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Generation III External Engine
LS1 | LS6 | Bolt-Ons | Intakes | Exhaust | Ignition | Accessories

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Old 07-19-2002, 10:30 AM   #1
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Default Intake Manifold Plenum Volume...............

The LS6 intake manifold has a largert plenum volume and shorter intake runners. It makes more torque and power than the LS1 manifold. All of the sheetmetal intakes I have seen have been of a larger plenum and even shorter, but wider runners design.
When does a larger plenum and shorter runners become a liability on the stock 346 inch motor?
Would shorter runners (Like the sheetmetal manifold at LS1speed) but with a smaller cross sectional area (similar to stock) maintain some of the low-end torque??
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Old 07-19-2002, 07:51 PM   #2
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Default Re: Intake Manifold Plenum Volume...............

That's a interesting question. Not something I would expect to see here, anyways.

The plenum volume is not as crucial as the cross sectional area and the runner length.

Yes the runner length is going to move the power band up higher in the RPM's having a smaller cross sectional area will move it lower. Now we would need to know the exact sizes of the two intakes you are trying to compare to see if it would be beneficial to you. The best thing to say is that the runner lenght is going to be more important than the cross sectional area, but putting a smaller cross sectional area on a shorter runner will give you more TQ below power peak which is exactly what you want. It's relatively complicated but yes a smaller cross sectional area will make up for the shorter runners to a point.

Add on top of that the LS6 intake has much better flow over the LS1 intake and it will make more power because of that. Now if we can get a intake that will flow as much as the cylinder heads we have now we will get great tuning and flow and a 346 would be capable of making a 500ft lbs and 500 HP at the flywheel.

Another thing you would want to look at is the runner taper. This is the main reason why I feel that the LS1 with it's long runners can works so well at high RPM. A larger taper angle is going to give you higher RPM power, and a smaller one will give you higher low-mid range TQ numbers.

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Old 07-22-2002, 11:48 PM   #3
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Default Re: Intake Manifold Plenum Volume...............

Runner length vs cross sectional area will have a significant effect on engine torque tuning. <img border="0" alt="[judgement]" title="" src="graemlins/gr_judge.gif" />
SStrokerAce is correct in saying that runner length will move the powerband up or down, depending on length. The following is only an indicator of manifolding trends.
Long skinney runners = low speed torque. Short fat runners = high rpm torque. Long fat runners = mid to upper rpm torque. Short skinney runners = mid range torque. Long large runners without bends = difficult to make with a common plenum and throttle body.
FWIW bends in the intake runner will almost always cause a loss in flow and that will usually result in some loss of high speed torque (horsepower). The LS6 intake does an excellent job of bridging the gap between low speed and high speed torque. <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />
Remember, GM developed the LS6 intake as a way of tuning more horsepower without a significant loss of low end torque. Most will agree they did a pretty good job. <img border="0" alt="[judgement]" title="" src="graemlins/gr_judge.gif" />
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Old 07-23-2002, 10:46 AM   #4
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Default Re: Intake Manifold Plenum Volume...............

So based on the fact that we have limited hood/cowl clearance, the ultimate intake for all around power and torque would have slightly shorter runners; a cross-sectional-area equal to the intake port on the heads; and have a very slight taper angle (versus the LS6 intake).
That sound about right??
I guess that IF you can get an intake runner design that will flow the same as the heads, then you can dictate how plenum volume changes the tuning.
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Old 07-23-2002, 11:10 AM   #5
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Default Re: Intake Manifold Plenum Volume...............

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by 1CAMWNDR:
<strong>So based on the fact that we have limited hood/cowl clearance, the ultimate intake for all around power and torque would have slightly shorter runners; a cross-sectional-area equal to the intake port on the heads; and have a very slight taper angle (versus the LS6 intake).
That sound about right??
I guess that IF you can get an intake runner design that will flow the same as the heads, then you can dictate how plenum volume changes the tuning.</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Yeah, something like the C5R intake would be close to perfect.

The magic number for LS1 intakes is about 9 inches. Depending upon the application the cross sectional area will change, because you have 346 cube engines and 434 cube engines, both will need different cross sectional areas to tune the power band. The intake runner cross sectional area is very important. Next to head flow and size, then camshaft I feel it's probably the 3rd most important part of engine design.

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Old 07-23-2002, 11:10 AM
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