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How do you pick the correct flow rate for a intake manifold?

7K views 36 replies 14 participants last post by  WHT/73 
#1 ·
How do you pick the correct intake manifold (single plain in my case), with so many to select from. Most do show RPM ranges. So other than that, just ask around and find out what works?

My combo is in my sig.
Thanks.
 
#2 ·
Are you talking flow rates for each cyl ?

If you are, do a search on this site there is a guy who ports them so each runner flows very close to the same cfm.

A little pricey but... if your looking for that last few HP it's priceless.
 
#3 ·
How do you pick the correct intake manifold (single plain in my case), with so many to select from. Most do show RPM ranges. So other than that, just ask around and find out what works?

My combo is in my sig.
Thanks.
learn VooDoo like Bryce and Mark when it comes to porting the intake
OR
off knowledge of what works

Dart 4150 or Motown
Victor Jr.

Dart 4150 or Motown are almost identical and would be best, Victor Jr. would be good

Get the heads ported and use intake you have and the car will accelerate down the track with your rear gear
 
#4 ·
Here's what I'm thinking.
Look at any intake and it looks like the runners are more than big enough to support any size head intake port/valve size/cam lift/exhaust port. Providing the carb/ventures are up to the task . But as RPM increases, more air flow increases.

I understand the straighter the shot to the intake valve via the intake manifold is best. And the length of the intake runner must be of a length that will effectively mix the incoming air/fuel. And each intake runner must flow as close as possible to each other to supply equal amounts of mixed air/fuel to each cylinder.

Is there a point were a intake can't support this demand?
I think there is. But how do you pick the manifold that's correct for you application?

I'm not looking for anything for my build. Just trying to learn something here.
 
#5 ·
Here's what I'm thinking.
Look at any intake and it looks like the runners are more than big enough to support any size head intake port/valve size/cam lift/exhaust port. Providing the carb/ventures are up to the task . But as RPM increases, more air flow increases.

I understand the straighter the shot to the intake valve via the intake manifold is best. And the length of the intake runner must be of a length that will effectively mix the incoming air/fuel. And each intake runner must flow as close as possible to each other to supply equal amounts of mixed air/fuel to each cylinder.

Is there a point were a intake can't support this demand?
I think there is. But how do you pick the manifold that's correct for you application?

I'm not looking for anything for my build. Just trying to learn something here.
Theres a lot more to it than just flow, runner length, and cross section, there is tuning envolved.
 
#26 ·
This the best I can describe this, there are different harmonic frequentcys through a intake tract, if you can tune it correctly (by length and cross section) you get ram tuning at a certain RPM actually a small amout of boost, this also has to work in conjuntion with the exhaust tuning.
 
#8 ·
^ you know, I've not seen that many intake comparison on a flow bench or even on a given set of heads with different intakes. I recall a Vic JR 4150 flange vs a dual plane air gap bolted on to the flow bench on a 315 (maybe it was 305) head. The diff between the two were about 15-20cfm. The single was outflowing it, but, on the motor the intakes made close hp (496 I think it was). Of course the single plane made more on top, but on average, they were pretty darn close...if I recall correctly.
 
#9 ·
Ideally you want the intake manifold to act like an extension of the intake runners in your head. So when you bolt on the intake manifold there will be no loss of flow into head or at least kept to a minimum.

I doubt there will be any out of the box intake manifold that will fit that description. What you should of done when you purchased your heads was to also talk to a porter and ask him which intake manifold would work best for your application and have him work both the head and intake manifold at the same time in order to match them up.
 
#11 ·
Ideally you want the intake manifold to act like an extension of the intake runners in your head. So when you bolt on the intake manifold there will be no loss of flow into head or at least kept to a minimum.

I doubt there will be any out of the box intake manifold that will fit that description. What you should of done when you purchased your heads was to also talk to a porter and ask him which intake manifold would work best for your application and have him work both the head and intake manifold at the same time in order to match them up.
We all wish that :D
 
#10 ·
Is this purely a track deal, or will it see street time too?

You decide on the manifold design based on what rpm the engine will spend the most time at. Honestly.

For most street cars, a tall dual-plane like the Air Gap you've got are the best choice. The Air Gap loses less at high RPM than others, and the design is really good in the low/midrange as well.

If you want more peak high rpm power, by all means- look at the single-planes that are out there. The models suggested already are all good, and by having one ported to match your heads, you'll be doing about all you can do.

It also depends on how much torque and traction you've got. I've run single-plane intakes on street cars with big cubic inch engines because they had all the torque I could possible get traction with on the street. Even though they were not optimized for low rpm use, it didn't matter because I could overpower the tires at any moment just by hammering the throttle. I could use the high rpm power, and I didn't lose anything down low.

You've got a 406 in a relatively-heavy '69 Chevelle. I'd guess you could use the torque, but you know your combo and car better than any of us. Can you sacrifice a little low end to get more top end? Is traction an issue?
 
#12 ·
This is not such an easy question once you get beyond the single four barrel basics of single plane/dual plane, low rise/mid rise/high rise. You will find that even a lot of pro builders will try several different intake manifolds to find the best one for that combo of engine/drivetrain and car. And even then, they still fine tune with different size carb spacers.

It is too bad that there is no standardized rating of the runner and the plenum volumes.

If you really wanted to do the math. I would expect that you should match the intake manifold and intake port runner volume to the engine size and camshaft duration - and then size the plenum to match the sum total flow of those runners.

Thomas
 
#16 ·
Mike,

For a given combination there is an optimal range of runner length (total head + intake manifold), size at the head, size at the plenum opening, corresponding taper, proper radius at the plenum, and proper shape of the runner divider. Most of the top guru's agree that as long as you aren't racing the flow bench, the runner divider in the plenum should be radiused and not a razor edge as shown in Butch's pictures.

The problem with any cast manifold is that the runner length is pretty well set to what is cast give or take just a bit. You can grind some away and you can weld some on, but you aren't going to make 2-3 inches difference in most cases.

That is where sheetmetal or fabricated manifolds come in. They can be "tuned" exactly to what the combination should need and not start off with a bunch of compromises as you would typically be starting with on a cast manifold. That is unless you get lucky and a cast manifold dimensions just happen to line up with what your combination needs, but that is just about never ;)
 
#17 ·
Got some great posts. Lots to re-read."DragRacer" The sheet metal manifold has to be a ton better to tune in a system. No curves, all equal lengths.

"Bomber '67": It is too bad that there is no standardized rating of the runner and the plenum volumes.

I agree 100% Bomber. With flow numbers and valve sizes you have some sorta idea with heads. But with a intake.:noway:

Sorry I didn't post a reply sooner. My main computer burned up it's motherboard "EVGA nForce 680i SLI". Now I'm running on my backup "2nd" system. I do have a spare motherboard "ASUS P5N SLI". Again thanks guys.
 
#18 ·
Does anyone make a flow fixture that actually recreates the real flow through a head/manifold setup? Flow with the head, manifold or both through a static valve is one thing, but how about the flow through the setup with the valves cycling at a specific rate? This would bring into play the pressure waves that are constantly reverberating through the manifold, and show the specific rpm ranges that these waves are adding or subtracting from flow.

A single plain manifold is one by itself with no paint on it.
 
#20 ·
onovakind67.......that would be an engine ;)

The pressure wave tuning can be done by calculation for the most part. Again, how much can you really "tune" a cast manifold though. If the runners in the head/manifold are 3" shorter than optimal.....how do you "fix" that on a cast manifold. You don't.

how important is cross section width in an intake runner, just like in a head?

you want the intake to floe 30-40cfm more than the head, but wouldnt you want a properly sizes runner diameter so you dont loose velocity?


aaron
Aaron,

Yes, cross section in the manifold is absolutely important.
 
#19 ·
how important is cross section width in an intake runner, just like in a head?

you want the intake to floe 30-40cfm more than the head, but wouldnt you want a properly sizes runner diameter so you dont loose velocity?


aaron
 
#22 ·
Sheet metal intakes are mainly for specialized applications, especially for some engine and cylinder head combos and split dominators and other high end EFI setups.

Everything else will probably get very close results within a range of similar intake manifolds. I like to think of the intake manifold runner sizing like headers in reverse: that the further away from the valve, the larger the runner - that it is a funnel. This is why you can use a rectangular port intake manifold on oval port heads with good results.

Thomas
 
#25 ·
what if i had a set of heads that flow 248 cfm at 600 lift (my brodix ik180's). and i put an air gap intake on my car, thats rated at 210cfm on the best runner (using the data from dr j's site), would the intake need to be ported to at least 250cfm or more to make optimal power? but why does the intake work so good out of the box, and you hardly see any ported ones?


aaron
 
#28 ·
I agree withe ya. My AFR's are in the 280 range and i'm making just as much or more power with the Air Gap then with the big single.

I have a '70 lt-1 intake that I had on it and it's runners cross sec are larger then the Air Gaps. So much so that I thought it was going to hurt my et. I was wrong.

I didn't port match it. I just put it on. With the single, you could see to the intake valve and the ports matched up perfectly.
I still have more tuning to do. :thumbsup:
 
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