Induction/airbox design for dual carb system, inline engine

Ron Earp · Oct 19, 2010

I had some time to work on an airbox design for my Z car whilst my wife was recovering. The engine has two carbs, old SU knockoffs for all intensive purposes (square top 260Z US only carbs actually but easy to make work).

The car has had individual round filters for a very long time. But, they are cumbersome and really make working on the car difficult. I fabricated an airbox as shown in the attached picture. It is made from folded aluminum and mates up to both carbs on the front face. On the face of each carb is a "flow enhancer" which is basically a sort of thin air horn that provides a radius into the square mouth (yes, these carbs have a square mouth) and were dyno proven to provide four horsepower, repeatably. Box is fed through a large cone filter with 3.5" hole. Teh air horn is 5/8" tall, solid aluminum, and has a very gentle radius in the carbs. The air horns also hold the box to the carb by sandwiching one side of the box between the horns and the carb.

I was wondering if anyone has had experience with airbox design and might offer some comments. Will I see large changes in airflow to either carb based on them sharing a box compared to individual filters? Is this solution possibly better than the individual filter setup? I can get the box volume once I finish it up, as well as the dimensions.

I know that with the individual filters the car makes a repeatable 168-170 rwhp on a Dynojet. I have the box just about fabbed up and can compare it to the individual filters someone in the future, but since that is quite a ways away if there are changes I need to make in the design I'd like to learn about them.

JeffYoung · Oct 19, 2010

From the reading I did back in the TR8 carb days, you basically want (for Constant Depression carbs) as "still" of a mass of cold/cool air in the vicinity of the carb mouth as you can get. You don't want to force feed CD carbs as it can impact pressure over the needle and thus mixture.

I like the "carbs in the box" design better though. I think it will help stabilize the air flow and just basically cool the interior of the box.

Do you have some of the heat shield I left over there? That stuff is right on top of the headers isn't it? Definitely shield it up. If you don't have any, I will get some from Tires & Parts on Thursday.

Ron Earp said:
I had some time to work on an airbox design for my Z car whilst my wife was recovering. The engine has two carbs, old SU knockoffs for all intensive purposes (square top 260Z US only carbs actually but easy to make work).

The car has had individual round filters for a very long time. But, they are cumbersome and really make working on the car difficult. I fabricated an airbox as shown in the attached picture. It is made from folded aluminum and mates up to both carbs on the front face. On the face of each carb is a "flow enhancer" which is basically a sort of thin air horn that provides a radius into the square mouth (yes, these carbs have a square mouth) and were dyno proven to provide four horsepower, repeatably. Box is fed through a large cone filter with 3.5" hole. Teh air horn is 5/8" tall, solid aluminum, and has a very gentle radius in the carbs. The air horns also hold the box to the carb by sandwiching one side of the box between the horns and the carb.

I was wondering if anyone has had experience with airbox design and might offer some comments. Will I see large changes in airflow to either carb based on them sharing a box compared to individual filters? Is this solution possibly better than the individual filter setup? I can get the box volume once I finish it up, as well as the dimensions.

I know that with the individual filters the car makes a repeatable 168-170 rwhp on a Dynojet. I have the box just about fabbed up and can compare it to the individual filters someone in the future, but since that is quite a ways away if there are changes I need to make in the design I'd like to learn about them.

Ron Earp · Oct 20, 2010

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ontVertAlignInTxbx/> <w:Word11KerningPairs/> <w:CachedColBalance/> </w:Compatibility> <w:BrowserLevel>MicrosoftInternetExplorer4</w:BrowserLevel> <m:mathPr> <m:mathFont m:val="Cambria Math"/> <m:brkBin m:val="before"/> <m:brkBinSub m:val="--"/> <m:smallFrac m:val="off"/> <m:dispDef/> <m:lMargin m:val="0"/> <m:rMargin m:val="0"/> <m:defJc m:val="centerGroup"/> <m:wrapIndent m:val="1440"/> <m:intLim m:val="subSup"/> <m:naryLim m:val="undOvr"/> </m:mathPr></w:WordDocument> </xml><![endif]--> I like the carb in a box design too, but, when I put that heat shield between the carbs and intake you said that was illegal. Which is why I went to a heat shield between the carbs and exhaust that also attached to the radiator, because, radiator ducting is whatever you want it to be.

So, what you're saying is attach the "carb box" to the airbox, and since the airbox is legal and free then the carb box will be too. Interesting but would be a PITA to work on. I do have a little heat shield between the carbs and exhaust. Could use a bit more but isn’t bad.

Any other thoughts on two carbs breathing through a common plenum/filter? I don’t think it should cause any trouble. From messing around with multi carb motorcycles I was able to get more power from a common box than four individual filters, one on each carb (no I wasn’t using flat slides), but, I kept the four individual filters because it was cool and sounded mean. Quite a bit different though since the bike was one carb per cylinder, and this is three cylinders per carb.

JeffYoung · Oct 20, 2010

Well, I didn't say it was illegal, the rules did. But it's not the box design that it is illegal, it is the attachment points -- as you note. You can build the box around the carbs so long as it attaches to something free: radiator, exhuast, air filter, etc.

See your second paragraph. May be too hard to do I agree.

But I think we are making this too hard, aren't we? Diagram 1 is essentially a multi carb "box" it just doesn't enclose the carbs. But it does enclose the radiused intake trumpet, which is really all you need. Should get air temp down, and give the carbs a big area of "undisturbed" air to draw from.

Induction/airbox design for dual carb system, inline engine

Ron Earp

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