I forget now where i read this but i remember that this article said that the best size for a custom intake plenum was 60% of the engine's displacement. That seems awfully high to me, but i know very little about this topic. Anyone out there ether prove me wrong, or possibly support my best guess of 60%?

https://www.fiero.nl/forum/Forum2/HTML/081203.html

near the end of this thread - I was wondering the same thing - and someone posted a GREAT link to a PDF about intakes & plenums

Since you mentioned it (and I'm also in the middle of a custom intake) I looked into the numbers. Someone tell me if I am not doing the math correctly.

The 2.8L engine is approx 173 cubic inches. 60% is ~104 cubic inches. So an intake 4" wide X 10" long X 3" high is 120 cubic inches. This does not appear very large at all.

The 3.4L engine is approx 207ci. 60% is ~124ci. Again, not too big really.

quote Originally posted by Hudini: Well I just read the .pdf on plenum volume. A quote: The Helmholtz resonator theory does work well, however, it is limited to how many cylinders can operate off a single plenum. To be effective, no more than 4 cylinders should be used in a single plenum. This set up is very effective on 6 cylinder engine with two plenums, each plenum feeding 3 cylinders. Hmmm, guess we are outa luck unless you build a dual plenum.

yes, it really makes the "enterprise" intake look like the way to go. with at least a liter of volume for each "nacelle".

you may have read something about the 3.4 DOHC engine build... somebody built a cusom plentum for it, the total volume was 7.xx Liters (twice the engines displacement) with the intake and mild cam timing he went from 205hp to 260hp or something wild lke that. I cant find a link to that build, maybe somebody else? good luck with your build, be sure to fill me in on your progress.

------------------
1986 Silver 5 speed Fiero 3.4 DOHC
Bored .30, Fully balanced and blueprinted
13.93@101mph as it is on the street.
... ... !
350ci 400hp/tq SBC, 4 bolt main
Spec Stage 3, Nitrous Oxide, many extras.
87 GT 5-speed Getrag, power everything.

I'm working on a home made plenum out of aluminum. It will be 432 cubic inches of volume. I hope to have a before and after dyno slip. I really did not think it was that large until I ran the numbers this morning. Should be interesting to find out if this is worth it.

quote Originally posted by Hudini: In conjunction with the intake size, I looked into the best throttle body size. This article http://www.grapeaperacing.c...inductionsystems.pdf gives a number of no more than 180 ft/sec flow rate to get the benefits of the Helmholtz resonator theory. Using the example numbers for a 3.4L engine you get a intake pipe diameter of 2.25 inches which is just a hair over 57mm. Isn't this the size DM ports our 52mm throttle bodies? Anyone know of a similar size (57-60mm) throttle body that one could grab off a junkyard car?

First the 2.25 inches is all open. So you need a 60mm - 62mm TB once you take into acount the butterfly valve.
The stock Fiero TB is 52mm.
The Fiero Store bores them to 55mm
DM bores them to 57mm

You can buy aftermarket TB's for "J" body cars at 60mm and 62mm. I think 58mm is stock, but I don't know for sure.

damn, every time I hear numbers & calculations - I feel better & Better about going with a 3.1, 57mm TB & the upper plenum work I am doing right now. cant wait to dyno it when the plenum is done & get a new DarthChip

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1985 Fiero SE - Plain Red V6 Coupe
3.1 Crane272 MSD 4.10-4spd DarthChip WCF Long Tubes Borla
D.A.M.M. - Drunks Against Mad Mothers

Hudini and Pyrthian I'm eager to hear about those dyno results and intake spec's. I belive that i will take the track testing approach, that way i can get some real world performance.

I've done some calulations and some might be wrong and some could even be right.

From the grapeaperacing link
"We must take into account the intake
duration, but you want the pressure waves to arrive
before the valve closes and after it opens (air won’t
pass though a closed valve). To do this you must
subtract some duration, typically you take off 20-30°
from the advertised duration. 30° works well for
higher rpm solid cammed drag motors.

For a racecam with 305° of intake the ECD would be about
275°. The ECD must be subtracted from a complete
cycle of 720° to get the effective valve closed
duration.

The formula to figure EVCD for a race cam
with 305° advertised duration would look like this:
EVCD = 720 - (305 - 30)

The EVCD of that cam would be 445. For
smaller cams in the 270° range, subtracting 20° from
advertised duration will give better results. The
formula for optimum intake runner length (L) is:
L = ((EVCD × 0.25 × V × 2) ÷ (rpm × RV)) - ½D
Where:
EVCD = Effective Valve Closed Duration
RV = Reflective Value
V = Pressure Wave Speed
D = Runner Diameter"

For me i took the cam spec's from the fiero store the street strip cam which was 264*. So i subtracted the recomended 20* and 244* was the out come. Subtract 244 from 720 and i got 476.

L = ((476 x .25 x 1300 x 2) / (6500 x 2) - .8125 = 22.98"

So my runners would been 1 5/8" and they would have to be about 23" in order to get peak performace out of 6500rpm. That seems to be a huge number, but then again i'm really new to this subject.


I also used that links equasion for the diam of an Intake Ram Pipe.

D = ^ (CID x VE x Rpm) / ( V x 1130)


D = ^ (171 x 0.85 x 6000) / (180 x 1130) = 4.29

Now I really know i did that wrong becuase i have no idea what the ^ is in that equasion. Any help with that would be appreciated.

Not mine, just one I ran across - I also do not have any more real information other than it was custom made and uses the stock throttle bodies.

[This message has been edited by Mickey_Moose (edited 04-09-2007).]

^^^

while that looks great, you would really pick up some power if you ditched them 2 cones, and ducted back to the stock CIA.

Yes i agree, plus if it was routed the same as stock it would be a little farther away from the exhaust that would help somewhat

quote Originally posted by GT2efiero: D = ^ (171 x 0.85 x 6000) / (180 x 1130) = 4.29 Now I really know i did that wrong becuase i have no idea what the ^ is in that equasion. Any help with that would be appreciated.

Its square root so 4.29 = 2.0712315 inches or 52.6mm

Oh ok yeah my first thought was a little off. Thanks for the help with the equasion

Bump for any additional info

well, I am going to be finishing up my upper plenum today or thursday, and with this info, I plan on trying to add as much volume as possible. I am down to getting the bottom plate on, and I will put a "bow" in it now to give it more space.

quote Originally posted by Pyrthian: ^^^ while that looks great, you would really pick up some power if you ditched them 2 cones, and ducted back to the stock CIA.

hmmm...if you went back to stock for the air intake, you would not gain anything. The stock intake is fairly restrictive itself - you need to construct a whole new air intake to feed the volume of air needed to take advantage of the 2 throttle bodies.

...anyways, like I said, it's not mine - just posted the picture to show what someone else has done.

quote Originally posted by Mickey_Moose: hmmm...if you went back to stock for the air intake, you would not gain anything. The stock intake is fairly restrictive itself - you need to construct a whole new air intake to feed the volume of air needed to take advantage of the 2 throttle bodies. ...anyways, like I said, it's not mine - just posted the picture to show what someone else has done.

the motor will flow XXX air. max. thats it. the stock CAI will flow more than XXX air.
just because the 2 throttle bodies can flow twice as much air as one - does not mean the engine will be able use twice as much air. most of us only need 1.2X more airflow. thats it. and, the existing CAI will easily flow 1.5X the needed air. especially if you clear out the water seperator. or, just put a single cone in the fender well, like the CRX CAI. whatever. but, sucking air off the Y-Pipe is not a good thing. or build a box a box & baffle to the outside.

Is there any caculation to decied the best diamater for the runners? Also has anyone made any progress on their intakes perhaps some pictures?

I think the actual creation of my intake will have to wait till the summer. I've been way to busy to start anything other then the actual numbers. I've got a welder but i'm weary about welding aluminum, Oh well i might have to try steel.

From the pdf article:

quote In a well tuned intake system there can be as high as 7psi of air pressure at the intake valve due to these pressure waves and sometimes even higher. So you can see that it can have a very large influence on the volumetric efficiency of the engine. This is how a normally aspirated engine can exceed 100 % volumetric efficiency.

Anyone remember that debate? heh. I win.

So what exactly is that saying. That with 7psi at the intake valve, there is 100% VE? Where is this other thread that you've posted a peice from? Is it the http://www.grapeaperacing.c...inductionsystems.pdf ? I'd rather plan things out the best i can rather then start building stuff and guess on where i need to go.

[This message has been edited by GT2efiero (edited 04-16-2007).]