2.8 Dual throttle body intake manifold question (Page 1/5)
Boot MAY 30, 10:46 AM


Ive been thinking about making something similar to this. But theres one thing about this one in particular that confuses me a little. Arent both of those throttle bodies stock fiero ones? Because everything ive ever heard says that two 52mm's would be way too much throttle body for the engine and theyd outflow it by a mile. I did some calculations and the 40mm throttle bodies from a late nineties GSXR750 crotchrocket would be about the right size. Or would it even make a noticeable difference? From my understanding its better to have throttle bodies that match the max cfm of your engine when theyre open all the way, because the butterfly is flat which causes less turbulence than one that outflows your engine when its only 1/2 or 3/4 open.

Am I overthinking this, or no?

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quote
Originally posted by Frank2:

here's the goal: Build a mid engine car 1/2 as good as a Lotus Elise at 1/10 the cost.



I have no idea what Im doing, but Im doing it.

IG: @giveintogravity
Blacktree MAY 30, 11:39 AM
If it were two throttle bodies feeding a common plenum, then yes it would be grossly oversize. But those are two separate plenums. So you only have one throttle body feeding each cylinder. It may seem counterintuitive, but that's how the math works out.
2.5 MAY 30, 12:01 PM

quote
Originally posted by Blacktree:

If it were two throttle bodies feeding a common plenum, then yes it would be grossly oversize. But those are two separate plenums. So you only have one throttle body feeding each cylinder. It may seem counterintuitive, but that's how the math works out.



You mean each cylinder bank (of 3) right?
But each still doubled it's input correct?
Boot MAY 30, 01:06 PM

quote
Originally posted by Blacktree:

If it were two throttle bodies feeding a common plenum, then yes it would be grossly oversize. But those are two separate plenums. So you only have one throttle body feeding each cylinder. It may seem counterintuitive, but that's how the math works out.



2.5 is right... When I did the math I calculated it as a 3 cylinder 1.4L engine with a common plenum and it came out to between 38-42mm depending on the application. Since this engine could be thought of as two 1.4L 3's stuck together wouldnt that work?

------------------

quote
Originally posted by Frank2:

here's the goal: Build a mid engine car 1/2 as good as a Lotus Elise at 1/10 the cost.



I have no idea what Im doing, but Im doing it.

IG: @giveintogravity
fieroguru MAY 30, 02:44 PM
As long as you can modulate the opening rate to get smooth throttle tip in performance, I don't think there is such a thing as too large of a throttle body.
2.5 MAY 30, 03:03 PM

quote
Originally posted by fieroguru:

As long as you can modulate the opening rate to get smooth throttle tip in performance, I don't think there is such a thing as too large of a throttle body.



'Cause its only gonna suck as much air as it can suck?

The computer/injectors just need to know how much fuel to match it with I suppose.
Patrick MAY 30, 04:30 PM

quote
Originally posted by fieroguru:

I don't think there is such a thing as too large of a throttle body.



I think where some of us get tripped up on this is that in the good ol' days, there was such as thing as too large of a carburetor. It's my understanding that this doesn't apply to a throttle body as (in the case of the 2.8) it's only supplying air and not an air/fuel mixture. Out of curiosity though, can throttle body injection have an issue with being "too large"?

[This message has been edited by Patrick (edited 05-30-2017).]

Blacktree MAY 30, 04:36 PM
With dual throttle bodies, throttle input will be more aggressive than the stock setup*. So the fuel response to throttle input needs to be more aggressive, as well. Otherwise, the engine will go lean on acceleration.

I realize the math says that a 1.4 liter engine needs a smaller throttle body. But if you look at cars with small engines, most of them have larger throttle bodies. I spent quite some time combing through salvage yards looking for cars with small engines, so I could grab a couple small throttle bodies to use on a dual plenum project. Almost everything had a throttle bore of 50mm or larger, even the Hondas with 1.5 liter engines. As a matter of fact, the only car I could find with a smaller throttle body was a Geo Metro with a 3-cylinder. And it was a TBI unit, so I couldn't use it anyway. So if Honda can get away with a 50mm throttle body on a 1.5 liter engine, then we should be able to make a 52mm throttle body work on a 1.4 liter engine (i.e. half of a 2.8 V6).

* To make the car more driveable, you would use a progressive throttle mechanism that opens the throttles slowly at first, then ramps up.
Patrick MAY 30, 05:30 PM

quote
Originally posted by Blacktree:

So if Honda can get away with a 50mm throttle body on a 1.5 liter engine, then we should be able to make a 52mm throttle body work on a 1.4 liter engine (i.e. half of a 2.8 V6).



Are you sure you're not comparing apples with oranges? Comparing a DOHC engine that flows and revs quite a bit higher than a much more archaic pushrod engine?
Boot MAY 30, 05:42 PM
You're right about the small engines using larger throttle bodies. But often those small engines rev a lot higher than our pushrod v6's... A honda might have its throttle body sized for peak power at 6k rpm while ours makes its peak at 4.

Here, I made a crude paint drawing to show what Im saying with the larger tb causing turbulence.



In that drawing, both are flowing as much air as the engine can breathe. The 52mm is open halfway and the 40mm is open fully, because the 52mm flows more overall than the 40mm.

[This message has been edited by Boot (edited 05-30-2017).]