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Determining optimal plenum ram tube diameter

Determining optimal plenum ram tube diameter

Determining optimal plenum ram tube diameter

To prove a theory that the aftermarket intake systems being sold were too large for one of my cars, I used the following formula

D = SQRT((CUI x VE x RPM) / (V x 1130))

which I found in the book, "How to Tune and Modify Automotive Engine Management Systems" by Jeff Hartman.

The math as I plug in the numbers always shows that the optimal tube diameter is around 2.5" while all of the aftermarket systems are 3.5".

D = SQRT((230 x 0.85 x 6600) / (180 x 1130))

230cui (3.8L)
.85 Volumetric Efficiency (I used various VEs with little change on the result)
6600 is where peak power occurs on this engine
180 is the target ft/sec.

The problem is that on the dyno, the 3" was actually the best option by a lot out of 2.5", 2.75", 3" and 3.5" tubes tested, with the 2.75 almost the same as the 3.5 and the 2.5 just taking a big hit.

I reevaluated the math and was able to get 3" diameter as a result from the formula by changing 180 to 120. 180 used because in the text, Jeff states that you should always use 180. The problem is the text doesn't cover much more on the topic other than that.

My question is, How do you determine what the target ft/sec is? Or, is there a different formula that can be used?

RE: Determining optimal plenum ram tube diameter

It might be worthwhile for this discussion to elucidate what principle you are trying to effect.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Determining optimal plenum ram tube diameter

It seems strange that the formula includes total engine displacement but not the number of cylinders. Does the formula assume 8 cylinders whilst your engine is a 6? A V8 with your cylinder size (and probably needing the same runner diameter) would be 307 cu in.

je suis charlie

RE: Determining optimal plenum ram tube diameter

"2.5 just taking a big hit."

Was that referring strictly to peak HP, or with some consideration for the power band and torque curve ( kind of the same thing, I think) .

Is this the inlet tube from K&N airfilter to throttle body? What was the condition of the does the inlet "bell?"

RE: Determining optimal plenum ram tube diameter

@hemi - Not sure if there is a principal, just trying to determine optimal tube diameter.

@gruntguru - The formula example given in the book was for a 4cyl and yes, I'm applying it to a V6 (obviously with my own numbers plugged in).

@TMoose - Both peak hp/tq as well as the curve. Yes, the inlet tube is from filter to throttle body and for these tests, there was no bell and all tubes were the same length.

RE: Determining optimal plenum ram tube diameter

Quote (Titan2782)

@hemi - Not sure if there is a principal, just trying to determine optimal tube diameter.
...Ay, there's the rub.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Determining optimal plenum ram tube diameter

Sounds like the tube we are discussing is the (single) inlet to the plenum. The word "ram" in the thread title suggests runners.

je suis charlie

RE: Determining optimal plenum ram tube diameter

The ratio of the main intake area to the runner area might be a significant factor. To be honedt I've not seen more than 0.5 kps pressure drop across the entire intake including air filter so any friction effects from the tube are small beer.


Greg Locock

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RE: Determining optimal plenum ram tube diameter

@gruntguru - "Plenum ram tube" is how it's described in the book and in my opinion is appropriately names since the goal is to help fill the plenum as fast as possible.

@GregLocock - Interesting thought, I'll run some numbers and see what I get comparing runner area to intake tube area.

RE: Determining optimal plenum ram tube diameter

Hi Titan2782,

What was the OEM's diameter choice?
Have you driven with all the various inlet diameter variations?

Were things like A/F ratio, ignition timing, and absolute inlet manifold pressure being monitored in any way during the tests ?
Is variable valve timing involved ?

Do you have a picture of the actual inlet layout?
I'm kind of surprised that a large diameter inlet could have an effect "across the board" that overpowered whatever "science" had been applied between the throttle and the engine.
Even if a sharp cornered step resulted from the 1" variation in inlet tube diameters I'd expect the reduced flow coefficient to only show up at the upper end of the rpm/airflow range.

In the dark ages "ram" tuning meant a mild supercharge was achieved as a result inertial effects in the intake manifold. The intake "velocity" was much more than a buzz word, and separate from acoustic "organ pipe" influences.

RE: Determining optimal plenum ram tube diameter

@Tmoose - The tests were all performed back to back on a dyno and the curves, peak hp/tq and AFRs were observed. The OEM size is 3.50", but is very short (and goes into an air box) and is corrugated which puts the effective inner diameter at 3.25". Each tube was tested with a 45 degree elbow since 1) that's the best angle to direct the tube down into the fender and 2) closely matches the OEM elbow

As the tube diameter got smaller, the AFRs did get richer as expected and of course, with proper tuning the results of each test would be better, but the 3" tubing was richer than the 3.5" tubing but had much better result even though it was further away from the desired AFR of 12.8:1.

I'm not sure if the "science" was overpowered or if I was sciencing incorrectly, which is why I'm reaching out to people smarter than I.

RE: Determining optimal plenum ram tube diameter

Maybe 12.8:1 wasn't the AFR the engine desired.

RE: Determining optimal plenum ram tube diameter

Unless you are futzing about with the engine during testing, a given pressure and temperature into the manifold should deliver the same power and torque curves regardless of intake plumbing. JMO but unless your plumbing has some serious bends I'd be surprised if it yielded much of a pressure loss. Temp (and yes humidity) into your plumbing should be controlled during testing. Therefore I'd suspect you've either got a couple unrecognized variables in your test or aren't considering realistic variation. What is the difference in power between the three? A couple percent variation between runs is normal, even within the best test cells. Engine (not chassis) dyno, correct? Only conditioned (controlled temp, pressure, and humidity) air into the engine correct?

Personally I try not to worry too much over others' rule of thumb engineering such as the above equation. If it seems to work then great, if not then look elsewhere.

RE: Determining optimal plenum ram tube diameter

Formulas and computer programs are nice, but calculations only go so far when the variables are stacked against it.
I guess there is a reason that actual testing still is used.

RE: Determining optimal plenum ram tube diameter

"As the tube diameter got smaller, the AFRs did get richer as expected "

From the armchair in front of the fire, and assuming a moderately sophisticated mass air flow sensor and an EFI system, why would he AFR be expected to get richer ?

RE: Determining optimal plenum ram tube diameter

I wouldn't have expected it either, but maybe the tubing change effected the MAF sensor operation. At any rate, a change in AFR skews the results. As I hinted at before, even though he wanted the AFR to be 12.8:1 maybe the engine actually produces more power at the richer AFR the 3" tubing caused.

RE: Determining optimal plenum ram tube diameter

I think it's good that the original poster is trying to reference some theoretical theories when sizing up plenums.

In practice, these along with some of Vizards relationships I have never really found to work definitively.

The guidelines I've developed over the years include:
• 20% bellmouth radius everywhere except the floor, which is often flush
• No kinks or sudden expansions except in balance tubes/IMTTs
• 1 D code derrived runner taper

Packaging allowing all engines I design now have some for of taper- but it must be optimised to specific application-we're not talking much- maybe 1-2 degrees.

For a given cylinder and engine speed- if we compare a taper duct (which converges down to the diameter of the inlet port) versus one which is of constant cross sectional area ( the same area as the port- for sake of argument).For both these cases let us assume that the ports , valves and port flows in the head are identical.
Assuming we haven't gone above the 7 degree taper so that seperation isn't encouraged also.
For the taper runners the entry conditions will be less "lossy".
This lowering of entry CD will benefit top end more than bottom end- that adds up. The effective inlet tuned length is now altered however and that is why often you must lengthen slightly to retune. However what doesn't add up is that you can achieve the same lowering of entry losses by simply putting a fully eliptical bell mouth on a constant cross sectional runner. This will NOT achieve the broadening of the VE curve apparent in tapering runners.

I would pay attention to the R/D losses of the intake runners- R being the radius of the runner and D being the diameter. If tight packaging dictates a tight radius R- this will incur a loss. This can be rectified somewhat by increasing D - although this will change the tuned frequency of the runner AND increase the throttle volume of the engine. You can also change the intake runner cross section SHAPE to make it less sensitive to imposed bend radii.

That brings me to the final point- the throttled volume of the engine- larger plenums and therefore throttled volume (except ITB engines) will typically increase performance- witness the BMW S54 M3 for instance- which uses a 16 litre plenum but uses ITBs to keep the throttled volume down. However a large throttle volume will adversely effect your throttle response. If transient performance is important to you- this must also be considered


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