Heads / int cc’s? Vs flow Vs cross section Vs air speed
01-16-2022, 06:24 PM
#21
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I’ll be 100% honest. I have no fking idea. Just putting my heads/builds into the hands of a guy who’s made power all his life. I don’t necessarily agree with the end product (port volume) but I’m going with it. What fun it be to be in R&D for roush racing.
Last edited by getrdunn; 01-17-2022 at 11:24 AM.
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KWright (01-16-2022)
01-16-2022, 09:04 PM
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I think theres alot more to it than port "volume" per say. Manufacturers came up with port volumes pretty much as a buying guide for the average guy. People ran with it as gospel.
Lets look at two heads from Dart for example. The 335 CNC and 355 CNC
Darts published flow numbers on the intake ports, are within 5 CFM from .200 thru .700 lift. At .800 lift, the 355 flows 20cfm more. Both have identical exhaust ports, and 2.300 intake valves.
In my opinion, the 355's would offer nothing over the 355's unless you're running an .800 lift cam. The fact the 335 flows the same air up to .700, tells me that the port is simply more efficient. So, whether its a 598ci, or 540ci, the 335 seems a better fit, again, if not over .800 lift.
But lets say you're comparing a 310cc dart as cast head. That head is down 20cfm to the 335cc cnc version in the mid lift range. Now the question is, does the smaller head have better airspeed than the larger one? Maybe it does, maybe it doesnt. Thats not going to be able to be measured on paper, or on a forum discussion.
The GM rectangles that came on most Merc engines, were what, 325ish cc? Big port. Small flow. Yank those things off, and bolt on a set of 335cnc heads, and you're going to absolutely see an increase in power, pretty much across the board, including low end torque. It had nothing to do with port volume, but rather port architecture.
One thing is for sure in my opinion. There is no bigger overhyped discussion, besides maybe cam talk, than port volume. If a guy puts a 335cc head on a 502ci, people freak out. They say "youll have no low end , it will be a turd!". Think about how many low compression 454 mags or 420's were out there with 325cc iron boat anchors, that ran just fine. They had decent throttle response, pulled skiers, and went in some big heavy slug boats. Im not suggesting head volume doesnt matter, but I think its often blamed for performance issues, when in reality it was cammed wrong, carbed wrong, tuned wrong. Over the years I've seen guys sell their aftermarket 325ish cc heads, tear their engines all apart, just to go with a head with a port 10-15cc smaller volume. Just to find out it didnt make an ounce of a difference on the boat's speedo.
Lets look at two heads from Dart for example. The 335 CNC and 355 CNC
Darts published flow numbers on the intake ports, are within 5 CFM from .200 thru .700 lift. At .800 lift, the 355 flows 20cfm more. Both have identical exhaust ports, and 2.300 intake valves.
In my opinion, the 355's would offer nothing over the 355's unless you're running an .800 lift cam. The fact the 335 flows the same air up to .700, tells me that the port is simply more efficient. So, whether its a 598ci, or 540ci, the 335 seems a better fit, again, if not over .800 lift.
But lets say you're comparing a 310cc dart as cast head. That head is down 20cfm to the 335cc cnc version in the mid lift range. Now the question is, does the smaller head have better airspeed than the larger one? Maybe it does, maybe it doesnt. Thats not going to be able to be measured on paper, or on a forum discussion.
The GM rectangles that came on most Merc engines, were what, 325ish cc? Big port. Small flow. Yank those things off, and bolt on a set of 335cnc heads, and you're going to absolutely see an increase in power, pretty much across the board, including low end torque. It had nothing to do with port volume, but rather port architecture.
One thing is for sure in my opinion. There is no bigger overhyped discussion, besides maybe cam talk, than port volume. If a guy puts a 335cc head on a 502ci, people freak out. They say "youll have no low end , it will be a turd!". Think about how many low compression 454 mags or 420's were out there with 325cc iron boat anchors, that ran just fine. They had decent throttle response, pulled skiers, and went in some big heavy slug boats. Im not suggesting head volume doesnt matter, but I think its often blamed for performance issues, when in reality it was cammed wrong, carbed wrong, tuned wrong. Over the years I've seen guys sell their aftermarket 325ish cc heads, tear their engines all apart, just to go with a head with a port 10-15cc smaller volume. Just to find out it didnt make an ounce of a difference on the boat's speedo.
01-17-2022, 09:29 AM
#23
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I think theres alot more to it than port "volume" per say. Manufacturers came up with port volumes pretty much as a buying guide for the average guy. People ran with it as gospel.
Lets look at two heads from Dart for example. The 335 CNC and 355 CNC
Darts published flow numbers on the intake ports, are within 5 CFM from .200 thru .700 lift. At .800 lift, the 355 flows 20cfm more. Both have identical exhaust ports, and 2.300 intake valves.
In my opinion, the 355's would offer nothing over the 355's unless you're running an .800 lift cam. The fact the 335 flows the same air up to .700, tells me that the port is simply more efficient. So, whether its a 598ci, or 540ci, the 335 seems a better fit, again, if not over .800 lift.
But lets say you're comparing a 310cc dart as cast head. That head is down 20cfm to the 335cc cnc version in the mid lift range. Now the question is, does the smaller head have better airspeed than the larger one? Maybe it does, maybe it doesnt. Thats not going to be able to be measured on paper, or on a forum discussion.
The GM rectangles that came on most Merc engines, were what, 325ish cc? Big port. Small flow. Yank those things off, and bolt on a set of 335cnc heads, and you're going to absolutely see an increase in power, pretty much across the board, including low end torque. It had nothing to do with port volume, but rather port architecture.
One thing is for sure in my opinion. There is no bigger overhyped discussion, besides maybe cam talk, than port volume. If a guy puts a 335cc head on a 502ci, people freak out. They say "youll have no low end , it will be a turd!". Think about how many low compression 454 mags or 420's were out there with 325cc iron boat anchors, that ran just fine. They had decent throttle response, pulled skiers, and went in some big heavy slug boats. Im not suggesting head volume doesnt matter, but I think its often blamed for performance issues, when in reality it was cammed wrong, carbed wrong, tuned wrong. Over the years I've seen guys sell their aftermarket 325ish cc heads, tear their engines all apart, just to go with a head with a port 10-15cc smaller volume. Just to find out it didnt make an ounce of a difference on the boat's speedo.
Lets look at two heads from Dart for example. The 335 CNC and 355 CNC
Darts published flow numbers on the intake ports, are within 5 CFM from .200 thru .700 lift. At .800 lift, the 355 flows 20cfm more. Both have identical exhaust ports, and 2.300 intake valves.
In my opinion, the 355's would offer nothing over the 355's unless you're running an .800 lift cam. The fact the 335 flows the same air up to .700, tells me that the port is simply more efficient. So, whether its a 598ci, or 540ci, the 335 seems a better fit, again, if not over .800 lift.
But lets say you're comparing a 310cc dart as cast head. That head is down 20cfm to the 335cc cnc version in the mid lift range. Now the question is, does the smaller head have better airspeed than the larger one? Maybe it does, maybe it doesnt. Thats not going to be able to be measured on paper, or on a forum discussion.
The GM rectangles that came on most Merc engines, were what, 325ish cc? Big port. Small flow. Yank those things off, and bolt on a set of 335cnc heads, and you're going to absolutely see an increase in power, pretty much across the board, including low end torque. It had nothing to do with port volume, but rather port architecture.
One thing is for sure in my opinion. There is no bigger overhyped discussion, besides maybe cam talk, than port volume. If a guy puts a 335cc head on a 502ci, people freak out. They say "youll have no low end , it will be a turd!". Think about how many low compression 454 mags or 420's were out there with 325cc iron boat anchors, that ran just fine. They had decent throttle response, pulled skiers, and went in some big heavy slug boats. Im not suggesting head volume doesnt matter, but I think its often blamed for performance issues, when in reality it was cammed wrong, carbed wrong, tuned wrong. Over the years I've seen guys sell their aftermarket 325ish cc heads, tear their engines all apart, just to go with a head with a port 10-15cc smaller volume. Just to find out it didnt make an ounce of a difference on the boat's speedo.
same to me regarding the port size. He said don’t get all worked and other relevant factors involved as it’s more of a reference. I need to confirm he cc’d a raised port also.
Last edited by getrdunn; 01-17-2022 at 09:54 AM.
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JaniH (01-17-2022)
01-17-2022, 10:00 AM
#24
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If you can get them cheap or free, maybe worth trying. Otherwise just have someone work your current heads into something like the 335cnc dart pro 1.
01-17-2022, 10:13 AM
#25
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I think theres alot more to it than port "volume" per say. Manufacturers came up with port volumes pretty much as a buying guide for the average guy. People ran with it as gospel.
Lets look at two heads from Dart for example. The 335 CNC and 355 CNC
Darts published flow numbers on the intake ports, are within 5 CFM from .200 thru .700 lift. At .800 lift, the 355 flows 20cfm more. Both have identical exhaust ports, and 2.300 intake valves.
In my opinion, the 355's would offer nothing over the 355's unless you're running an .800 lift cam. The fact the 335 flows the same air up to .700, tells me that the port is simply more efficient. So, whether its a 598ci, or 540ci, the 335 seems a better fit, again, if not over .800 lift.
But lets say you're comparing a 310cc dart as cast head. That head is down 20cfm to the 335cc cnc version in the mid lift range. Now the question is, does the smaller head have better airspeed than the larger one? Maybe it does, maybe it doesnt. Thats not going to be able to be measured on paper, or on a forum discussion.
The GM rectangles that came on most Merc engines, were what, 325ish cc? Big port. Small flow. Yank those things off, and bolt on a set of 335cnc heads, and you're going to absolutely see an increase in power, pretty much across the board, including low end torque. It had nothing to do with port volume, but rather port architecture.
One thing is for sure in my opinion. There is no bigger overhyped discussion, besides maybe cam talk, than port volume. If a guy puts a 335cc head on a 502ci, people freak out. They say "youll have no low end , it will be a turd!". Think about how many low compression 454 mags or 420's were out there with 325cc iron boat anchors, that ran just fine. They had decent throttle response, pulled skiers, and went in some big heavy slug boats. Im not suggesting head volume doesnt matter, but I think its often blamed for performance issues, when in reality it was cammed wrong, carbed wrong, tuned wrong. Over the years I've seen guys sell their aftermarket 325ish cc heads, tear their engines all apart, just to go with a head with a port 10-15cc smaller volume. Just to find out it didnt make an ounce of a difference on the boat's speedo.
Lets look at two heads from Dart for example. The 335 CNC and 355 CNC
Darts published flow numbers on the intake ports, are within 5 CFM from .200 thru .700 lift. At .800 lift, the 355 flows 20cfm more. Both have identical exhaust ports, and 2.300 intake valves.
In my opinion, the 355's would offer nothing over the 355's unless you're running an .800 lift cam. The fact the 335 flows the same air up to .700, tells me that the port is simply more efficient. So, whether its a 598ci, or 540ci, the 335 seems a better fit, again, if not over .800 lift.
But lets say you're comparing a 310cc dart as cast head. That head is down 20cfm to the 335cc cnc version in the mid lift range. Now the question is, does the smaller head have better airspeed than the larger one? Maybe it does, maybe it doesnt. Thats not going to be able to be measured on paper, or on a forum discussion.
The GM rectangles that came on most Merc engines, were what, 325ish cc? Big port. Small flow. Yank those things off, and bolt on a set of 335cnc heads, and you're going to absolutely see an increase in power, pretty much across the board, including low end torque. It had nothing to do with port volume, but rather port architecture.
One thing is for sure in my opinion. There is no bigger overhyped discussion, besides maybe cam talk, than port volume. If a guy puts a 335cc head on a 502ci, people freak out. They say "youll have no low end , it will be a turd!". Think about how many low compression 454 mags or 420's were out there with 325cc iron boat anchors, that ran just fine. They had decent throttle response, pulled skiers, and went in some big heavy slug boats. Im not suggesting head volume doesnt matter, but I think its often blamed for performance issues, when in reality it was cammed wrong, carbed wrong, tuned wrong. Over the years I've seen guys sell their aftermarket 325ish cc heads, tear their engines all apart, just to go with a head with a port 10-15cc smaller volume. Just to find out it didnt make an ounce of a difference on the boat's speedo.
01-17-2022, 10:27 AM
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Just for reading.
https://www.streetmusclemag.com/tech...al-port-magic/
https://www.motortrend.com/how-to/su...-engine-build/
Last edited by getrdunn; 01-17-2022 at 10:48 AM.
01-17-2022, 11:18 AM
#27
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I do know they put a ton of time in on port development these days and it does make alot of difference. Below are the two dyno sheets besides heads and headers the only other difference is 2nd one being dry sump and running 10 inches of vacuum.
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getrdunn (01-17-2022)
01-17-2022, 11:39 AM
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Alot of truth here. Sometimes you have to trust your guy. When we went from the cnc bb2 to the cnc 24b I was concerned about losing low end torque. Especially since we were also increasing header size. 1 7/8. To 2 1/8. Larger volume bigger header on a na by old school usually means loss of low end torque. But I was assured because of port design and moving the valves slightly I would gain.
I do know they put a ton of time in on port development these days and it does make alot of difference. Below are the two dyno sheets besides heads and headers the only other difference is 2nd one being dry sump and running 10 inches of vacuum.
I do know they put a ton of time in on port development these days and it does make alot of difference. Below are the two dyno sheets besides heads and headers the only other difference is 2nd one being dry sump and running 10 inches of vacuum.
I followed your builds and their sweet. Also watched your vids. Impressive! Nice to see that torque curve from 3,500 all the way through. Great peak torque also!!! How do you like your boat overall. I’ll be happy to get my straight bottom on the water this year. John SR’s 93 trip sc 42 was a blast back in the day. Wonder how much dif the boat handles etc going from trips to twins.
01-17-2022, 12:09 PM
#29
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I followed your builds and their sweet. Also watched your vids. Impressive! Nice to see that torque curve from 3,500 all the way through. Great peak torque also!!! How do you like your boat overall. I’ll be happy to get my straight bottom on the water this year. John SR’s 93 trip sc 42 was a blast back in the day. Wonder how much dif the boat handles etc going from trips to twins.
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getrdunn (01-17-2022)
01-17-2022, 12:18 PM
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We absolutely love the boat. I would like to whipple it so we could run with the outboard cats. My concern is being able to sustain 8 to 10 minute pulls. If you watch my videos all the boats we ran with are supercharged. They just can't sustain the long consistent pulls. Not sure why. Maybe heat, inconsistent fuel supply? In the last video they all took off before us so we had to run them down then they weren't able to run with us after. I do know that their top speed was just as fast as us and a couple faster, but they just couldn't maintain. I don't have alot of experience with supercharged boats so it makes me a little nervous. Thanks for the kind compliments, maybe we will get to run together some day.
