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Thread: Intake manifold options

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Posts: 191-200 of 217
2015-09-06 14:17:11
Is 7.8" not enough?

I just reread this entire thread and theres a lot of conflicting info here. From 1" longer than stock ve runner bottoms to very long runners. I'm shooting for 8.5-9k peak.
2015-09-06 16:33:56
It's been way too long since I've done the N/A runner length math.
2015-09-08 17:56:56
I know the math and theory can only take you that far, but do you guys include the intake port length when spec'ing out the intake runners? Couple of the shorter lengths people are throwing around make sense for the 3rd harmonic when you consider say 4.9" length intake port.

For 184c cams and 8500 rpm peak I'm getting theoretical "correct" pulses at 5,9", 8.6", 9.1" and 14.4" intake RUNNERS
same cams with 9000 rpm peak seem workable at 5,2", 7.8", 8.4" and 13.4" intake runners.
Last edited by Doctor G on 2015-09-08 at 18-14-37. Reason: measured the intake port and updated #s
2015-09-09 13:17:57
Very interesting to see also how intake cam affects the correct length. Might also explain why different people are getting different results and opinions on the same intake manifold. There's not much in it between hitting the sweet spot and getting it completely wrong.

Some very good illustrations on this page about hafway down. Especially the molecule diagrams.

Open Source Physics @Singapore: Longitudinal Sound Wave in Pipe Model by Juan M. Aguirregabiria
2015-09-09 18:53:02
Your above calculations look within the ballpark to me. With the cam your major/only concern is going to be inlet valve open time or number of degrees open. This will determine when the pressure wave generated from the piston begins to travel up the inlet, and the amount of time needed (length) for your compression wave to return at whatever harmonic you so choose before the inlet valve is closed.

On any properly designed or "tuned" inlet (NA or FI does not matter), you should clearly see the harmonics develop on the torque curve. Peak torque will hit at the 4th harmonic, with HP following at the 3rd (every engine will deviate slightly where these "peaks" develop). This is always evident with most stock engine manufacturers who know what they are doing (Honda, BMW, Mercedes...etc). This is why most any BMW/Honda intake has longer runners. When the general public installs the "short" runners, they usually wash out the harmonics since 5-8'' runners do not hit their 3rd and 4th resonance until 10-13k+ RPM and you see the dreaded flat torque curve. If you want to do it the way the professionals do it, tune for the 4th harmonic which will determine your peak torque (highest volumetric efficiency of engine) you can then extrapolate where your peak HP will fall.

In answer to your question about calculation length - include the distance from the inlet valve seat to the opening of the bellmouth (total centerline distance).

If you want I can look up my notes from the engine classes I took in college and what work I had done for a sr20ve inlet in my spare time. It has been a few years so I am rusty as well, but I have some killer inlet stuff since our instructor worked/works as an induction engineer for Honda Indy/F1. I could suggest runner lengths and talk theory all day with you, but he told us if he ever caught us disclosing formulas he would find us and kill us . Hope some of this helps.

Walker Clark
2015-09-09 20:10:20
sounds good Dudeman. By all means yeah please share what you're comfortable with.

And let me mail you my numbers then you can comment on them...
2015-09-10 00:26:10
I know of a man who has an intake with a 15.x" runner length from the valves to the "bellmouth" on the intake. Mhm. That was not designed for all-out, high RPM racing either. It was meant for the most usable power from idle to redline. Not peak power. N/A runner lengths have a plethora of "ifs", "ands" and "buts".
2015-09-14 12:27:18
I'm considering having a feeder plenum/tube feeding the main plenum. Similar to this setup from Serz:

I want to divide the feeder tube with baffles to try and distribute the air more evenly between the 4 runners. Picture this feeder tube as side feed, opening into the plenum from below over the entire length of the plenum as per the picture above.

Now, when dividing the feeder tube into 4 sections, would you think it's sufficient that the area for each baffle is equal at the entry of the feeder, or should the volume be equal over the entire body of the feeder tube.

This pic, although different may help explain the concept.

Last edited by Doctor G on 2015-09-14 at 12-40-36.
2015-09-15 12:44:28
I can't see the first image because it's blocked here, but I can see the second image.
I assume you're talking about something like what the 1980's turbo Audi race cars used and such?

There is no need for all that internal baffling if you pinch the opening between the plenum and the feeder tube, and add some strakes (tiny baffles) inside the pinch area. It will even out the flow for you.
There was a guy who was making a replica of the Audi manifold and found this adequate using computational flow analysis.

However, I've never seen an N/A car with a plenum like that. I don't think it has the benefits that the turbo cars saw. Although you might have found the only place where it would provide a benefit? In that it lets you have a throttle body on the side, and still get the benefits of a center fed plenum? For packaging reasons...
Last edited by BenFenner on 2015-09-15 at 12-52-53.
2015-09-15 13:05:05
Hey Ben. Similar but almost integrated into the plenum vs a narrow slot on those turbo plenums.

Yes I'm trying to create better distribution to the 4 cylinders but using the side feed. I'd go centre feed if I had space but it's not that easy without very sharp bends or odd angles into the plenum.

Let me try again. Can you see this image now?

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