Intake manifold flow

[Boport]

How important is intake manifold flow in relation to the intake port flow?

Reason I ask is I am limited of cross-sectional area in the intake manifold (oem unit) for a 4 cylinder ford engine. The intake port flows 235ish cfm with a 1/2 in. radius, but with the intake manifold attatched it drops flow to about 215 cfm. The intake manifold by itself flows about 220 cfm. The intake manifold ports are quite small and round.

Should I be more concerned with the length of the runners or is my flow issue something that is definitely robbing some power? There is not much I can do to change the runner length, but I may consider welding on the exterior of the manifold to gain more cross-section. What is a acceptable drop in flow when an intake manifold attatched and flowed with the head?

Thanks,
Bo

[bill jones]

-most of the single 4 barrel open plenum manifolds that I have worked on, about the best average losses seem to be about 1% for each .100" lift.
-This means that if you test the head at .600" with the radius inlet and then change over to flow the manifold (and carburetor, carb spacers and air filter etc), that I would expect to see a 6% average loss among the manifold runners.
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-If your bare head 235 flow was at .600" then I'd expect to see about 221 with the entire intake system installed.

[Boport]

Great info Bill, thank you.

I guess I may not be that far off anyway.

Bo

[Joe Mendelis]

I have seen manifolds pick up airflow before as well as hurt it. I flowed a BBC Victor head that flowed 390 and with the manifold flowed 408. LT1 stock head with valve job flowed 235 and backed up at .440 lift. With the manifold attached it flowed 215 from .400 up and never backed up. The worst one I've ever seen was a GT40 Ford. It took a 1.94 valve port flowing 280 and knocked it down to 206!! One thing to mention about very long runner manifolds is that they will hurt flow as well as murdering your torque at higher piston speeds. An SB2 manifold for the most powerful Chevy Cup engines lose 20 CFM with carb attached as well.

[bill jones]

-We did some MPH tests on a 393ci SBC in a rear engine dragster years ago and all we did was port the single four manifolds.
-We started with a set of heads that I adjusted the flow on each head port to 268CFM (at 25") using a radius inlet, then installed the intake and the carburetor and the air pan for the scoop.
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-First manifold was a Victor JR with a $150 fluff & buff that averaged 246.75cfm and the car ran 159.6MPH.
-then I spent all summer and ported on several different manifolds and for each "average" CFM gain that I got averaged of the 8 runners, the car went one mph faster.
-When we got done we had improved the various manifold runners up to an average of 257.25CFM and the car was then running just under 170mph.
-The only other thing we did was to jet the same carburetor around to suit what we thought we were seeing with the sparkplugs as we found the porting changes also changed where the fuel needed to be within the engine.
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-This was a pretty regimented and about as scientific a group of tests that we could do so that we could prove to ourselves the exact gains of manifold runner porting.
-NOTHING else was changed.
-The manifolds iincluded 3 Victor jr's, a Victor 4+4 and a GM bowtie.
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-Altho I haven't flowtested a ton of manifolds I have yet to find any manifold runner that ever flowed more than the port did with a radius inlet.
-in this group of tests where the head ports were adjusted to 268cfm, the absolute best I ever saw thru any one of the manifold ports was 260cfm.
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-When testing swirl of the head VS swirl of the manifold runners there is defintely a measurable difference and if you happen to test 2bbls VS 4 barrels on the same manifolds the differences are huge.
-Most of the swirl observations I've done were using 180 style manifolds and 2bbls vs 4 bbls.

[Boport]

That's some very interesting findings Bill, thanks again for taking the time to post your results. Great info!

Bo

[b73]

I have seen manifolds pick up airflow before as well as hurt it. I flowed a BBC Victor head that flowed 390 and with the manifold flowed 408. LT1 stock head with valve job flowed 235 and backed up at .440 lift. With the manifold attached it flowed 215 from .400 up and never backed up. The worst one I've ever seen was a GT40 Ford. It took a 1.94 valve port flowing 280 and knocked it down to 206!! One thing to mention about very long runner manifolds is that they will hurt flow as well as murdering your torque at higher piston speeds. An SB2 manifold for the most powerful Chevy Cup engines lose 20 CFM with carb attached as well.

The LT1 you refer to, is that the early LT1, or the ~1992-> injected LT1?

[Joe Mendelis]

B73, that head was off of an LT1 Camaro engine, I think it was 1996. It was an aluminum head.

[358T]

-We did some MPH tests on a 393ci SBC in a rear engine dragster years ago and all we did was port the single four manifolds.
-We started with a set of heads that I adjusted the flow on each head port to 268CFM (at 25") using a radius inlet, then installed the intake and the carburetor and the air pan for the scoop.
------------------------------
-First manifold was a Victor JR with a $150 fluff & buff that averaged 246.75cfm and the car ran 159.6MPH.
-then I spent all summer and ported on several different manifolds and for each "average" CFM gain that I got averaged of the 8 runners, the car went one mph faster.
-When we got done we had improved the various manifold runners up to an average of 257.25CFM and the car was then running just under 170mph.
-The only other thing we did was to jet the same carburetor around to suit what we thought we were seeing with the sparkplugs as we found the porting changes also changed where the fuel needed to be within the engine.
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WOW! 10mph just from intake manifold porting. That is a BUNCH. Were the weather/track conditions the same for these tests? Was the combo optimized before you started porting the manifolds? Which manifold ended up being the best? Were the runner lengths the same on all the manifolds?

Scott

[bill jones]

-It all started out when the car owner was playing with some desktop dyno and his real life results were showing up on the desk top dyno like if his 276CFM heads only flowed about 246 or so.
-I knew the manifolds hurt the flow like I have described so we took the manifold off and I flowed it on a set of nearly identical heads that really matched up nice and had nearly identical flow numbers.
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-Once we got the REAL story that his manifold was indeed hurting the flow I had several manifolds here that I owned, that also fit his engine and heads like they were made for each other, so I flowtested about 3 of them all at the same time and we started changing manifolds.
-We had the engine and the car tuned in really good to where the ET's were something like just under 8.50's like 8.48's 8.49's etc and we felt it should be going a quarter second quicker than that but we were up against a wall.
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-The heads were Brodix 10's done by Mikey Chapman and the first manifold was a fairly simple fluff and buff and had been port matched by Chapmans guys also.
-The manifolds I had flowed significantly better than the fluff and buffed piece (but these were my PERSONAL stuff), so that's what we started with.
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-We run in a very dry enviroment and we feel like our tests were pretty regimented and conditions were all hot and dry and at 4200 base elevation.
-We didn't ever keep track of the weather with a weather station but we do know and observe the absolute barometric pressure which is typically 25.65" mercury.
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-I have a method of tuning where I cut the sparkplugs apart so that I can view the porcelain from the side and view the ring at the base of the plugs, and this has taught me how to look for what to do to get all 8 cylinders making closer to the same power so we wasted about 4 sets of plugs every weekend to get their stories.
-So I use that information and I mainly worked on getting the low flowing ports up to meet the best flowing ports, never wasted much time on the good flowing ports.
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-This all took place in the late 1980's and the best manifold at the time was the GM Bowtie which was taller than any of the Edelbrocks.
-But that bowtie was something like $350 when the Edelbrocks were $150 and nobody was really interested in spending the big bucks but I bought one anyway just to see what is was all about.
-When we got done with these tests we had never ran a 170 anything and that really bothered us so we used a another bowtie manifold done by Bud Harper that had the top modified with a welded on a 4500 funnel type spacer and we tried an old Nascar 1050cfm 4500 that I had and the car went 173 and change.
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-During all of this testing the ET's came down from 8.48's to 8.08 was about the best I remember.
-Problem was we had a friend who was 1/10th quicker than we were and every time we went faster and quicker so did he, and that provided some extra incentive to whip up on him but we never got there.
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-There is no way that I can see where you can ever get the runners equal in length on a single 4bbl cast manifold.

[shawn]

That's really interesting stuff, Bill.
Has anyone got the speedtalk disk that has Phil Martin on it? It says it covers some intake porting info. I was just wondering how in depth it gets.
thanks,
shawn

[70MC]

I have a brodix intake for a sbc part# HV1002, it has a 4500 seies top. What i see is that the 4 inner runners are narrower than the outer runners, about .100 or so. If my memory is correct my Victer JR had the 4 outer runners narrower, what is best and why? Or should all 8 runners be the same size?

[Boport]

Thats to keep the volumes closer between the short port and the longer outside port I believe.

[LilRacr]

When flowing manifolds, which port do you flow? Short or long port?

I have never really found power in a intake unless it was really restrictive, several years ago I ported a standard Vic Jr (2975) and was able to get an even 300 cfm @ .700, the head alone flowed 314 @ .700 lift, I wanted more but could not spend the time in it.

You have now given me the inspiration to look more closely at intakes and flow. I have tested a few intakes and all seem to make about the same power when prepared to match the heads. I do not have direct access to a dyno so I am somewhat limited to testing.

[cboggs]

Bobby,

I flow all the ports, .. head & manifold.

There's a fair amount of power in the manifold, .. it's part of the intake port.
If you pay attention to the cross sectional areas in the manifold and look
at the manifold and head all as one port you'll find power.

Take a standard Edelbrock Vic and a good head like a 225 AFR ( for example )
and map the CSA's from the runner opening in the plenum to the valve, ..
you'll see what I mean.

Curtis