Intake/exhaust flow ratio as important as overall flow?
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From: Dyess AFB, TX
Intake/exhaust flow ratio as important as overall flow? I just received the flow sheets for my heads and I'm disapointed to say the least. To add fuel to the fire, they were flowed on a 4.0 bore instead of a 3.9. These are LS6 heads with stock valves.
.200--145.8--104.2
.300--209.2--139.4
.400--250.0--164.2
.450--266.0--175.8
.500--280.6--185.1
.550--291.6--193.2
.600--291.4--197.6
From stock LS6 heads, my flow approved approximately 40cfm on the intake and 20 cfm on the exhaust. Well now that my I/E ratio is completely fucked, I'm wondering if I should get a cam that has alot more exhaust duration to try to help this. I'm currently running an F13 which is a 230/232.
If the heads weren't already on, I'd send them back, but since they are, I'm kind of stuck here because I'm not going to take them off. So should I get a different cam or will it not make much of a difference? How bad is my I/E ratio?
.200--145.8--104.2
.300--209.2--139.4
.400--250.0--164.2
.450--266.0--175.8
.500--280.6--185.1
.550--291.6--193.2
.600--291.4--197.6
From stock LS6 heads, my flow approved approximately 40cfm on the intake and 20 cfm on the exhaust. Well now that my I/E ratio is completely fucked, I'm wondering if I should get a cam that has alot more exhaust duration to try to help this. I'm currently running an F13 which is a 230/232.
If the heads weren't already on, I'd send them back, but since they are, I'm kind of stuck here because I'm not going to take them off. So should I get a different cam or will it not make much of a difference? How bad is my I/E ratio?
I am going to assume your numbers are on intake side are less intake bolted up and your numbers on the exhaust side are less pipe. If so your I/e ratio is actually better then you think. Bolting the intake to the head on these engines useally cost the intake side around 10%. Bolting your header or a pipe to simulate your primary ID size usally helps the exhaust side by 5% or better. So if you take this into consideration your ratio is not that bad....but really it doesn't matter what it is as long as you cam for it.
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Explain what you mean by "overall flow"...Int/exh ratio IS important. There are ALOT of things that are important when looking into a set of heads. What CC did the intake runner end up being? Remember, those heads have STOCK valves in them. Ferreas or manleys, or ANY aftermarket larger diameter, tapered stem valve would improve those numbers. Was the exhaust flowed with a pipe or wide open? Was the intake flowed with clay, a radiused inlet, or was it also wide open? Or was there an intake manifold bolted to it? Those look like pretty decent stage 1.5 to stage 2 numbers. One thing you can rest assured about, is that the porter doesn't lie about his numbers...That in itself is hard to find...
Check out this link:https://ls1tech.com/forums/showthread.php?t=256055
Check out this link:https://ls1tech.com/forums/showthread.php?t=256055
Thread Starter
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From: Dyess AFB, TX
Originally Posted by TIM@APENGINE
Explain what you mean by "overall flow"...Int/exh ratio IS important. There are ALOT of things that are important when looking into a set of heads. What CC did the intake runner end up being? Remember, those heads have STOCK valves in them. Ferreas or manleys, or ANY aftermarket larger diameter, tapered stem valve would improve those numbers. Was the exhaust flowed with a pipe or wide open? Was the intake flowed with clay, a radiused inlet, or was it also wide open? Or was there an intake manifold bolted to it? Those look like pretty decent stage 1.5 to stage 2 numbers. One thing you can rest assured about, is that the porter doesn't lie about his numbers...That in itself is hard to find...
Check out this link:https://ls1tech.com/forums/showthread.php?t=256055
Check out this link:https://ls1tech.com/forums/showthread.php?t=256055
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Teching In
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To figure out your intake/exhaust ratio, you simply do it like this: exh / intake =....for example..at .600, it came out to 0.6781056966369..........so that would be almost 68%. You want to try and be above 72% at least. You should have had them put some SS 1.6 exaust valves in them and opened up the exhaust bowl a little more. That would have more than likely put them at around 72%. Overall, they WILL be a noticable improvement from the stock numbers. Stock int/exh ratio @ .600 is 71% roughly. They arent far off. I think you will be pleasently surprised with them especially if you get a cam to work with those specific numbers...Comp Cams can pick you a great cam to work with those heads. I know there are other companies on this board that can help you pick a great cam as well.
Good luck. Hope i helped a little.
Good luck. Hope i helped a little.
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60-70% intake to exhaust flow ratio with a normally aspirated engine with the heads flowed without a pipe is fine. Bigger exhaust valves rarely make more power and usually make less. I have a small block engine that makes 900+ HP to the crank on motor and the intake to exhaust flow is 63% without a pipe. FWIW, pro stock truck engines ran 2.20"+ diameter intake valves and the exhaust rarely exceded 1.58", they have almost exactly the same intake to exhaust flow ratio.
We did some back to back testing with a set of LS6 heads with a 1.57 exhaust valve with a port exit size that is almost stock, then with a 1.60 valve with a big exit and 20 cfm more flow. The small exhaust port made 10 RWHP more and carried the power further. Take most everything you hear, and throw it out the door. Hope this helps.
We did some back to back testing with a set of LS6 heads with a 1.57 exhaust valve with a port exit size that is almost stock, then with a 1.60 valve with a big exit and 20 cfm more flow. The small exhaust port made 10 RWHP more and carried the power further. Take most everything you hear, and throw it out the door. Hope this helps.
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From: BFE
Originally Posted by Brian Tooley
60-70% intake to exhaust flow ratio with a normally aspirated engine with the heads flowed without a pipe is fine. Bigger exhaust valves rarely make more power and usually make less. I have a small block engine that makes 900+ HP to the crank on motor and the intake to exhaust flow is 63% without a pipe. FWIW, pro stock truck engines ran 2.20"+ diameter intake valves and the exhaust rarely exceded 1.58", they have almost exactly the same intake to exhaust flow ratio.
We did some back to back testing with a set of LS6 heads with a 1.57 exhaust valve with a port exit size that is almost stock, then with a 1.60 valve with a big exit and 20 cfm more flow. The small exhaust port made 10 RWHP more and carried the power further. Take most everything you hear, and throw it out the door. Hope this helps.
We did some back to back testing with a set of LS6 heads with a 1.57 exhaust valve with a port exit size that is almost stock, then with a 1.60 valve with a big exit and 20 cfm more flow. The small exhaust port made 10 RWHP more and carried the power further. Take most everything you hear, and throw it out the door. Hope this helps.
That is one reason why your heads which usually portray big exhaust numbers do so well with them.
Gotta match the cam VE's to the head flow at lifts.
