A camera phone picture of my intake fresh out of the bay. I figured that since my old build thread is archived (https://www.fiero.nl/forum/Archives/Archive-000001/HTML/20090907-2-059801.html), I would start another thread about this here.

I still have to polish/paint and route hoses and pipes to make it look nice, but it works great. I'm not the first person to change to the 96-97 style intake, or cut the runners short and weld on a piece of aluminum pipe for a plenum and make power gains. But I haven't seen anybody use physics to design the important parts (runner length, plenum volume, etc) to achieve a specific goal. I set out to cut the runners down to where they would resonate at 6000 RPM, boosting power there, and propping up the power band where the engine would be weak. If you watch the video, you can clearly hear the intake tone change drastically exactly at 6000 RPM - proof that it works:

http://www.youtube.com/watch?v=WkCUodmhn7E

[This message has been edited by Blue Shift (edited 10-20-2009).]

Sweet video. Sounds cool. Nice mod, looks fun. It does sound crazy at 6k. Thanks for posting

[This message has been edited by LZeppelin513 (edited 10-20-2009).]

Now for some of the math behind all of this:

1.) David Vizards Formula 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.

For 6000 RPM, Length = 35cm
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2.) Another Formula to Calculate Runner Length for a Specific Peak Torque RPM: from Steve Magnante at Hot Rod magazine N x L = 84,000 where N represents the desired engine rpm for peak torque and L is the length in inches from the opening of the runner tube to the valve head.

For 6000 RPM, L = 14.0" or 35.56 centimeters from the plenum to the valve head

So, the runners need to be cut back, so that the total length of the runner from the back of the valve to the plenum is 14 inches long - a bit shorter than stock. Next I found a rule of thumb in the book "How to Tune and Modify Engine Management Systems" by Jeff Hartman. Look it up on Google Books, pages 119 to 121 are all you need. The book suggests a plenum size of 40-50% for a V8 and 50-60% for a 4 cylinder. My assumption is that ~50% would be suitable for a V6. The book goes on to suggest subtracting 10-15% for engines operating at 7000-7500 RPM, and to add 30% for a torque boost at 2500-3500. The width of the runners is approximately 13.5", so using 3.5" pipe spanning all 6 runners = 129 Cubic Inches, or 62% of engine displacement, which is in the ballpark.

The book goes on to show you how to select size and length of ram pipe between throttle body and plenum. The length determines when it becomes resonant (and gives a natural ram-charging effect of its own), and the diameter determines the highest air velocity will get. It turns out GM's stock 2.75" diameter is dead on for 180 ft/second max airflow for this engine (which they recommend not to exceed), so I built my ram pipe from 2.75" pipe, to 12" in length - which is what the deal is with the goofy looking long throttle body arm.

And, even with all of the math and guidelines, I was still surprised to see that it does exactly what I was hoping for, and then some. If you're considering building a custom intake manifold, do some research and take a scientific approach, and you can achieve great results!

[This message has been edited by Blue Shift (edited 10-20-2009).]

Actually, I'd like to replicate what you've done, since you've already experienced the good results. Would you be interested in doing another one for $? Or, can you detail the build up, possibly with sources to the materials? At some point, I'll be putting a 96 DOHC in my 88, and I think your intake is a nice solution!

[This message has been edited by dsnover (edited 10-20-2009).]

Sounds yummy.

I built the intake at a friends shop, so I won't be able to make another one, sorry. You'll need the following:

From 1996-7 3.4 DOHC:
-lower intake and bolts
-fuel rail w/injectors
-upper intake and bolts
-throttle body arm
-throttle body w/throttle cable bracket
-gasket kit

-a piece of aluminum pipe at least 13.5" long for the plenum. 3.5" is what I used.
-a 12" long, 45 degree bend 2.75" aluminum pipe for the throttle body ram pipe.
-3" cold air intake piping (from a Honda civic will do) and cone air filter.
-access to TIG welding and cutting machinery
- a bunch of hose, fittings and piping to plumb it all.

And most importantly - the above calculations and the book I pointed out to determine where your power band will be depending how you build it.

Nice job, it looks great. Loved the high RPM sound in the video. I'd give you a + but you already have one.

[This message has been edited by Sourmug (edited 10-20-2009).]

NICE!!!!

any other work done to the motor? any plans to dyno that thing?

JUST WOW!!

The engine was a fully rebuilt, balanced, and blueprinted 3.4L DOHC from a 1991 Olds Cutlass Supreme. I used 1996-7 pistons (debateable whether they're good for extra compression or not). It has a lightweight aluminum flywheel which seems to help acceleration from a roll some at low speed. Exhaust manifolds and crossover are stock, going into a super short, out the back exhaust system with a cat and Flowmaster 80 muffler. It just barely barely fits without cutting the trunk out.

The difference between the stock 1991-5 intake manifold, stock Fiero air pot, and intake silencer, and this intake is amazing. I'm dying to know what it's putting down but am out of money at the moment. A dyno session is definitely in the plans.

[This message has been edited by Blue Shift (edited 10-20-2009).]

Do you have factory valve timing?

Yeah, I'm still running 0/0 factory cam timing (which may be a little out due to belt stretch now). Michael Smith achieved killer numbers using a crazy setup, but needed to dial in his cams for maximum effect. It's quite possible that I could see gains by adjusting my cam timing... Something I may play with next time I slap on a fresh timing belt.

I'm also running a mostly stock 91' chip (and very slightly larger 96 injectors), that probably needs some tuning for the new VE curves and such. Gains could possibly be made here as well. I hope to buy tuning equipment in the not too distant future, and start getting it dialed in perfect.