Phil99vette

iTrader: (11)

Here's our setup:
346" ~12.75:1 CR
254/260 .650 lift 11x + 0 Solid roller
ET 245 heads 11 degree
Target RPM: 5000 - 7500
300 shot Nitrous

Plenum size and runner shape/size are my two biggest questions. We've got a hood with a 4" cowl so size is not a problem.
Phil

J-Rod

Ok, this is about as mucha black art as anything. There are so many factors involved here, that you really have to be a mechanical engineer and understand fluid dynamics to even scratch the surface. I've been doing some research so that I could use the valve event spreasheet I developed to calculate some of the ROUGH parameters.

So, the best advice is to cosult the experts, or do some "light" reading, and be prepared to experiment.

A rough, rough, rough rule of thumb is you'll need some taper on the runner (say about 7 degrees if memory serves me correctly). Plenum volume is one of those things that it depends on who you ask. The plenum is giving you runner length to some extent, and allows you to rob one cylinder to feed another. I've seen numbers of 1- 1.5x displacement to 4x displacement. Then you get into things like reversion chokes, etc...

Good place to start:

Professor Gordon Blair

http://www.profblairandassociates.com/

Now, to some of the math and some of the opinions about how all of it works...

David Vizard's Rule for IM Runner Length

The general rule is that you should begin with a runner length of 17.8 cm for a 10,000 rpm peak torque location, from the intake opening to the plenum chamber. You add 4.3 cm to the runner length for every 1000 rpm that you want the peak torque to occur before the 10,000 rpm.

So, for instance, if peak torque should occur at 4,000 rpm the total runner length should be 17.8 cm + (6 x 4.3 cm) = 43.6 cm.

Vizard also suggests that you can calculate the ideal runner diameter by the equation :

SQRT [ (target rpm for peak torque x Displacement x VE)/ 3330 ]

SQRT = square root

VE = Volumetric Efficiency in %

Displacement in Liters

Another formula (don't remebr who):

optimum intake runner length (L) is:L = ((ECD × 0.25 × V × 2) ÷ (rpm × RV)) - ½D

Where
Effective cam duration (ECD) = 720 - (Adv. duration - 30)
RV = Reflective Value (which set of pressure waves do you want to use 1st, 2nd or 3rd, etc…)
D = Runner Diameter
RPM=RPM range you wish to tune port for
V= Velocity - The velocity in the plenum intake pipe should not be higher than 180 ft/sec at maximum rpm.



Helmholtz Resonator Calculations

Remember at the start of the article I mentioned that the dimensions of 3 parts of an IM can affect where peak torque can occur? Well here is another way we can calculate estimates for our IM dimensions for the peak torque location we want.

A Helmholtz resonator is an acoustic resonance chamber (as described by our plenum above) that modifies the acoustic frequency of a sound wave like a spring oscillating with a mass attached on the end.



where f = the rpm at which you get peak torque ( the natural frequency of pressure oscillations in the acoustic chamber ) , c = the speed of sound (= 340 m/sec.) , S = runner area, L = runner length, V = displacement per cylinder

A simplified version of this is using the Englemann formula for the above which also takes into account static CR of the engine:

RPM for peak torque =

642 x c x [ SQRT (S/[L x V] ) ] x [ SQRT { (CR-1)/ (CR+1) } ]


= 218,280 x [ SQRT (S/[L x V] ) ] x [ SQRT { (CR-1)/ (CR+1) } ]


For a more detailed explanation on the application of Hermann Ludwig Ferdinand von Helmholtz's acoustic resonator theory applied to intake systems, please check out:

http://enaf1.tripod.com/teche.html#helm

http://www.mecc.unipd.it/~cos/DINAMO...suonatore.html

Hemholtz calculator:
http://www.phys.unsw.edu.au/~jw/Helmholtz.html

Another:


The Helmholtz resonance model is an electrical circuit analogy developed by H.W Englemann for manifold design applications. It is not as complete as more recently developed models, as it does not include the effects of plenum volume and other resonant volumes such as secondary runners, but is nevertheless enough for our purposes. Data for our basic design is as follows:

L = Primary runner length: 14 cm

A = Primary runner area: 5.8 cm2

V = Displacement per cylinder: 150 cc

C = Compression ratio: 13.0:1

c = Speed of sound is taken as 340 m/s.

The Helmholtz peak, in RPM, will be given by the following equation:



642 * c * sqrt(A/L * V) * sqrt(C-1/C+1)

Replacing the variables yields a tuning peak of 10621 RPM.

This is higher than our target peak, but the intake port length was not taken into account. Considering an intake port length of 6 cm, with constant area, the equation yields a lower peak of 8886 RPM, which is close to our target range.



A Helmholtz resonator is used not only in an automotive induction sytem but also in the designing of exhausts to suppress sound and many other non-automotive designing that involves amplifying sound like in the music industry.

RAM INTAKE TUBE DIMENSIONS

What are the best intake tube dimensions for the IM that we have just designed for a particular peak torque rpm?


III a./ INSIDE DIAMETER (D) of a RAM INTAKE TUBE


First Method:


D in inches = SQRT [ ( Displacement x VE x Redline) / (V x 18.5) ]

Displacement = Total Displacement in Liters, VE = Volumetric Efficiency in %, V is the velocity of the air flow in the IM plenum for resonance (usually estimated at 180 ft/sec max.)


eg. SQRT [ (1.8 x 85 x 8500) / (180 x 18.5) ]

= SQRT [ (1,300,500)/ (3330) ]

= SQRT (391)

= 1.98 in.


Second Method:

Throttle Body Size is Determined by IM Plenum Size.


Once we have determined the optimal TB size for our IM, we can then determine the best intake inner diameter.

The ideal diameter for an intake is when the intake has 25% more cross-sectional area than the TB's bore cross-sectional area . Your TB diameter (overbored or not) dictates your intake diameter.

Remember that the area of a circle (your TB bore) is pi x radius squared and the diameter = 2 x radius. If you calculate your TB's area and then multiply it by 1.33, you will determine the intake's area. Then, use the area of the circle equation to determine the intake's radius.

Therefore, for example, with a 64mm (plate side bore) TB, the calculated "best" intake diameter is 2.8 in. ID.

III. b/ LENGTH OF RAM INTAKE TUBE

A suggested starting point for the length of a tube with peak torque at 6000 rpm is 13 in.

You add 1.7 in. for every 1000 rpm that you want to move the peak torque below 6000.

Or subtract 1.7 in. for every 1000 rpm you want to move the peak torque above 6000.

For more info on specific intakes (short rams versus CAI's etc.) please refer to my intake tech article over at hondavision.com :

http://www.automotivetech.org/forum/...5&pagenumber=1


----------------------------------------------------------------------


Please remember that formulas only serve as starting points. To get the actual best IM runner dimensions and intake dimensions for your particular engine package takes a cut and try approach to zero in on the best dimensions for you.

J-Rod

Vacuum:
Intake manifold vacuum is a very complex matter, affected by factors not readily apparent. Vacuum is caused by the ratio between 2 separate volumes, which are “connected” when the intake valve is open, and the piston passes TDC on the intake stroke. During this interval, the (theoretical) 14.7 lbs. of vacuum in the cylinder is “satisfied” (partially returned to atmospheric pressure) by the volume of the entire intake tract before the remaining vacuum reaches the carburetor. Let “E” = the effective vacuum, “V1” = the volume of 1 cylinder, measured at full stroke length, and “V2” = the total volume of the entire intake path: combustion chamber, intake port, manifold runner, and plenum.

E ~ V1 ÷ V2

(Effective vacuum varies as the product of the cylinder volume divided by the combined volume of the intake tract)

Obviously, as the cylinder volume goes up (larger motor displacement), or intake port, manifold runner or plenum volume go down (dual plane manifold instead of single plane, etc.), the ratio V1 ÷ V2 is high, and vacuum is improved, and a relatively high percentage of the original vacuum is available at the carburetor to draw in fresh mixture.

The reverse is also true: as the cylinder volume goes down, or intake port, manifold runner or plenum volume go up, the ratio V1÷ V2 is low, and vacuum is reduced, and the vacuum present at the carburetor is a smaller percentage of the original. The signal (demand) at the venturi and discharge nozzle is lower, resulting in a lean condition.

Another factor is combustion chamber volume. Although this may be only a small percentage of the V2 total, it’s always present, and its size affects vacuum. To demonstrate the practical range of chamber volumes, it may be as small as 7.7% of the cylinder volume (CR: 14-1), or as large as 16.7% of the cylinder volume (CR: 7-1). If the intake manifold is very small (“IR” (individual runner) type, with minimal internal volume), the effect of a change in combustion chamber volume will be much greater than if a tunnel ram style manifold is used (where the plenum volume is typically larger than the cylinder volume). This is one reason why why using a carb, a tunnel ram motors typically don’t require major re-jetting for compression ratio changes, but for instance a single-cylinder motorcycle with a single Mikuni will experience a sharp drop in response if compression is reduced (exactly as if intake manifold volume had been increased), unless jetting is revised to cure this lean condition.

The above analysis is only relevant under theoretical conditions, where the cylinder volume is constant regardless of cam timing. Add the dynamics of the real world, and it’s no longer accurate. The “effective” (true, or corrected) cylinder volume is dramatically affected by changes in the closing point of the intake valve. For more detail on this subject, click here to see my “Cam Timing vs. Compression Analysis” article: .


Of note to all of us, late (radical) intake valve closing therefore adds another negative effect on vacuum: reduced intake manifold vacuum at low to moderate speeds. Even though the theoretical cylinder volume remains constant, the “effective” cylinder is actually much smaller at the point where it’s “sealed” - the intake valve closing point, which has the same effect on vacuum as on compression: a cylinder of smaller volume. The volume of “effective” cylinder varies with the intake valve’s closing point; it’s always smaller than the theoretical cylinder because all modern cams close the intake valve ABDC.

In slow motion, at low to mid-range engine speed: there will be “normal” vacuum during the movement from TDC to BDC with any cam timing, but as the piston rises ABDC, some of this vacuum is satisfied by a smaller “positive cylinder” (the piston’s motion from BDC to intake valve closing point), rather than by incoming mixture from the manifold. This “positive cylinder” (its bore is the cylinder diameter, its stroke is measured from BDC to the intake valve closing point) may be as much as 1/3 of the total cylinder volume with very late intake valve closure (e.g. 80° ABDC). The result is reduced ability to produce vacuum in the manifold, and therefore less accurate metering at the carburetor and a generally lean condition. A delay in intake closing of only 5° will probably not be noticed in most applications, but a delay of 30° will almost certainly require more accelerator pump discharge, richer primary jetting, revised primary throttle disc position at idle, etc. At higher speeds, this factor has much reduced effect, and almost completely vanishes with large-volume manifolds.







Sources:
http://www.team-integra.net/sections...?ArticleID=466
http://victorylibrary.com/mopar/intake-tech-c.htm
http://enaf1.tripod.com/teche.html#intake
http://www.truckpulls.com/Tech%20Fil...%20Systems.htm

SStrokerAce

iTrader: (2)

Easy answer..... Call up Keith Wilson or CFE, write him a check, call it a day.

As for the vacuum that's not as critical in a EFI motor since the motor doesn't need a high signal to mix the fuel into the airstream.

Most times the runner diameter or CSA is determined by the cylinder head, so that's always the first place to start, that's also going give you part of the total induction length as well so working the head and the intake together is a important factor in the total design. I'd suggest googling Jim McFarland and reading some on his site as well. J-Rod talked some about Larry Widmer, his Endyn site is full of goodies as well.

Remember the engine is a SYSTEM of comprimises and like camshafts the intake should be worked into the total plan.

Bret

DRI_Z28

His first sentece sums it up PERFECTY!

Pat7272

Wow, badass post J-rod

Old SStroker

As Paul Harvey might say, "Here's the Rest Of The Story."

http://victorylibrary.com/mopar/intake-tech-c.htm

J-Rod

You're right. That is why you'll see that was cited as a source. I took the portion I felt was more on topic. Most of the stuff related to single plane dual plane, etc.. is interesting, but is not necessarily as important to this specific part of the topic.

ls1408cp

iTrader: (37)

how about a good intake that i dont have to win the lottery to buy

MrDude_1

iTrader: (4)

in that case..... have a good welder? cause you're going to need it.

ls1408cp

iTrader: (37)

i do i have been debating on doing my own

Cary@Perf-Induction

iTrader: (1)

j-rod,

That is pretty slick. A lot of people don't see it that way. even a lot of the popular manifold fabbies out there. good post.

J-Rod

Another thread from another site which might help out with design criteria.

http://www.rx7club.com/showthread.ph...hreadid=199788

Greg Fell

iTrader: (9)

i'd like to make one with a removable top, and adjustable runners so you can see what makes power. math is great, but sometimes as we all know, reality is different.

you could do a run, pull the top off, change runner length, reseal it, and re run it.

you could also adjust plenum volume.

Valkyn

iTrader: (8)

Is there any information out there regarding optimal plenum volume and other considerations like plenum shape and inlet sizing/location?

Big-DEN

Greg Fell, with the newer designs where the runner extends into the plenum, there
could be stacks with length from 1 to 4". you could run a 0" with radius entry for
super high rpms.

The newer designs with the runner extending into the plenum are pretty good
because its enough runner to support a broad enough torque curve and its short
enough to enhance high end HP.

For most with the big cube higher rpm a runner in the 6-8" range is where its at. 3" aint going to cut it.

Ryan@ABear

Richt-click, add to favorites.

Thank you very much.

300bhp/ton

How about something that uses individual runners for each cylinder with a TB on each (8 TB's)? I've seen this sort of thing used in the past on other motors, don't know too much about designing one though.