow rates being produced in car exhausts. Shock diamonds sure seem to be rare occurrences too. Unless maybe you drive a turbine powered car at full tilt.. or like to run ultra high rpm big blocks with zoomies pointing up towards the night sky.

[MadBill]

There was a big discussion about it here a couple of years back. As usual a unanimous decision was far from forthcoming, but one point that didn't arise then was the one I mentioned above. To restate: What would we do differently if we knew that there was or was not momentary sonic flow in the throat during exhaust blow down?

[GARY C]

I've heard the name, but don't get the implication..

That was his favorite response to unsubstantiated talking point.

[randy331]

Knowing your ex vs in flow ratio is like knowing your "dynamic comp".

Both are easy to figure but are of little value.

Randy

[digger]

From everything I've read, it could only happen at EVO for perhaps less than 20° after effective flow begins and so would have ended long before BDC.

for an extended period of time its sonic through the valve aperture (curtain) but i doubt the average port throat velocity is that early in the blowdown phase because the area ratio is large (curtain much smaller than throat), according to EngMod4T the peak mach number in ex port throat is much later and close to BDC and its high until after BDC to. it would depend exactly where the "throat" is and exact cam profile but a few engines ive looked at its kinda in some way proportional to the lift profile of the valve

[digger]

Knowing your ex vs in flow ratio is like knowing your "dynamic comp".

Both are easy to figure but are of little value.

Randy

if you have no exhaust flow data how do you spec a cam?

i agree its pretty hard to prove that there is a magical in/ex ratio to target as being best/ideal even if were somehow true as there are lots of ways to skin the cat, but knowing it means you know something about the port flow characertics, better than nothing

[groberts101]

Knowing your ex vs in flow ratio is like knowing your "dynamic comp".

Both are easy to figure but are of little value.

Randy

Seems kinda funny to make blanket statements like that. I thought BOTH of those were derivative aspects of the engines overall design considered by cam grinders to help decide the best location for all the valve timing events.

IOW, wouldn't that type of data be utilized on, maybe say a 9:1 vs 11:1 SCR motor.. both identical engines but with different SCR's. Maybe slightly differing cylinder pressure profiles which seemingly lead towards differing induction port speeds/flowz, and exhaust port speeds/flowz.. might produce different final cam spec's?

[randy331]

Knowing your ex vs in flow ratio is like knowing your "dynamic comp".

Both are easy to figure but are of little value.

Randy

Seems kinda funny to make blanket statements like that. I thought BOTH of those were derivative aspects of the engines overall design considered by cam grinders to help decide the best location for all the valve timing events.

IOW, wouldn't that type of data be utilized on, maybe say a 9:1 vs 11:1 SCR motor.. both identical engines but with different SCR's. Maybe slightly differing cylinder pressure profiles which seemingly lead towards differing induction port speeds/flowz, and exhaust port speeds/flowz.. might produce different final cam spec's?

Maybe cam companies do use both flowz and the so called "dynamic comp" to spec you a cam, I just call Comp cams with lobe numbers, lsa etc and order a cam.

Let me ask you a question,..If you quit calling Cam companies with your flowZ and asking them for a cam, and started using things like piston position vs open/close events, rpm, bore stroke, results of past engines, etc, to spec your own cams, and your engines improved, would you reverse direction?

2 recent engines had very high ex/in flowz, yet when we decided where to put valve events, both ended up with 20* more ex duration. Both are some of the better engines I was involved with. Very nice power curves. Better than past engines.

We've learned when you call up and ask for in and ex lobes that are 20* apart, your gonna get one of those questions like,... oh so you got a blower on it?
To avoid confrontation,... just say yes and go on.

On a side note, my daily work truck has a 9.75-1 "dynamic comp" my fasted 383 has 9.02-1. guess which one lives happily on nothing but 87 fuel ?

Randy

[Ron E]

The big throats work well with high lift cams. Probably because sonic flow is said to be more dependent on area than on seat geometry. Sims show some of the combos we've done stuff for to remain sonic across the seat through around .400"or so lift. So, area-wise the big throats may very well flow better on the opening side. Big throats also tend to flow good high lift numbers and poor low lift numbers on the closing side. All of that works well for us. As already mentioned, you have to figure out what the bench is telling you. There's much more than CFM to learn once combined with dyno and track results.
Randy, if I'm talking to a cam guy, I can't really give him piston vs. valve positions prior to having a cam. We try to give them what they ask for. On exhaust, we'll give him with pipe and without exhaust. flow as W/O pipe numbers are the only ones consistent. But, we develope the head using a pipe even though those numbers are only useful to us.

[Newold1]

Let me try to put forth a certain scenario in a normally aspirated engine using 93 octane gasoline as fuel and at a certain V-8 pushrod OHV engine with a 53 cubic inch cylinder with a bore of 4.125" and a stroke of 4.0" a static compression ratio of 11.0 to 1 and a total combustion space of 120cc . This engine has an intake tract that is 14" in length that provides 46 cubic inches of total area from throttle plate to intake valve seat. The curtain area for the intake valve is 3.45 sq.in. This intake tract will flow 330cfm at the camshafts maximum valve lift of .650" and will open at 20 degrees BTDC and close at 50 degrees ABDC. With an air fuel ratio of 12.5 at WOTand ignition starting at 29 degrees BTDC, at 6000rpms after fairly decent combustion, how much volume of exhaust will be created when the exhaust valve begins to open at 60 degrees BBDC and at that volume and pressure how much cfm of exhaust gas is going to flow until the exhaust valve closes at 18 degrees after TDC on the exhaust stroke @ sea level of course. It seems as though this is said to be a mathematically produced number that can in essence determine the required size of the exhaust valve, its curtain area, port length and volume of the port to its exit? I realize this might be a stretch for a calculation but is it possible to calculate this requirement based on givens, or as some here are insisting that this is just a guess at best, or not really important at all? I guess what I am also wondering is do cylinder head designers or builders use formulas and math or do they just kind of guess at a valve size, port size and cfm they feel they need to outfit their cylinder head with based on previous designs and testing?