## Determining optimal plenum ram tube diameter

## Determining optimal plenum ram tube diameter

(OP)

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?

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

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

## RE: Determining optimal plenum ram tube diameter

je suis charlie

## RE: Determining optimal plenum ram tube diameter

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

@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

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

## RE: Determining optimal plenum ram tube diameter

je suis charlie

## RE: Determining optimal plenum ram tube diameter

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

## RE: Determining optimal plenum ram tube diameter

@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

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.

http://wildaboutcarsonline.com/members/AardvarkPub...

## RE: Determining optimal plenum ram tube diameter

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

## RE: Determining optimal plenum ram tube diameter

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

I guess there is a reason that actual testing still is used.

## RE: Determining optimal plenum ram tube diameter

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

## RE: Determining optimal plenum ram tube diameter

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

www.auto-scape.com

Sideways To Victory!