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Old 10-19-2000, 07:11 AM   #1
JaiMak
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Post Intakes connected to plenum VS. Intakes Connected to throttle body - opinions pls.

Just like the header states, many people are using and like very much the intakes that connect up to the plenum like Larry's and ISR's, but then there are also the other intakes that hook up to the TB such as the weapon R, JC, or rallispec intakes.

Which one has better gains overall? What are the strengths and weaknesses of either kind?
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Old 10-19-2000, 07:15 AM   #2
JaiMak
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Oh, and before anyone tells me to do a search, I did. I also wanted NEW views.
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Old 10-19-2000, 07:36 AM   #3
SCRAPPYDO
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look I will never tell you to search.. quite frankly it does not take that much time to type a new message.

I am using the plenum type of intake. I probably did not get the max gains in the top end that a TB intake will give you, but I have very good throttle response at all RPM ranges. You will loose some low end with a TB type intake, but you get better top end with it as well. Like everything else in engine management, its a give/take relationship. Also, I dont know how you feel about engine noise, but if you use a TB type intake, it will generate a lot of noise at WOT. Where as a Plenum intake is relatively quite, at least the one I made is.

Either way you are going to notice good gains. These cars respond to an intake very well, so I do belive that you wil be alright either way. My advice is to ride in a car with both kinds and listen and compare them for yourself.

I hope this helps. If not maybe somebody else will give you some input that will..

have a good one

SCRAPPYDO
stephen
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Old 10-19-2000, 07:52 AM   #4
Matt
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I have a K&N right on the throttle body, and I know that I am losing quite a bit of low end. But I like the sound and there is a small gain up top.

Honestly I am waiting on the Cobb intake to get done (HINT HINT). The one I am using is pretty much just something to have there while I wait for a real one to get made.

So for now I have it right off the throttle body, but I will end up with an airbox in there somewhere when I buy a real setup.
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Old 10-19-2000, 07:54 AM   #5
WRC666
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My intake mounts to the MAF housing, the rest toward the engine is stock. It's pretty loud, but response is good.
Just get an S-AFC , if you have a 99.
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Old 10-19-2000, 08:12 AM   #6
STiShawn
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Okay, since non of you responded to my other post concerning types of intake and required fuel additions here it is again. If I run an ISR intake on a 99RS do I need further enrichment?
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Old 10-19-2000, 08:17 AM   #7
8Complex

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There are theories behind this all that get pretty complicated...

Supposedly with a tube directly from the TB you get more velocity on the intake air, which only benefits high-end and hurts the low-end torque because it has to suck through that tube harder to get air.

While a plenum intake leaves a reserve of pre-filtered air just within reach of the engine, it doesn't have very good flow characteristics and causes high-end loss (which you don't notice since the car has always driven like that).

Basically whatever you use it for is what type of intake is right for you. If you use it for high-speed racing like drag and such, a tube intake would probably be better for you since you're keeping the rev's up almost all the time. If you're using it for autocross (!!) or towing a boat or something, the low-end torque of the plenum being there will benefit you the most.

My intake is a slight combination of the two and draws the coldest air possible. It is definetly not recommended for anyone not knowing what they are doing or willing to deal with switching filters often or checking it after heavy rainstorms. http://8complex.mwic.org/intake/
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Old 10-19-2000, 08:19 AM   #8
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STiShawn - I've never seen someon running the ISR on a 99, but you should see if you can get your hands on an A/F guage to tell if you need enrichment or not. I think you should feel it pull a lot less hard up top if that intake doesn't make enough resistence in the flow tract, in which case you'd need an AFC.
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Old 10-19-2000, 08:20 AM   #9
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You shouldn't shawn. The plenum will help curb the air flow turbulence which causes the rich/lean mixture to your MAF.

Wow 8C. Your errr....cardboard hood scoops is so amateurish that it looks dang kewl!
Reminds me of how I use playdohs to stick more fans to my monster computer to cool that o/c chip. hehehehe.....
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Old 10-19-2000, 08:21 AM   #10
legacyGT00
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The 99s are known for running lean on aftermarket intakes, if I'm not mistaken.
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Old 10-19-2000, 08:37 AM   #11
8Complex

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Gambit - LOL yeah I know... I keep meaning to make an actual mold for fiberglassing a couple of them, but never get around to it. They drain perfectly though they have to be remade every 2-4 months cause the water seeps through the duct tape.
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Old 10-19-2000, 08:51 AM   #12
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If it leans out do I need an AFC, or can I skimp and use a fuel pressure riser?
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Old 10-19-2000, 08:56 AM   #13
8Complex

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I think you'll need to get an AFC because you'll only need to add from 5-5.5k and up. Not sure... I've heard of people fixing it with FPR's as well, but I imagine they run rich in the mid-RPM's.
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Old 10-19-2000, 08:57 AM   #14
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From all the reading I have done, it WILL lean out with a full tube style intake. But I was under the impression that with the greater reserve of air provided by the 2nd air box, it will curb the lean run condition. If it leans out I will sell the intake, I cant drop $3 to $400 for an AFC, that will buy an anti lift kit,sway bar end links and other chassis goodies.
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Old 10-19-2000, 09:52 AM   #15
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As a very satisfied Rallispec intake owner on a 99 equiped with SAFC and A/F gauge, I can tell you this. The tube intake will run lean due to MAF misreading above 5300 rpm at WOT. This can be compensated by the SAFC. But there is no driveability loss w/o the SAFC. Driving the car at part throttle, just short of WOT, I can redline the car with no lean-run, MAF misreading problems. There is some low end loss from using the tube intake, but I've compensated for that by running the engine leaner under 5000 rpm. My 99 runs strong from idle to redline to the point a certain late model Honda Prelude won't play anymore.

Here is the reasoning to my maddness.
As 8Complex mentioned the tube adds velocity to the incoming air at higher rpms. At lower rpms, the air velocity is much lower and the engine recieves less air than the stock ecu expects. This causes the engine to run a little rich at low rpms which I confirmed with the AF meter. Near mid-rpms the flow rate is fairly similar and no real change is required. At high rpms the MAF misreads due to a combination of factors. The higher airflow and the return vibrations from the motor. All that noise the engine generates at high rpm is reverberating down the tube and disturbing the MAF readings. Easy to fix. Use the SAFC to lean the engine low and rich the engine high to compensate for the new tube flow rate. My AF meter stays in the "rich" area (as opposed to very rich) throughout WOT-redline.

The plenum-box intakes seem to have the only advantage of reducing losses from trying to suck air through a small surface area filter. Any velocity gains from the tube are immediately negated by the large plenum-box.

The 8Complex intake is another matter by itself. It has no tube to provide increased velocity air, but it is open to cool air and is wide open. ie no narrow tubes to restrict airflow to that HUGE filter element. The 00 are suppose to run rich, but that is probably compensated by the colder air. Drive the MY00 long enough and the ECU will eventually correct for the rich run condition. There is also the consideration that 8Complex's engine is constantly steam cleaned whenever it rains.. (its a wonder he hasn't hydrolocked the engine).

I've had the RalliSpec intake for 4 months and 5000 miles. No problems, because I believe that MAF failure is caused by 3 primary conditions. Oil, vibration, and extreme temperture change. The Pipercross dual foam element in the RalliSpec intake requires no oil, or is it recommended. Engine vibrations (not noise vibrations) are well damped by their bracing setup. The extreme temperature changes cannot be helped as the MAF is handling air flow in excess of Subaru's design. But it should be able to handle it.
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Old 10-19-2000, 09:58 AM   #16
8Complex

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*cough* I've only gotten that filter soaked once and that was while it was parked during that huge storm. My dumbazz decided it was a good idea to park the car FACING the rain so the filter got soaked... it only ran bad above 4500 RPM though until I got the new filter in the next morning.

BTW, I've got about 3" of tube before the throttle body so I think it adds a little velocity there... just enough to even out the gains, I think.
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Old 10-19-2000, 10:04 AM   #17
JaiMak
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Thanks guys,

Revision, that's more what I was looking for, I have a 2000 and AFC, I'm going to put the AFC in before the intake because I want to monitor the so-called "normal" conditions.

I was more concerned with the air velocity issue, which is why I wasn't altogether sure about the gains stated from the plenum type intakes, I suppose however, the best way to do it would be to try both.
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