Custom Intake Manifold Theory

[1993-Talon-TSI]

i built a custon manifold out of sheet metal (stainless) 240Ci plenium (square) with 5.75 in runners that are not tappered, same size as my inatke ports (ported out a bit so they are slightly larger than stock). The only difference from mine to others like the magnus or BJs is the material type and the tappered runners that mine does not have. My question is i put it in the car and it don't seem to be any better than my stocker! Any of you guys have to tune everything completly different when you put on an aftermarket manifold?
If anyone can offer me some insight or the theory behind High performance intake manifolds on a 4g63T would be of great help.
My setup is a tuned 14b 93' awd, ported head, 3", 255 with 6an, fmic,maft,act 2600

thanks for the help

 

[Defiant]

It's forced induction. As such, much of what matters in normal intake design no longer does, other than when in out-of-boost driving.

Still, those guys don't go to the hassle of tapered runners because they're playing around. They just might have some kind of clue.

 

[highpsi93tsi]

The plenum AND the runners HAVE to taper. Or its probobly making LESS power than the stocker. If you want yout UICP to bolt up you also have to make the runners taper...and bend. Nearly impossible.

 

[kpt4321]

I'd say that even without tapered runners, a manifold with a big plenum and short runners is going to show solid power gains over stock at higher engine speeds.

However, power gains of this type tend to show themselves as a percentage, and also only will manifest themselves at higher engine speeds and higher airflow levels.

A low power 14b car isn't going to see many gains.

 

[highpsi93tsi]

Exactly, a large plenum short runner intake will make no bottom end but from 6000-9000 hang-on. It takes ALOT of dyno testing to make a good intake, and the stocker does quite well from 2000-6200ish then its dead. Think of the V8s, Take a small block chevy for example. They have the performer idle-5000, Performer RPM 2500-6500 and the Victor JR 3500-8500+. They havent done staging with the 4G63...yet. It'll come, took ten years untill they started this with fuelie 86-95 Mustangs.

 

[1993-Talon-TSI]

went to the track wed and i went a ten slower than my best, went [email protected], my best of [email protected] air was so humid and damp, bad thing. One difference i noticed was it had lots of midrange torque??? i posted my best 60' time of 1.7 flat!!!! I'm going to try making another manifold, BJs manifold does not have a tappered plenuium at all, but most of the others do??

 

[hostile]

If your going to copy a manifold I would suggest taking a good look at the hogan/extreme manifold.

It scored really well on the rmdsm manifold shootout.

 

[bjones18]

Welcome to the world of airflow.

I've tested air flow with manometers & pitot tubes, computational fluid dynamics, and in Lockheed's wind tunnel. but have yet to design intake manifolds nor port cylinder heads. It is just as easy to screw it up as it is to make it better (especially for a wide RPM band).

Do you want torque or HP, and at what RPM?

If you want to do it right, you must either copy someone else's COMPLETE system or to invent your own, it will be easiest & most successful to make a flow bench complete with manifold and cylinder head. This will require CFM! (regenerative air compressor).

This answer falls into the rhetorical category, since the answer is a flow bench, dyno, and race track.

 

[Defiant]

Just an easy, quick, real-world experiment:

While you're watching television, and you're sitting there nice and relaxed, put the pad of your thumb on your cheek next to your nose, right above your lip and above the bone holding your upper teeth, just along where you can feel your cheekbone fair into your nasal opening along the front of your skull. Without changing your breathing, move the meat around just slightly as you inhale, and feel how little movment of your nostril makes how much difference on the drag of the air being pulled into your lungs. That's porting. That's how delicate the flow into a cylinder is. Get a small booger in there, or a bit of mucus built up when you have a cold, and see how much it ruins your intake and exhaust.

There's much more to it, of course- supercharging the intake affects things, running without boost affects it, every damned thing changes it. You said your manifold had a torquey band, and almost all will. It's like you have a 1-pound pail of torque plaster, and you can arrange where on the wall you want it how thick- a big old lump in the middle, or a nice even layer all the way across, or a tapered layer that's thicker in the middle (or, as in street Hondas, on the bottom) or toward the top. But for a given engine size, you only get a certain sized bucket. You're never going to have as much material as you get with a 5-liter V-8. And figure the same situation for horsepower.

 

[highpsi93tsi]

Best flow bench around Toronto is Rolling Thunderz in Concord. Ned And Serge. Runners are tappered to compensate for diff RPM levels Un-tappered your shootin at the sky. If you cant tapper the runners play with the plenum. Good at tig welding?

 

[bOOstd GSX]

If your going to copy a manifold I would suggest taking a good look at the hogan/extreme manifold.

It scored really well on the rmdsm manifold shootout.

I thought Hogan made custom, one off manifolds?

Also thinking about making my own intake manifold... Why doesn't someone with a magnus or bj's (or even another high performance intake mani) intake manifold just get dimensions e volumes of it then we can copy it?

Oh and sorry for reviving such an old thread, just looking for the help on my intake mani.

 

[Jon Lane]

A lot of useful old-time V8 type airflow thinking in here, which is good.

Remember the variable length velocity stacks in early V8 fuelie intake design? Makes me wonder if individual runners (tubes) right from roughly the throttle body location to each of the four DSM cylinders would broaden the power curve. Which naturally raises the question about TB location vs. the entire upstream intake tract length/volume. I assume the "downstream" plenum volume has a significant relationship to the TB location, right?

Anyway, seems too easy; somebody must have tried this already and given up...

 

[herostar]

What are the advantages and disadvantages of little "velocity stacks" inside of the plenum on a sheet metal intake?

 

[Dauntless]

Jon, those V-8 velocity stacks had to be different lengths to even out the total intake tract length, due to the differing lengths of the runners inside the manifold. And has been pointed out above, once the aircharge is boosted, tuned intake runners no longer make much sense.

 

[LaserRST]

Dunno if boost or no boost will change the performance of any intake. From what I understand the only difference boost brings is air density. Higher pressure brings more air in but doesnt change the physical properties of gas, it will still react the same. Does that really change things like port velocity and rarefraction? Im not gonna argue that there is no difference between the two, but I will say the difference if probably not enough to eliminate the function of tuning design in an intake. The air volume moves at the same rate when the piston is in its downward stroke and changes with speed, which is some of the principles in intake design. Now how much mass is in the cylinder is probably a group of equations and fairly complicated. I think some one who is fluent in fluid dynamics can help explain this.

Any ways. I would definately consider runner and plenumn volume when designing your own. It takes a great deal of math, understanding air flow, and how engines work to actually create a "tuned" intake for a specific RPM range. Other wise its a great deal of hit and miss. To help I would add these things to consider. Every cylinder is .5 liters on a 2.0 engine, but that doesnt mean half a liter will have the same aircount at every stroke. You have things like flow (fluid dynamis), the lift and timing of the valves,taking advantage of port velocity (a phenomenon in the runners that change with engine speed), rarefraction (which is still being fully understood),time, and how much air is in the plenumn at a specific given time. In my opinion for a street car stuck on a 14b stick with stock or Cyclone intake. Otherwise find out how much lb/min or cfm is rated for all the parts from the filter to the throttle body and see if you intake is whats keeping you engine from breathing freely. Remember the engine is an air pump and efficient fuel consumption is your power.

 

[Jon Lane]

Velocity stacks were individual direct port intakes; there was no manifold. Half were one length, the other half another length, the theory being this averaged high and low rpm efficiency.

Since there are performance differences between racing and stock DSM manifolds, presumably due to short vs long runners, this theory would still apply to pressurized intake engines, no?

 

[Dauntless]

Okay, got it. I thought you referred to stacks on a manifold, but whether or not one uses velocity stacks has little impact on the breadth of the power curve on fixed-cam NA engines. (Variable cam engines are another story, of course.) Deviating from the ideal length actually decreases the tuning effect, and hence power from that cylinder. One good reason to vary stack length on adjoining stacks is to prevent interference, where airflow patterns into the engine become disrupted when all the stacks are the same length and closely adjoined in a line.

You asked if the tuned effect works on boosted engines, and the answer is basically no. There is a tuned effect under boost, yes, but the effect is miniscule compared to the flow effects of being boosted. Once an engine is boosted, the shortest and fattest stacks will flow the most, assuming they aren't grossly mis-designed. And of course, our engines aren't boosted most of the time...we're driving down the freeway on cruise control sucking up 15 or 20 hp. Under these conditions the tuning of the intake manifild has a measurable effect on power, efficiency and throttle response, so you don't want to screw too much with it on a daily driver.

Hope that helps.

 

[Maglin]

I think a tapered plenium would benifit as the no 1 cyl is farthest from the TB and air takes the path of lease resistance. So it will go to cyl no. 4 then 3 and so on so you will have a slight pressure difference from the No. 4 cyl to the No. 1 cyl. I think a tapered Plenium would help offset that somewhat. Has anyone ever tried different length runners ie. no 4 has the longest and no 1 has the shortest runner length? Or maybe moving the TB to the Center of the Manifold which would require a lot of fabricating and might not even be very possible but would allow for better air distrobution. Baffles would be needed to direct air to both sides if TB location was moved. I'm just guessing and thinking of various things.

 

[BlknBlue2G]

"Dunno if boost or no boost will change the performance of any intake. From what I understand the only difference boost brings is air density. Higher pressure brings more air in but doesnt change the physical properties of gas, it will still react the same. Does that really change things like port velocity and rarefraction?"

The velocity of the air will always be the same no matter what boost you are running. It is based mostly on engine speed but also depends on the manifold design. Tapered runners are a must since the decreasing cross sectional area increases the air velocity. Increasing air velocity increases power everywhere in the rpm range, not jsut where your runner is tuned to.

Also, the pressure differential across the valve opening with boost is what is pushing the air into the cylinder, but taking advantage of the reflected pulses couldnt hurt. Otherwise why would these short runner manifolds make big power top end while the long stockers are choking at the top but decent on the lower end. I find it hard to believe that these manifold builders are not tuning their lengths.

 

[bjones18]

"Dunno if boost or no boost will change the performance of any intake. From what I understand the only difference boost brings is air density. Higher pressure brings more air in but doesnt change the physical properties of gas, it will still react the same. Does that really change things like port velocity and rarefraction?"

The velocity of the air will always be the same no matter what boost you are running. It is based mostly on engine speed but also depends on the manifold design. Tapered runners are a must since the decreasing cross sectional area increases the air velocity. Increasing air velocity increases power everywhere in the rpm range, not jsut where your runner is tuned to.

Also, the pressure differential across the valve opening with boost is what is pushing the air into the cylinder, but taking advantage of the reflected pulses couldnt hurt. Otherwise why would these short runner manifolds make big power top end while the long stockers are choking at the top but decent on the lower end. I find it hard to believe that these manifold builders are not tuning their lengths.

1. Higher density air will have more inertia...might change flow pattern into the runners at different velocities (RPMs).
2. high velocity by decreasing port area...but not resricting more than the "open valve."
3. "runner length tuning" is specific to rpm. how much length is required so air entering the runner is not "hampered" at runner exit by temporarily closed valve. (high rpm = shorter runners)
4. plenum designed to supply runners equally (plenum c-section decreases by runner cross-section after each runner.

These are "general" concepts. When air enters the plenum at high velocity it may form vorticies which can affect the "runner-distribution" at different RPMs.

I am not an expert...but I have worked with high velocity air. All the "pre-theory" in the world can fall apart when you start measuring air-flow for the first time.