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Engine Plenum Size Q's after days of searching....

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Inspired1

Automotive
Jan 11, 2006
2


Hi all,

After tons of searching I think I'll just ask before my eyes explode. I realize there isn't a prefect plenum size for a given motor but I'd like a professionals opinion about my specific application. Because I am no pro on the subject.

We're talking about a 4.3l turbo charged V6 that originally came out of a GMC Syclone. Stock Plenum is a single throttle body style with a LT1 Throttle Body mounted on the side, off centered toward the front.

I've built the motor to be around 550hp with ported vortec heads with 2.02/160's,larger hybrid turbo and the list goes on and on.

New plenum design is aluminum sheet metal with a front mount throttle body.{not much room for anything else}

Here are the stats: 4.3L V6 Turbo...not a drag racer...

Bore--4.030 / Stroke--3.480
Cyclinders---6 / Rod length--5.7"
Combustion Camber size--65cc / Piston Dish size--neg22cc
Maximum Rpms--around 6-7K / maximum boost--25psi
Compression ratio--8.2:1

My Q's:

1.I've heard soooooo many different percentages like 50-60 of engine displacement, 80-90 and even 100% on turbo motors....Can someone elaborate{cause I'm a newbie}on what plenum volume is supposed to be on a turbo motor with a maximum 7K RPM's?? Or at least give me approximation with explanation.

2. I resonation tuning really that practical in a turbo sheet metal intake??{For guy with a small wallet}

3. How do i test the plenum for even flow distribution to each intake runner??


4. What is the actual formula for figuring out intake volume??

Thanks for your time,

Malik






 
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You are correct when you said that there is no defined equation for plenum volume.

With that in mind here's what I know from my experience.

Regarding non-turbo engines, if the engine carb. is "too small" you generally want a large plenum. If the carb. is "too big" you want a small plenum. I have heard the number 150% of one intake runner and cylinder head intake port volume as a good starting point. Heavy cars like small plenum volumes, light cars like large plenum volumes.

I would measure the stock plenum and add XX% to it (maybe increase the volume by 30% to 50%). If it is too big you can add a "stuffer" in the plenum to reduce the size. Make sure and leave at least 2" above the runner so the air can turn the corner into the runner. If the top of the plenum gets too close to the runner entry it will impede air flow.

Since the engine is fuel injected, if the runners are the same shape and there is a good radius on the runner entry you should not have a problem with equal flow. This type of problem is more critical with a carb. where air and fuel have to turn.

Resonate tuning is DEFINATELY important on any high performance engine. There are formulas to calculate the correct runner length.

In my opinion, a turbo charged engine plenum volume is less critical than a non-turbo engine because the entire manifold is pressurized.
 
V6 needs more volume than V8 of same displacement, etc.
 


Thanks for your replies,

The stock Plenum is 3,903 cc's.....I'll post some pics of the new one I designed....

Just got the pieces back from laser cutting and i just need to get'em welded up.

This topic of plenum volume is a broad one....And even more so when it comes to turbo engines. Also theres alot of acceptable knowledge about intake runner size and configuration but apparently less when it comes to plenum volume!!

Why is this?? Thats my real Question....

This new plenum design will be around 2,000 cc's and take i'll it from there.

Thanks again,
Malik
 
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