Tech Discussion Intake manifold development
08-08-2005, 11:17 PM
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Tech Discussion Intake manifold development Ok I realize this isnt technically the right forum and its not LS1 specific but I know theres alot of intelligent people in here so here it goes
I’m looking for a good in depth tech discussion for intake manifold development. For my senior design project Id like to design a new intake. Now this is going to be for use on Vortec SBC heads. But the discussion can be more general as no need to tailor since concepts still apply. Its going to be a sheet metal intake fabricated from 3003 because well that’s what I have. The intake base will stay bolted on the engine at all times with separate runners and plenum which can independently be replaced to allow fine tuning. I’ve got some design questions as far as plenum volume, shape, and runner design and length. Now I’m just kind of throwing out some ideas here and I’m just typing everything as my thoughts comes to me. Scary I know, right
As far as runner lengths I’ve done a couple of quick calcs for two test engines a 350 and a 383 as I feel these are the most commonly built. Using some different rpm’s for peak power to occur at I calculated runner lengths and assembled them into this table using this equation
L = [((720-ECD) x .25V x 2) / (RPM x RV)] - .5D
Where
L=Runner length
ECD = Effective cam duration (adv dur – 20 to 30 deg based on type of cam)
V = Velocity of sound in hot air (approx 1250 – 1300 ft/s)
RPM = Self Explanatory
RV = Reflective Value
D = Diameter of intake pipe
Helmholtz Resonator
D = sq. rt. [(CID x VE x RPM) / (V x 1130)]
Ok using those equations there really isn’t much difference in length between 350 and 383 so to simplify things I just averaged the numbers and I came up with this for values based on a 270 degree adv. Dur cam.
Using a RV of 4
Rpm Length
3000 24.2
3500 20.4
4000 17.5
4500 15.3
5000 13.4
5500 12
Using a RV of 3
Rpm Length
3000 32.8
3500 27.8
4000 24
4500 21
5000 18.6
5500 16.6
Using a RV of 2
Rpm Length
3000 50
3500 42.5
4000 36.8
4500 32.5
5000 28.9
5500 26
Using this equation as the ECD goes down the runner length needs to get longer. So with this in mind and these numbers I’m thinking of making two or three different lengths of runners. Probably 24 in, 28 in, and maybe 26 in. The 28 would account for using about a 10 deg shorter cam and still hitting tuned lengths at approximately the same rpm values as the 24 in runners. Now yes these sound long but this length includes the length of the intake runner in the head. So with a small block head were figuring subtract 6” to find actual runner length. Also another thing these are all effective lengths aren’t they. So things like curves are going to make the runners effectively longer. At which point we could go with a little longer runner as the real length is shorter then it seems since we are looking at effective length. Knowing cam duration effects runner length what effects does cam LSA have on it?
Now I’ve got some questions about runner shapes and entry angles at the head and plenum. I’m debating about using a curved runner approach similar to TPI in that each head pulls from the other side of the intake but it would probably draw from the bottom of the plenum vs. the sides so the curve isn’t quite as sharp unless there is a distinct advantage to drawing from the sides but I don’t see it if there is. I’m not sure if I can run a straight runner and keep overall height decently low enough. But with some longer runners the entry angle at the head may end up being sharp and abrupt especially if each head draws from its own respective side of the plenum. So with that in mind I may go for a long curved runner since the effective length would be longer then actual length that way so it could shorten overall height. Id imagine that you want the port entry angle at the head to be perpendicular to the face or parallel with the heads intake runner or at least semi close say plus minus 10-15* would probably be ok. So with all this in mind runner shape is going to significantly shape plenum design or plenum design is going to significantly shape runner design. Though I feel like its better to change plenum shape since Id think its more volume dependant in a dry flow manifold application then shape dependant this would allow optimization of runner shape and design. Aside from general shape I’ve always been under the impression that a taper of about 1.5* is the best for the runners. It makes sense but I’m looking for other peoples experiences/thoughts. Another thought was what about pairing side by side runners together for the majority of the runner length but say for example the last half or the runner before the head would have a divider in it between each respective runner. Does the runner entry angle at the plenum play a significant factor in power production? If I were to pull the runners from the same side the feed if there were to draw from low on the sides I could kind of have a V shaped lower half just put an extension in to spread the sides of the V- apart a bit and then add some vertical sides if needed to the manifold in order to achieve my desired volume. If any of that makes sense. Also another thing to consider is runner cross-sectional area as I know this can directly affect port velocity. I’d figure to start off at the intake face with the port floor of the intake about .050 higher then the head floor (though this isn’t as important in a dry manifold) but other then that semi closely matching the heads intake port and then going on from there with a 1.5* taper. I’ve also heard of the general rule of thumb that runner volume should be about 80% of valve area. Though I’m sure there is a better way to do this.
As far as injector placement Id figure you want it as close to the head intake face as reasonable possible pointing near the back side of the valve. Any other thoughts? Also, what about nitrous fogger nozzle placement. I’m thinking that coming in from the bottom of the runner would be preferred since the majority of the air flow is at the top of the port. Any thoughts.
Another crazy thought I had was what about anti-reversionary tactics in order to help clean up idle and low speed characteristics of a cammed up engine. Do something similar to anti-reversion plates in a header (but not exactly the same) so incoming air flow wouldn’t really be impeded but anything trying to backflow would be (especially since I like to run tight LSA’s in NA motors) Don’t know if it would work.
Moving on to plenum volume. I know that boosted engines like a larger plenum as you don’t rely on the induction pulse to fill cylinders since the manifold sees pressure instead of vacuum. I’m also thinking in a dry flow manifold (well ok mostly dry flow manifold) that to an extent we can use a larger plenum volume as well. Yea there will be some induction pulse dampening but we also don’t need it to atomize fuel or pull fuel from the carb boosters. On a CFM induction limited engine this may be a way to make more power up higher by giving the cylinder more volume to draw from. I’m thinking that this would work since you always have the TB filling the plenum which has more then enough volume versus a smaller plenum in which some of the air for each cylinder has to pull through the TB as say it’s pulling so much out of the plenum. Don’t know if that makes sense. But being as this is going to be a custom developed intake we won’t necessarily have a CFM problem. I’m also thinking about running a rather large plenum with a divider door to split it in half and then using like a vac referenced valve to open it when you romp into it. Think 70 Chevelle SS 454 cowl on the hood. Same concept just a different use. This could provide higher operating vacuum during cruise, good strong induction pulse, better part throttle acceleration, power, and better part throttle fuel economy. I’m thinking of building a couple different plenums to test these different ideas especially since I plan on have two different test engines (350 and 383). I’m thinking about using a plenum volume of about 230 cu. in. give or take some. This large is wanted as we’re trying to tune for good numbers a little lower in the rpm range.
How much can the plenum volume increase with boost. I want a plenum that can be semi-versatile in operation. Though custom tailored plenums could be built to suit each individual combination.
I’m looking for a good in depth tech discussion for intake manifold development. For my senior design project Id like to design a new intake. Now this is going to be for use on Vortec SBC heads. But the discussion can be more general as no need to tailor since concepts still apply. Its going to be a sheet metal intake fabricated from 3003 because well that’s what I have. The intake base will stay bolted on the engine at all times with separate runners and plenum which can independently be replaced to allow fine tuning. I’ve got some design questions as far as plenum volume, shape, and runner design and length. Now I’m just kind of throwing out some ideas here and I’m just typing everything as my thoughts comes to me. Scary I know, right
As far as runner lengths I’ve done a couple of quick calcs for two test engines a 350 and a 383 as I feel these are the most commonly built. Using some different rpm’s for peak power to occur at I calculated runner lengths and assembled them into this table using this equation
L = [((720-ECD) x .25V x 2) / (RPM x RV)] - .5D
Where
L=Runner length
ECD = Effective cam duration (adv dur – 20 to 30 deg based on type of cam)
V = Velocity of sound in hot air (approx 1250 – 1300 ft/s)
RPM = Self Explanatory
RV = Reflective Value
D = Diameter of intake pipe
Helmholtz Resonator
D = sq. rt. [(CID x VE x RPM) / (V x 1130)]
Ok using those equations there really isn’t much difference in length between 350 and 383 so to simplify things I just averaged the numbers and I came up with this for values based on a 270 degree adv. Dur cam.
Using a RV of 4
Rpm Length
3000 24.2
3500 20.4
4000 17.5
4500 15.3
5000 13.4
5500 12
Using a RV of 3
Rpm Length
3000 32.8
3500 27.8
4000 24
4500 21
5000 18.6
5500 16.6
Using a RV of 2
Rpm Length
3000 50
3500 42.5
4000 36.8
4500 32.5
5000 28.9
5500 26
Using this equation as the ECD goes down the runner length needs to get longer. So with this in mind and these numbers I’m thinking of making two or three different lengths of runners. Probably 24 in, 28 in, and maybe 26 in. The 28 would account for using about a 10 deg shorter cam and still hitting tuned lengths at approximately the same rpm values as the 24 in runners. Now yes these sound long but this length includes the length of the intake runner in the head. So with a small block head were figuring subtract 6” to find actual runner length. Also another thing these are all effective lengths aren’t they. So things like curves are going to make the runners effectively longer. At which point we could go with a little longer runner as the real length is shorter then it seems since we are looking at effective length. Knowing cam duration effects runner length what effects does cam LSA have on it?
Now I’ve got some questions about runner shapes and entry angles at the head and plenum. I’m debating about using a curved runner approach similar to TPI in that each head pulls from the other side of the intake but it would probably draw from the bottom of the plenum vs. the sides so the curve isn’t quite as sharp unless there is a distinct advantage to drawing from the sides but I don’t see it if there is. I’m not sure if I can run a straight runner and keep overall height decently low enough. But with some longer runners the entry angle at the head may end up being sharp and abrupt especially if each head draws from its own respective side of the plenum. So with that in mind I may go for a long curved runner since the effective length would be longer then actual length that way so it could shorten overall height. Id imagine that you want the port entry angle at the head to be perpendicular to the face or parallel with the heads intake runner or at least semi close say plus minus 10-15* would probably be ok. So with all this in mind runner shape is going to significantly shape plenum design or plenum design is going to significantly shape runner design. Though I feel like its better to change plenum shape since Id think its more volume dependant in a dry flow manifold application then shape dependant this would allow optimization of runner shape and design. Aside from general shape I’ve always been under the impression that a taper of about 1.5* is the best for the runners. It makes sense but I’m looking for other peoples experiences/thoughts. Another thought was what about pairing side by side runners together for the majority of the runner length but say for example the last half or the runner before the head would have a divider in it between each respective runner. Does the runner entry angle at the plenum play a significant factor in power production? If I were to pull the runners from the same side the feed if there were to draw from low on the sides I could kind of have a V shaped lower half just put an extension in to spread the sides of the V- apart a bit and then add some vertical sides if needed to the manifold in order to achieve my desired volume. If any of that makes sense. Also another thing to consider is runner cross-sectional area as I know this can directly affect port velocity. I’d figure to start off at the intake face with the port floor of the intake about .050 higher then the head floor (though this isn’t as important in a dry manifold) but other then that semi closely matching the heads intake port and then going on from there with a 1.5* taper. I’ve also heard of the general rule of thumb that runner volume should be about 80% of valve area. Though I’m sure there is a better way to do this.
As far as injector placement Id figure you want it as close to the head intake face as reasonable possible pointing near the back side of the valve. Any other thoughts? Also, what about nitrous fogger nozzle placement. I’m thinking that coming in from the bottom of the runner would be preferred since the majority of the air flow is at the top of the port. Any thoughts.
Another crazy thought I had was what about anti-reversionary tactics in order to help clean up idle and low speed characteristics of a cammed up engine. Do something similar to anti-reversion plates in a header (but not exactly the same) so incoming air flow wouldn’t really be impeded but anything trying to backflow would be (especially since I like to run tight LSA’s in NA motors) Don’t know if it would work.
Moving on to plenum volume. I know that boosted engines like a larger plenum as you don’t rely on the induction pulse to fill cylinders since the manifold sees pressure instead of vacuum. I’m also thinking in a dry flow manifold (well ok mostly dry flow manifold) that to an extent we can use a larger plenum volume as well. Yea there will be some induction pulse dampening but we also don’t need it to atomize fuel or pull fuel from the carb boosters. On a CFM induction limited engine this may be a way to make more power up higher by giving the cylinder more volume to draw from. I’m thinking that this would work since you always have the TB filling the plenum which has more then enough volume versus a smaller plenum in which some of the air for each cylinder has to pull through the TB as say it’s pulling so much out of the plenum. Don’t know if that makes sense. But being as this is going to be a custom developed intake we won’t necessarily have a CFM problem. I’m also thinking about running a rather large plenum with a divider door to split it in half and then using like a vac referenced valve to open it when you romp into it. Think 70 Chevelle SS 454 cowl on the hood. Same concept just a different use. This could provide higher operating vacuum during cruise, good strong induction pulse, better part throttle acceleration, power, and better part throttle fuel economy. I’m thinking of building a couple different plenums to test these different ideas especially since I plan on have two different test engines (350 and 383). I’m thinking about using a plenum volume of about 230 cu. in. give or take some. This large is wanted as we’re trying to tune for good numbers a little lower in the rpm range.
How much can the plenum volume increase with boost. I want a plenum that can be semi-versatile in operation. Though custom tailored plenums could be built to suit each individual combination.
