308 carb airbox restriction discovered

Discussion in 'Technical Q&A' started by snj5, Feb 20, 2005.

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  1. snj5

    snj5 F1 World Champ
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    #1 snj5, Feb 20, 2005
    Last edited by a moderator: Sep 7, 2017
    I can't believe I didn't see this before. Must give credit to Mr Morgan at Durable1 for pointing me in the right direction.
    I had always noticed a large dyno hp loss, in excessf 12 hp, using the stock 308 airbox over open trumpets. Well, it turns out that on later 308 airboxes with the noise insulation, Ferrari did not re-engineer the airbox size, but rather just the snout and added the mesh and sound deadening. While it did lower the diameter of the feed duct, the key thing was that the way they fastened it on the inside almosts abuts against the air cleaner element, effectively limiting the filtered area to just the diameter of the opening. The first clue I had was that the filter showed discoloration only around the entry in the shape of the entry area. The pictures show just how tight the fit is. So tomorrow, I'm going to take it to a shop and have the screen and foam silencer removed to allow airflow around the whole of the element. This has just gotta increase airflow.
    Until then, I am back to the somewhat extereme experiment of using the low profile high flow velocity stacks with the individual Pierce Filters. Should be interesting and will report back.
    So if you have a muffled airbox and want to make 10+ easy hp and improve the sound, I suggest removing the restrictive sound deadening in the airbox snout.
    best
    rt
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  2. snj5

    snj5 F1 World Champ
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    Quick note after a short highway run.
    Well, now I remember why they muffled the airbox - the Weber carb howl and air induction hiss actually is louder than the exhaust at some settings. With the deep sounding free flowing muffler and the Webers the difference in sound over the previous Kjet+Tubi is amazing.
    This car sure revs freely and strongly over 5000 - 7000 rpm really fast in every gear... it seems so happy over 5000 rpm....
    best
    rt
     
  3. Sloan83qv

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    Leave the box off, even with the sound material removed it is restrictive as hell and doesn't allow the air horns to act as designed. The sound of webers breathing is the sound of a Ferrari.
     
  4. pma1010

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    Dremel tool exercised.
    Philip
     
  5. Peter

    Peter F1 Veteran
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    My GT4 never had that perforated screen and the filter was always equally dirty around. You are using a mod'ed GTB/S air-box, correct?
     
  6. hanknum

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    Russ,

    I thought the low profile high flow "stacks" were designed for use with the OEM airbox. It is my understanding that the limited height of the airbox did not allow sufficient room with the OEM velocity stacks. Are the individual filters about the same height as the OEM filter? If they are, then I guess the same problem exist. If they are taller, isn't it prefered to have the taller stacks (more torque)?

    I've also often thought that the original airbox is restrictive. I know that colder air makes more hp, but is it better to have a restrictive airbox with a CAI, or less restrictive intake with warmer air. Are you ducting the side intake onto the filters at all or just leaving it alone? Does anyone know if the side intake provides any "ram air" effect?

    Thanks

    Henry

    p.s. you got the individual filters at Pierce Manifolds, right?
     
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  8. snj5

    snj5 F1 World Champ
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    Peter
    The airbox is a bone-stock later model 308 GTB airbox. Your statement that in your airbox the filter was equally dirty acts to confirm my suspicions.
    Many thanks
    Russ
     
  9. snj5

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    #8 snj5, Feb 21, 2005
    Last edited by a moderator: Sep 7, 2017
    Henry
    Thanks for the note. Yes, the Pierce Manifold individual filters shown are approximately the same height as the stock airbox being used, but the individual filters provide far more total surface area. Of course, one does loose the plenum effect of the single airbox. Until I get the stock airbox opened up, will be using the Pierce boxes with the cold air ducting blowing at them, but not a closed system.
    While the shorties do increase high end flow shown in the development dyno, part of the original mod was also to open up this airbox screening. There's no doubt that the shorter stacks increase high end flow. In theory just as you say, taller stacks would improve response and move the power band down. I am a bit inspired to a possible solution using Ferrari's own technology taken from Napolis' 0846, which uses slightly curved and belled velocity stacks. If I could fab a set of these that added some length, and then fit inside the airbox, it could possible solve the bell to to top area problem and also pick up some response down low by increasing flow velocity. Our Alfa buds also commonly use similar curved stacks on 40/45 DCOE installations to move the power band down (more low end). Also interestingly similar curved velocity stacks are also used on the Alfa Montreal small displacement V-8 in a similar filtered airbox which also acts as a Helmholtz resonator.
    Shown below is Jim G.'s engine with the curved and belled velocity stacks and a photo of curved Alfa DCOE velocity stacks.
    More as things develop
    best
    rt
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  10. thecarreaper

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    i really like the sounds from the car with the indivudual boxes. i may try the shorty stacks as well.
     
  11. snj5

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    #10 snj5, Feb 21, 2005
    Last edited by a moderator: Sep 7, 2017
    I'm not a CAD/CAM designer, but here's a rough sketch side view of the air horn I am thinking of fabricating, that would fit in the stock or Pierce airbox. May look at making it a bit longer.
    best
    rt
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  12. cavallo_nero

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    For the individual air cleaner setup (one for each carb), i think it would be a good idea to have the air cleaner covers be a filter material, that way, air could enter thru the top. this is done on some of the american V8 muscle car models where the lid to the air cleaner is acutally a filter element. anyone ever see this for the pierce individual air box setup - any comments??
    i bet these can be fabricated, thus allowing the taller velcity stacks..
     
  13. snj5

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    #12 snj5, Feb 21, 2005
    Last edited by a moderator: Sep 7, 2017
    Here's the 'after'photos after removing the sound deadening material and restrictive mesh - Wow. A lot more room to flow air to the filter and around it. also as a side benefit, it really brings the carbs into a nice sound - more than the muffled airbox but not as raw (better for a GT). Compare with 'before' pics at start of thread.
    Will take it out a bit this week for a report with and without the short stacks.

    And for Weber purist Paul Sloan, I've been fabricating removable wire mesh covers for the stock trumpets in case I want to go 'al fresco'. :) :) It looks pretty impressive. And can use in conjunction with the airbox sans filter and lid for cold air ducting.

    More later
    Russ
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  15. chrismorse

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    GOD,

    I just HATE that awfull wailing - maybe if i opened the windows and took off the muffler, i could better hear the engine over that horrid webber screaming - ya and maybe i'll buy a citroen 2cv.

    LET IT SCREAM,
    chris

    thanks for the tip, I'll check it out.
     
  16. Spasso

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    I am in the last stages of completing the modification on a 1976 GTB box. I will post pictures to show the difference between my stock 1978 box and the '76 box.

    The '76 box had mesh and fiberglass in it but not as intrusive as the '78 box.

    The snout was shorter on the '76 box so I cut off the end and fabbed a new "smoothie" snorkle to match the length of the '78 box to hook up to the rubber bellows. Everything is opened up, tapered and smooth. I will be searching for material to make a smooth bellows to complete the package.

    In answer to one question about ram air.

    Yes, at speed there is a ram air effect. I would say beneficial over 40 MPH. Most importantly, cool, direct and smooth airflow.

    I have read that aftermarket induction on front engine cars that draw air from under the hood instead of from behind the grill or airdam suffer in HP gains. It does make a difference.

    With luck I can snap a few pictures of my "new" airbox tomorrow and post a few.
     
  17. pma1010

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    20 minutes with the dremel and a chisel and the mesh and the associated insulation are out.

    Making a smooth "floor" to overcome the "step" in the airbox would be useful, unless of course the step is there to get the air tumbling(?)

    Feeding cool air from both sides (like the Michelotto cars) would be the way to go but then how to duct air to cool the oil?
    Philip
     
  18. Spasso

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    I thought about bridging that step but didn't want to reduce the volume of the cavity. I too wondered about the air tumbling but thought again that maybe the worst that could happen is a high pressure area would develop in that area (at speed. See "ram air") in effect bridging the step with high pressure air.

    Good idea but I'll settle for a single inlet for the carbs just for the sake of keeping the oil cool. The Michelotto cars used a NACA type air intake in front of the left rear wheel for the oil cooler if I remember correctly.

    It's cold enough around here I have that side plugged with a foam piece right now.
     
  19. pma1010

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    The Michelotto 308 I have seen "up close and personal" has the oil cooler up front in the "mouth" of the car (committing one of Carroll Smith's "cardinal sins"). Of course, some of these cars have been modified extensively since delivered my Michelotto...

    I believe the ducts you reference were for brake cooling.
    Philip
     
  20. Gary48

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    snj5, don't throw away that air box yet! Here's another approach that I used on my 246. I would alway notice how damed good my Dino ran when cold and then performance dropped when it fully warmed. I proceeded to study the large (lots of tin) of the original carb cover. This darned thing was absorbing all the heat from the engine and engine compartment. My thinking was if I insulated the air box including everything to the outside inlet, including the air cleaner housing that it would help alot. I discovered this great self adhesive, thin, fiberglass insulation covered with .010 quilted aluminum. Its made for transmission tunnels and I saw it in eastwood's catalog. I sealed all joints with aluminum tape and the whole thing looks great. I can't wait for the overhaul to be completed to give it the test. Those individual K&N cleaners look great and show the engine better but its very difficult to get cold air to them.

    Gary
     
  21. hanknum

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    I know many shy away from the individual filters due to higher temps in the engine compartment, but it seems a few here have them. For those of you who do, what do you think? Is the benefit of less restriction enough to override the effects of the high temps? I would assume that the higher temps are more of a problem at idle and lower speeds. As speeds increase, the side opening would "blow" over the filters and the temps would decrease. I know the intake sound would be nicer. What do you guys think? I'm seriously considering these.

    Anyone else other than Russ have the low profile high flow "stacks"? Are they worth it? I know they aren't too expensive.

    Thanks

    Henry
     
  22. Spasso

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    Because of debris?.

    More than likely.
     
  23. Spasso

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    I am in the process of installing them, rejetting from 125's to 135's and doing the airbox modifications.

    I will dyno on March 12th and compare with last year. Film at 11:00.

    '78 GTS
     
  24. pma1010

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    Oil cooler placed in front of water radiator.
     
  25. Spasso

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    #23 Spasso, Feb 22, 2005
    Last edited by a moderator: Sep 7, 2017
    To elaborate on Russ's (snj5) airbox mods.

    Disclaimer for alarmists!;

    Let me first say that the box I modified came off of a burned out '76 GTB. The box was heavily pitted and rusted. Otherwise junk.



    Enclosed are pictures of the stock airbox from a 1978 308 GTS.

    It incorporated the perforated metal and fiberglass insulation.
    Restricted inlet bore size.
    Vacuum operated damper/flap in the snorkle
    Restricted clearance between the filter and inlet area as noted by Russ.

    The 1976 box had a shorter intake snorkle so I took the opportunity to fabricate a new one to match the original length of the old box at the same time smoothing and opening the internal bore.

    The finished product weighs about HALF what the stock box weighs.

    Stock airbox pictures first then the modified box.
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  26. Spasso

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    #24 Spasso, Feb 22, 2005
    Last edited by a moderator: Sep 7, 2017
  27. Spasso

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    Ooops!
     

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