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Discussion Starter · #1 ·
OK, this is from the crazy-ideas-on-the-commute-home department.

The D16A6 manifold is basically what old-school rodders call a "log" manifold, meaning that it has a long round plenum (the log) with a runner going to each pair of intake valves. So, though I, instead of going to a larger diameter throttle body, what if you got a second 56mm TB and attached it to the other end of the plenum?

You'd grind off the other end of the casting and weld, bolt, or epoxy (JB weld of course) an aluminum adapter plate on, to which you would bolt the second TB.

You could use a progressive linkage like multiple carb setups sometimes use, so that perhaps the second TB doesn't start to open until the normal one is 25% open. This gives you more controllable part-throttle response so it doesn't drive you nuts in stop-and-go traffic. The linkage would have to be set up so both TBs would be fully open at WOT.

Now since you've effectively doubled the area of the TB this is equivalent to a 79mm TB (assuming my math is right). An aftermarket intake should be easy to adapt for an intake pipe and filter for the second TB.

Probably you have to either a) electronically adjust the throttle position signal or (easier) b) chip the ECU so that you can adjust for the slightly strange flow vs throttle position characteristics of this setup. I think since this is a speed-density ECU it's got to have a table that you can modify for intake efficiency vs RPM which is probably further modified by a lookup table (or maybe a simple function) based on throttle position.

Probably this isn't going to help much unless you've also got at least a header and improved exhaust.

Is this a crazy idea? Or has somebody already tried something like this?

It wouldn't be very expensive to implement, as all you'd need is another stock TB, intake pipe and filter, and a piece of stock to make the TB mount from.

The progressive linkage should keep you from having drivability problems that can come from a larger TB.

Airflow in the plenum might be reversing in the middle as the different cylinders sucked air. But log mainfolds for v8's with multiple carbs seem to work ok, and the same thing would happen there.

Maybe the limiting factor would be the size of the runners in the stock D16A6 manifold, and more air into the plenum wouldn't really help. What kinds of gains do people see going from 56mm TB to 60mm (+14% area)?

Any omments from the D16 hackers out there?
 

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First, i think the mods to the intake wouldn't be worthwhile. You could machine off the end, but there really isn't enough meat there for mounting bosses.

Second, its a bit more of a fluids air flow problem that that. The first thing to remember is that your engine (air pump) can inly pump ir through it at a certain mximum rate depending on your build. So long as your TB can flow that much air at the proper velocity, you don't really gain anything by increasing the TB cross section. Plus you would have air coming in at opposite sides hitting each other creating a lot of turbulence which is not what you want for a speedy intake air flow. You want a smooth flow.
I think that if you just slapped another TB on you would also have to make them staged, like a carburetor, where the second one starts to open after the first one has opened all the way.

Just my thoughts after thinking about it for 2 minutes.
 

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It seems like it would make more sense if you mounted them to the back of the manifold, so each runner would have 2 throttle bodies. Thats how dual weber carbs for the rex are set up, so I would think that would be a better way to mount them
 

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Discussion Starter · #7 ·
rex2nr said:
It seems like it would make more sense if you mounted them to the back of the manifold, so each runner would have 2 throttle bodies. Thats how dual weber carbs for the rex are set up, so I would think that would be a better way to mount them
Assuming you meant each throttle body would serve 2 runners of course...

This'd be a bit more work, cut 2 holes in the back of the manifold with a hole saw and make mounting blocks for the TBs to bolt to. The mounting block with a curved back to fit against the manifold could be u-bolted in place with appropriate gasket material. Probably you'd have to angle them down at least 45 degrees to clear the firewall. And you'd have to come up with some fiddly solution to provide the EACV with a connection to the intake.

Yeah all right ITBs is looking better. The main advantage to the original scheme is that you could do it really cheap.

jmart, the main reason I don't think you'd want to stage them sequentially is you'd have to synthesize a TP signal for the ECU. However, you could do this with a simple analog circuit (op-amp and a couple resisters) that averages the TP signals from the two TB's. Then when the first is at WOT and the second has not started to open, you get a TP signal that's 1/2 WOT, and you don't get the full WOT level until both TB's are all the way open. The same circuit would also handle an overlapping staging as well.

Let's go back to the question of air reversal in the manifold. The firing order is 1-3-4-2, and #4 is closest to the stock TB. During the period of time when you have #3 intake, and then #4 intake, you'll get some reversal of air flow within the plenum anyway, as the air from the TB is headed first for #3, and then for #4. During this time period the lowest pressure in the plenum will be at the end of the #3 and then #4 intake runner. There will be extra air at the #1/#2 end that had started moving there before the #1 intake cycle ended. After #1 closes, this air in motion has no where to go and will pile up there resulting in a slightly higher pressure at that end.

Hmm, back-of-the-envelope time. The engine needs 1600 cm^3 of air every 2 revolutions (assuming 100% volumetric efficiency which is optimistic) so at 7000 rpm it would need 3500*1600=5600000 cm^3 of air per minute. This has to pass through the 56mm TB which has area 2.8*2.8*3.14159 cm^2 = 24.63 cm^2 which implies an intake velocity of about (5600000/60)/24.63 = 37.9 m/s. This is well below sound speed (344 m/s) so I'm not sure how much of a reflected shock wave we'll see when when #1 intake closes. Probably there will be some reflection though. [edit to fix embarrasing snafu with rpm vs rev/sec]

Anybody know how close the peak MAP sensor readings get to ambient (i.e. what the MAP sensor says when the engine is not running)? That would give some indication of what the volumetric efficiency of the TB is.

Getting to the point of all this, when #1 intake reopens, the air in the plenum may be moving away from it. Adding the second TB at the other end could help the induction into #1 and #2. In fact, then you have a symmetrical situation. #3 and #4 have their intake one after another, moving toward the stock TB. Then, #2 and #1 have their intake one after another moving toward the 2nd TB.
 

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sorry, that is what I meant, 2 runners per TB, lol

shut it, I'm sick as hell right now... had a fever of 101 when I posted that, lol. That, and I was mixing meds I probably shouldn't have, especially in the amounts I took, lol.
 

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Anything is possible, but i think i'd rather close off the current TB end, and form a new plenum, and then aim the two TBs straight up in the air... then just connect the two linkages together. Then, create a cowl for it using the high pressure at the lower edge of the windshield to force-feed while driving.

Its just time, $$ fabrication and time... will it work? Who knows, thats what experiments are all about 8)
 
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