The results of a Google search I did on "plenum size" suggest that all other things being equal, the larger the plenum size, the smaller the size of the throttle body needed. Maybe I'm overlooking something very basic here, but I'm not sure why that would be true. Some of the threads I found also stated this is a "well-documented" relationship, but I couldn't find said documentation, so I'm wondering if this relationship between plenum size and optimum throttle body size makes sense to you, namely:

• Is it the case that all other things being equal, the larger an intake manifold's plenum size, the smaller the throttle body size needed?

I don't know the answer to that question, and again, maybe I'm overlooking something very basic here, but offhand I don't see why that relationship between plenum size and throttle body size should hold. What do you think?

grapeaperacing.com

tb size does not really coralate with plenum volume. check out 60degreev6.com and search there. we covered it many times there

It doesn't make sense to me, but I'm not a physicist. From all that I've read, there is no direct relation between the two.

Throttle body sizing is more a function of peak engine airflow, which is itself a function of overall engine volumetric efficiency. Intake design plays an important part in VE, but it isn't the only thing. Cylinder head design, cam profile and timing, exhaust system, even ignition system function all play their own roles in determining the VE of an engine.

Intake manifold theory is very complicated, and is made more when applied to a street engine. Street engines are a compromise. You can't design for just low RPM operation, and you can't design for peak output, since the engine needs to operate adequately over such a broad range of speeds.

The intake plenum is actually a Helmholtz resonator. It actually functions using pressure inversion waves to in effect supercharge the air without any actual compressor. Now it only does this in a very limited range of engine speeds, perhaps as little as 1000 RPM or less. The pressure inversion waves result when the intake valve opens, causing a pressure drop in the immediate area of the valve, which then travels back up the intake runner. Upon reaching the end of the runner, the air molecules in the plenum will rush into the runner to fill the vacuum created when the valve opened. Under certain conditions when the air begins moving into the runner it will move with enough speed that when the intake valve closes it will allow the air column to "stack up", or pressurize behind the valve as it's closing generating a small supercharging effect. The runners and plenum must be specific sizes (dependent upon VE, displacement, and engine speed) for the effect to be maximized. One of the cylinder head books I have around here somewhere has all the equations for it, but I'm not sure where it is.

Another thing that needs to be considered involving intakes is port taper. The intake runner needs to neck down slightly as it approaches the head to increase air speed to encourage better cylinder fill, particularly at part throttle.

So, with the above, one may be tempted to simply install the largest throttle body you can find, but that too has it's own drawbacks.

With a TB that's too large a slight twist or the throttle blade will expose a large area to the engine, which can cause very abrupt throttle response. So the "right" throttle body for a particular engine needs to be decided on TB airflow vs. desired engine airflow for the targeted power level.

[This message has been edited by FieroJimmy (edited 03-20-2008).]

The throttle body needs to be large enough to supply the maximum overall airflow of the engine regardless of plenum volume.

quote Originally posted by project34: The results of a Google search I did on "plenum size" suggest that all other things being equal, the larger the plenum size, the smaller the size of the throttle body needed.

That "information" is wrong. Two counter-examples will illustrate:

1) Imagine hat you could increase the plenum size (I presume "size" here means "volume") as much as you want. Do you think that by doing this you could eventually reduce the size of the throttle body to that of a soda straw? No way.

2) Consider situations where there is no plenum at all ... "plenum size" = 0 ... such as throttle-per-cylinder intake designs. (The Porsche 911 and many racing engines come to mind.) Whether carbureted or fuel injected, the size of the throttle bores is matched to cylinder displacement, not to plenum volume.

Plenum volume and geometry do play a significant part in modern intake design, but in general throttle bore is much more highly correlated with engine displacement than anything else. You can think of it this way: The throttle bore is sized to supply the engine's peak airflow requirement, while plenum volume may be only one factor in a complex tuned intake design.

[This message has been edited by Marvin McInnis (edited 03-16-2008).]

quote Originally posted by merlot566jka: tb size does not really coralate with plenum volume. check out 60degreev6.com and search there. we covered it many times there

Unfortunately, I didn't find any information about the topic of this thread on the website you mentioned. Maybe I was searching in the wrong sub-sections of that website; I don't know. In any case, if you've a link to any threads on that website that discuss this topic of plenum size and throttle body size that you say was covered many times there, I'd certainly appreciate it. That in part is because as I mentioned at the outset of this thread, some of those on the Internet who say there is a relationship between plenum size and the throttle body size needed claim this is "well-documented," but I didn't find any of that "well-documented" support for that assertion either.

Overall, however, others who've posted on this thread so far seem to agree with you, so thank you everyone, for your opinions on this issue:

quote Originally posted by FieroJimmy: Throttle body sizing is more a function of peak engine airflow, which is itself a function of overall engine volumetric efficiency....So the "right" throttle body for a particular engine needs to be decided on TB airflow vs. desired engine airflow for the targeted power level.

quote Originally posted by Steven Snyder: The throttle body needs to be large enough to supply the maximum overall airflow of the engine regardless of plenum volume.

quote Originally posted by Marvin McInnis: ...but in general throttle bore is much more highly correlated with engine displacement than anything else. You can think of it this way: The throttle bore is sized to supply the engine's peak airflow requirement, while plenum volume may be only one factor in a complex tuned intake design.

Plenum size is not simply determined by a formula. On the face of it, one may look at an intake with a rather large plenum and think; hmm, under sudden W.O.T. it will have lots of air to provide. And that would be true to some extent, only thing is, that extra air will be gone in an instant and not noticed. Plenum size should be such that the runners have space between them to allow for the use of velocity stacks. It also should be large enough not make the plenum a tight bucket of turbulence and vortexes.

While ensuring plenum size (or more accurately shape) is adequate for proper flow into the intake runners (i.e. not too close to the inlets, smooth transitions, etc.) to maximize power output (through reinforcement of pressure wave reflections within the intake) a specific size is needed. Too big, or small will lessen the effect.

All of this advanced intake theory is really overkill on a street motor, though. Unless you are clean sheet designing a complete intake system, like for a race motor, there are only small benefits to be had and, even with all the calculations, empirical testing is the only way to know that the cylinder heads and cam are compatible with what has been done. There are almost too many variables to design an engine (or parts of one) only on paper, and have them work the way you are envisioning.

For anyone interested in learning more about intake theory (and carburetors, ugh ) I recommend "How to Build Horsepower Volume 2, by David Vizard"

http://60degreev6.com/forum...&highlight=helmholtz
http://60degreev6.com/forum...&highlight=helmholtz
http://60degreev6.com/forum...hlight=intake+design

Some info i found...

http://www.cartechbooks.com...e/s...=3048&CATID=21


Quote:
One effect method of determining the worth of a throttle body, or any airflow restriction for that matter, is by monitoring the vacuum present behind the component. In the case of a throttle body, taking vacuum measurements at wide-open throttle from a source inside the intake manifold as close to the throttle body as possible gives you a good indication that a restriction is present. It takes an accurate vacuum gauge, preferably one that reads small changes, since you might be looking for 1 to 2 inch. The scale on your test gauge should read from 0 to 5 inches, and no boost. The typical vacuum/boost gauge will not likely be accurate enough unless the restrictions are sizable. If you record vacuum readings that exceed 2 inches at wide-open throttle, chances are a throttle body will be worth some power. If the vacuum reading is less than 1 inch, there isn’t much power to be had with a larger throttle body. The vacuum present behind the stock throttle body will increase with engine speed and power output. It’s a lot less expensive to check the vacuum than buy a throttle body only to find out the stock one is fine for your current power level.


I'll try that later on tonight

http://60degreev6.com/forum...38308&highlight=75mm

http://60degreev6.com/forum...35365&highlight=75mm

http://www.grapeaperacing.c...inductionsystems.pdf


there are a few good books too. i dont have time right now to list them, but pm me and ill send ya the names. you have to get the whole picture before you start to compare things like plenum vs tb area

Below is the key to a good intake not spacific volume. If it don't look right, bad angles, runners too close too small etc, it probably is bad.

[QUOTE]Originally posted by FieroJimmy:

While ensuring plenum size (or more accurately shape) is adequate for proper flow into the intake runners (i.e. not too close to the inlets, smooth transitions, etc.) to maximize power output (through reinforcement of pressure wave reflections within the intake) a specific size is needed. Too big, or small will lessen the effect.

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[IMG]



Trueleo.com/fiero.htm
RSpiderII@aol.com

Francis T

I have been looking at your intake manifold. I have a 3.4 built for more HP now. You might want to change your sig to http://www.trueleo.com/ because now it does not go to your true website.

Ed

------------------
Fino
www.feroce.net
Member National Kit Car Club www.kitcarclub.com
Lifetime Member CFOGi (Lot of good that does me)

Thanks Fino

this is all rough, use actual math to get tighter - but:
the plenum volume should be about 1/2 the displacement of motor
the runners, roughly the diameter of the intake valves, length 1 to 1.5 of the piston stroke
the neck - another variable - roughly same length as 1 runner
the T/B - sized by peak HP output of motor

useing them guidelines, you can make an awesome plenum for any motor.

^ huh? ive always agreed with ya, but ill dissagree on that. not trying to bust ur balls but thats way off. the 3500 plenum is over 3500cc and it makes more power than the 3400 plenum which is about 2000cc....

The plenum area is where the intake
runners meet. There can be one plenum that all
runners meet, or two smaller plenums with 1/2 the
runners meeting in each. The plenum volume is a
very importing tuning aid. As high velocity gasses
flow through the carburetor or throttle body, the
plenum give the gasses a chance to slow down, as
the velocity is reduced the pressure rises. Higher
pressure means that the air will be denser, and of
course that means more power.
As rpm goes up you need a larger plenum,
but a larger plenum will reduce throttle response and
low-end power. A plenum also reduces peak air
velocity through the carburetor (or throttle body).
The induction pulses in an intake cause velocity to
rise and fall with every pulse. The plenum helps to
reduce them by acting as an air capacitor. Average
velocity will remain the same, but the highs and lows
will be closer together. Since you need a carburetor
that will flow enough air at peak velocity, a larger
plenum will allow you to run a slightly smaller
carburetor without losing airflow, but it will also
reduce the peak signal strength, which is why large
plenums tend to reduce low-end power

Plenum Volume
There is not going to be a simple answer to
the needed plenum volume for a given application or
rpm range. The good thing about plenum volume is
that there is a pretty wide range that it can be and
still be effective, so general rules work well. The
following guidelines are for engine operating in the
5000-6000 rpm rage.
V8's with one large plenum feeding all 8
cylinders does not work all that well as far as the
Helmholtz resonator goes, but if this is the case,
plenum volume should be about 40-50% of total
cylinder displacement. On a four cylinder engine 50-
60% works well. For 3 cylinders (6 cylinder engine
with two plenums), each plenum needs to be about
65-80% of the 3 cylinders it feeds.
If a boost is desired in a higher rpm range,
closer 7000-7500 rpm, the plenum will need to be
10-15% smaller. To get a boost in the 2500-3500
rpm range, it will need to need about 30% larger.
The plenum size of a Helmholtz resonator may go
against the typical plenum size rules, but the rules
change when the resonator is being used. The
whole Idea of a plenum is to allow the gases to slow
down and gain density. The Helmholtz plenum
makes a dense charge by use of pressure waves, in
the same way tuned port intake runners work.
This plenum sizing method does not apply to
engines that to not use a tuned intake pipe. Many
engines simply have the air cleaner assembly
directly on the carburetor or throttle body having
very little intake length. In those cases the
Helmholtz resonator system does not work.

Intake Ram Pipe
The last thing to adjust is the length of the
intake ram pipe. It is possible to make an adjustable
pipe that can be made longer or shorter for testing
purposes. For a starting point figure a 13" long pipe
will help at about 6000 rpm. For each 1000 rpm
drop in rpm add 1.7" and subtract 1.7" per 1000 rpm
increase. This is just a starting point.
The inlet of the pipe should have about a
1/2" radius for smooth flow. Once you get a
baseline (you must do a power pull and get a
baseline), which can be done at the track or on a
dyno. Then try moving the pipe 1/2" in either
direction as see how power improves. The dyno
may be a little deceiving, since peak hp my go up
but average power may drop. Track testing will be
best, since you will be testing in actual racing
condition and can tune the pipe for the best times. It
is usually best for average power if the intake ram
pipe is tuned about 1000 rpm lower than the intake

Intake Port Area
Unlike intake runner length which effects
power over a narrow rpm range, the size (area) of
the runner will affect power over the entire rpm
range. If the port is too small it will restrict top-end
flow and flow, and if it's too large velocity will be
reduced and it will hurt low-end power. The larger
the port is, the less strength the pressure waves will
have.
Since the intake valve is the most restrictive
part of the intake system, the intake runners should
be sized according to how well air can flow through
the valve area. Most decent heads will have an
equivalent flow through the valve area as a
unrestricted port of about 80% of the valve area, this
is if the camshaft it matched to the heads. In other
words a 2.02" valve, which has a 3.2 square inch
valve area, in a decent flowing head will flow the
same amount of air as an open port with about 2.56
square inches of area (80% of 3.2). So the port area
should be about 2.56 square inches just prior to the
valve (this is in the head port). Some well ported
race heads may have an actual flow of an area up to
85%, but for the most part it is around 78-80%.

from grapeaperacing.com

[This message has been edited by merlot566jka (edited 03-18-2008).]

quote Originally posted by merlot566jka: ^ huh? ive always agreed with ya, but ill dissagree on that. not trying to bust ur balls but thats way off. the 3500 plenum is over 3500cc and it makes more power than the 3400 plenum which is about 2000cc.... ......

as I said - my guidelines are rough. and, if you read your post - you'll see they are valid - except the neck - your post suggests a longer neck. I just dropped many details, and stuck to the simplistic. yes - there is more, just didnt find it necessary for this thread.....

[This message has been edited by Pyrthian (edited 03-19-2008).]

quote Originally posted by merlot566jka: there are a few good books too. i dont have time right now to list them, but pm me and ill send ya the names. you have to get the whole picture before you start to compare things like plenum vs tb area

What books do you recommend?

quote Originally posted by Will: This is not correct... the actual effect is in fact exactly the opposite.

I agree with the first part, but not necessarily with the second. Given adequate plenum volume, negative pressure pulses propagating upstream (at the speed of sound) from an opening intake valve will be reflected back downstream as a positive pressure pulse. At some RPM (depending primarily on intake runner length and intake air temperature), this positive pressure pulse will arrive back at the intake valve just as it it closing, and the net effect will be a slightly higher mass of air in the cylinder than would otherwise be expected. Interestingly, organ pipes work exactly the same way.

[This message has been edited by Marvin McInnis (edited 03-20-2008).]

quote Originally posted by Marvin McInnis: I agree with the first part, but not necessarily with the second. Given adequate plenum volume, negative pressure pulses propagating upstream (at the speed of sound) from an opening intake valve will be reflected back downstream as a positive pressure pulse. At some RPM (depending primarily on intake runner length and intake air temperature), this positive pressure pulse will arrive back at the intake valve just as it it closing, and the net effect will be a slightly higher mass of air in the cylinder than would otherwise be expected. Interestingly, organ pipes work exactly the same way.

That is the correct description.

What it sounded like the first guy was describing was that the air would "stack up" behind the intake valve while it is closed and create a high pressure pulse into the valve as it opens, which is a common falacy.

quote Originally posted by Will: What it sounded like the first guy was describing was that the air would "stack up" behind the intake valve while it is closed and create a high pressure pulse into the valve as it opens, which is a common falacy.

That's the way I read it, too. But I guess I didn't follow your logic to the point of "... exactly the opposite ...." Anyway, I think we agree on what really happens.

quote Actually, for throttle per cylinder intake systems, plenum volume ranges from the size of the airbox to infinite... depending on how the ductwork is run. Putting the throttles between the plenum and the cylinders doesn't eliminate the plenum. A zero plenum manifold would be something more like the LO Q4 or TC (I think) intake manifold, which is basically a header with a throttle body on it.

I agree. I should have said "plenum size = approximately infinity." The air filter (if present) and any ducting upstream of the throttle plates (if present) should generally have minor effects. The air filter will provide some damping, and the geometry of any upstream ductwork may introduce some minor resonances. How significant these effects will be is open to question and experimentation.

[This message has been edited by Marvin McInnis (edited 03-20-2008).]

quote Originally posted by Marvin McInnis: That's the way I read it, too.

That's the way I typed it, actually, but I know it doesn't work that way. I'm not sure what I was thinking. I corrected the post.

wow all good info here, i like!!

the helmholtz resonator is a brilliant idea. tunning the intake and exhaust track to take advantage of the pulses of presure in an engine. not every engine uses this tunning. not everyone is aware there are 3 different pulses you can take advantage of here. with no plenum you have a strong signal strength (pulse) but you have to be very good at your math and fabrication or have a dyno in your garage.

there is so much more that goes into this. the book that i want but cannot afford (over $200) is written for SAE about exactly whatwere talking about here. i dont even remember the title, but amazon has it.

the books i have and have read are:
scientific design of exhaust and intake systems, isbn 0-8376-0309-9
maximum boost, 0-8376-0160-6
how to tune and modify cylinder heads isbn????
grapeaperacing.com
physics text books (ive got a few round here somewhere)

things to study, wave motion, acustic engineering, thermo-dynamics, fluid-dynamics, and gonna have to beef up to atleast calculus to digest it all. i havent had much schooling after high school, just some nuclear engineering junk and some more military crap to go with it. the most ive learned is from reading books, and going out and giving things a try.


im going to edit this in a bit to add the isbn's for the books on my toilet tank that i read daily. (i read for half an hour a day and research all day long)

[This message has been edited by merlot566jka (edited 03-20-2008).]

ttt

Since the 60s, yeah I'm that old, I've always wanted to build an intake with tellscopic runners that would match the runner lenghts to the RPM. Even made a full set of plans on how to do it that addressed vacuum sealing etc. Just wanted to throw that out there for the armchair-engineers to trash around. Oh, and you can also do the same to a lesser extent with exhaust headers whereas you can tellescope the end of the collectors.

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[IMG]



Trueleo.com/fiero.htm
RSpiderII@aol.com

ive seen it with velocity stacks, but to actually beable to seal the vacuum in....that would be awesome.

on a seperate 'armchair engineering' plot, varible valve timing.....use servo motors on every valve controlled by a computer to open and close the valves. no cams. imagine that, unlimited cam profiles, sounds perfect for varible intake and exhaust. add in a computer controlled waste gate and bov, and imagine what you could do!! 40mpg and 800hp..... and the most broad flat torque curve ever. unlimited rpm range (well the bottom end).

ahh hell why not have varible length con rods and varible combustion chamber volume (saab already has that technology).
reverse polarity crank and rods, main bearing too...a floating rotating assembly, rpms ensue!


ok im done with all my nonsense

quote Originally posted by merlot566jka: ive seen it with velocity stacks, but to actually beable to seal the vacuum in....that would be awesome. on a seperate 'armchair engineering' plot, varible valve timing.....use servo motors on every valve controlled by a computer to open and close the valves. no cams. imagine that, unlimited cam profiles, sounds perfect for varible intake and exhaust. add in a computer controlled waste gate and bov, and imagine what you could do!! 40mpg and 800hp..... and the most broad flat torque curve ever. unlimited rpm range (well the bottom end). ahh hell why not have varible length con rods and varible combustion chamber volume (saab already has that technology). reverse polarity crank and rods, main bearing too...a floating rotating assembly, rpms ensue! ok im done with all my nonsense

Would likely be easier to have the cylinder head raise and lower to change compresion. If each was a cylinder shaped unit within the head and all tired to gether/bank they could move up and down and still retain a good seal. The valve train would have be OHC with idler pullys on the the belt or chain. Arm-chair you can come with anything!

BMW has done continuously variable intake runner length on their Valvetronic V8's in the 550, 650 and 750. They also do something approaching continuously variable valve timing. The Valvetronic technology is basically a variable ratio rocker arm that allows the rocker ratio to go so low that valve lift is so curtailed that the engine doesn't need a throttle. This supposedly reduces pumping losses. The Motorsport engines still use throttles because valvetronic apparently doesn't have the crisp response that throttle per cylinder does.


The technology for camless valvetrains is in existence. Pneumatics would accomplish this easily using an engine driven compressor. However they would be for racing use ONLY because continually blowing off the high pressure air would be VERY LOUD. Hydraulic actuation would be preferrable, but the speed of the hydraulic actuators currently available limits engine RPM to <2000 (would work for over the road trucks). There may even aleady be diesels on the market with hydrauliv valve actuation.