Exhaust / intake flow ratios on LS heads

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Ron E
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Re: Exhaust / intake flow ratios on LS heads

Post by Ron E » Fri Jan 13, 2017 12:46 pm

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.

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Re: Exhaust / intake flow ratios on LS heads

Post by Newold1 » Fri Jan 13, 2017 2:28 pm

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? :?:

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Re: Exhaust / intake flow ratios on LS heads

Post by RW TECH » Fri Jan 13, 2017 8:55 pm

Newold1 wrote: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? :?:
Calculations may exist but the "common gist" in racing is to basically ignore the exhaust port & crutch it along with the cam. Maybe not all of the time but some of the time engines will respond to a combination of a bigger/better exhaust port and less exhaust duration. Sometimes this kind of thing happens on engines that are notorious for big duration splits.

And sometimes flow measured on a bench at 28" H20 depression isn't any better or even as good in some places with a bigger/better exhaust port but the engine makes mucho-more power after peak, especially when events are narrowed up a little.

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Re: Exhaust / intake flow ratios on LS heads

Post by Rick360 » Fri Jan 13, 2017 10:07 pm

Why do people think that exhaust duration should be the same or close to the intake duration on a race engine? The process and dynamics are totally different between intake and exhaust. Maybe a smaller exhaust port and longer duration is the RIGHT way to do it??? Why is 20º considered a LOT of split? Why is it considered ideal if the intake and exhaust end up sized where they need the same amount of time to fill/empty the cylinder? The highest hp/ci engines always have a much longer exhaust duration. Why???

Things to think about...

Rick

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Re: Exhaust / intake flow ratios on LS heads

Post by swampbuggy » Fri Jan 13, 2017 10:15 pm

Warp Speed ?????????

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Re: Exhaust / intake flow ratios on LS heads

Post by Newold1 » Sat Jan 14, 2017 9:45 am

I am not trying to infer that an exhaust port needs to be closer to the performance of the intake port. What I was asking in this example I posted was how does one calculate what size an exhaust valve and port needs to be to optimize the performance of the intake valve and its port in a given set of parameters. If it can't be calculated then "HOW DO IT KNOW?"

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Re: Exhaust / intake flow ratios on LS heads

Post by HDBD » Sat Jan 14, 2017 10:27 am

Why does the exhaust port need to be tied to the intake at all? Why not just optimized to suit the need of the air pump, the engine application. Smaller than some may expect I bet and leaving more real estate to use a larger intake valve.

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Re: Exhaust / intake flow ratios on LS heads

Post by MadBill » Sat Jan 14, 2017 11:56 am

HDBD wrote:Why does the exhaust port need to be tied to the intake at all? Why not just optimized to suit the need of the air pump, the engine application. Smaller than some may expect I bet and leaving more real estate to use a larger intake valve.
You answered your own question. Considered on its own, a race engine exhaust valve probably would be larger than typical practice, and opened later to extract more work from the expanding gases. Since the chamber size is finite though and for a high RPM two valve engine intake valve size is a limiting factor, finding effective ways to shrink the exhaust valve sets the stage as you say for a larger intake. One of the trade offs, especially for high compression engines, is to make the exhaust smaller but open it sooner to compensate.
By way of contrast, highly supercharged engines need less 'help' for filling but have more pressure to potentially waste late in the stroke and also must deal with a greater volume of exhaust products. Such engines use exhaust vales approaching the size of the intakes.
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Re: Exhaust / intake flow ratios on LS heads

Post by shawn » Sat Jan 14, 2017 11:58 am

Ron E wrote: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.
You must have done this before :)
shawn

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Re: Exhaust / intake flow ratios on LS heads

Post by Ron E » Sun Jan 15, 2017 6:09 pm

shawn wrote:You must have done this before :)
shawn

A time or 12. Hope all is well your way.

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